DIY CR250 engine porting

[dmcca]

Ive been waiting to open up my 07 CR250 engine for about 18 months now, pretty much since the day i bought it. I have been fascinated with 2 strokes for years but never knew much about how they really worked (ports, squish, head design, reeds, etc, etc). So after reading as many books as i can and trawling the internet for info i am ready to tear into my engine with the hope of modifying it for a better spread of power. Specifically i race enduros so i am looking for more bottom end torque and a nice broad spread of power.

First of all let me just say that this thread could get quite long and may take me a few weeks to complete, and the end resut may very well be that i have trashed my engine! But you can't choose what you're fascinated in, and i am determined to learn about 2 stroke porting, etc even if it means learning by my mistakes. I am by no means an expert, far from it, i am a relative beginner who is happy to sacrifice an engine if necessary. I'm more than happy to be corrected along the way if you think i'm doing something wrong, and welcome input and suggestions at any time of the 'surgery'.

I have learned a huge amount of theory over the last year or so and i won't explain it all here (cause its just too involved and i'm still struggling with it) but i can recommend 2 sources of info for anyone who is interested... join the site macdizzy.com, or at least check out the info outside the forum there (this can be done for free, and before you ask, no i don't have anything to do with that site!)), it really is brilliant if you're into this stuff, secondly i can highly recommend the book "Two Stroke Performance Tuning" by A. Graham Bell. He takes a slightly different (simpler) approach that others, which i found fairly simple to understand (after about the 100'th time i'd read it!). I have also been buying equipment to do the job with... i bought a metal lathe, a basic rotary tool with grinding bits, and have several other things needed to disassemble and reassembel the engine. Although i have done dozens of top end rebuild i have never split the cases of any bike so that alone will be a big achievement for me.

As there is so much info to consider when making changes to an engine I'm going to go through it in the order that i do things in the shed... first of all measure and assess the current set up on the engine to determine where its power is intended to lie, secondly make a plan of what new power characteristics i want and how each element of the engine needs to be modified to meet those characteristics, thirdly make the necessary changes, and finally cross fingers that it works!

So lets begin.....

So some things i know about the engine...

- Its has a good mid range hit with the stock pipe

- It is a pain to jet properly

- It often comes with an overly large squish clearance which i have already fixed)

- its crankcase volume is too large (at least thats what the experts say) but i dont really know how this affects things.

- its transfer port tunnels are too large (this also adds to the crank volume, once again this is the experts talking)

- it has less bottom end than say a yz250, but similar top end i think.

- some people say that the case reed setup is to blame for some of its problems but KTMs use case reed induction and they are some of the best 2 strokes ever made, so there is hope.

- aparently once set up properly the cr engine can be made to rip!

OK so that what i started out knowing, now lets find out more...

- i need to measure the crankcase volume to see why people say its too large

- i need to measure the cylinder and its ports to find out the durations of each port so i can determine where its power should lie (using A Graham Bells book as a reference)

- i need to look closely at the head and its design

- i need to look for anything which will restrict flow or cause unwanted turbulence (smooth things out)

Crankcase Volume:

Crankcase volume is calculated by measuring the volume inside the cases when the cylinder is at TDC. It includes everything underneath the piston, including the volume of the transfer tunnels. It is measured by filling the cases with liquid. To be honest i still dont fully understand its importance, other than that fact that people say its important!

To measure it i removed the head and installed an old piston which i drilled a hole in. I removed the reeds and filled them with grease so they sealed, then re-installed them. I also filled the exhaust port with grease so that nothing could pass beyond the exhaust port. I then used a 250cc measure to pour in water (i should have used 50/50 kerosene and oil but didnt have any handy).

In total it took 895cc of water until the level inside the engine was just touching the underside of the piston...

Filling the cases with water...

The old hole in the old piston trick...

The exhaust port packed with grease to hold the water in...

Now i have read that a good crank case compression ratio (CCR) is around 1.4 or 1.5:1, with the equation being:

CCR = CCV/(CCV-displacement)

So in this case...

