Low compression forged piston for a 600?


20 replies to this topic
  • HeadTrauma

Posted March 09, 2007 - 11:18 AM

#1

Does anyone make such an animal? By "low compression" I mean less than 9:1; the closer to 8:1, the better.

  • cleonard

Posted March 09, 2007 - 08:22 PM

#2

The lowest that I know of is the stock 9:1 compression ratio. Why do you desire a lower ratio?

I'm reaching here, but what about the XR650L piston? The 650L is more like 8.6:1. Is it possible to bore to 100mm, use 650L piston and get lower compression that way? Again, just guessing, but does the 650L get lower compression by locating the piston pin closer to the top of the piston? There are probably reasons that this will not work, but it is worth investigating if you really need lower compression.

  • HeadTrauma

Posted March 09, 2007 - 08:41 PM

#3

Why do you desire a lower ratio?


I have two reasons;
Posted Image

Haven't settled on one or the other yet.

I'm reaching here, but what about the XR650L piston? The 650L is more like 8.6:1. Is it possible to bore to 100mm, use 650L piston and get lower compression that way? Again, just guessing, but does the 650L get lower compression by locating the piston pin closer to the top of the piston? There are probably reasons that this will not work, but it is worth investigating if you really need lower compression.


You're right; that probably won't work. The 600 has a BxS of 97 x 80mm, the 650L is 100 x 82mm. That means the compression heights are most likely different. If the 650L's in shorter, then any existing quench will be gone.

  • ringding

Posted March 09, 2007 - 08:45 PM

#4

thats awsome.

  • cleonard

Posted March 09, 2007 - 09:49 PM

#5

I was hoping that was your reason. Forced induction was the only reason I could come up with for wanting to lower compression. Sounds like a really cool project. Just make sure that the rest of the engine is up to the extra torque.

I think that the 650L cylinder is the same height, but the stroke is longer. Either the crank is lower in 650L or the pin is higher on the piston. If it is the pin then wouldn't the piston be 2mm lower if used in the 600? The piston would be 2mm from the top. This would change the shape and dynamics of the combustion chamber, but wouldn't any change that lowers the compression ratio change the combustion chamber some?

I bet if you ask here someone would send you a 650L piston. With all the people who put the higher compression pistons in, there must be a lot of them laying around.

The other option is to machine the head and/or the piston to increase the head volume.

  • HeadTrauma

Posted March 09, 2007 - 10:43 PM

#6

1.) I was hoping that was your reason. Forced induction was the only reason I could come up with for wanting to lower compression. Sounds like a really cool project. Just make sure that the rest of the engine is up to the extra torque.

2.) I think that the 650L cylinder is the same height, but the stroke is longer. Either the crank is lower in 650L or the pin is higher on the piston. If it is the pin then wouldn't the piston be 2mm lower if used in the 600? The piston would be 2mm from the top. This would change the shape and dynamics of the combustion chamber, but wouldn't any change that lowers the compression ratio change the combustion chamber some?

3.) I bet if you ask here someone would send you a 650L piston. With all the people who put the higher compression pistons in, there must be a lot of them laying around.

The other option is to machine the head and/or the piston to increase the head volume.


1.) That's something that remains a gray area. I know the transmission is a weak point, but I'm not sure about other things like the crank and rod. It's expensive being the first person to find out.

2.) AFAIK, only the crank/rod/piston are different between a 600 and 650L. The cases are the same and external dimensions are too. That just leaves the stroke, rod and piston. My reason for getting compression down below 9:1 is so I can run pump gas and maximize knock resistance. Running 9:1 and 10-14psi is easy if I want to shell out for 116 leaded race fuel. I don't have an easy source for that, though.

3.) That might be useful, but I don't think I need an actual 650L slug unless no one knows the compression height and dish volume. In that case I can measure it myself if I have one.

In desperation I could machine my own custom piston. I'd like to avoid that, though.

  • creeky

Posted March 10, 2007 - 06:04 AM

#7

1.) That's something that remains a gray area. I know the transmission is a weak point, but I'm not sure about other things like the crank and rod. It's expensive being the first person to find out.


Third gear is the weak point in the transmission. Somebody used to market a stronger one, might have been XRs Only. An HRC crank with a Carillo rod makes for a pretty strong bottom end. Interesting project, keep us posted.

  • Jon-D

Posted March 10, 2007 - 07:58 AM

#8

Travis_37 had a company make a custom piston for him (I think it was J&E)but it was a little pricey from what I recall. He was doing some experimenting with a KTM motor to get the character he wanted. His project turned out pretty good and the pistons worked well. the only downside is the pistons were considered "one-offs" and when it comes time to rebuild this adds to the cost. very interesting project...

  • HeadTrauma

Posted May 04, 2007 - 10:08 AM

#9

I'm bringing this thread back.

I poked around some and Ross will make customs with a 4-piston minimum order(at least $100+ each). It would be more practical to do a group buy, but I'm not sure enough people would be interested. I'd want at least two for myself.

I have a dual carb head I can cc to find the needed piston dish volume...

  • Kawabuggy

Posted May 04, 2007 - 04:22 PM

#10

My guess would be that unless you are REALLY knowledgable on building turbo charged motors, you might just need at least 2 of that minimum 4 order.

I have heard that turbos make great power. The caveat is, and has always been, knowing how to properly set the motor up to deal with the additional boost. If you make a mistake, you are sure to burn or melt a piston.

I am NOT calling your knowledge into question. Simply making a point that you might want to hang onto one of those additional pistons just in case. It would suck to buy in and get 1 piston, and then fry it the first time out. You would be back at square 1.

