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Squish Angle Cheetah PV question

7337 Views 12 Replies 8 Participants Last post by  REDLINE
Calling all 2 stroke gods,

I have a technical head question. My piston to head clearance is convergent. The OD measurements are about .028 in. and the ID measurements are about .026 in. They vary slightly but this is a close average. The bike is a 472 Trinity Cheetah PV motor. I was under the impression that you always wanted divergent piston to head measurements. Any thoughts on this people?

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I got David Noss's input on this. Thanks David. I would also like to hear from some others before cutting new domes.

I like to run twice that much squish, if that's in fact what you're talking about....on the CP cylinders.
Actually...on the Cheetah's cousin, the cub cylinders. Mid to high .040's to low .050's has worked fine for me...

Of course fuel type, port layout, dome step and design ALL play a part in the correct clearance...etc.
I was also wondering what most people were running on average for a squish clearance. That is the info you provided. Thank you. My question was actually about divergent vs. convergent squish measurements. If you measure the piston to head clearance you will find it is usually a little different on the outside of the squish band vs. the inside of the squish band. If it get smaller as you move closer to the center then it is considered convergent. If it gets larger as you move toward the center then it is considered divergent. I believe most two stroke motors have divergent squish angles but I wanted to hear from anyone who has these motors and has taken these measurements. This is a pretty technical question that even some builders don't understand it.

Thanks again for your input,


QUOTE (dajogejr @ Aug 11 2008, 03:22 AM) index.php?act=findpost&pid=7895I like to run twice that much squish, if that's in fact what you're talking about....on the CP cylinders.
Actually...on the Cheetah's cousin, the cub cylinders. Mid to high .040's to low .050's has worked fine for me...

Of course fuel type, port layout, dome step and design ALL play a part in the correct clearance...etc.
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wow.. i really dont understand squish... im lost.
QUOTE (kawiking @ Aug 12 2008, 10:21 PM) index.php?act=findpost&pid=8117wow.. i really dont understand squish... im lost.

I understand it. BUT....I couldn't tell you how to design one for a particular motor setup in general.

I can tell you this...most builders agree...different fuel types (gas or methanol) require completely different dome designs.

I asked Dan Wade @ Patriot to send me a set of domes when I went back to a shell/dome type head. (I ran a cut stocker most of last year)
The way the domes are cut and shape are TOTALLY different than what I've used in the past.

I was skeptical, but the bike has been a runner all year, bullet proof damn near....and did very, very well with a A1 pilot at Gilbert this year.

I would agree with you that not all builders subscribe or understand/know what works. Dave Noss is probably one of the top guys that knows this.
He's been in this game long enough...and a true good guy for the sport...

Very good discussion....
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Pretty much all the literature out there states to run a divergent or parallel angle or radius for the squish. Ive ran all of them, and they all do different things. Most interesting was a convergent angle- by several degrees- and what it did to the power curve- in some cases it was desirable.

In the geometry aspect of squish design: One issue that has to be accounted for is how is the squish cut. A piston crown doesnt have an angle as its actually a radius. Most people calculate the angle of a coincident tangent or secant line to the radius of the piston crown. This is because most cut the squish band on a compound set-up in the lathe. As such there will be a tiny little "pocket" on the outside edge of the squish band (in addition to the ring periphery volume). In highly developed engines this can present enough additional end gasses to promote detonation. Many of the software programs out there that people use to calculate squish velocity calculate a radius type parallel squish band, which will have higher values than a more typical "straight" angle squish band to a crown radius.

