C.R. ratio vs top end

7 replies to this topic
  • Le_Racer

Posted February 05, 2002 - 09:05 AM



I've got to find some more top end out of my 444.
Right now I've got it running with the 13.5 Wiseco piston, ported head with standard valves, EVO mid-top pipe, free flow air filter (ice racing: doesn't need much protection) with no screen and the standard airbox, my carbon fiber one is lost somewhere between here and Switzerland or hasn't made it thru customs.

Has anybody tried lower compression ratio and/or lobe center set-up?

I can sacrifice some low-end for more top, but I want to keep the mid I have now.

Throw your suggestions and ideas, I need help!


  • Shawn_Mc

Posted February 05, 2002 - 09:47 AM


If ya got money to try it, go with a thunder alley pipe. It never stops pulling. It uses a reversecone meg, and it just pulls and pulls clear to the rev limiter. Just my two cents. If you can find cams with a little more over lap You could pick up some top. I dont know if its possible but if you can retard the entire cam profile 3 or 4 degress you would pick up some top end also. You can always go with a bigger carb too. Id be lookin at the air box too. Some large holes with screen across them may be in order too. Good luck and post your results.

  • Hayseed

Posted February 05, 2002 - 12:55 PM


I keep hearing that overbores, especially w/ higher compression pistons, widen the squish band and the engine cannot rev out like it should due to this increased resistance.

Of course, this point is never accompanied with advice on how you are supposed to adjust the squish band, barring shipping your motor off to some engine builder somewhere. Perhaps you should ask the shop that ported your head, but I would imagine that, somehow, material (or maybe extra base gaskets) would have to be added to the barrel to provide for increased squish (of course this would necessarily reduce you comp. ratio unless accompanied by machine work to the head). I’ve heard of guys using some type of clay to sit on top of the piston and determine what the squish band’s measurements are by assembling the head with clay on the piston at TDC. I believe Taffy posted this information some time ago.

I too had the 13.5:1 97 mm in my ’00, and I too was not happy with the over rev. There wasn’t any. Sure, it was pure evil anywhere below about 7k rpm, but it wouldn’t bounce off the rev limiter like the stock motor. It would just sort of gradually lean up against it.

Higher compression in general is known to perk up the left hand side of the torque curve, while lower compression pushes it back to the right. I was, in fact, told by Gary Jones of White Brothers that it was difficult to get any top end out of the 426 with a higher compression piston.

In a few weeks I’ll have my ’01 back together with adjusted cam timing (although I don’t know what I’m going to try first here) and a 97 mm 12.5:1 piston. I’m hoping that is a good compromise. I had to buy the piston but already had the barrel, I’ll be sure to post how this works relative to the 13.5:1 piston. Of course that won’t be the only thing to have changed, I’m currently leaning towards cam timing that will help the bottom end. I like to ride a gear tall and am seeking an accommodating torque curve but I’d still like to have some rev.

But the lack of rev could simply be the engine (head and valve train) having trouble filling the now bigger cylinder at higher rpms.

In short, I have no $&*# idea.

Hope this helps.

  • Le_Racer

Posted February 06, 2002 - 06:59 AM


I'll try to augment the squish with an extra base gasket, I just don't have the time to get a 12.5:1 piston.
I've been sick for some time and now I can just try to salvage my season, so my testing goes as I can find the time between the races and we race every week-end...

As for results, I'm leading the 125 class on my 250F, in the 250 I'm something like 5th, not good, in 2000 I won that championship (beating Pascal Picotte) and finished 3rd last year.
In the open 4 strokes I'm 4th despite the crash I had in the qualifier last sunday.
In the 600 I'm also in 4th place now.

Thank you

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  • Boit

Posted February 06, 2002 - 12:57 PM


LeRacer: have you degreed your cams? The only way I know of to do this reliably is with the cam sprocket adaptors from Falicon. These little gems allow you to time the cams pretty much as you'd like. In my '00 manual, it says that the sprockets are bolted on so it's pretty clear that Yamaha had initially intended to go with adjustable cams and not these pressed-on units. Why they chose to press them on instead is a bit of a mystery to me. Perhaps this design saved a little bit of assembly cost?¿

  • Shawn_Mc

Posted February 06, 2002 - 02:14 PM


This idea is an old hot rod trick, but take the muffler section off you pipe and rev/ride it for a lap and see how it revs then. If you get more top, its backed up in the exhaust and probably choking for air on the carb side. When you run out of RPM its because your running out of air. Squish band may have something to do with it but not as much as plain old air flow.

  • Le_Racer

Posted February 07, 2002 - 04:37 AM


I was to try different cams and timing but I barely got enough time to put the bike back together before the first race, I rode my 444 one afternoon before the first race so I'm still fiddling with suspension set-up and such.
I was supposed to have plenty of time for developpement, I ended up being barely ready!
I'm looking at getting the Falicon gears and playing with timing/overlap on the dyno, it's just a matter of finding the time.

About the pipe I have on it, it's definitely a top end pipe, it does kill some bottom, but for ice racing that's ok.
Last year on my 2001, I was running the same pipe with a 2000 ported head with over size valves and the the 2000 ignition (the 2001 cdi didn't worked that good on the very top).
With that set-up I was at just over 53 hp at the wheel.
I had a different set-up for shorter tracks, 2001 cdi with EVO Supermotard pipe: about the same hp, but more grunt down low and barely more on top than with the stock pipe, it liked to be short shifted and I pulled holeshot with it at least 90% of the time!

The head I have now is the 2002, ported but standard valves, seems that nobody have O.S. valves in Ti. On the flow bench we went from 85 (at 140cfm) to 99, a very good gain.
I have a decent mid-range and on top it rev's out not that bad, but I'm looking on getting the edge on top.
Right now I'm about on par with the factory KTM 520 (size unknown) that Picotte and Beaulac rides.

[ February 07, 2002: Message edited by: Le Racer ]

  • Jon_Escombe

Posted February 13, 2002 - 12:20 PM


I'm in a similar situation with my '00 Supermoto bike - 98mm piston, flowed head, oversize valves, falicon camwheels. I need some more dyno hours to finalise the cam timing, should be done by the end of the month.

The best I've seen out of it is 52.5 with the standard pipe. The Thunder Alley pipe is great for bottom-mid but loses about 2hp from the top end.

Still looking for a good top-end pipe. Akrapovic have just released a Supermoto pipe for the YZF, which we should have on the dyno in the next couple of weeks..

Cheers, JonE.

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