The man I purchased my mustang from told me that the 302 highcompression engine im my 70 fastback needed 99 octane fuel to run. At the first fill up I just went with premium fuel (93 octane). Im getting low on fuel again and I dont like paying tons of money for fuel. Im thinking of downgrading to mid grade (91). Do you think this will cause me many problems? WillI lose reliablility or power. If I lose power how much?

 

Whats your compression ratio?

 

I cant really tell you.... The guy i bought the car off of was a Dumb a**. He had recently bought the car and was going to jail bc of god knows wat and needed money for the lawyer so I got the car for 2000. I know the engine was rebuilt because I have all the recipts but I dont have all the specs. Im not sure if it is just me making things up but Im pretty sure I could feel the differentce between the 99 octane fuel that was in there and the 93 octane I put in. But I figure if it still runs nice on 91 octane then Ill run 91 on the road and 99 when i go to the track

 

The things i do know are that the whole engine was rebuilt (302). I got a 4V holley carb. Edlebrock intake. Mallory ignition. Detroit posi rear end.

 

My compression ration is a smidge over 10:1, and all I run is premium octane. I wouldnt do any less if I were you.

 

How can I find my compression?

 

The correct way is to pull a head and do a little deck height measuring and some 'cc' work. In other words buy the 93 octane - or take a chance.
Jim

 

yeah our older engines like 93because the engines then were built with higher compression to help with power and gas was dirt cheap too, but remember paying 2 bucks more per fillup is alot less expensive than a motor rebuild because of detonation...

 

I ran 100Low Lead avitaion gas in my old 289 once. It didn't like it very much.

 

Even though gas is expensive, the 93 octane is still only 20 cents or so more than 87 octane. The higher gas prices get, the better a deal the 93 is.