Keep pressing on, and share with us your project. I for one would love to hear how it works out. Will offer help where/when I can as well.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • HeadTrauma

Posted May 04, 2007 - 07:28 PM

#11

Well, the minimum order is four slugs, but they would happy to make more than that. Their profit margin improves with bigger numbers because they get more mileage out of their production setup.

No sweat, didn't take anything to be an attack on what I know. I'm not worried about tuning a turbo engine because deep down they share most of the same nuances. With wideband exhaust gas oxygen feedback and starting on a really rich tune, all you have to do is lean it out until you reach the target air-fuel ratio(somewhere safe for an aircooled engine like ~11 to 11.5:1). I still want to have a spare for emergencies or at least one rebuild down the road. It doesn't have to be a tune-related failure; a fuel filter could clog or the fuel pump could go weak and the engine would go lean and bye-bye custom piston.

  • Kawabuggy

Posted May 05, 2007 - 11:47 AM

#12

Here is another option; What is the rod length on an XL600? If it is shorter, you could easily run an XL rod which will drop the piston in the bore. Depending on the length of the shorter rod, if it is too short, you could simply then use a high compression piston which would increase compression hopefully to where you need it to be. Otherwise, if the compression is too high, you could easily mill the crown off the piston to get it down to where you want it to be!

In this manner you could take an off-the-shelf high compression piston and mill it to get it where you need it to be. My guess is it would be cheaper to go that route, than specifically having a set of 4 custom made. Just an idea.

A shorter rod would be the answer.

  • NMike

Posted May 05, 2007 - 03:46 PM

#13

thicker head gasket?

  • HeadTrauma

Posted May 05, 2007 - 05:01 PM

#14

Unfortunately a shorter rod or a thicker gasket will both eliminate the squish area and make the engine more prone to detonating.

  • Kawabuggy

Posted May 07, 2007 - 10:33 AM

#15

True, what I doubt you are going to be able to find a piston that will give you a compression ratio that you can work with without losing quench.

The other option would be to get the center of the piston milled out. This would leave a depression in the center of the piston, but still leave the outter ring of the piston crown to force the air/fuel mixture into the center of the piston/head chamber. My guess would be that this would introduce quite a bit of turbulence into the chamber which may help spread the flame kernel, and/or would promote flame propagation.

The other thing to think about here is that if you don't mill the edges of the piston, you will leave the "meat" between the top of the piston & the first ring. If you are familiar with turbo pistons, or even diesel pistons, you will note the placement of the rings is further down on the piston. They do this to keep the extra heat away from the top ring. My initial idea of milling the entire piston crown would make this worse instead of better. However, milling the center of the piston-depending on how much available material there is for removal-would keep you out of hot water all the way around. In the car world, we call these dished pistons.

Also, it has been written that having a pocket in the center of the piston helps eliminate piston rock in the bore as it supposedly channels the expanding gas.. I don't know if I believe all of that, but it made for good reading.

My next question is-what are you going to do about timing? If you believe as I do that timing is tied to how well the cylinder head is designed. The better the head/quench/piston design, the LESS total timing the engine needs to make power. Since you are altering the head/chamber/piston significantly, not to mention adding a turbo, how do you plan on retarding the timing, and how do you plan on testing to see which timing curve will work best?????

  • HeadTrauma

Posted May 07, 2007 - 02:07 PM

#16

1.) However, milling the center of the piston-depending on how much available material there is for removal-would keep you out of hot water all the way around.

2.) My next question is-what are you going to do about timing? If you believe as I do that timing is tied to how well the cylinder head is designed. The better the head/quench/piston design, the LESS total timing the engine needs to make power. Since you are altering the head/chamber/piston significantly, not to mention adding a turbo, how do you plan on retarding the timing, and how do you plan on testing to see which timing curve will work best?????


1.) That's the problem; everyone is anal about having a lightweight piston so the domes are usually really thin and might have strengthening ribs, but not a thick total cross section.

2.) Well, I'm not sure what kind of advance curve the stock ignition has, but if I can find out then I would try to duplicate it minus maybe 5 degrees across the board to start with. I'll be using a standalone programmable EFI computer that can pull timing based on boost.

  • cleonard

Posted May 07, 2007 - 02:47 PM

#17

2.) Well, I'm not sure what kind of advance curve the stock ignition has, but if I can find out then I would try to duplicate it minus maybe 5 degrees across the board to start with. I'll be using a standalone programmable EFI computer that can pull timing based on boost.


The ignition pickup under the right crankcase cover is not adjustable. However with a little machining I'm sure there is enough room to allow for five degrees of movement.

  • HeadTrauma

Posted May 07, 2007 - 10:04 PM

#18

Actually, I meant mapping the stock advance curve in the EFI controller and them subtracting 5* from all values that fall under boost. I most likely don't need to adjust the pickup unless it triggers too close to TDC.

Then again, I could retard the pickup and not use spark control at all; just do fuel injection only. It would be a bummer not being able to use the anti lag, though. :applause:

  • HeadTrauma

Posted November 23, 2007 - 07:10 PM

#19

It has come to my attention that some individuals both on here and elsewhere on the web have machined their own pistons out of billets, bar stock, or something of that nature. If anyone can shed some light on what non-forged alloys are being used, I would be very thankful.

  • cleonard

Posted November 23, 2007 - 09:46 PM

#20

Not a simple task to machine a piston that will have the characteristics that Wiseco piston will have. The cast pistons use alloys that are difficult to machine and the forging process makes the meta stronger for the application.

I found this thread. Look at post #3. http://www.cnczone.c...ead.php?t=24313

This one too, again at post #3.
http://www.cnczone.c...read.php?t=3192

CNCZone is a really cool site for info like this.

Like the guy says on that second thread. Good luck if you accept this mission.





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