Fuel for stock engines

[Duke W.]

Re: Fuel for stock engines

Has anyone rebuilt their engine to original specs with 10.25:1 compression or higher and used the current octane fuel?

Download and study the following pdf:

http://www.metroli.org/pdf/2012%20Nationals%20tuningseminar.pdf

Duke

[Mark E.]

Re: Fuel for stock engines

I for one do get knocking if set up to factory specs with 93 octane. A few adjustments will take care of almost all knocking but I don't really run her hard for fear of causing damage when running 93. Now with a blend of racing fuel and 93 octane she runs about perfect and I can run more timing. To say it wakes the old girl up is an understatement. She loves the additional octane.
Bill

My stock '70 LS5 with 10.25:1 behaves the same way.

[Mark E.]

Re: Fuel for stock engines

By the way, all of GM's crate, all cast iron big blocks are built with a compression ratio of 8.25:1 and require minimum of 91 octane fuel. The way I see it, if GM thought that big blocks could satisfactorily run on a compression ratio of higher than 8.25:1 with 91 octane fuel, they'd build them that way. There's a message in all this.

I understand what you're saying, Joe. It's a good idea to be conservative with the compression ratio for a street motor.

ZZ crate big blocks sport 9.6:1 compression so maybe we can go a bit higher if the ignition timing is tuned for it.

[Joe L.]

Re: Fuel for stock engines

I understand what you're saying, Joe. It's a good idea to be conservative with the compression ratio for a street motor.

ZZ crate big blocks sport 9.6:1 compression so maybe we can go a bit higher if the ignition timing is tuned for it.

Mark-------


Those big blocks have aluminum heads. This allows somewhat higher compression. VERY FEW of original 60's and 70's Corvettes have aluminum heads.

[Paul Y.]

Re: Fuel for stock engines

I understand what you're saying, Joe. It's a good idea to be conservative with the compression ratio for a street motor.

ZZ crate big blocks sport 9.6:1 compression so maybe we can go a bit higher if the ignition timing is tuned for it.

They also have aluminum heads which are known to ease the detonation.

[Mark E.]

Re: Fuel for stock engines

Why are aluminum heads less prone to detonation, all else (comp. ratio, piston/chamber configuration, fuel, etc.) assumed equal?

[Joe L.]

Re: Fuel for stock engines

Why are aluminum heads less prone to detonation, all else (comp. ratio, piston/chamber configuration, fuel, etc.) assumed equal?

Mark-------


I believe it has to do with heat dissipation properties. Generally, one can run about 1 point higher compression with aluminum heads versus cast iron everything else being equal.

[Duke W.]

Re: Fuel for stock engines

Aluminum has one-third the thermal resistance of cast iron so it transfers heat more readily, which keeps the combustion surfaces cooler, which reduces the tendency to detonate. The number I've heard over the years is half a point, but it's real "squishy" and will vary with the basic engine design and specific configuration for a given design architecture.

Anecdotal evidence I have is that most OE engines don't detonate (and the actual CRs are up to half a point less than advertised) on 91-93 PON fuel, especially with the typically lazy OE spark advance maps, and those that do can usually tamed with some spark advance map adjustments.

That's why it's important to measure the true CR on disassembly, and if it didn't detonate before, the CR does not need to be "lowered" and can sometime even be raised.

Duke

[Mark E.]

Re: Fuel for stock engines

Joe, Duke- Thank you.

[Douglas L.]

Re: Fuel for stock engines

My experience with my factory spec L36 pretty much mimics what the others have said.

I'm in California(91 octane) and during the winter (40-70 degrees) setting the timing to factory specs works okay, its not really ideal and sometimes there is some light pinging (long inclines/ transitioning to secondaries in 4th at low rpms)but for the most part it works. In the summer the pinging gets a bit too frequent/loud so I take a few degrees of timing away.

[Enzo C.]

Re: Fuel for stock engines

I add 3 gallons of moose juice (MS-109) to a tank and my 67 small block runs like a clock... Just as it should

[Timothy B.]

Re: Fuel for stock engines

Some of these older engines that detonate could have some carbon build up and need a good cleaning out.

Also, the A/F ratio could be a issue with today's fuel.