Octane rating

[dlrshort]

I was wondering what octane rating you are running in your lower compression LT-1's (1971 and 1972)? Do you notice any difference in perfomance and fuel economy with respect to different octane fuels? Have you found any advantages to using a very high octane unleaded fuel?

Dave S.

 

[Robert N]

If my memory serves me, the 71 had a 10.8:1 CR and the 72 was 9:1. The 70 was 11:1 and a mix of 100+ and 91 octane resulted in huge increase in performance. The higher the octane, the later the detonation (power stroke) at that allows the engine to take full advantage of the compression. That will result in greater power.

 

[Rab563]

I maybe wrong about this but I believe running a higher octane in a lower compression engine will not result in more power. The higher octane rating raises the flash point of the fuel. In high compression engines the act of compressing the molecules can produce enough heat to trigger the flash point of the fuel. This is called pre-ignition. Pre-ignition can damage the motor. What we need to do is raise the flash point above the combustion temperatures. The higher the compression ratio equals higher temperatures and the higher the octane rating needs to be. Older engines might benifit with higher octane because of the buildup of carbon in the cylinders. Carbon can act like a glow plug and cause detonation. Higher octane helps reduce detonation. You might need to run a slightly hotter plug. One way higher octane might produce more HP is allowing more advance in the timing. THE REAL REASON MOST DO NOT USE HIGH OCTANE FUEL UNLESS THEY NEED TO IS THE COST. 100 OCTANE IS GOING FOR $4.00 A GALLON. OUCH!!!

 

[dlrshort]

As a follow up, does anyone know what octane rating did Chevrolet recommend for these engines?

Dave S.

 

[MidShark]

Robert N- The '71 comp. ratio was also 9:1.

dlrshort- I run 91 octane in my '71. That is what it recommends in the owners manual. I think, too, that anything more in the lower compression engines is probably not going to make much of a difference. I read a good article that said the most octane you really need is the least "number" it will run on without detonation.
I do agree it will probably make a big difference in the higher compression engines though.
Dick

 

[Robert N]

Rab563 - you are correct from what I read as well. There should be little or no improvement when using higher octane on low compression engines. Your post was great. 100 octane a month ago was $4.29 a gallon by me. Hate to see it now and I need the stuff.

dlrshort - I think 98 was the octane requirement for the 70 LT-1. That is only based on previous owner info though.

MidShark - Thanks for the right numbers on the 71.

 

[LIRacer]

Increasing octane will result in more power as it allows more advance. The 71 and 72 both had 9.0:1 CR. The rating difference in was due only to a apurer rating method in all GM cars starting in 72.

That all being said, I have a mostly stock 72 except for headers, MSD and a dyno tune where I had the carb rejetted. I am pulling 275 at the rear wheels on 92 octane with a +104 booster and lead aditives. With the advance dialed in, it detonates a bit on anything else other then the brew I mentioned.

I have a 7000 RPM rev limiter and the Vette pulls hard...really hard over 3500rpm all the way up to 7Gs.