Fuel needs for hi compression engine

Roldman

New Member
Mar 9, 2022
8
I have a 70 Camaro SS with a Stock 350-300 in it. The engine has a 12:1 compression ratio so it requires high test gas. I have been running High test with ethanol fuel and so far no pings. I don't know if there is any internal problem with the ethanol. I just saw the local Racetrack has an ethanol free gas available. Should I look in to this or since I haven't heard anything just go on oblivious to possible problems. There are a lot of people on this site that know a lot more than I about this subject. Does anyone have any comments? Ethanol high test or non ethanol lower grade fuel? Thank you.
 

mallard

Veteran Member
Oct 3, 2013
1,956
SK, Canada
Do you know if your heads have hardened valve seats installed? You could have a problem with seat recession with no lead gas. IIRC hardened seats weren’t used until ‘71. However a lead substitute additive should alleviate that.

I’ll let others chime in regarding their thoughts on ethanol. I haven’t been doing anything special and have had no problems. (So far)

I also use an octane booster as I’ll get slight detonation without it.
 

2ndGenCrazy

Veteran Member
Aug 18, 2011
893
Upstate New York
The "dynamic" compression ratio will dictate the need for higher octane fuel. In other words, your cam will be the determining factor, up to a point. Wallace Racing has a calculator to help you with this. Then you can read about what ratio people are getting away with for pump gas. Just remember, all engines are a bit different and yours may respond differently. http://wallaceracing.com/dynamic-cr.php
 

biker

Veteran Member
Gold Member
Dec 7, 2014
5,068
Canada
There isnt much ethanol in pump gas, it says "up to 10%" on the pump, meaning it could be less. It's worth trying the non to see what happens, but if it is sold at a racetrack, it's probably alot more expensive.
My static CR isnt as high as yours, but with my cam, timing and jetting, I do fine on 93 with ethanol, zero issues. Switching to a tank of non ethanol actually causes run-on and a slight rattle under load. So, 93 ethanol it is.
 
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70Znut

Veteran Member
Lifetime Gold Member
Feb 5, 2016
342
Sarasota,FL
I have 11:1 compression and have been using both VP racing 93 from the track and Sunoco gt260 100 available locally at the pump. Both seem to work well with do ping/detonation issues.
My biggest concern with ethanol use is with a carbureted engine that sits long enough for the fuel to drain or evaporate from the carb it leaves behind undesirable deposits in the carb that are problematic.
 

tom3

Veteran Member
Aug 1, 1999
14,663
ohio
A stock 350/300 doesn't have near 12 : 1 compression. Factory rated (as blueprinted) at 10.25 : 1 IIRC. But today's 93 octane premium seems to be plenty good enough for that engine. My mild build 350 has right at 220 psi cranking compression and it's fine with current premium. Some race gas does smell a lot better from the exhaust end though.
 

Gary S

Administrator
Lifetime Gold Member
Apr 14, 1999
24,581
Bismarck, North Dakota
I had my 70SS for 51 years. It has 10.25 to 1 compression. Mine refused to run on any fuel with ethanol in it so I always ran pure gasoline in it. I used premium leaded gas as long as it was available and after that, I either added a few gallons of leaded racing gas to premium unleaded or added some lead additive to premium unleaded. Both mixes worked just fine in it.
 

G72Zed

Veteran Member
Sep 8, 2015
4,072
Canada
I have a 70 Camaro SS with a Stock 350-300 in it. The engine has a 12:1 compression ratio so it requires high test gas. I have been running High test with ethanol fuel and so far no pings. I don't know if there is any internal problem with the ethanol. I just saw the local Racetrack has an ethanol free gas available. Should I look in to this or since I haven't heard anything just go on oblivious to possible problems. There are a lot of people on this site that know a lot more than I about this subject. Does anyone have any comments? Ethanol high test or non ethanol lower grade fuel? Thank you.

Well, if it has 12:1 compression, it's not totally stock now, if so, on a 4.00 x 3.48 OEM 350 ci, you need only 65cc at TDC to hit 12:1 CR by way of approx. 13cc domes or heavy milled heads down to 51cc in a typical 350 sbc, if it's the later, you are better in many ways.

How "high" is the high test pump fuel octane you are using, do you know how much Ethanol it contains, what's the octane of the fuel at the track?

And if it has the "stock" cam, your cranking compression must be up there, I would properly test this out to see what you have first and document and then decide the plan of attack from there. A good fuel, with perhaps a proper additive and proper timing/ignition/tune will keep you in the safe zone.

Sometimes you don't hear what's hurting the engine, kind of a silent killer, just saying.

As an example, my iron headed 350 based sbc (4.060 x 3.500) has 10:8 static compression ratio and runs great on pump 94 Octane that contains 9.2% Ethanol (tested last week) cranking pressures in the 210 psi range with a 249 @ .050 solid cam. I don't bother with dynamic compression ratio's.....because it's dynamic and ever changing.
 

1320feet

Veteran Member
Oct 23, 2019
514
Calgary, Alberta.
I had a 383 in my 79 Camaro with 10.3 comp and a 244°@50 Hydraulic cam. I was running 87 octane without any issues. Just sold that 383. Now I have to get that damm 434 together.
 




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