Squish & Compression & Head gasket thickness

[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)!

Mun And John,

You cover it very well!

A tips:

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

//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

[mun]

Doordie is right.
Less squish velocity > less turbulense > slower mixture burning > more time to detonate. This is theory.
However i think that you can try with two gaskets with on you risk. If isn't work, go back to one gasket.

When you have too much squish clearance, you have less power on every rpm. Squish clearance is one most important thing on tuning.

[quincyman]

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

I have the same book you are looking at. I made this mod years ago because I ride my KX500 in tight woods situations and wanted more tractable power.  It still is a beast but more controlable. I was also sick of buying kick starters. I broke 3 in a 2 year period. Doubling the head gasket did as the book suggest. Lowered the compression so I no longer break kick starters and made it more tractable. You will have to change the jetting a bit. I just lowered the needle one clip and all was well. I don't know much concerning the science of squish but do know the bike is a better woods bike. I also sent my flywheel to Aloop to have a permanent weight welded to the flywheel. This also made it more woods worthy. If I ever go back to desert or sand dune riding I will buy a stock flywheel and use only one of the thick head gaskets.

Well that's my 2 cents on this.

[doordie]

You loose about 6-10 % overall-power with a squishclearance over 2mm,sure it?s make it more like a "woodsbike" ,but if you turn the throthle wide open for a while it will for sure "pinging".
Better to reduce the advance timing to make it not so "peaky"!
IF you want to get less compression you have to "digg-out" some material in cylinderhead,but still have the right squishclearance in range 1,5mm.

//doordie

[KXcam22]

I have to agreee with Doordie & Mun. Squish clearance is extremely important for both performance and a properly running engine.  For those seeking a softer starting/running engine I would suggest a flywheel weight would be more effective.  All theory aside, the function of the squish band is to force the combustible mixture into the top portion of the head for ignition by the spark plug. Any mixture left behind in the squish band (caused by too much squish clearance) is not ignited and represents lost HP and efficiency.  The optimum shape for a head should have a minimum squish clearance and then the combustion chamber is adjusted (machined) to provide the correct compression ratio.  As an example, my last 2-stroke roadrace bike (RD350LC) had a .200" squish clearance (huge).  I had to mill the head .100 and toss the .080" headgasket to get me down to .020".  But wow what a difference it made!!  I was lucky in this case that the original compression ratio was low enough that I did not have to machine the chambers to reduce compression.   I doubt I would do the 2 headgasket thing.  My KX is a woodsbike and I love the crisp throttle response the optimum squish clearance gives me (adjusted with Doordies help of course). Runs nicely on 94 pump gas.  Cam.

[IridiumRed]

hey Guys

I posted a long reply about this and subsequently it didn't get posted... argh

Short version - will replicate the longer version later if necessary

I rebuilt my motor, full rebuild, used the nice "top end rebuild" sticky (thanks guys!!)

I measured the piston to head clearance a bit differently though

I assembled the top end as it would be with the motor fully together, except - no rings on the piston (less drag), no base gasket, no sparkplug (again less drag) BUT with a piston ring on top of the piston

Then i could spin the crank over by hand (using the ignition/stator rotor), and feel for when the head/piston interfered/locked up

Then i used feeler gauges under the cylinder block, at opposite ends (to eliminate "rocking" or tilting)

I kept using thicker gauges until no interference could be felt

Then i looked at the thickness of the piston ring, and the thickness of the feeler gauges, and from that I could determine what base gasket thickness i needed to run.  

Ended up at .055 (1.4mm approx) = .045 ring thickness - .020 feeler gauge (where the interference JUST went away) + new base gasket thickness of .030 (made my own)

Took me a while to think up how to do this (probably not new idea, just new idea to me!), but it was easy to do, very repeatable (within .001")

Bike runs great!  180 psi compression.  But getting the powervalves clean and reset (great idea about flipping the pins on the main/center powervalve), I can kick it over in tennis shoes, no prob, starts 1st/2nd kick.......

What do you guys think of that method?

[KXcam22]

Iridium,
  That method sounds pretty good. Sometimes you might want to factor in a thou or 2 for gasket compression. The only issue with messing with the base gasket thickness is that you are changing your port times a bit.  With the thinner base gasket it is always a good idea to use lots of good quality gasket sealer.  I had one blow a hole in the gasket and it sucked enough sand in a few minutes (time it takes to notice it) to ruin a good engine. Cam.

[IridiumRed]

Actually, the base gasket that came with the gasket kit (got a full gasket set for the whole engine) was about .021

That would have given me a head to piston clearance of .046 (1.2mm approx), instead of the .055 (1.4) I went with, using the supplied head gasket

The gasket I made (using good gasket material of course) was .030, so it was actually 50% thicker.  

But still, you make good points about the sealant (which I did use, very careful to cover all the surface, let set up a bit before assembly) and also gasket compression

The gasket compression is one reason I aimed a little over the nominal 1.3mm squish height - i suspected the gaskets might crush a bit, reduce the figure

I do think there was some gasket compression - after the first running period (15 minutes or so of high idle, blipping the throttle, then short easy ride) i let it cool and retightened all the head/cylinder bolts, and there was a small amount more tightening on the cylinder bolts than the head bolts (head gasket was steel, should have low crush)

You're right on the

[Brett]

The best and safest method to measure squish clearance is to first of all setup cylinder height to get desired port timings via changing base gasket thicknesses, bolt cylinder down on the base gaskets you are going to use to correct torque, install head with head gasket and torque down.  Then with a piece of 2mm solder bent into an "L" shape put through spark plug hole and ensure is all the way to cylinder bore and then slowly turn motor over by hand untill solder has been crushed once, then measure with vernier callipers. Ensure you measure in multiple locations in case cylinder and head faces have not been machined on the piss.  Once taken the minimum measurement adjust head gasket thickness to give desired squish clearance.  When i rebuilt my motor i had a .25mm head gasket and that gave 1.7mm squish clearance but i do not know what the base gasket thickness was or if the head or cylinder faces had been modified.  The way i see it if you are using 2 x 1mm head gaskets to make the motor tame you should have bought a 200cc 4 stroke for woods riding.

[quincyman]

The way i see it if you are using 2 x 1mm head gaskets to make the motor tame you should have bought a 200cc 4 stroke for woods riding.

No 200cc 4 or 2 stroke has what my KX500 has with two 1mm head gaskets. I am monitoring this thread and if it wasn't for breaking kick starters would go back to the narrower squish. I may still go back to 1mm head gasket which is not quite as bad as the thin metal head gasket. Any ways this is a good thread. I am learning more all the time. I have a friend who suggest increasing the base gasket thinkness over doubling the head gasket. Sounds like he was closer than I am as far as performance goes.

We can't get 94 octane gas here. At least not from the pump and that is where I get my gas. 92 is the best I can get from the pump. Not sure a woods bike needs more than that. I rarely get out of third gear.

[Timbowe]

Are you using a later model kicker? I managed to bust one on the 92 once. then replaced it with the later model one with thicker base. No worries at all. You must be doing something wrong to bust one of them..