TheSamba.com :: View topic - Talk to me about Dynamic Compression Ratios

Sun Oct 26, 2008 7:57 am

I'm trying to get my head around what compression ratio to use. I will be building a basically stock 1500 or 1600cc engine. The cam will be either stock or something very close to it. My goal is to build an efficient engine with good mpg. Stock BHP or close to it will do. Engine will not see above 5000rpm and will normally cruise at 3000rpm. Pushing a 72 squareback. Engine will have D-jet FI hardware, but running modern sensors with megasquirt.

I appreciate that the stock CR is 7.3-7.5ish static. I know most of you will say keep it at that for a daily driver. I just wanted to check to see if it is practical to push it up slightly with normal unleaded gas? I don't want to put premium in it. I've done a search and so far it seems that people say shoot for a max dynamic ratio of 7.5. Is this the general feeling, or has anyone pushed the 7.5 boundary a bit further?

How far could you push the boundary if the piston and chamber were coated?

Thanks
Craig

Sun Oct 26, 2008 10:48 am

I know pushing the compression ration up is free HP, but on a stock build where your going to make about 60hp the bump from 7.5 up to 8 wont get you more than 1 hp.

With all the possible variable there can be in an engine, what you set your total spark timing at, what the atmospheric conditions are no one can really say what you limit is. (a knock sensor could though)

Sorry to have no answer for you other than to say dont get hung up on half a point of compression ratio, you dont want to run premium stay conservative with the stock comp ratio.