I am having my 394 hi compression engine rebuilt and I like to keep the same compression ratio but the guy doing the work says to do that it would be to expensive to drive because of the high octane fuel u would need. I was wondering what u guys driving these cars r using in these cars and what was the octane when these cars were built.

 

Why is the guy telling you what you can afford? Who is the customer here?

 

Long 64, just run the highest octane pump gas you can find in your area.There is a chance you may need to use a octane booster(bottle to the tank)but I didn't notice much difference with or without it.This was on a HC 394 in a 63. I would get a little ping on very heavy acceleration,mostly at low RPM during a sloppy shift from second to third.I think most of the blame goes to the shifting of the sorry Slim Jim....Tedd

 

Most engine builders would suggest a compression ration at 9.5:1 max. I don't know what the ratio is on your Starfire, but I am guessing it is above that. Some older high compression engines do okay with todays lower octane gas which is usually 93 max, but others have constant overheating and pinging issues. Back in the 60's premium was rated 95+, up to over 100 in Sunoco. I had constant pinging problems with a 69 Grand Prix 400 10.5:1, 70 Bonneville 455 10.0:1 and a 69 Corvette 350 11.0:1 that I hjave owned during the past 20 years. It was a constant pain to try to adjust advance curves, trying lead additives, etc., to eliminating it. All my car mentioned had factory A/C and running it increased operation temperature a bit and made the problem worse. I eventually changed heads to a 9.25:1 on the Bonneville and the pinging went away. When my engines pinged, they lost power, not to mention rattled the engine parts. Backing off the timing would help, but it also decreased efficiency and power and tending to increase the operating temperatures.

Any old cars I have rebuilt engines on, I have lowered the compression either by changing the heads or the pistons.

I don't know what is referred to by "what you can afford". Did he mean buying and mixing aviation fuel, which is a pain if you take a trip that requires a fill up, and in all or most states, illegal. You can try a bunch ot octain boosters that yield marginal improvements at best, or changing pistons or heads during the rebuild. Since you can not buy the 95+ octane fuel the engine was originally designed to run on, if it were me, I would lower compression. With today's technology, power can still be maintained or increased by better camshaft technology, head work etc.

 

I believe your engine had either 10.25 or 10.5:1 compression depending on model. It will run on standard hi-test found at your corner gas station. When he reassembles your engine and uses a standard head gasket vs the old steel shim, it will probably lose .5!

They rated octane differently up till the early 70's, todays number is an average of the other 2 old ratings, to eliminate confusing the consumer.

 

I know someone was making custom thickness copper head gaskets to help the high compression in the older motors.
I know they've been used without problems, but couldn't tell you where or if they're available for your motor.
Could make all your problems go away!!

 

I am planning on putting new pistons in it for the olds ones were not to good he is boring it .30 over. He was talking about the price of the aviation fuel and the problem with getting it. The 394 I have is the 10:25:1 compession ratio.

 

Never mind!!!

 

The trouble is the damn EPA forces so many different fuel formulations depending on location, that a high-compression engine may run perfectly well on fuel from one zone, but drive a hundred miles and get into another formulation and the car might barely run.

If you have to replace pistons anyway, might as well drop the CR down to something that can tolerate this **** gasoline. With the corn lobby trying to push E15 and E85, it is only going to get worse.

With the gas formulations currently available here, even me old 8.5:1 Cruiser wagon has to have 89. It used to run happily on 87. BP gasoline has gotten crappy enough it takes 93 to stop the pinging in that engine, and needless to say I said the hell with BP.

I can put 87 octane in it in Charlotte or Richmond and it runs fine and delivers better gas mileage to boot.

 

The engine in my 63 was rebuilt before I got it. Not sure of the compression ratio but a reading of 180 psi in all the holes indicates it is fairly high. I use 93 octane premium and have not had pinging problems yet but who knows what the future holds. If you intend to drive the car a lot under high load conditions the low-comp pistons might make sense but the performance will suffer - tough call.

 

Be sure your vacuum advance is functioning properly, and use the best gas available. For a healthy, stock 394, you shouldn't have a problem. I agree w/ Don: If you intend to drive the car a lot under high load conditions the low-comp pistons might make sense.
But for "show and go," you shouldn't have a problem ... unless they mill the heads and block significantly -- but then if they're using modern, thick head gaskets, that will be offset anyway.... Plus, a "clean" combustion chamber is less prone to detonation ... tell the guy to deburr the combustion chamber.
Are you having the heads "converted" for use of unleaded gas?
Actually, the only thing "octane" rating really indicates is resistance to detonation. I know this doesn't always prove infallible, but theoretically, 93 octane is 93 octane is 93 octane, at least for detonation resistance (as long as the octane measurement method is consistent).

 

Does anybody make an ignition system for olds that controls the timing?

I know VW had a knock sensing system that would advance the timing until it heard engine knock, and then retard the timing a little. The system would do this continuously. I adapted this system to one of my VW's because I put a turbo on it. The great thing was I didn't have to worry about what grade of fuel I used, and when I did use premium the difference in power was noticeable.

 

I had one that I bought new in the box at a flea market and installed on one of my 455s about 20 years ago. The kit was from about 1980 (bad-gas years).

It didn't do crap, so I disconnected it. Sold the car with it still attached.

I'd also be interested in knowing if there's anything out there that works.

- Eric

 

You can also put a gallon or two of 116 per tank.Its expensive but your your probably not driving it that much anyway.
railguy

 

Retarding your ignition setting a couple of degrees will also decrease detonation . It does, however, quickly cut into your throttle response and acceleration.

 

It will run just fine on today's fuel. As advised, just by the highest octane available.