Octane vs. Compression Ratio
Thread Starter
Pro
Joined: Apr 2013
Posts: 575
Likes: 56
From: Placitas, New Mexico
Octane vs. Compression Ratio
Hi folks. I tried doing a bit of research on the web and didn't get any straight answers. Perhaps you folks know!
For a naturally-aspirated LS1, what is the minimum octane necessary for a compression ratio of 11.1? Or better still, does anyone have a chart that shows the relationship of octane vs. compression ratio?
I live in New Mexico, and the highest octane pump gas available is 91. What compression ratio would be supported by 91 octane gas?
I've thought about having a higher performance engine built, but it would be nice to know what compression ratio I should shoot for. Thanks for your thoughts!
For a naturally-aspirated LS1, what is the minimum octane necessary for a compression ratio of 11.1? Or better still, does anyone have a chart that shows the relationship of octane vs. compression ratio?
I live in New Mexico, and the highest octane pump gas available is 91. What compression ratio would be supported by 91 octane gas?
I've thought about having a higher performance engine built, but it would be nice to know what compression ratio I should shoot for. Thanks for your thoughts!
Advanced
Joined: Feb 2006
Posts: 54
Likes: 0
From: Reading Pa
You didn't get a straight answer because it's not that easy. If you want it that easy, stay below 10.5 to 1. There are many variables that contribute to detonation suppression. As compression goes up, more of the suppression techniques must be perfected. The power increase becomes insignificant to the risk of reliability if some slight imperfection can cause catastrophic failure from detonation. For the purpose of "straight answer", if you make say 430 hp at 10.5 compression and choose to squeeze it up to 11 to 1, you will probably only gain 10 to 15 horse power. You probably can run an Ls1 at 11 to 1 all day long on 91 octane. You definitely can on 10.5 to 1. If you want to make the power you decide the risk you will take to get it just like deciding how many rpm's you will shift at. Good luck.
Melting Slicks
Joined: May 2008
Posts: 2,606
Likes: 355
From: Rochester New York
High octane will probably be OK. I would not try any lower than 91. You can always get cans of octane booster as a safety net. Your engine's tune will also change things, high advanced timing calls for higher octane. Temperatures and exhaust restriction should be considered as well.
There are tons of high performance LSx motors on 91/93 pump gas.
There are tons of high performance LSx motors on 91/93 pump gas.
Melting Slicks
Joined: Aug 2004
Posts: 3,003
Likes: 89
From: Indy IN
So long as you have a knock sensor you shouldn't damage the engine, but you really don't want to be retarding the spark all the time. OTOH, if you are just cruising on the highway as low throttle openings you don't need much octane
Another thing you could add is water injection and spray a very small amount of water when you are at wide throttle openings. Since you are in Mexico you shouldn't have to worry about freezing so you don't need any alcohol with it.
I'm surprised that the big car companies haven't gone back to that approach since it is simple and works so darn well.
Another thing you could add is water injection and spray a very small amount of water when you are at wide throttle openings. Since you are in Mexico you shouldn't have to worry about freezing so you don't need any alcohol with it.
I'm surprised that the big car companies haven't gone back to that approach since it is simple and works so darn well.
Le Mans Master
Joined: Apr 2012
Posts: 7,774
Likes: 2,251
From: Brighton MI
Hi folks. I tried doing a bit of research on the web and didn't get any straight answers. Perhaps you folks know!
For a naturally-aspirated LS1, what is the minimum octane necessary for a compression ratio of 11.1? Or better still, does anyone have a chart that shows the relationship of octane vs. compression ratio?
I live in New Mexico, and the highest octane pump gas available is 91. What compression ratio would be supported by 91 octane gas?
I've thought about having a higher performance engine built, but it would be nice to know what compression ratio I should shoot for. Thanks for your thoughts!
For a naturally-aspirated LS1, what is the minimum octane necessary for a compression ratio of 11.1? Or better still, does anyone have a chart that shows the relationship of octane vs. compression ratio?
I live in New Mexico, and the highest octane pump gas available is 91. What compression ratio would be supported by 91 octane gas?
I've thought about having a higher performance engine built, but it would be nice to know what compression ratio I should shoot for. Thanks for your thoughts!
Can you run lower octane? Yes.
However, the ecm will begin making adjustments (retard timing, etc.) to compensate for the lower octane, thereby preventing detonation but also losing power in the process.
Here is a pretty exhaustive read on the topic if you are interested...
http://www.contactmagazine.com/Issue...ineBasics.html
Pro
Joined: Nov 2006
Posts: 627
Likes: 33
From: Omaha NE
You need to do more research. Look into dynamic vs static compression. There are two different types and dynamic compression is what affects what fuel you can run. A huge part of it is also in your tune. There is no magical graph or chart. It all depends on your combo, tune etc.
James Shor put together a nice excel file that when inputted with your engine combo information will spit out your static and dynamic compression numbers. All of this is available by just searching using google.
You can go pretty radical on 91 if you stay N/A. My combo puts me around 11.3:1 static and 8.4:1 dynamic and I don't have any problems with 91 at all.
James Shor put together a nice excel file that when inputted with your engine combo information will spit out your static and dynamic compression numbers. All of this is available by just searching using google.
You can go pretty radical on 91 if you stay N/A. My combo puts me around 11.3:1 static and 8.4:1 dynamic and I don't have any problems with 91 at all.
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Thread Starter
Pro
Joined: Apr 2013
Posts: 575
Likes: 56
From: Placitas, New Mexico
Wow...there are many variables in this equation! Thanks everyone for your input and suggestions. I'll continue to do my research. But it appears that the compression ratio that suits my needs would be 10.5:1. I don't plan on doing any racing, just daily driver duty and road trips for fun.