CCV = 895/(895-249cc)

= 1.385 (or 1.385:1)

So this number is slightly off the scale of what is considered 'good'. Aparently a large CCV will lead to a lower pressure blast of mixture up into the cylinder via the transfer ports as the piston comes back down from TDC, so it makes sense that a lower CCV will give a larger pressure blast of mixture, therefore assisting with scavenging of the burnt gases out of the cylinder through the exhaust port (the pipe and transfer port shape also has a big impact here but i'll leave it at that for now). From what i have read a lower CCV generally gives better bottom end power due to this rise in pressure of the incoming charge... so i'm heading in the right direction if i'm looking at reducing the CCV.

So if we were aiming at a CCR of say 1.45:1 we can work backwards to get the desired CCV...

1.45 = CCV/(CCV - 249cc)

blah, blah, blah, the answer ends up being... CCV = 800cc.

So i will aim to get the CCV down to around 800cc's, but that depends on what i find inside the cases and how easy it is to take up 95cc of volume! I think that 95cc could be a big ask. Before i undertake this work ill try to find out the CCR of a bike like a YZ250 as this will give me a good idea of where to aim. To be honest i am guessing that a reasonable number may be more like be 30-50ccs, simply because 95ccs is a lot of epoxy!

[dmcca]

Port Durations...

Now is the time to check out macdizzy.com if you havent already... there is an excellent article on creating a port map and then calculating the time area of the ports. I wont actually use this info here, but its good to have an understanding of how its done.

I'm going to use A Graham Bells approach simply by looking at the duration (in degrees of crank rotation) of each port to determine the characteristics of the ports. This is also a good time to look at his book if you havent done so already. By using a degree wheel (which i found a picture of on google,, then printed it out) stuck to the outside of the stator you can easily calculate how many degrees of crank rotation each port is open for, obviously one full rotaion of the crank = 360 degrees. First of all you have to align the zero mark on the degree wheel to correspond with TDC (use a bit of ware bolted to the side of the case and bend it to point at the zero mark). If you then rotate the engine by hand and stop exactly at the point where the top of exhaust port is just visible beside the top of the piston you can then read off the measurement from the degree wheel. In this case i got 86 degrees after top dead centre (ATDC), so it stands to reason that this port is open for a total of 188 degrees (as it keeps moving the piston travels a further 4 degrees until it hits 90 ATDC, then logically another 180 degrees until its back at that point, then plus a further 4 again to get back to the top of the port = 188 degrees)

You can do the same with the top of the transfer ports... they opened 61 degrees before BDC (BBDC), so once again you can work out that the time from when the piston uncovers the top of the transfer port until when it comes back around and covers it up again is 122 degrees.

Now using A Graham Bells book as a reference it is fairly well known that a certain duration of opening corresponds to the maximum potential of that port being reached at a certain rpm, regardless of engine size. Looking at the tables in his book it shows that an exhaust port duration of 188 degrees corresponds to max power at around 9500 rpm. That means that an engine with an exhaust port duration of 188 degrees will develop its max power at 9500rpm assuming all other elements of the engine are tuned for the same rpm, regardless of whether its 80cc or 500cc.

It also shows that a transfer duration of 122 degrees should give max power at about 6500-7000 rpm, so the ports on this engine are mismatched in their duration and are not working together as they should.

From what i have read a good max power for offroad riding on a 250 two stroke is around 7000-7500rpm. Using the book once again, it shows that for max power at 7-7500 rpm exhaust duration be about 180 degrees and transfer duration about 122 degrees. So our transfers are ok but the exhaust port needs to be modified.

There are two ways to decrease the duration of the exhaust port, one is to weld up the top of the port so that the piston uncovers it later in the stroke, the other is to machine a little off the bottom of the cylinder so that it drops the whole cylinder (exhaust and transfer port) in relation to the piston. The added benefit of doing either is that you increase the 'power stroke'... the mixture in a 2 stroke isnt really compressed properly until the exhaust port is covered by the piston, so by lowering the roof of the exhaust port you effectively increase the compression, which also adds to bottom end... exactly what i want. The second option is best as it means you dont have to re-line the cylinder (yet), and it can easily be done on a lathe, however it also drops the transfer port therefore reducing its duration, and you also need to machine the head to account for the fact that the piston will extend further into the squish area... i will deal with these later.