In the tense of this discussion, I think its going to wind up a discussion of squish velocity, as that is the factor influenced by the band set-up. There's considerable argument regarding MSV for given fuels. I have my opinion and it differs from some and others hold the same. My always standard disclaimer would have to apply that if we all did it the same you'd just go to Wal-Mart to get your stuff. Speaking of Banshee motors- typically the pipes are not highly developed enough and the stock cylinder porting layout is crappy enough that many have not ran into issues with too high of an MSV creating detonation. Its getting closer for certain and with the CP Ind. cylinder line it moved much closer as evidenced by how many people had detonation issues early on. In addition I hold the opinion that too many people view combustion as this homogeneous event that is the same cycle to cycle and can be calculated. The modeling involved would require astounding computing power to calculate whats going on not only in a mere snap-shot of several cycles, but at something with enough RPM resolution to show whats up from 6000 RPM to 11000 RPM. AND- In an effective enough manner to make well enough sense to try and effect a worthy change. Most of my head designs come from empirical data. I have found the software handy to do the geometry for me, but the old adage- 1 test is worth a thousand expert opinions works for me. I typically always strove for high to very high MSV numbers in my head designs, but as of late, Ive been trying some different set-ups that have a lower MSV. Mainly because what I think was happening in a convergent set-up. In a divergent angle squish band set-up, the squish velocities as the bowl edge will be lower than at the squish OD, but it does expose a larger curtain area of moving gasses to the contents of the bowl. In a convergent set-up, I cant help but think that the band acts narrower than it actually is in that only the inner "half" of the squish band is creating the turbulence. Volume and velocity are inverse of each other, so thats something to think about in how a squish band may be working. Especially when you get into very wide bowls either by design(narrow band) or by configuration(big bore).
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backcountry, i have thought about the same thing especially on the saber and other cylinders with the huge 95-98 bores. i all so was wondering how much turbulence the outer part of the bowls was seeing compared to center with the wide bowls found on a 95 or 98 mm bore . i all so Ben thinking a bout what the dome sees in forced induction and nos . the forced induction i have figured some things out but the nos thing has me baffled because the incoming air velocity is not sped up by nos ,like a turbo but the flame front is sped up considerably with nos , ha the 2stroke engine the simplest engine and most complicated engine to understand and probably will never be mastered lol any thoughts on a nos dome.
Thanks for your input backcountry and dajogejr. I think it could turn into an endless theoretical discussion, but a good one for sure. It is true though that by doing some experimental cuts and comparing runs on a dyno, you will find out a lot more sooner than with a program. I am going to see if I can get Noss to cut some domes for me. For what it's worth, the 472 (+4 stroke, 72mm pistons) Cheetah PV is on gas (vp110). I originally asked because the dyno runs were showing some inconsistent cycles in the higher rpms. My original domes were designed by Trinity. The new domes were the same basic Trinity domes but with about 2 less cc's. Trinity designed their domes with a convergent squish which I found odd. I don't know if they put in the R+D time or just "threw darts". Knowing Trinity, the latter is probably true.

Thanks again for the input,


By the way backcountry, would you have any domes for this motor for sale that you think would work well?

If Dan will make the domes for you, it would certainly be a good way to go.

He is one of those guys whose posts I read and re-read.

i have a 480cc and have been wanting to get better domes for it as well backcounty and i have talked about it a lil bit..

i have a set of back's 350cc domes and they make considerabley more and wider hp range than a similar alky" specific dome i have had to run lately from head shell problems.

with his domes and NOTHING else it puls MUCH harder out of the hole and has much stronger overrev.

if he'd set us up with cheeta domes it would be sweet. i have a 535 ( with rkt head) and a 480 i'd like to do.

and also a cr500 that begging for a correct head , pipe design!!

also nate mccoy can cut some for them also. but there are other things going on in that motor configuration that causes problems..

out of curiosity what did yours dyno on gas?
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My 472 Cheetah with only Trinity porting and domes on gas had its best run showing 88hp. The dyno I am using is pretty fair. After the HJR porting and domes, we got an honest 93 hp out of it. We had a run where it showed 95 hp but the low end of the curve did not look good enough. Kevin at HJR feels this motor should put out 100hp or a little more.

I am having someone locally design some domes for me but we will have to dyno them to see if they produce what we are looking for.

Thanks David for the advise. Most people in this sport are too ignorant or too proud to suggest others do the work. You could probably design a set of domes better than 99% of the builders out there.

I would love to have backcountry (Dan?) design some domes for this motor.

Dan, what do you think????

Does anyone have any opinions on the max hp from this motor without going strickly drag?

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There are just a shiz load of variables here, and a lot of ways to accomplish the same thing. For example; if the pipe wants more heat, a convergent squish angle is one way to accomplish it. My personal choice would be to cut a divergent angle and have the flexibility to introduce more timing to create the heat, and pick up the bottom end at the same time. A convergent angle is going to FOR SURE!!!! build a motor that won't take much timing before it starts predetonating, the light ends are going to get caught out at the edge of the piston and light. Bottom line; Detonation is the enemy, and it usually starts at the edge of the piston, in the hottest part of the cylinder, just above the exhaust port. The reason we have a squish band is to focus the heat into the center of the piston to prevent detonation. It becomes more of an art than a science with two strokes, which is why it is so rewarding to make them run.
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