I'm currently rebuilding my L18 stroker engine for my rally car. What is the best compression ratio to run with premium unleaded fuel (Optimax or Synergy)?

The engine currently runs 10.5 : 1 compression ration, and around 6 to 8 degrees static advance timing, to stop it pinging. It currently runs a thick (2.8mm) head gasket for decompression, which I will be changing to a standard thickness head gasket in the rebuild (much cheaper) along with machining material from the pistons to suit the thickness change.
My delima is what compression ratio I should achieve? Should I machine extra material from the pistons to lower the compression further?

Will I pick up MORE POWER by reducing the compression slightly so I can advance the ignition slightly (12 degrees)?

Most rally engine builders I know are aiming for 10.3 to 1. Some "naughty" people still seem to be at higher compresions (12:1) and AVGAS.

From what I have been told, PULP is a fast burning fuel and u should expect more power at 10.3 than you would get out of AVGAS (which is designed for slow reving aero engines - hnce slower burn rate) and higher compresion.

But if your talking about 10.3 vs 10.5 - prob more critcal on combustion chamber design for timing than compression ratio.

"all though recently had a debate with someone on another site who was convinced that 12:1 on pulp was fine - gave up casue figured he had never built a motor run on pulp"

Andrew,

I can honestly say I have no idea when it comes to compression but it sounds like around 10.5:1 is fine. It also depends on the level of control you have. If you want to start pushing things I'd definitely be going EFI, or if you can't afford it, DEFINITELY programmable ignition control. The timing requirements around max torque will vary from those everywhere else. At max torque you need to pull timing out I believe to stop it from detonating (this is the case with a turbocharged engine anyway, I would think about why and try to explain but I've only had 3 hours sleep).

Get programmable fuel and ignition if you can.

Dave

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