So far i have learned that two possibilities for getting more low end power from this engine are:

reducing the CCV and,

reducing the duration of the exhaust port.

They are two things i didnt know yesterday so i think i'm heading in the right direction...

More pics and more info to come in the next few days as i keep pulling the engine apart.

Dave.

[jzook]

I love seeing threads like this on TT. Great work and keep it up. Thanks for citing the your sources so others, like myself, can read up on how to do this kind of work.

[PowerFiend]

I'm very interested in the data that you are gathering. Please also take some rough caliper measurements of the cases. I will be doing an 02 topend soon and would like to see how the case volume/design differs from the 07. I will try to add the 02 port timing to this thread if I can get some good measurements.

Also, why not compare apples to apples, and compare the CR to a KTM rather than a YZ?

[Canyon 701SM]

Dang! You don't have to reinvent the motor. If you look around, there is plenty of info on getting these engines to run as good as they can.

[rayivers]

I just had a few comments - take them for whatever they're worth to you:

1) I would re-read pp. 48-49 of "Two-Stroke Performance Tuning" while keeping in mind that a) your '07 CR motor probably breathes much better stock than any of the modified motors described in the book, your transfers are designed to work with the existing CCR, and c) modern exhaust systems perform the scavenging function much better than the ones in common use when most of this book was written. By increasing the CCR substantially in a motor that already has excellent transfer and scavenging action, you may end up with an 'over-scavenged' engine blasting most of the fresh crankcase charge well down the pipe, too far for the convergent cone to return it to the cylinder.

Here's a sentence from page 49, describing the results of the Japanese companies' extensive efforts to increase CCR: "True, power outputs rose to levels previously unknown from two-strokes, but the power bands became razor-thin and engine speeds rose to incredible levels". He then goes on to mention 18,000 rpm, 18-speed road racers. That doesn't seem to describe a low-end boost to me.

2) I'd highly suggest purchasing a genuine degree wheel, instead of one printed out from an on-line picture. And surely at least the 2007 CR250 port-duration specs, if not the actual port diagrams, are available somewhere? They used to be included in the Honda service manuals.

3) If you're going to modify the exhaust port duration, why not try doing it using the power valve actuator and/or linkage if possible, rather than welding or shimming the cylinder?

Don't get me wrong - I'm sure that there's more power to be had from your motor - but I think the Bell book's techniques are more applicable to motors of the 60's and 70's than those of today, many of which are making close to the peak road-racer power levels mentioned in the book, but with wide powerbands and high reliability as well. It would really suck to do a whole bunch of work and end up with a motor making less power than stock over a narrower rpm range.

If it were my motor, I might look into slightly more compression, different ignition maps, changing the power valve engagement point if possible, more responsive reeds, perhaps a different pipe, getting the jetting just right, gearing, etc. - all fairly easy to do.

Ray

[dmcca]

I'm very interested in the data that you are gathering. Please also take some rough caliper measurements of the cases. I will be doing an 02 topend soon and would like to see how the case volume/design differs from the 07. I will try to add the 02 port timing to this thread if I can get some good measurements.

Also, why not compare apples to apples, and compare the CR to a KTM rather than a YZ?

Good point regarding the KTM. I will try to do that, but it will probably rely on someone else measuring and posting it here. I'll see what i can find.

Dang! You don't have to reinvent the motor. If you look around, there is plenty of info on getting these engines to run as good as they can.

I know that there is a lot that can be done to get more from this motor without tearing it down, am i've already done a lot of it before. To be honest my engine is already probably the best i have ridden on a CR in terms of being set up for the style of riding i do.

There are two things that come up time and again though... the experts say that the CCV is just too large for it to have good bottom end power, and the transfer tunnels are too wide... I want to understand why they say this, not just take their word for it, thats just the sort of person i am. Besides, you can't fix either of those with a bolt on part!

To be honest the main reason why i am doing this is for personal interest. It has more to do with learning that it has to do with getting 110% from the engine.

