High octane gas?

I have a 1952 Mercury with a newly rebuilt engine and I am wondering what octane level of gas I should run in it. It is a high compression motor so I’m not exactly sure if that makes a difference for the kind of gas I should use. Also, would the octane level make a difference on the mileage I will get?

What’s the compression ratio? Is there any boost (supercharger or turbocharger)? How much? How’s the cam (a modified cam allows more fuel in and more fluid in on the downstroke means more real compression on the compression stroke)?

Offhand I’d say the highest octane you can get your hands on.

OK4450 has (I believe) built lots of racing engines. In this area I’d follow whatever he recommends. OK???

the motor is the original flathead 255. I don’t know the other specifics right off hand.


Again I’m going to ask OK4450 to join us and refer to his expertise. But without more info it may be hard. It’ll never hurt to go high-test.

According to this web site, the stock compression ratio is only 7.20:1 (not very high)


I would try regular or mid-grade and see if you hear any pinging or detonation.

When the engine was rebuilt, were hardened valves and valve seats installed for unleaded fuel?


Oh Yes! The machanic we had rebuld it said he wouldn’t have done it unless we had him install hardened valves and valve seats!

I don’t believe it required high test, but I could be wrong. If OK says otherwise just ignore my comment.

High compression on a flathead is pretty low compared to most overhead valve engines and I don’t think you’re going to have any problems at all. If you hear any rattling on acceleration when the engine is warmed up then step up the octane or retard the timing by a degree and see what happens.

My late brother in law had a 1950 Mercury and my next door neighbor has a 1952 Hudson Hornet (very similar vehicles) and neither one of them has had any fuel caused problems.
I do not own a sidevalve powered car but have had a sidevalve motorcycle for about 34 years. It’s a large displacement flathead Harley but the compression on these is considerably lower than your car even. The high compression model in this case was 5.5 to 1 but mine has the lower compression engine at 5 to 1 (believed to have been a WWII Navy shore patrol bike which relied on very low comp. due to much idling and pulling loads (sidecar w/passenger). It has not had any fuel related problems up to this point.