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Discussion Starter · #1 ·
Those of you who have cut your heads, what is your squish. How close to spec can you go? Anybody go below spec? What fuel are you running? Spec is .059 on an 03 F7. Can you bring the squish right down to like .060 and still run 93 octane?


Also do you check squish while the engine is hot or do it cold?? I have always wondered.
 

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<div class='quotetop'>QUOTE (snoman820 @ Feb 10 2007, 09:24 AM) <{POST_SNAPBACK}></div>
Those of you who have cut your heads, what is your squish. How close to spec can you go? Anybody go below spec? What fuel are you running? Spec is .059 on an 03 F7. Can you bring the squish right down to like .060 and still run 93 octane?


Also do you check squish while the engine is hot or do it cold?? I have always wondered.[/b]
if the spec is .059" and you run .06"(.06 squish is bigger than .059, therefore safer) your already on the safe side and if the .059 is factory spec then yes you can run 93 octane because they configure their squish with pump fuel.
 

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Discussion Starter · #3 ·
So does anybody know how far below spec you can safely go and still run pump fuel?
 

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Could you safely cut .020 off on a '06 laydown 700? Please explain squish; I'm a little confused, is it the space between the piston and dome?
 

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<div class='quotetop'>QUOTE (BlewByU @ Feb 10 2007, 04:13 PM) <{POST_SNAPBACK}></div>
Could you safely cut .020 off on a '06 laydown 700? Please explain squish; I'm a little confused, is it the space between the piston and dome?[/b]
sort of, basically the "squish zone" is the very outside edge of the piston dome next to the cyl wall and extends inwards toward the center of the piston about 1/4". its very doubtful you could cut .020 off without running either race gas or very short distances, there is no set amount for all sleds the best thing is find a guy that knows his stuff in your particular brand and go by what they say. FWIW AC sleds seem to like a little MORE squish zone than other brands, usually around .065-.070"
 

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[attachment=64610:setup_037.jpg]Squish is the flat band on the outer diameter of the head, this is where squish velocity and dome/head volume are calculated to give optimum performance and economy. .058-.078 is factory specs, but if you think you can cut your head down to factory spec and still run 93 because the book says its spec then you will be looking for parts and jugs soon. If you do happen to have a motor with stock squish of .058 then you have one of the few early editions that made big power. All other 700's are .078 squish. If your head is cut down to .058 your head volume will be less than 28 cc's and comp will be over 13.5:1 and 110 will be required as we run in 1 of ours. You never want to run squish band les than .060 on this sled , unless you have specific use like 500' drags with proper octane.
 

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I am pretty sure RKTek will not only cut the head to decrease the squish but will also increase dome cc's to keep the compression to any ratio you want.
I would think the other big shops are offering the same service.
The high end moto bike / atv shops have been doing this for years.

There are program's that figure the correct dome cc's, squish band width and angle perpendicular to piston crown to achieve desired squish velocity and compression ratio.
Just cutting a head will do more than increase compression ratio, squish velocity is increased also.

Squish Velocity is a term for the speed of the trapped end gases being push toward the center combustion chamber.
The gases being forced toward the center of the combustion chamber help burn the fuel mixture faster.

Decreasing distance between the cylinder heads squish band and piston crown will increase squish velocity thus increasing the fuel mixtures burn rate.
The faster the burn rate the less you need to advance ignition timing because the same amount of fuel is burned in less time.
Retarded ignition timing means less pumping pressure the piston fights on the compression stroke, thus free horsepower!

Tighter squish band clearance's allow for smaller squish band widths, and less total trapped end gases.
The trapped end gases are cooled and somewhat shielded from the combustion flame front that they never fully burn until much to late in the power stroke.

So the less wasted fuel mixture you have trapped in the squish band area the more power you can make.

But wide squish bands have there place to, low rpm motors need them.
At low rpm's squish velocity is very low making for a lazy combustion chamber, but a wide band will have a higher flame speed when compared to a high rpm narrow band

Max rpm of the motor is the major influence on the final width of the squish band.