I just had a few comments - take them for whatever they're worth to you:

1) I would re-read pp. 48-49 of "Two-Stroke Performance Tuning" while keeping in mind that a) your '07 CR motor probably breathes much better stock than any of the modified motors described in the book, ? your transfers are designed to work with the existing CCR, and c) modern exhaust systems perform the scavenging function much better than the ones in common use when most of this book was written. By increasing the CCR substantially in a motor that already has excellent transfer and scavenging action, you may end up with an 'over-scavenged' engine blasting most of the fresh crankcase charge well down the pipe, too far for the convergent cone to return it to the cylinder.

Here's a sentence from page 49, describing the results of the Japanese companies' extensive efforts to increase CCR: "True, power outputs rose to levels previously unknown from two-strokes, but the power bands became razor-thin and engine speeds rose to incredible levels". He then goes on to mention 18,000 rpm, 18-speed road racers. That doesn't seem to describe a low-end boost to me.

2) I'd highly suggest purchasing a genuine degree wheel, instead of one printed out from an on-line picture. And surely at least the 2007 CR250 port-duration specs, if not the actual port diagrams, are available somewhere? They used to be included in the Honda service manuals.

3) If you're going to modify the exhaust port duration, why not try doing it using the power valve actuator and/or linkage if possible, rather than welding or shimming the cylinder?

Don't get me wrong - I'm sure that there's more power to be had from your motor - but I think the Bell book's techniques are more applicable to motors of the 60's and 70's than those of today, many of which are making close to the peak road-racer power levels mentioned in the book, but with wide powerbands and high reliability as well. It would really suck to do a whole bunch of work and end up with a motor making less power than stock over a narrower rpm range.

If it were my motor, I might look into slightly more compression, different ignition maps, changing the power valve engagement point if possible, more responsive reeds, perhaps a different pipe, getting the jetting just right, gearing, etc. - all fairly easy to do.

Ray

Ray, Thanks for your comments, i do appreciate your opinion. I have already done every one of the things that you mentioned above... pipes, compression, jetting, adjust power valve, adjust ignition, looked at preformance of the reeds, gearing, squish band design, combustion chamber design, etc. As i said above the bike already runs very well for my style of riding. I race state level enduros and thoroughly set up my bike for each race by changing pipe, jetting, gearing, muffler, suspension, etc, so i'm not doing this due to a lack of performance.

Regarding the other points you made:

1. You are right that the book was written for earlier model engines, and my CR would perform better than most already. But that doesnt mean its performing at optimum level. As i have found the ports are tuned for different speeds... although i wont go into it here i have also calculated the time area of the ports and this data supports what i discussed in my post above. So there is room for improvement in this area. I feel that p49 of the book is also really referring to much much higher crank pressures (and transfer durations below 110 degrees), where almost all area within the crank is filled, the CR is way off this level of CCR, so although there are limits to the benefit of decreasing CCV at this stage i dont think the CR is near those limits. Also, regarding scavenging it may already be good already but i'm going to see if i can improve the scavenging at lower rpm if possible. p49 also says that a bike which is 'pipey' with a narrow power range is often caused by too low ports or incorrectly aimed ports. The CR stock is defintely pipey... not much bottom end, huge mid and signs off early up top. The transfers are very short compared to the exhaust duration and if you look at the kicker on the rear transfers they almost point straight out the exhaust port... this is pretty much what the book describes as a pipey engine.

2. I have searched for 18 months to find the oficial port durations, even Honda in Australia can't give me a definite answer and its not in the manual. Although my degree wheel is a bit dodgy it does the trick, just. However i have re entered my measurements into a port duration spreadsheet that i have and i think i'm going to revise my exhaust duration to about 186-187 degrees based on the results this gave (from 188 i got using the degree wheel). The spreadsheet is a more accurate way of doing it i think as it relies on maths rather than hand/eye coordination.

3. I have already played with the PV actuator before and had good results. However this will only affect the power up to the poibnt that the PV is fully open, from then the power characteristic of the port is determined by its height (and shape) so it would still have the same peak potential (in terms of rpm)

I can understand your advice to avoid potentially costly engine damage but i am not going into this lightly... it's a passion of mine and as i've already mentioned i'm doing it more for learning than anything else. Besides i already have a spare cylinder and there is very little that i will de doing that can't be undone with a little effort.

