Variable compression ratio for different octane fuels? HELP!
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Teching In
Joined: Feb 2005
Posts: 5
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Variable compression ratio for different octane fuels? HELP! Hi, bit of a different style question for you guys.
I'm looking to run an LSx in my off-roader. I'd like a reasonably exciting engine most of the time (400 & 400 ish), but if I drive it down to Africa for a bit of desert fun I can't rely on having decent quality fuels (octane rating will drop).
Do I have ANY options for running different compression ratios without rebuilding the thing?
I thought about forced induction - so I'd lower the static compression ratio to maybe 9:1 (or possibly even less!...) and then somehow use the forced induction when I'm running on higher octane fuels (bypass the rest of the time?).
Or 2 sets of heads? But I'd like to avoid spanner time where possible!
Interesting problem, huh? Any ideas???
Anyone know how much the performance is affected by reducing the compression ratio by, say, 2 whole points?
Is there something else I can compensate with - intercooling / water injection??? (At reasonable cost if poss).
Cheers, Al.
I'm looking to run an LSx in my off-roader. I'd like a reasonably exciting engine most of the time (400 & 400 ish), but if I drive it down to Africa for a bit of desert fun I can't rely on having decent quality fuels (octane rating will drop).
Do I have ANY options for running different compression ratios without rebuilding the thing?
I thought about forced induction - so I'd lower the static compression ratio to maybe 9:1 (or possibly even less!...) and then somehow use the forced induction when I'm running on higher octane fuels (bypass the rest of the time?).
Or 2 sets of heads? But I'd like to avoid spanner time where possible!
Interesting problem, huh? Any ideas???
Anyone know how much the performance is affected by reducing the compression ratio by, say, 2 whole points?
Is there something else I can compensate with - intercooling / water injection??? (At reasonable cost if poss).
Cheers, Al.
Teching In
Joined: Jan 2006
Posts: 19
Likes: 0
The PCM, using the knock sensors, will pull timing when you use low octane fuel. The worst operating condition for allowing knock to occur is when you open the throttle all the way (WOT) when the engine RPM is low. This gets a practically full charge of air into each cylinder raising the Dynamic Compression Ratio to its maximum possible. You can also get knock at higher RPM's where timing will get pulled by the PCM. This probably occurs because of heat retained by the cylinder head. So, if you were merely cruising with low octane gas, you could probably get away with it. If you want to get sporting, then you'll be down on power. I can't remember any posts on this and other forums about detonation damage.
If you are building an engine for the future, yes you could start with low compression for supercharger. Some of these allow you to change the driven pulley to raise or lower boost, and this might be the simplest adjustment. Remember, lower boost means lower power.
If you are building an engine for the future, yes you could start with low compression for supercharger. Some of these allow you to change the driven pulley to raise or lower boost, and this might be the simplest adjustment. Remember, lower boost means lower power.