Not sure if many people know but the reason a 2 stroke has all its timing in at idle and retards itself as rpm increase is because of slow combustion speeds.

The slow combustion speeds at low rpm's are because of low squish velocity, residual exhaust gases in combustion chamber cause largely by poor port velocity's and exhaust pipe scavenging.
But as rpm's increase and combustion chamber fuel burn rates increase because of less residual exhaust gases and higher squish velocities, less timing is needed.

Proper squish velocity is a science in itself the factory spends countless hours figuring out what works and what doesn't.
The things I have wrote above are theory's of how they work but as we all know testing is truly the only way to prove something.
So you have to ask yourself are you smarter than Arctic cat R&R dept along with Suzuki's.
You can close up the safety margins that they build in to there products to make them idiot proof and see gains.
But I would not stray to far from the beaten path because they probably have forgotten more than we will ever know.

But if I were to spend money on a head mod it would probably match the factory's squish velocity and compression ratio with a squish band clearance of only .040-.045
I know that's a tight number but with a good crank and fresh cast piston's keeping tight wall clearance there will very little rock in the cylinder.
I have ran as little as .025 squish clearance in a 250R motor with out ever having piston to head contact.
 

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Many factors go into calculating "squish". If you are just skinning a stock head, as long as you run premium fuel, than the above suggestions would probably be fine.

But saying a number that "works" is not correct. Bore size, head volume, squish angle, piston crown, pipe style and timing all work with "squish"

I have seen sleds run "zero" squish on 89 octane (less than .001 squish with a cold engine) and run just fine and I have seen engines burn down at .080" due to not enough squish.

IMO the answer to power is not in just dusing the heads .010". On older engines yes, but the newer engines are tweeked allot further right from the factory. The only time you should change squish IMO is if you are porting the engine and or pipes thus you are re-working the "timing" of the engine.

On a stocker it is just a demon that will get ya the first time you get bad fuel or forget to jet up when it gets cold
 

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Discussion Starter · #9 ·
Thanks for all the info. My cold measured squish on my engine was right at .59. I had the engine burn down on me the other day. I had enough predet to turn the spark plugs out about half way. The pistons were not "sandblasted", but I had no signs of wash what so ever. I had one of the ring groove pins come out and toast the one side. I take it the pin came loose from mileage and the heat from detonation. Now I wonder if I should even have the squish that close to spec. The head on the one side has a lot of dings in it from the pin and pieces of ring banging around inside. I suppose I could take it to a machine shop and have them finese this out and increase the comp ratio a tad. It's just a hard decision. On the other hand, the sled went 4600 miles with this head (which is stock) and never had a problem. All of a sudden I am detonating and ****s falling apart. Gotta love the two stroke. :wacko:
 

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<div class='quotetop'>QUOTE (rsss396 @ Feb 26 2007, 09:05 PM) <{POST_SNAPBACK}></div>
I am pretty sure RKTek will not only cut the head to decrease the squish but will also increase dome cc's to keep the compression to any ratio you want.
I would think the other big shops are offering the same service.
The high end moto bike / atv shops have been doing this for years.

There are program's that figure the correct dome cc's, squish band width and angle perpendicular to piston crown to achieve desired squish velocity and compression ratio.
Just cutting a head will do more than increase compression ratio, squish velocity is increased also.

Squish Velocity is a term for the speed of the trapped end gases being push toward the center combustion chamber.
The gases being forced toward the center of the combustion chamber help burn the fuel mixture faster.

Decreasing distance between the cylinder heads squish band and piston crown will increase squish velocity thus increasing the fuel mixtures burn rate.
The faster the burn rate the less you need to advance ignition timing because the same amount of fuel is burned in less time.
Retarded ignition timing means less pumping pressure the piston fights on the compression stroke, thus free horsepower!