One thing you may be very interested in regarding your comments on the transfers and scavenging, etc... When comparing my two cylinders (both are currently stock) there is a 1.5mm difference in the height of the transfer ports between the two! This is a huge difference in duration. Also one of the cylinders has all transfers and boost ports open at exactly the same height, the other has the transfers on one side open first, the boost port open about .75mm later and then the other transfers open last... this defintiely won't lead to good scavenging at all! In fact it was this mismatched cylinder that came with the bike when i bought it new! Also there is a HUGE mismatch of the transfer tunnels where they meet the cases on both cylinders.

One thing that has been drilled into me by a couple of engine tuners that i know is that just cause its made by a big manufacturer doesn't mean its made well! My cylinders prove that point. The fact is the CR in theory is a fantastic bike, but it does suffer from excessive manufacturing tolerances that can give very different results in performance between two bikes. Basically i am learning that every engine is differnt, even if they share the same model number and year of manfacture.

Dave.

[rayivers]

Dave,

OK, I understand better now what you're trying to do - and I think it's pretty cool! I tend to think of the latest factory engines as "the best that can be done", but who knows if that's true or not? Certainly not me.

From surfing the Web I agree with your observation that 'identical' engines from any given make/model/year will exhibit different levels of performance, reliability, vibration, etc. Your findings of 1.5mm port height differences between cylinders (!) and port mismatching certainly bear this out. I've read so many times that current factory bikes are often basically blueprinted production bikes tweaked for individual rider preferences.

There's one thing I've wondered about increasing CCR since the first time I read about it (in the Bell book); if there's less crankcase space, wouldn't there be less mixture available to transfer to the cylinder? I guess it would only be an issue at absolute max power, if then.

I had two ancient 175 cylinders ported recently, in a very similar way to what you're thinking of doing; transfer duration increased to match the time/area of the exhaust, transfers re-aimed and staggered (rears opening first) along with a lowered intake and widened (but not raised) exhaust. Like an idiot I put the first one on without double-checking the work, and I noticed very little improvement over stock. The 2nd cylinder I went over carefully, and had to re-do a lot of it (the intake and exhaust weren't bad, but the transfers and passages were pretty much a joke), match to the cases. etc. I can't wait until warmer weather (or at least no snow!) to finally see what it's like.

Ray

[dmcca]

Port Durations continued...

After some careful checking of my measurements (and double checking with a spreadsheet) I have revised my exhaust port duration to 186 degrees from 188 which i said it was before. This still has a max potential at around 9000rpm, which is higher than the 7-7,500 i am hoping for.

Splitting the cases...

This was a very easy job, the whole thing only took a short while and there were no surprises along the way. I don't know why i waited all these years before attempting it!

Grinding and filling time...

Here are two very useful threads on this topic...

This one by Doctor D about matching cases and smoothing flow lines...

https://www.thumpertalk.com/forum/showthread.php?t=669610&highlight=cr125+hidden+horsepower

And this one by Crazyced with some great photos of his professional porting work...

https://www.thumpertalk.com/forum/showthread.php?t=702087&highlight=cr250+complete+overhaul

Upon opening the engine up it became obvious that the entrance to the transfer tunnels (where the case meets the cylinder) is very large compared to the port windows themselves, and that this area could definitely be tidied up and smoothed out for better flow. Having read a couple of other posts here on TT i had already seen other people match the cylinder to the cases as well as use epoxy to smooth the flow in this area. You can see in these photos where i used epoxy to create a beter flow path and also the areas where i removed aluminium to match the ports and smooth some of the rough areas.

Check out the huge transfer tunnels!

Right side case before any work

Marking out where to cut

Left side case half way through the operation

Slowly getting better flow lines into the cylinder

Marking out where to cut to match the ports

Just doing this stuff took about 3-4 hours and I ended up removing a lot of material as well as adding a lot of material to the cases. The end result is that total volume would be only slightly lower now but the flow lines look much better and the transition to the transfer tunnels is much better.

