Squish Gap

[Tony Di BenedettoParticipant]

Will altering the squish gap change RPM ?   Running a ROK and currently maxing out at 14,573rpm.   If I drop base gasket of cylinder will increasing compression boost rpm to 15K+ ?

[Mike BurrillParticipant]

Tony really good question.  Do you know what your squish gap is currently?  If you close it up too tight you run the risk of the piston contacting it.  I think there are a lot more variables then squish to effect RPM.  Are you racing on tracks with long straights or short?  How long are you at wide open throttle with the maximum RPM reached?

 

Mike Burrill -
Karting Kartel

[tony zambosParticipant]

Agree with Mike. Also, by changing the base gasket, you’re changing the port timing.

LAD Specialties customer / tony kart / rotax / kt100

[Christian FoxParticipant]

Mike and Tony are correct, and to Tony’s point, a thinner gasket/tighter squish will lower your exhaust port and retard timing a bit. Yes, you will get more compression, but with a lower exhaust port, you will be moving the engine’s peak hp to a lower rpm band. So, if anything, tightening the squish in the absence of any other adjustment will give you more bottom and mid and hurt your top end. You should be targeting 15k in a Rok, so you aren’t far off. Just add a tooth on the rear.

[Tony Di BenedettoParticipant]

So, on short tracks with more corners the way to go is dropping size of cylinder base gasket thickness to shift power band down and vice versa for long / fast tracks.   Correct ?

[Walt GiffordParticipant]

I seriously doubt you will see any performance difference you can feel or measure by changing the thickness of the base gasket. I would not consider it a tuning tool.

The head clearance is where you will see an increase. If it’s optimized to the minimum without hitting and, centered on the bore, it will burn more efficiently and that counts for allot.

Increases in compression ratio (to a point) will benefit the lower rpm band but the increase in compression ratio from a gasket change is minuscule.

Gif

FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience

[Gordon DuaxParticipant]

Tony Di Benedetto wrote:

Will altering the squish gap change RPM ? 

It’s actually the other way around…

Altering your rpm will change the squish.
I’ve had a 125 bike engine that was set up with .016″ squish, but if revved above 16k, would go to zero, due to crank flex.

Would start making a ringing sound above 16k, and when torn down both the piston & head would show light contact.
It never hurt anything, but I didn’t dare let it rev past that point.
I just used it as a shift indicator :)

I am sure this is happening on most engines to some extent.

[Author]

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