Tighter squish band clearance's allow for smaller squish band widths, and less total trapped end gases.
The trapped end gases are cooled and somewhat shielded from the combustion flame front that they never fully burn until much to late in the power stroke.

So the less wasted fuel mixture you have trapped in the squish band area the more power you can make.

But wide squish bands have there place to, low rpm motors need them.
At low rpm's squish velocity is very low making for a lazy combustion chamber, but a wide band will have a higher flame speed when compared to a high rpm narrow band

Max rpm of the motor is the major influence on the final width of the squish band.

Not sure if many people know but the reason a 2 stroke has all its timing in at idle and retards itself as rpm increase is because of slow combustion speeds.

The slow combustion speeds at low rpm's are because of low squish velocity, residual exhaust gases in combustion chamber cause largely by poor port velocity's and exhaust pipe scavenging.
But as rpm's increase and combustion chamber fuel burn rates increase because of less residual exhaust gases and higher squish velocities, less timing is needed.

Proper squish velocity is a science in itself the factory spends countless hours figuring out what works and what doesn't.
The things I have wrote above are theory's of how they work but as we all know testing is truly the only way to prove something.
So you have to ask yourself are you smarter than Arctic cat R&R dept along with Suzuki's.
You can close up the safety margins that they build in to there products to make them idiot proof and see gains.
But I would not stray to far from the beaten path because they probably have forgotten more than we will ever know.

But if I were to spend money on a head mod it would probably match the factory's squish velocity and compression ratio with a squish band clearance of only .040-.045
I know that's a tight number but with a good crank and fresh cast piston's keeping tight wall clearance there will very little rock in the cylinder.
I have ran as little as .025 squish clearance in a 250R motor with out ever having piston to head contact.[/b]
TTT, cause that's just good reading.

So, the BMP head mod would do more than just "mill off 0.010"?

I want to do the head mod, but I am "leery" of what it will get me, if anything.

I was going to do it myself, so you just saved me a rebuild (hopefully).
 

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Most "Head Mods" are actually taking material off of the head along with re-cutting the squish band back to proper clearance brought about by the factory. This will increase compression by bringing the dome and spark plug closer to the piston. The dome has actually gotten smaller and the compression has increased, without the squish changing much other than maybe going to the smaller side of spec.

~Fast
 

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"TTT, cause that's just good reading.

So, the BMP head mod would do more than just "mill off 0.010"?

I want to do the head mod, but I am "leery" of what it will get me, if anything.

I was going to do it myself, so you just saved me a rebuild (hopefully)."





I would guess that they are doing more than just chopping .010 off but PM them, through this board and I am sure they will explain in a little more detail what is envoved in their head mod.

Or talk to Kelsey at RKTek(site sponser), he does head mods and does nice work.
 

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Anyways Snowman in your case. You might have just gotten some bad fuel. Not enough octane will do exactly what happened to you no matter what head you have. The sleds that have the .058 squish are built to last , just like your 4600 miles. Now If I were to cut my .078 stock squish down to meet yours, I will be in trouble, there is a difference in motors. You do have to clean all of the dammage out of your dome. It needs to be smooth in order to not have any hot spots and deto again. If your a sea level rider then Speedwerx or D&D would be the best to contact as they do have the most expirience with what you'lll need. If your an elevy rider then maybe contact an out west shop. I personally would just clean what you have at a local, but familiar with sleds machine shop. If you have to take more than .010 off to clean it up then have it redone. I have fixed many marks with just 400-800-1500 grit sand paper and do it under water and palmolive soap. Just never touch the shut off areas at the edge of the chamber where it seals.


I sure would like to know what motor runs with no squish or one that is as tight as .001. I can't imagine why the piston at rpm wouldn't contact the head in those cases. There is as much as ..010 play just in the 700 motor or .003 in a tighter one at rpm. So you'd be takin out some ring lands before you even got off idle. Some BMP head mods are just whacking .010-.020 off, but it depends on you package
 
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