The other area where i think i could take up volume was the transfer tunnels themselves. They are huge compared to the port windows and as long as i keep the tunnels at least the same area as the windows themselves i should be ok. Think of it like a garden hose... it will only flow as fast as the tightest point so having a 2 inch hose with a 1 inch outlet wont actually flow any faster than a 1 inch hose with a 1 inch outlet. Based on that analogy the CR looks to have about a 3 inch hose!

Using epoxy in the transfer tunnels is one area where i can definitely gain low end power too. Not only will it help to reduce the CCV a little, but if i do it correctly i should be able to improve the low rpm scavenging (2000-5000 rpm). This is something Eric Gorr talks about in one of his books... that low end scavenging (and therefore power) can be improved by directing the transfer ports directly at each other from opposite sides of the cylinder. This helps to clear out more of the exhaust gasses at low rpm. Fortunately for me he also has a couple of photos of this in his book so i kind of know what to do here...

The cylinder before any work

After some work (not fully completed)

Close up after some grinding of the epoxy

Close up of the rear transfer hook filled with epoxy

Smoothing the flow into the boost port

I can use a line of epoxy all the way down the side of the transfer tunnels (to take up volume) and then also a small amount just near the port window to fill in the 'hooks' in the tunnel which direct the mixture forward or backwards... you can see the effect of these hooks in the following photos... using a hose you can see the flow of water to get an idea of how the port directs the mixture...

It is obvious that these hooks have quite an impact and that the rear transfer ports actually direct a lot of mixture straight out the exhaust port which is not good! So my aim here is to take up space in the tunnels and re-direct the ports so that the flow goes directly across the cylinder (or similar)... you can see all of this in these photos...

Rear transfer port before epoxy

Rear transfer after epoxy

Front transfer port before epoxy

Front transfer after epoxy

Boost port

I am confident that the transfer ports now direct their mixture directly across the cylinder and so i am hopefull that these changes will help low end power... if it doesn't i might have to have a quiet word with Eric Gorr! Just kidding EG. If i had to guess at how much material i had added in total to the cases and cylinder i would say it was only around 20-30cc at best, a long way off what i had first assumed. Although the CCV will only be reduced by a small amount i am fairly confident that the changes i have made so far will make a very noticeable difference... even though i have never done this before it was intuitively obvious when i first saw the cases and the cylinder that these areas could be improved upon, and i am confident that the changes i have made are still within reasonable limits.

More to come soon...

Dave.

[98cr250r]

Dave,

My bike is still ripped apart, but I havn't had time to do anything with it ?

I'm wondering about large CCV... I havn't been able to check mine yet, but... Does a large CCV mean high end? Could be mistaken, but it seems like every bike that has "too large ccv", when its corrected, they boast improved low end. Does that in anyway mean that large CCV helps top end, and small CCV helps low end? CCV isn't the only thing corrected in the bikes I'm talking about though, so its not answering the effect of changing the CCV only.

I've wondered about the hook on the rear transfer. Its a heckuva hook, and I figured it would do what your picture with water showed. (wish I woulda thought of using water to see it). I've thought about getting rid of the hooks on the rears, but not too sure if I should be using epoxy on the front side to keep the window the same, or what the effect would be of having the larger window.

I'm not scared of messing up a cylinder... I'm scared of gaining a lot of low end... but losing my top end in the process!

::edit:: what are you using for epoxy?

[dmcca]

Dave,

My bike is still ripped apart, but I havn't had time to do anything with it ?

I'm wondering about large CCV... I havn't been able to check mine yet, but... Does a large CCV mean high end? Could be mistaken, but it seems like every bike that has "too large ccv", when its corrected, they boast improved low end. Does that in anyway mean that large CCV helps top end, and small CCV helps low end? CCV isn't the only thing corrected in the bikes I'm talking about though, so its not answering the effect of changing the CCV only.

I've wondered about the hook on the rear transfer. Its a heckuva hook, and I figured it would do what your picture with water showed. (wish I woulda thought of using water to see it). I've thought about getting rid of the hooks on the rears, but not too sure if I should be using epoxy on the front side to keep the window the same, or what the effect would be of having the larger window.

I'm not scared of messing up a cylinder... I'm scared of gaining a lot of low end... but losing my top end in the process!

