Fuel for stock engines

[Justin S.]

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

[Bill L.]

Re: Fuel for stock engines

HI:

My 66 Corvette Big Block coupe with 390HP engine and 10.25:1 compression ratio ran just fine on 93 octane pump gas.... car is in Georgia----

(Original engine --- never rebuilt)

Hope this helps

Regards
Bill

[Gene M.]

Re: Fuel for stock engines

65 with 10 to 1 runs on 10% corn juice 91 octane. but best on 93 octane all gas.

[Donald O.]

Re: Fuel for stock engines

My 67 L79 at 11:1 runs great on the stock timing map with E10 93 octane.

or

you can waste some money and only buy racing fuel.

Don

[Michael W.]

Re: Fuel for stock engines

If for no other reason, 2016 is memorable as the 40th anniversary of the requirement for all cars to run on unleaded gasoline. I would have thought that four decades would be enough time for the old myths to die away about the 'need for lead' in gas- yet there are still some diehards.

This thread has given me hope.

I also take comfort that no one has brought out the octane and ethanol myths for yet another beating.

Hiya Don!

[Bill L.]

Re: Fuel for stock engines

If for no other reason, 2016 is memorable as the 40th anniversary of the requirement for all cars to run on unleaded gasoline. I would have thought that four decades would be enough time for the old myths to die away about the 'need for lead' in gas- yet there are still some diehards.

This thread has given me hope.

I also take comfort that no one has brought out the octane and ethanol myths for yet another beating.

Hiya Don!

Bully for all those whose Corvettes with higher compression that will run on today's gas. My 70 L46 will not when set up to original specs. It needs more than today's 93 Octane.

Yes it will run on 93 Octane by making some adjustments. Still runs best with the higher octane racing fuel mixed with 93 Octane. Most would not notice the difference, but those that have driven with both tunes will absolutely know the difference.

Bill

[Mark E.]

Re: Fuel for stock engines

Bully for all those whose Corvettes with higher compression that will run on today's gas. My 70 L46 will not when set up to original specs. It needs more than today's 93 Octane.

Yes it will run on 93 Octane by making some adjustments. Still runs best with the higher octane racing fuel mixed with 93 Octane. Most would not notice the difference, but those that have driven with both tunes will absolutely know the difference. Bill

Ditto with my rebuilt as stock '70 LS5 with 10:25 compression. It runs ok with 93 octane E10 with the timing retarded a bit, but it runs better when 100 octane avgas is added and the timing is advanced.

[Larry M.]

Re: Fuel for stock engines

Bully for all those whose Corvettes with higher compression that will run on today's gas. My 70 L46 will not when set up to original specs. It needs more than today's 93 Octane.

Yes it will run on 93 Octane by making some adjustments. Still runs best with the higher octane racing fuel mixed with 93 Octane. Most would not notice the difference, but those that have driven with both tunes will absolutely know the difference.

Bill

My 1967 L79 327/350 HP car will run on 93 octane non-leaded with or without ethanol. When doing this I retard the initial advance a few degrees to around 6 BTDC just to be safe......as otherwise total mechanical advance with the factory distributor is 40 degrees (Chevrolet spec). However, it much PREFERS to run with more advance and a 50/50 blend of 91 octane non-ethanol and 110 octane VP Leaded Racing Fuel. Really wakes the old girl up: both at idle and at redline.

Larry

[Wayne D.]

Re: Fuel for stock engines

Bully for all those whose Corvettes with higher compression that will run on today's gas. My 70 L46 will not when set up to original specs. It needs more than today's 93 Octane.

Yes it will run on 93 Octane by making some adjustments. Still runs best with the higher octane racing fuel mixed with 93 Octane. Most would not notice the difference, but those that have driven with both tunes will absolutely know the difference.

Bill

Same with my 327/350

[Michael W.]

Re: Fuel for stock engines

So all you guys are hearing detonation when running on 93 octane and stock timing?

[Al R.]

Re: Fuel for stock engines

My 59 290HP FI car with 10.5 to 1 actually produced more HP with the 93 octane 10% ethanol on the dyno than with 110 racing fuel or 93 non-ethanol fuel per the rebuilder and the graphs. Who would have figured? He sure didn't!! Anyway, I use 93 non ethanol and/or aviation fuel in hot weather to avoid percolation.

[Bill L.]

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

[Joe L.]

Re: Fuel for stock engines

Here's what I have to say on the subject:

First of all, in many states, including California, 93 octane is not found at the vast majority of gas stations. In fact, I've NEVER seen it at ANY west coast gas station. The common premium grade is 91 octane. So, folks that find that 93 octane runs OK might not find that 91 octane works as well.

Second, having to live with detonation is a MISERABLE thing. When one rebuilds an engine, a wise person is conservative in this regard. Better to "leave a little compression on the table" and be relatively assured of a detonation-free engine than "roll the dice" and end up with a detonating engine that's miserable to drive. After the engine is built, it's too late to decide that you need to reduce the compression ratio. Having to use racing fuel, aviation fuel, or octane booster is way too inconvenient, especially if you ever plan to take the car out on the open road. I want to be "free as a bird" when I'm out on the open road, not "shackled" to the need for special fuels or additives. I want to be able to pull into any old fuel station and "fill her up".

So, here's what I recommend:

For small blocks-------no more than 10:1 with 9.5:1 being a better choice

For big blocks---------no more than 9:1

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.

[Bill L.]

Re: Fuel for stock engines

Here's what I have to say on the subject:

First of all, in many states, including California, 93 octane is not found at the vast majority of gas stations. In fact, I've NEVER seen it at ANY west coast gas station. The common premium grade is 91 octane. So, folks that find that 93 octane runs OK might not find that 91 octane works as well.

Second, having to live with detonation is a MISERABLE thing. When one rebuilds an engine, a wise person is conservative in this regard. Better to "leave a little compression on the table" and be relatively assured of a detonation-free engine than "roll the dice" and end up with a detonating engine that's miserable to drive. After the engine is built, it's too late to decide that you need to reduce the compression ratio. Having to use racing fuel, aviation fuel, or octane booster is way too inconvenient, especially if you ever plan to take the car out on the open road. I want to be "free as a bird" when I'm out on the open road, not "shackled" to the need for special fuels or additives. I want to be able to pull into any old fuel station and "fill her up".

So, here's what I recommend:

For small blocks-------no more than 10:1 with 9.5:1 being a better choice

For big blocks---------no more than 9:1

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.

Pretty much sums it up for a stock build.

Thank you Joe!


Bill

[Joe C.]

Re: Fuel for stock engines

Compression ratio is only a small part of the overall picture and using it ALONE as an octane requirement criteria is meaningless. In reality, the "low horsepower" engines which have lower static compression ratios actually need higher octane than the "high horse" engines. Why? Because by virtue of their camshafts they actually build higher cylinder pressures during normal driving then do their "high horse" counterparts. Spark timing as well as individual engine build variations, coolant temperature, and air/fuel ratio make a big difference as well.

What if I told you that a 327 with 11.7:1 static compression ratio runs detonation free with 93 PON gas. My engine does so as a result of all of the above as well as a very carefully engineered build and close tolerances. Yes it makes close to 500 horsepower @ the flywheel and has more torque at the bottom/midrange than many big blocks.