Is it possible to base fuel octain requirement on cyl pressure?
- Thread starter Rick
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There are other factors, such as head design, localized cooling, etc...
My LT1 runs 87 pump gas and 10:1 compression, with an optimised SBC design and reverse cooling. Previous generations could only handle that with a 9:1 ratio. My Yamaha Genesis 700CC motor does the same thing with an 11:1 comp ratio.
My LT1 runs 87 pump gas and 10:1 compression, with an optimised SBC design and reverse cooling. Previous generations could only handle that with a 9:1 ratio. My Yamaha Genesis 700CC motor does the same thing with an 11:1 comp ratio.
Rick, as per Vader's response, combustion chamber design is the one thing that you can't measure to help you out.
The distance that the flame has to go to get to the other side of the cahmber, and the amount of swirl (i.e. air movement in the chamber) just prior to ignition makes a huge difference.
A 4.5" bore with an open chamber, and a plug mounted at the side will detonate long before an advanced "clover" shaped chamber with a central plug, on a 3" bore.
The distance that the flame has to go to get to the other side of the cahmber, and the amount of swirl (i.e. air movement in the chamber) just prior to ignition makes a huge difference.
A 4.5" bore with an open chamber, and a plug mounted at the side will detonate long before an advanced "clover" shaped chamber with a central plug, on a 3" bore.
A typical B-body LT1 like what Vader mentioned will crank around 200psi
even at 100K which is beyond what conventional "wisdom" says os OK for even 93 octane. While the things he mentioned are very big parts of why that compression is OK for this motor and not other there is also the very accurate port injection and again very accurate optispark distributor that help too. Throw a carb and an HEI(both possible) on the same engine and I would be willing to bet you will need above 87 octane even if you got it dialed in well.
Cranking compression is probably the most important factor.......but as vader said.....there are other factors at work.
I thought that LT1's had 10.5-11.0 compression. 11.0-1 on the LT4.........but I could be wrong.
Cam profile and overlap have alot to do with this as well. In the early days of when Mr. Crower was a racer he developed cams that allowed him to run a static compression ration as high as 16:1 on pump gas. I am sure it was premium but this is still a feat to this day. You problem to day is one of design. The engines and their PCM/ECM's have been pureposefuly designed and caliprated to run hot. You can not run hot and run sheap full without a lot of retard wich will kill performance! WHile it is common rule of thumb to expectt a higher octane need as static compression goes up it is not a linera function!
10:1 for the iron head, B/D body LT1, 10.4:1 for the al head, F/Y body LT1. 10.8:1 for the LT4.
I was kinda close.......
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