C/R ratios with alternative fuels


New Member
With various alternative fuels it's adviced to change the C/R. Now what are some general rules of the thumb / good ideas for compression when you run these fuels? I'm talking about E85, CNG, LPG primairily but also racegas like C12 and C16. Also turbo and N/A.
In stock form the N/A UZ is like C/R 10 or 10.5. Anyone played with this already?


The pent-roof head with correct tune can tolerate high compression on moderate forced induction.. With better fuels which have less detonation tendencies.. Again tune where more advance in timing can be built into tune to take advantage of slower burning fuel with correct lamda..
Dropping compression on a tough blown engine run around 9.1 compression on pump fuel.. With C16, std compression will work fine..


"Supra" Moderator
Staff member
Arnout, there's a very knowledgeable guy named Franz on this board: http://fuelsforum.rasoenterprises.com/viewforum.php?f=5. He's authored several books on alternative fuels such as LPG, LNG, CNG, etc. and has done a lot of laboratory research & testing with various engine configurations, both injected and carbureted.

We all know that the higher "octane" ratings (actually methane numbers) of these gases permit higher compression, but as regards the timing of the engine and the points about head geometry, squish, etc. these would all be good questions for him.