Premium gas needed?

Tom's36coupe

Tom's36coupe

Well-known member
With the low compression of the flatheads is the use of premium gas, 91 or higher needed or a good thing? I am running stock aluminum Edlebrock heads with about 8 to 1 and normally put in 89 or midgrade. Now that my distributor is better setup was just wondering if it would make a difference. Thanks!
 
I've always regular 87 octane in my '40 with Offenhauser heads with no problems. If your getting some pinging, you can use 89 octane.
 
Truthfully, if your engine will operate on the lower octane rating you will notice no increase in power with the higher octane numbers. The higher octane fuels will actually have a slower burn rate than the lower numbers. This is what prevents detonation with the higher compression engines and detonation is what cause the pinging or fire rattle. In general, relatively stock side valve engines do not require the higher octane ratings of the premium fuels.
 
My engine in my 40 is a 59A bored to 3 3/8 X 4 1/8 stroke,forged Ross pistons with Navarro heads milled to give .030 clearance. I have to burn the higher octane or I will get detonation .My other flathead is stock and the 87 octane is fine.
 
Pinging could be caused by improper timing as well. I don't think higher octane is all that it is cracked up to be.
 
Tom's36coupe said:
With the low compression of the flatheads is the use of premium gas, 91 or higher needed or a good thing? I am running stock aluminum Edlebrock heads with about 8 to 1 and normally put in 89 or midgrade. Now that my distributor is better setup was just wondering if it would make a difference. Thanks!
Click to expand...

All of the post so far are correct....technically, your engine will run OK on 87 or 89 octane gas. But.....I think you will find that it will run "smoother" on 92 or 93 octane gas, and will run much cleaner, mainly due to the additives in it. I tried an experiment using 87 vs. 93 octane on a stock flathead that was in near perfect running condition. It ran very well on 87, but had a noticeable improvement at idle rpm with 93 octane. Also, the cost difference will be a few dollars a year more....try it, listen to the difference......you might like it......
 
I thought I knew all about octane, the more the better, I used to run cars with 13.5 compression ratios. Well those cars had very poor head design so I needed 13.5 compression ratios to get anything out of them. Now I run engines with 9.5 compressions and hi flow heads and the cams are cut to support hi flow. Well since I knew it all I figured that more octane was better, same same, so I would buy the highest octane fuel I could get and also add 104 +, I just could not get a tune. I kept fouling plugs, so I made a call to the engine builder and asked them what was going on.
I was really sort of amazed that they said to just use 87, use 87 and if you have more problems you may have to tinker with your jetting, most carbs are still set up to run higher octane and the octane available can affect fuel mix and thus jetting.
I did the switch changed the jets and the guys were correct, I actually am running cleaner smoother and have better plug color and more power.
 
Always use the lowest octane that the engine will run on. You will gain nothing by using higher octane than needed. I have used 87 octane for many years in my 276 cu. in engine.
 
My '52 F1 runs better on regular (87 octane) than either of the higher octanes - guess it is a compression thing and I can't really get the benefit of the higher octane. Never use any lead additive, either.
 
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