MikesRed68

Hi All .I have a question about type of gas you all use in your C3 . Is regular unleaded good enough or do u use the plus { half regular half premium) or is it worth and make a any difference if u use the Premium. Does any also add any gas treatments? Thanks everyone

jim2527

It'll vary by location, time of year and build. Use the minimum octane that doesn't ping/detonate/knock. Anything more is a waste.

93 in my car.

jb78L-82

Yes...I have an L-82 OEM block bored .030 (355) with a Roller Cam (.525/.525 Lift, Duration 219/225, LSA 110) AFR aluminum Heads and 10.2:1 compression and I use 89 octane with zero ping....The same combo with iron heads would probs require 93 octane...depends on you motor

news2me

My '77 with the L-82 runs just fine Regular 87 grade. The only problem I have is when I fuel up at a BP Station, my car doesn't like there 87. For some reason I get Pre-ignition (aka dieseling), when I turn she off. That may sound strange but its the only gas that does it..

MikesRed68

OK My car is a stock 68 427/390

MikesRed68

She is tempermental like a woman

MikesRed68

I have a 68 stock 427/390

dmruschell

It will say in the original manual what kind of fuel it requires (regular, premium, etc). Just remember that the octane number quoted back then is the RON, and current gas is measured using AKI ((MON+RON)/2).

Most C3s 1971 and newer that is still stock was designed to run on 87 unleaded, but that can change if things are no longer stock. Most performance engines made before 71 had high compression and needs premium (93).

drwet

I only run premium in my '79. I started doing that because at one time premium was the only fuel around here that didn't have alcohol in it. Since then they have started putting alcohol in premium as well. Alcohol is hell on our cars. If you can still get alcohol free gas, use it. If you can't, buy the cheapest gas you can on which your car will run well. IIRC your car in stock configuration should have about a 10.25:1 compression ratio. With cast iron heads, I would expect you would need to run premium fuel.

MikesRed68

Would my 68 L36 stock engine be considered a performance high compression ?

theandies

93 in my car only because I have 10.5:1 compression and like an aggressive timing curve. Anything less and it's ping city.

MikesRed68

Not sure i understand you reply. U say to use the cheapest gas which will be reg unleaded then u say with cast iron heads u would expect i should run it on premium fuel. Did i miss something ?

MelWff

You have a high compression engine, 10.25:1, compared to latter C3's. You need 93 octane at all times, no mixing with lower octane.

v2racing

This is what I would do. I would recurve the distributor to have a little more timing at idle and come in full by 3000 rpm. Put the timing at about 36 total degrees with vacuum advance disconnected, then reconnect vacuum advance. This will make the engine feel a lot peppier and make it's best power. This will then require premium grade gas, but will be well worth it.

Mike

theandies

That's the timing I want to run but with the crappy gas these days (even 93) I actually had to back it off a couple of degrees just the other day because of a slight ping off idle (not WOT). I think it's because my vac can puts too much timing advance (50) in at cruise. I'm going to get Lar's vac can limiter to fix this then I can go back to my aggressive curve that I like.

footloose42

I put 93 in my '73 350. Not because of the high octane, but because the 93 at my local gas station is ethanol-free.

theandies

Must be nice. The only station that has ethanol free gas near me only has it in 87.
The ethanol BS is something Trump needs to take care of and soon.

jim2527

89 here in ethanol free.

But we can get 100 at the pump....

7T1vette

Grade of gas needed depends on the compression ratio of your engine and how well it is tuned.

Engines with 8:1 or 8.5:1 CR do fine with regular unleaded; 9.0-10.0:1 CR usually needs mid-grade; and 10.5-11.5 CR engines need premium fuel.

You might get by with a little lower grade of fuel by retarding the timing a bit, but you will be risking a "pinging" problem with that approach.

drwet

The reason to use higher octane fuel is to prevent detonation. This is where the mixture ignites prematurely under compression. The higher the compression ratio, the higher the octane rating that will be required to prevent detonation. Detonation is also known as 'ping'. It is that rattling sound you hear under heavy load. It will also occur if the timing is advanced too far for the fuel you are using. If you are running aluminum heads, you can usually tolerate about a point higher compression with the same octane fuel. This is all a gross over-simplification, but it should give you a feel for the factors involved. So the bottom line is, if you can run regular fuel without the engine pinging, you would be wasting your money to buy premium or mid grade fuel. The alcohol contamination problem is the only factor I can think of that would change my approach to selecting a fuel grade.