::edit:: what are you using for epoxy?

From what i have read i think there is a very loose connection between low CCV and Low end power and vice versa. Of course this relies on other things being changed in the engine and there are certain limits to CCV in terms of what is too low and what is too high. I believe that in stock form the CR is on the too high limit, but i have very little to back this up other than what i have read some professional tuners say, as well as reading that the magic number for CCR being somewhere between 1.4 or 1.5:1 (The CR was 1.3 something).

There is some good info on CCR and CCV on www.macdizzy.com, but you have to pay a membership fee to be able to view that section.

Regarding changing the transfers on your bike, what are your goals for improving your engine? Knowing that will determine what things you should look at changing. Re-directing the transfers will definitely gain low end power and may lose a little top end depending on how much you re-angle them. Obviously doing this only affects scavenging, it has no impact on when the pressure waves act to draw mixture into the cylinder (that is determined by CCV, port duration and expansion chamber mainly)... so although it will have an impact it shouldn't be as significant as if you changed the durations of the ports. If you are conservative you should be fine, besides it is very easy to grind away epoxy if you find you have too much in there! Worst case is that you get rid of all the epoxy and you're back to a stock cylinder! Changing the port durations of the transfers will definitely cost top end if you go in the wrong direction.

I am using two types of epoxy... for the cases i have used a 2 part liquid epoxy (its a rip off of JB Weld) and it is easy to use and easy to grind once set. I used this stuff in several layers to get the final result. On the transfer tunnels and port windows i have used a 2 part putty epoxy (i forget the name, something like Quiksteel) but it comes in a tube and you kneed it together then just push it into place. This stuff sets really well, much quicker than the liquid stuff, so only take off as much as you need for each small area at a time. I actually carefully measured out what i was using so i knew i was adding the same volume to each transfer tunnel on each side.

[Altune]

Your post is interesting.

I have been involved with porting over 10 years now.

I think you may have found the problem on this engine, large crankcase volume, large runners, ie transfers = flat or no low end.

what is called for on cfm vrs rpm and hp output ?

strangley on large cid race engines we make as much flow as possible.

depending on compression , cam profile etc.

[PowerFiend]

An observation that I have is if you have a low CCR, high CCV, your air fuel will be shot back into the cylinder at a low speed. With the large transfer tunnel compared to the transfer port size you also have an area for the a/f mix to lose velocity. A low velocity air fuel mix can cause the fuel to drop out of a vapor state and become liquid. This is bad for combustion efficiency(ie. power).

[dmcca]

An observation that I have is if you have a low CCR, high CCV, your air fuel will be shot back into the cylinder at a low speed. With the large transfer tunnel compared to the transfer port size you also have an area for the a/f mix to lose velocity. A low velocity air fuel mix can cause the fuel to drop out of a vapor state and become liquid. This is bad for combustion efficiency(ie. power).

Thats a good point... i hadn't thought of it in terms of vaporisation of the fuel.

Im glad that so far all the things that have come out during this project all seem to support one another and also possibly explain why the CR is know as having a poor low rpm performance. Your point just further supports all this:thumbsup:

I wonder if this also helps to explain some of the jetting issues with the CR. I have always found it very difficult to get a good clean, burn, even with correct squish, correct fuel, etc, etc. The burn marks on the head have never been as clean or as clear as say on my old KTM200. They are always off centre (bad port alignment probably) and there are always extreme contrasting areas of light and dark burn marks on the head showing the areas of high and low velocity during the burn... as opposed to a nice tan colour burn marks the KTM used to have evenly over the head.

[98cr250r]

As far as what I'd like to see out of it, I'd like to get more bottom end, losing as little top as possible. What I've been thinking about doing, is to put in a thin base gasket, which brings my squish to just under .050" (I don't remember exactly). I was thinking about grinding off the hooks on the rear transfers (havn't looked close enough to decide whether or not I should epoxy the front side of the rear transfers), to let the rear transfers blow almost directly across at each other. Small changes, but... too small? Not small enough?

