so what about bigger valves?


9 replies to this topic
  • ThumperKid250F

Posted June 11, 2008 - 06:49 PM

#1

i no ive been asking a lot of questions on making my motor fast and crap but i was talking to my dragracing buddys today and it got me thinking.. would bigger valves be worth it in my 06 yz 450..

so far my mods will be

long rod kit with all the crank mods
Hi comp piston.
hot cams
port and polished head


so i no that the head is where you would make most of your power, more air more power..

so who does this for dirtbike heads and how would my reliability be with bigger valves..

thanks

  • grayracer513

Posted June 11, 2008 - 09:11 PM

#2

Making the valves bigger is not always a way to gain power. Port configuration and layout is an extremely complex matter that can only be done correctly either by carving up and ruining several heads while testing what works on a flow bench and on the track, or buy coughing up the big bucks to someone who has already done that and has it figured out. In any case, you are as likely to loose 2 hp by incorrectly modifying the ports as you are to luck out and gain one.

Engines that produce the most power are often not the most suitable for track racing. For example, builders of half mile circle cars using the Ford 351 Cleveland engine very often preferred the "2bbl" heads with 2" intakes to the "4bbl" heads with bigger ports and 2.3" intakes.

Contact Ron Hamp Cycles

  • ThumperKid250F

Posted June 12, 2008 - 07:40 AM

#3

Making the valves bigger is not always a way to gain power. Port configuration and layout is an extremely complex matter that can only be done correctly either by carving up and ruining several heads while testing what works on a flow bench and on the track, or buy coughing up the big bucks to someone who has already done that and has it figured out. In any case, you are as likely to loose 2 hp by incorrectly modifying the ports as you are to luck out and gain one.

Engines that produce the most power are often not the most suitable for track racing. For example, builders of half mile circle cars using the Ford 351 Cleveland engine very often preferred the "2bbl" heads with 2" intakes to the "4bbl" heads with bigger ports and 2.3" intakes.

Contact Ron Hamp Cycles


hey thanks gray you been a big help with this motor..

as far the port work done on my 450 there is no need to worrie about it being done wrong. this guy has been doing it his whole life. first started in boat dragracing back in the day then he moved to motocycles. hes 96 years old and he still got it. its like a science with porting..


BTW gray is doing a long rod the same as stroking it? to me it seems like its the same thing but just two diff. ways of doing it?

give me some insite on this please..

thanks a lot

  • sparkyz

Posted June 12, 2008 - 09:25 AM

#4

I believe what gray was pointing toward is what is called valve shrouding. Once the valves are too large they take up too much of the head surface, and a side of the valve is close to the combustion chamber wall. This tight fit to the cylinder wall makes it harder for air to travel past than if you were using a smaller valve. Generally a shrouded valve will have a lower flow number somewhere in the lower lift range of the cam and will then have an increased flow in the higher lift range of the cam, once the valve gets pushed past the shrouded area.
Here's a note...The reason hemi's are a good design is that the combustion chamber is hemispherical. This means when the valve is pushed it always moves away from all parts of the combustion chamber, thus hemi's can use large valves and not run into shrouding issues.

  • ThumperKid250F

Posted June 12, 2008 - 10:10 AM

#5

yea that all makes sense..

thanks guys for your help:thumbsup:

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  • KJ790

Posted June 12, 2008 - 10:12 AM

#6

hey thanks gray you been a big help with this motor..

as far the port work done on my 450 there is no need to worrie about it being done wrong. this guy has been doing it his whole life. first started in boat dragracing back in the day then he moved to motocycles. hes 96 years old and he still got it. its like a science with porting..


BTW gray is doing a long rod the same as stroking it? to me it seems like its the same thing but just two diff. ways of doing it?

give me some insite on this please..

thanks a lot


A long rod is not the same as a stroker kit. A stroker is when they put in a crank with a longer throw, so the piston moves up and down farther, thus increasing the displacement of the engine. A longer rod is just a longer rod, the displacement of the engine doesn't change. The compression ratio may change with a longer rod, that depends on the piston used with it though. A longer rod changes the side load on the piston as the crank goes around.

