Fuel type

[BobE]

For those with tuned SBC engines what petrol grade do you use. I've seen some conflicting advice regarding whether regular unleaded should be used or something like Shell Optimax - or even alternating between the two.

[confusionhunter]

I tune for 95 so its and shove in premium whenever I can. I have a lumpy cam and some trick bits and around 500bhp with some other special bits. I'm not running massive compression (9.5) or aggressive timing (36) so it will run on anything. Its not like its a high compression, turbo charged, knock sensing, 2 bar of boost, engine that will melt on 95!!
If you have a high power 12:1 highly strung motor on the verge of detonation with mega power, then that gets a bit trickier.

So the answer is really dependent on your engine set up. Do you know what it is?

[BobE]

confusionhunter said:

I tune for 95 so its and shove in premium whenever I can. I have a lumpy cam and some trick bits and around 500bhp with some other special bits. I'm not running massive compression (9.5) or aggressive timing (36) so it will run on anything. Its not like its a high compression, turbo charged, knock sensing, 2 bar of boost, engine that will melt on 95!!
If you have a high power 12:1 highly strung motor on the verge of detonation with mega power, then that gets a bit trickier.

So the answer is really dependent on your engine set up. Do you know what it is?

It's an American Speed 6.3L giving around 550bhp measured by the engine builder so less in the car installation. I'm guessing the compression ratio is 9.8/10.0. The timing is 40 at 3000rpm from the documentation that came with the car which also indicates a 92+ octane fuel should be used. I think most normal unleaded fuel is 95 octane and premium fuels are 98. Having had cars in the past where using too low an Octane risks major problems I wanted to check what others use. It seemed to me regular pump fuel wasn't right even if the engine builder indicated you could go even lower. Comments?

[confusionhunter]

OK so thats not too extreme IMHO. Regular pump gas in the US is 92, so makes total sense its tuned for that. So basically I think you are safe as long as its UK petrol!
Provided the plugs are in good condition and not to 'hot' and to spec then 95 would seem to be fine. But even if the engine can run 92/95 you will get a better margin of safety with the higher octane and it might feel a bit more spritely on the bum dyno.

If the car was forced induction my answer would be different, but on a big old carbed American V8, its not so much a fuel management system, more bucket and a hose...:lol:
Love em, nothing snarls like a Carbed SBC.

[humble]

Fuel octane rating depends on where you're located. If you tune, or have your car tuned for a specific fuel in your area, you can ignore this, but expecting an engine tuned for a fuel in the US to behave similarly in the UK could give you some problems. Octane ratings are not the same in the US vs the UK or rest of Europe. In the UK, RON 95 is roughly equivalent to our Californian premium AKI (RON+MON/2) 91 octane which we'd give up in a heartbeat if we had access to Super 98, or AKI (R+M/2) 93 octane here. As mentioned previously, unless your engine is highly strung RON 95 is fine, but for high compression or boosted applications it's Super 98 or better.

Some light reading for the curious:

https://en.wikipedia.org/wiki/Octane_rating

Will

[JoulesCanAm]

Agreed, I'm fortunate I'm at 5,000 ft elevation and can get away with Premium (AKI (RON+MON/2) 91 octane) if pottering around locally, it was a different story when I took the car over to CA and sea level. Fortunately I can get AKI (RON+MON/2) 100 octane here locally at the pump, but it comes at UK prices, around $8/gallon.

Julian

[BobE]

Thanks for all the contributions - I thought it was a simple question but I've learnt a lot more about fuel technology. So if AS reckon 92+ in AKI for the UK with RON that means 97 octane. So that is BP Ultimate 97 or Shell V- Power Nitro at 98 octane. That's until their marketing or engineers go a change the names and I have to recheck again.

[V8Dom]

optimax or if desperate 99 octane from tesco

never used unleaded

[845ste]

i use 95. with 100 i have not see difference apparence

[UltimaCH]

V8Dom said:

optimax or if desperate 99 octane from tesco

never used unleaded

Because you can still find leaded in Brexitland?

[BobE]

UltimaCH said:

V8Dom said:

optimax or if desperate 99 octane from tesco

never used unleaded

Because you can still find leaded in Brexitland?

Not since 2013 and up to then only via specialist companies. The way fuel is marketed in the U.K. is that 95RON is called 'unleaded' where ever you buy it - the higher octane fuels have names to try and justify the extra cost....

[V8Dom]

845ste said:

i use 95. with 100 i have not see difference apparence

you wont see any difference in appearance, most petrols look the same, just smell differnt and burn differently..lol

[845ste]

Dom ok, but I meant as a performance for the gtr