How do you know when you need a higher octane fuel for max performance?I have a 350 bored 60 over with flat top Hypereutectic pistons and an Extreme comp cam with .468-480 duration.Roller tips with a Wiand al. intake and a dual line 750 Holley 4160 street performer carb. I am running hooker comp shorties and shorty exhaust w/o cat. I haven't been able to time it because it run erratic and there is a "dead" spot when you hit the throttle. I don't want to start messing with stuff if the problem is low (87)octane fuel.Any thoughts?

 

do you konow your compression ratio? sounds to me your in my boat and need to run 93 or highest you get in your state(93 in ga)

 

I am pretty sure it is under 10:1 but not sure where. A guy that has a similar set-up to mine had to plug his vacuum advance so he could get the timing close and I noticed that mine ran better plugged at the v/a and carb. I guess I could get a bottle of STP 104+ octane and water it down with some 93 pump gas and see if it helps.

 

How old or new is the motor // distrib // carb // plugs // wires // intake etc?

 

It is a complete new rebuild . They are the stock 882 heads with roller tips and stiffer springs for the cam(.468-480) Bored .060 over.Weind spreadbore with a 750 Holley 4160. Hooker shorties and true duals. The only component I reused was the distributor and I had it tested.

 

 

With 882 heads youre compression is going to be pretty low and I bet that cam bleeds off more compression too. I'd bet you can run 87 no problem but it just depends. That cam sounds small enough it might not bleed off too much compression.

 

 

What adjustments were made to the carb, if any?

Are you sure there are no vac leaks?

Valves are correctly set?

Should still fire & run fine on 83 for tuning purposes, maybe go 87 or 89 ( whatever is available in your area ) if you have any self-insurance issues.
Bogging // dead spots indicate carb needs some fine tuning.

Sorry, but is the carb vac secondary, and does distributor have vac advance?

 

Octane is the fuels resistance to detonation by slowing down the burn speed of the fuel. Detonation is when you compress the air/fuel mixture in the cylinder and the heat of compression ignites the fuel before the spark plug fires. A high compression engine or an engine with a power adder needs a higher octane rating to resist this detonation.

If your engine doesn't get detonation with a lower octane fuel, you won't make any power with a higher octane. Too much octane means the fuel burns slower and if the engine doesn't need the slower burning fuel, you can lose power with a higher octane.

Modern EFI engine can easily run on lower octane however they won't always make the most power. When the knock sensor detects detonation that you can't hear, the ECM will retard the timing until the knock goes away. When the timing is retarded, you'll make less power. Claims of more power with higher octane is simply because the computer isn't retarding the timing as much. Using a scan tool is required to monitor the knock count.

Running a carb, you won't have the benefit of an EFI systems electronics to monitor the engine. Without some way to detect engine knock under load, you'll never know how much octane is enough for the engine.

Don't waste your money with those "octane boosters". With claims of raising the octane 3 points, that means increasing 87 octane to 87.3. By the time you add enough of the booster to increase the octane a few full points, it's cheaper to just buy higher octane fuel.

I ran 93 octane in a 10:1 carbed SBC for a few years with no problems. After 2 years of racing it, I did finally blow up the engine but it wasn't because of the octane rating. It was those 7000 rpm shifts with a stock bottom end and cast pistons.

 

That is some serious info.Glad I read this before I bought anything other than some 89 octane gas. I have just begun the fine tune process and my mechanic agrees that it is probably carb issues. Today I play and see what happens.