octane

[jim cear]

With a small block rebuilt to a compression ratio of 9.5:1 would there still be a reason to use 93 instead of 89 octane?

[Joe L.]

Re: octane

Jim-----

It's very hard to say for any particular engine; there are just too many variables that affect the tendency of a PARTICULAR engine to "ping" or detonate. Compression ratio is just part of the total picture. However, it's a VERY easy thing to determine after the engine is running and the car on the road. First, use 87 octane fuel. If the engine "pings" or detonates more than a very small amount, run that tank down and fill up with 89 octane. Make the same determination. Basically, you want to use the lowest octane fuel that keeps the engine from significant "pinging" or detonation. Using higher octane is a waste of money.

[Duke W.]

What cam? *NM*

[Roy B.]

Re: octane

I use middle grade and some times high grade and low now and then , Have had no problem in 25 years 1967 327-350hp 4speed

[jim cear]

Re: What cam?

097

[Duke W.]

Re: What cam?

As stated, there are many variables in the detonation equation - inlet air temperature, timing map, ambient pressure (altitude), and another very important parameter is the point that the inlet valve closes. The later it closes the lower the "dynamnic compression", and the more static CR the engine will take on a given octane fuel.

The Duntov cam has a fairly late closing inlet valve to enhance the high speed inlet inertia pressure for more top end power. It closes a lot later than the base cam, which is one reason why late Duntov cam engines have higher compression than base cam engines. From earliest to latest closing it goes base cam, Duntov, L-79, LT-1, 30-30, and the L-46/82 cam is about the same as the LT-1 cam.

Given all the above it still comes down to specific operating conditions for a specific engine configuration, but my suspicion is that with only 9.5:1 and a Duntov cam, it will be happy on a diet of mid-grade and might even run on regular.

When the tank gets near empty, put in a few gallons of mid-grade. If it doesn't detonate, repeat when it's near empty again. If it does detonate, fill it up with premium

If there is no detonation with mid-grade, you could run the same experiment with regular.

Back in the mid-sixties when I was working in a gas station a chick with a 270 solid axle came in one day and order regular. I warned her that a 270 needed higher octane or it would ping, but she said it never did, and she became a regular customer for awhile. All the 270s had an advertised CR of 9.5:1, but as with most SBs whose decks were OE machined 10 to 15 thou higher than the 9.025" (crankshaft axis C/L to deck) nominal blueprint dimension, the actual AS BUILT CR was probably closer to 9:1.

Duke

[jim cear]

THANKS *NM*