  • grayracer513

Posted June 12, 2008 - 11:00 AM

#7

I don't care if he's been porting 4 strokes since their inception, the only thing he will be able to do correctly with a head he has never worked on before is to do some simple cleanup work. ANY reshaping carries the risk of negative results, especially with multi-valve heads and most especially with a three valve intake tract. The dynamics of the 3-valve intake port are very tricky to work with, and far more complex than you might imagine.

What percentage of the fuel will flow through the center versus the outer? What percentage of the air charge? What is the effect on combustion chamber air flow of changing this balance? What will the effect of altering the port divisions be? Are the ports already too big for optimum output, or will they be if the shape is altered? Perhaps most importantly, what makes you think the valves are too small now?

What is your target engine like? Are you willing to put up with an engine that will barely run at 3000 rpm to get 2-3 more hp at 9000? Stuff you need to think about.

The length of the rod has nothing to do with the stroke of an engine.

Stroke is controlled entirely by the crankshaft. An engine's stroke is the distance the piston travels between TDC and BDC, and that depends on the distance the crank pin is offset from the shaft centerline.

If the stroke on an engine is increased, let's say by 2mm, and no parts are changed, the piston will travel up 1mm farther than before, and down 1mm farther than before, so usually, something must be altered to compensate. The piston in the unaltered engine with the longer stroke will likely hit the head, and may strike the crank at BDC. The ways this can be corrected are to use a custom piston with the wrist pin moved up by half the added stroke, put a spacer under the cylinder to move it up half the added stroke, or shorten the rod by half the added stroke. (It should also be pointed out that increasing the stroke can easily increase the compression ratio, which is calculated as Vtdc/Vbdc by altering Vbdc, assuming that Vtdc is matched to the original engine)

Rod length has a completely different effect on an engine's performance. The rod length controls the point in the crank rotation where the piston can apply maximum force to the crank by changing the rod angle. The downward force of the piston will push hardest on the crank when the rod and a line drawn between the centers of the crankshaft and crank pin form a right angle (90 degrees). With a longer rod, this occurs later in the stroke in terms of crank degrees than with a shorter rod.

Now consider that gasoline burns at fairly close to the same speed in a combustion chamber more or less regardless of anything else. ( I know there are variables, but the range of speeds is narrow, and arguing the point will cloud the issue) The engine may operate at a variety of different speeds. To get the most power we can from an engine in a certain RPM range, we want to match the point in the crank's revolution where the rod is at right angles to the crank with the point in the combustion cycle where the cylinder pressure is highest. If the engine must run at very high RPM, we would want a relatively long connecting rod, so that the rod/crank will be at 90 degrees later in the stroke, which would give the combustion pressure more time in the stroke to reach maximum. At lower RPM's the piston will not have traveled as far before combustion pressures peak, so we need a shorter rod to creat the 90 degree angle earlier.

That's a nutshelled look at rod length, and it may be a little confusing, but to repeat the answer to your original question, a longer rod will not affect the stroke.

  • ThumperKid250F

Posted June 12, 2008 - 01:02 PM

#8

ok thanks guys for clearing things up with me..



does anybody know anybody that can put in new valveseats on my head..

i tryed calling ron hamp cycles to see if they would do it but i got no answer..

  • grayracer513

Posted June 12, 2008 - 02:04 PM

#9

If you want a stock seat replacement:

Millenium Technologies

Engine Dynamics

Race jobs:

Either Ron Hamp, or contact Eddie Sisneros at OTD Cyclesport (Denver)

Eric Gorr is good, too.

  • ThumperKid250F

Posted June 12, 2008 - 02:44 PM

#10

If you want a stock seat replacement:

Millenium Technologies

Engine Dynamics

Race jobs:

Either Ron Hamp, or contact Eddie Sisneros at OTD Cyclesport (Denver)

Eric Gorr is good, too.


thanks





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