cng engine

[smartineau]

I am building a Chevy 350 specifically for cng. I have measured cyl chamber cc and calculated what I need to achieve 11 to 1 compression. I am using Keith Black pistons (should run cooler) with the top ring closer to the top of piston which I believe will help with the dry fuel.
Question -- In hyperutectic pistons, how much would you recommend increasing the piston to wall clearance for the added heat of combustion? The recommended clearance on this piston for gasoline is .001-.0025 and I usually go for a closer tolerence, I am thinking .003 (for the increased octane)for cng would be where I should go.
Anyone out there have suggestions, I would appreciate any help.
Is anyone interesetd in the "after" results? I have already published the 5 gas analysis & timing advance changes form the "before" results.
Stan M.

[larrycng]

Re: cng engine

I'm trying to remember, but I think you will be safe on the high side of clearance. There is not that much difference in piston temperature if you driving in a civilized manner -- WOT is another story.

What you need to watch is cylinder finish for break in purposes. Use a 600 to 800 grit finish on the walls and work the engine.

For those who don't remember, gaseous fuels do not wash the cylinder walls as does gasoline therefore there is better lubrication for the cylinder walls during break in and therefore less cylinder wear. AND you have to have wear to break in. If you use a regular finish it will take you longer to seat the rings and you will probably have oil control problems which is not good for CNG engines (carbon and preignition.

Larrycng

[smartineau]

Re: cng engine

Excellent --- I really appreciate someone helping me!
I am using a plateau finish and will end with 800 grit stones & then do a soft hone finish like they do for low tension rings. That should allow me to use a chrome moly ring. I will also check valve clearance on the piston before assembly to verify no problems.
Stan