cymru232

What would be the best octane to use in a '79 Firebird w/ a 403 engine? (it's an automatic, if that makes any difference) -- sorry guys, I'm really new to this :)

73superduty

Using the highest octane would be best, but with that engine I think you'd be fine using whatever. With my collector cars I used highest grade and octane booster.
Chris

__________________



Check out my builds, model contests and more at my Fotki:
http://public.fotki.com/73superduty/

Philo

I have the '79 with a 400. Your manual should tell you the octane you need. The octane would probably be the same for both engines - the highest octane you can get at the pump (not Mobil's highest octane though probably unnecessary). BTW, all 403 engines were automatics and all 400 engines were four speeds in '79.

MrPbody

Philo,
Not "ALL" '79s with the 400 were 4-speeds. ALL WS6 cars were 400s. Many did have automatics. Look at the scoop. If it says "6.6 Litre", it's a 403. If it says "T/A 6.6", it's a Pontiac.
403s were designed to run on 87 octane, but we've found 89 works better. Using anything higher is a waste of money, and no gain. Too high an octane can actually hurt performance.

Jim

Philo

I have several sources of documentation that says all Pontiac blocks that went into '79 Firebirds (stamped "PWH", which were 400's), were four speeds. No automatics. And all Oldsmobile blocks with the 403's were automatics - no four speeds.

For example (Also supported by Pontiac Historical Services):
From: http://www.transamclub.com/july_news_002.html

"For 1979, the final year for the T/A 6.6 engine option, a total of 8,326 Trans Ams and 346 Formulas were produced, all with Borg Warner Super T-10 4-speed transmissions."

94gpse

I had a 78' Trans am with a 400 and I ran mid grade in it. Really you only need a higher octane if your car has high compression. Take for example the new mustang Cobra (I am not a mustang person this is just for example) they have about 10:1 compression so they have to run premium. In 79' a 400 had about 7.5:1 compression which isn't too much. I only ran mid because the car had a few miles on it. So If I were you I would regular or mid. Theres no point in wasting your money on premium if you don't need it.

Philo

You are probably right. If you are a leadfoot though, I would still use a octane grade higher than 'regular', since 'pinging' that you cannot hear can cause damage to the engine. The '79 400 has 8.1 compression ratio, and the owners manual says to use 91 octane or better.