As a forced induction newbie, I have a general technical question that I'd like to clear up. I've noticed that forced induction engines seem to be able to get away with higher effective compression ratios (i.e. CR x Boosted_Pressure / Atmospheric_Pressure) than naturally aspirated engines before encountering detonation. For example, it's not uncommon to read of setups on this forum that have CRs of ~8:1 running ~1 bar of boost, yielding an effective CR of 16:1, on pump gas. Yet I've never heard of a NA gasoline engine running a CR of 16:1 on pump gas, no matter how retarded the spark timing or how rich the fuel mixture. Why is that?

 

i want to say boosted cars from the factory were built to that spec vs. taking a stock na car and boosting will mess with the balance

 

I think you are confusing STATIC compression with Dynamic compression.

For the record, our motors are 11.1 compression, and running 15+ lbs of boost.

Put that in your math calculator

 

wait your running 15lbs of boost?

whats your setup? i'd like to up my boost from 11 to 15... well.. my tuner said 15psi...

 

well, an na motor w/ a static compression of 16:1 is pretty much always at around 16:1. i say "around", because a naturally aspirated motor HAS a dynamic CR that varies depending on valve size, valve lift, valve duration, cam timing blah blah blah etc.... now a turbo'd/supercharged engine with an effective CR of 16:1 only reaches it for a fraction of the time and usually occurs later in the rpm range after the engine's volumetric efficiency has already began to fall off, avoiding the type of cylinder pressures and combustion temps you would see w/ a static 16:1. so, 16:1 @ 8500rpm not as bad as 16:1 @ 1k-9krpm in terms of avoiding detonation.

can u imagine an 8:1 engine on 91 boosting 14.7psi at idle!! knock city!!

 

soon i will be trying to do a 11.5cr with 14psi..so we will see what happens

 

Hi, stefce.

New version was just released. Includes knock-finder LED's to show which cylinders are knocking, plus a headphone amplifier if you want to listen for knock.

Click the interceptor link at the top of my home page, or google eight channel interceptor.

 

I was just going to say