Maintenance & Technical > KX500 Original

Squish & Compression & Head gasket thickness

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IridiumRed:
Hi Guys

Working on a rebuild of my 97 KX500.  As always I try to read up and research as much as possible

One thing I haven't been able to understand yet is the relationship between squish / compression in the KX500 motor

What I'm getting at -

I hear thick headgaskets (1mm maybe even two @ that thickness?) make the motor a lot more manageable, less abrupt power delivery AND the motor requires less octane (less sensitive to detonation)

Thick headgasket should = greater piston to head clearance, which should reduce the amount of squish, yes?

I always thought that one of the great advantages of having a tight squish clearance (and thereby getting high squish effect/velocity) was that the extra turbulence helped the fuel/air burn, reducing detonation

What I'm trying to say - I can understand how thicker headgaskets reduce compression, which reduces octane requirements, but I'd expect some of that to be offset by the reduction in squish.  Does that make sense?


I notice in Eric Gorr's book (great book! but a lot of you know that I'm sure) he suggested the two thick headgaskets (11004-1186)

I hear that Eric does head mods on the KX500, but I have never heard what exactly they are.  Does he narrow the squish band?  enlarge the rest of the combustion chamber, etc??

Thanks!!
Steve

John:
I will try to answer this firing off a discussion and getting proper answers... I expect Doordie or Mun to chime in here.

Squish:
Is a designed feature (shape) that improves the overall effect of the combustion. As I understand it, the squish is not the actual distance but rather the part (shape) of the cylinder head that forces the air/fuel mixture to squeeze in to a smaller (concentrated) combustion zone.

Squish band:
The squish band is the distance between the piston in TDC and the lowest part of your cylinder head.

Compression:
It?s a relationship between two volumes. The combustion chamber volume - vs.- the cylinder volume (i.e. 499cc). The combustion chamber volume includes the cylinder head and the reminding volume caused by the squish band.
The effective compression is a different matter with two strokers. This is because of variable port timings.

A small squish band does not necessarily mean high compression and a large squish band does not mean low compression? it all comes down to the shape of your cylinder head. Imagine a flat cylinder head vs. a spherical one. These two different set-ups will under the same conditions produce a similar squish band but different compression.

A higher compression;
Is more likely to self detonate lower octane fuel
Will generate more heat
Delivers more power

A lower squish
Lowers the piston / cylinder head clearance (increases the risk of severe damage)
Increases the compression (assuming you don?t change the cylinder head shape)

My understanding is that you should never use more than one cylinder head gasket. The risk of leaks is using this setup is high.

Doordie, Mun???

//John

mun:
I accept all what John wrote. I will add some more, but i'm so poor to write english, that it's better put there only some links.

http://www.hpt-sport.com/blowopen.htm
" The central head type works best from 15-25 m/s for all-out race motors."
There is lot good info about 2T.
http://www.hpt-sport.com/images/123.jpg
"Things that raise MSV: Lower compression." This was first surprise to me.

http://www.2-stroke-porting.com/squish.htm > Squish Test
I measure squish same way, both side.

http://www.eric-gorr.com/twostktech/porting.html > Cylinder Head
"A cylinder head designed for supercross should have an MSV rating of 28m/s."
Supercross have very short straights, so MSV can be high. If you ride long flat out, desert or mile flattrack, so MSV should be lower.

doordie:
IridiumRed,

Use search and type squish(lots of info there)! :wink:

Mun And John,

You cover it very well! 8)

A tips:

Never use two headgasket!
Will detonate/pinging under heavy load, if squishclearnce is to much. :cry:
Better to lower barell and use 1 (one) thicker headgasket to make power
more "manageable" and use maybe a flywheelweight to. :wink:

//doordie

IridiumRed:
Two headgaskets on KX500 -

Eric Gorr's  book, "motocross & off road performance handbook", volume 3, page 193

"if you want to get a smoother low end pulling powerband with more over-rev, install two thick head gaskets (11004-1186)..."

I think he mentioned it somewhere else in the book, might be wrong about that, but he definitely said it there

its THAT quote that has me confused.  Two thick headgaskets = a lot more squish clearance = less squish velocity

Now, the compression ratio has fallen due to the greater volume in the combustion chamber, and that decreases the octane required, BUT the loss of squish velocity should require MORE octane

I just wondered, for the KX500 in particular, how those two effects interacted.  IE, in terms of octane required, if the drop in compression ratio more than offset the loss of squish velocity...



And the reason I ask is, I'd like my motor to run well on premium pump if possible, w/o having to mix in anything else.... its hard to get where i live

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