Yesterday I bougth a TransAm -84, 305 LG4. I belive that a stock engine required about 88-92 Roz fuel (am I right?).But the former owner has modified the engine tops so the comp is now about 10,3:1 (Is there a typical comp.value you get when "shortening" a top in a normal procedure?) But what octane must it have now? In Norway we have 95Roz and 98Roz, both unleaded. 95 is cheapest. Is 95 enough? The engine also have Edelbrock performer rpm, a metalfilter-intake. No headers, but a thick high performance exhaust (Simons) Cat and emissioncontrol-stuff is gone

 

TO be perfectly honest, those numbers seem weird. Is that 98 octane at the pump, if so thats sweet. Personally, I would run the highest octane you can get, then work your way down, but listen for pinging in the enigine, when you hear it you need to go back up an octane. Higher octane will never hurt an engine like that.
Also when you hear the ping, buy octane booster and put it in the tank

 

Run the lowest octane you can, that doesn't produce knock, or knock counts.

 

[QUOTE]Originally posted by 87LS


The numbers are correct In Norway we have two choice of octan: 98 octane and 95 octane, available at the pumps. At the pump we also have 98 with calium-additive (former lead) to lubricate older car valves.

Fuel price in Norway is 5,5$ pr gallon.... ...mostly tax..

I am aware how a 4-sylinder sounds when "pinging" (90% of cars in Norway are 4 syl) but a V8 isnt that easy to hear....? I have to get more experience with my new car. I have filled it with 95 now (Shell "V-power" 95 octane, with cleaning effects from tank to valves) http://www.sm.shell.no/Bilist/moonraker12.cfm

But since the comp is high (heads are planed), about 10,3-10,7:1, (what typical,someone knows?) i presumed that it at least it would demand 98. I knew that Mercedes and Bmw V8 and R6 from eigties(without cat) with comp from 9 to 10 demand 98 octane.....so i thought my TransAm with OVER 10 in comp would demand (at least) 98...

 

When these engines knock, it sounds like a steel hammer tapping on the block... a short 'ringing' kinda sound, like PINNNGGG....PINNNNGGG...

 

Just to inform all you guys. I found out that our Ron/Roz octane class dont compare to US gaspump octane. I have to reduce our numbers with #4.
So our 98 octane is equivalent to 94 octane. And our 95 octane compare to your 91 octane......

 

cool well i've allways go form any were up to 9.5:1 comp ration 82 octain. and the 9.6:1-10.9:1 on 90 and everything else as high as i can get. and also shell gas with those cleaning things isn't the best for you car cause it isn't ment for it and wears down things alot fast like fule injectores will go bad faster and so will other things i'll try to find the records of that

 

but the net effect is correct. ratings in europe are Research octane and US numbers are research+motor/2. The top grades in us and europe are basically the same, which is called pump octane and is 92-94

 

Yes, I know that formel..
But it was easier (from my language/translating) to explain it that way, by subtracting with 4 points....

 

It would be nice if you posted that record, or sent it to me

But, about that comp. ratio: I know f.ex. that an Opel Record -84, with 2,0 litre (120 Cid) carb-engine does need 98 octane RON. Or else it pings A LOT. And it has comp. of 9.0. So I think octane need it also has to do with the engine construction......(?) That makes the whole thing more difficult...