Honda GX35 Power Increase

[A. Devlin]

I got my GX31 piston delivered today and plan to swap it into my GX35 engine in the next couple of weeks. Does anybody know the clearance volume for the GX35 engine? At the minute, I am trying to find the exact compression ratio when the GX31 piston is swapped in. I know the swept volume but I cant find any information regarding the clearance volume. Thanks for your help guys.

 

[machanic]

Easy way to get an actual CR is by water displacement, set piston a BDC with spark plug out, fill cylinder with water to bottom of treads on spark plug hole drain into accurate measuring container, that is total volume, next set piston at TDC repeat process this is compressed volume total volume over compressed volume is CR . To adjust CR, volume removed from piston above rings is added to both volumes to effect CR.
P.S. add a small amount of alcohol 5% to water to lessen surface tension.

 

[Old Bob]

GX-31 piston in a GX-35 is 12.5 -12.8:1
On methanol its ok, on pump gasoline it may detonate.

With a given CR you can back plot the combustion chamber volume. I have it in my notes but can't access them right now.

don't use water! to measure volume use ATF, no rust problems with ATF

 

[hatem younis]

I have a chart somewhere on how much to remove from a GX31 piston to get 10:1 in a GX35, if you don't do anything to the top of it the CR is 12.5:1 too high for pump gas.

i need this chart so bad can you contact me via facebook ? https://www.facebook.com/hatem.younis.9 , thanks in advance

 

[Steve Best]

GX-31 piston in a GX-35 is 12.5 -12.8:1
On methanol its ok, on pump gasoline it may detonate.
With a given CR you can back plot the combustion chamber volume. I have it in my notes but can't access them right now.

don't use water! to measure volume use ATF, no rust problems with ATF

Well, I do use water because ATF will dissolve the grease I use to seal the cylinder and ATF will take the numbers off the syringe.

So, a bit of grease on the sealing surface of the head:

Then plank it down on a flat surface, such as this aluminum plate. Glass works very good.

Now fill up the combustion space with water, measuring what you put in, up to the bottom spark plug thread:

Easy peasy! This gives a good CC comparison between various heads but isn't very good for calculating compression ratio with a domed piston.
In this case, grease the outer edge of the piston top and measure the water to fill.
Combustion volume is this volume plus the bore area x squish distance.
Compression ratio is (displacement + combustion chamber volume + squish volume) / (combustion chamber volume + squish volume)
Squish distance is the closest distance from your piston at TDC to your head. Typically measured by crushing a lead wire.