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neo · Posted: Tue Sep 30, 2003 1:55 pm Post subject: Compression ratio

Hi!

What is the limit for the compression ratio when 98 octane fuel is used?

Thanks!

Peter D · Alfasud Joined: 15 Aug 2003 Posts: 60 Location: Denmark

There is no "right" answer to the question about compression ratio in relation to octane rating.

It depends on engine/cylinder size, 2 or 4 valve, combustion chamber design, single or twin spark, camshafts, ignition timing, injection system, and, and...

If you run hotter cams, you can also raise the compression ratio a little, because the filling at the low to medium rews where you usually get the pinking, will be not so good as before.

I do not have experience with boxer engines where you only raise the compression ratio, because I made a lot of other changes at the same time (forged piston 11:1 together with other cams, bigger valves).

If you shall hope to get other answers, I think that you must specify which engine you have in mind.

Peter (DK)

neo · Posted: Wed Oct 01, 2003 6:59 pm Post subject:

Hi!

I'm plannig to make some modifications like porting the heads, lighten the flywheel, intake valves, rods and maybe the crankshaft, fit larger venturis on the carbs, rejetting on dyno, raising comp. ratio to about 12:1 (i've calculated that for 98 octane fuel, 1.3 liter engine with volumetric efficency at about 80% that max. teoretical compression would be 12.8:1).
To get 12:1 comp. from 9.5:1 I have to remove 1.6 mm from the block. I don't know if i'ts possible because I don't know the lift of the cams so that my valves don't hit the piston.
I'll probably put digiplex ignition.
The exhaust will be modified.