At the moment, thats all I've been thinking about doing... But I'm curious to hear how yours runs, what changed in the top end after you've done everything your doing. I don't mind losing a little bit, but my goal is to gain what I can on the bottom, without losing from the top. Originally I hadn't planned on it, but with this discussion of CCV, I'll be at least looking at mine, whether I decide to alter it or not remains to be seen.

As far as epoxy goes... I've used JB Weld quite a bit, its pretty thin and "runs" easy, but you can let it sit for a while, and it will set up pretty good. In the back of my mind, although I know many use it, I wonder how JB Welds heat expansion differs from aluminum. Aluminum expands a lot as it heats up... and if the JB Weld dosn't, it will come lose. But like I said, with many using it, I guess thats not a problem, I just have a hard time convincing myself of it. Are you using the "quicksteel" only because its easier to work with in the spots JB Weld would be hard to use?

Hopefully before the weekend is over I'll have the time to write down the port durations and check the CCV.

[dmcca]

As far as what I'd like to see out of it, I'd like to get more bottom end, losing as little top as possible. What I've been thinking about doing, is to put in a thin base gasket, which brings my squish to just under .050" (I don't remember exactly). I was thinking about grinding off the hooks on the rear transfers (havn't looked close enough to decide whether or not I should epoxy the front side of the rear transfers), to let the rear transfers blow almost directly across at each other. Small changes, but... too small? Not small enough?

At the moment, thats all I've been thinking about doing... But I'm curious to hear how yours runs, what changed in the top end after you've done everything your doing. I don't mind losing a little bit, but my goal is to gain what I can on the bottom, without losing from the top. Originally I hadn't planned on it, but with this discussion of CCV, I'll be at least looking at mine, whether I decide to alter it or not remains to be seen.

As far as epoxy goes... I've used JB Weld quite a bit, its pretty thin and "runs" easy, but you can let it sit for a while, and it will set up pretty good. In the back of my mind, although I know many use it, I wonder how JB Welds heat expansion differs from aluminum. Aluminum expands a lot as it heats up... and if the JB Weld dosn't, it will come lose. But like I said, with many using it, I guess thats not a problem, I just have a hard time convincing myself of it. Are you using the "quicksteel" only because its easier to work with in the spots JB Weld would be hard to use?

Hopefully before the weekend is over I'll have the time to write down the port durations and check the CCV.

I'm using the putty epoxy in the places where the liquid stuff just won't stay put, and in the areas where i cant easily come back and grind the epoxy into the shape i want. With the putty stuff you can press it into a nice smooth surface but the liquid epoxy needs to be filed or smoothed over in some way once its set.

A thin base gasket it a good idea as it will also raise your compression slightly and reduce your exhaust duration slightly, both of which will help low end power. Its always a compromise though but i personally think that the vast majority or riders dont use the top end enough to really notice that much difference... besides most people will be faster with a little more bottom end rather than a little more top end. Bottom end just makes cornering, hill climbs, etc so much easier and you will just naturally ride where the power is anyway. Of course you can go too far and find that it signs off way too early, but hopefully i won't get to that point.

[dmcca]

Your post is interesting.

I have been involved with porting over 10 years now.

I think you may have found the problem on this engine, large crankcase volume, large runners, ie transfers = flat or no low end.

what is called for on cfm vrs rpm and hp output ?

strangley on large cid race engines we make as much flow as possible.

depending on compression , cam profile etc.

I'm glad you find it interesting. What sort of engines do you mainly work on?

To be honest i'm not sure what the cfm requirement is. I have only ever done a small amount of reading on 4 stroke porting but i have heard cfm talked about a lot there... not so much with 2 stroke porting that i have come across. I know its an extremely important measure but i just havent heard much about it.

Im about to buy some 2 stroke tuning software and this should give me all of those facts and figures. I think its just too hard to calculate and then measure on a 2 stroke without software... i could be wrong though.

[Altune]

Right now I build 1000 hp drag engines.

I used to build hi per jet ski's

and 2 stroke bikes.

need to study the 4 stroke mx bikes.

Al

[TGSMITHYZ250]

great thread keep the posts coming!

[Aido_DRZ]

So when is this engine going to be reasembled? im very interested in seeing how it turns out.