Safe RPM for a Flattie?

[Merle Coggins]

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I posted this over on the truck side first, but thought you guys may have some insite for me as well.

I've been thinking about tires today. From what I can find, I'm thinking of going with 235/75R15's. The only drawback may be that these tires are recommended for 6" wide rims. Aren't the stock rims 5"ers? This is one issue I've been contemplating. I'm thinking they would probably work.

But now to the title question. What's a safe / max RPM for these flat head engines? I've been playing with some calculators and have determined with my 3.73 gears and the 235/75 R15 tires at 28.8" diameter, I'd be spinning around 2500 RPM at 56 MPH. If I spun it up to 3000 RPM I could get 68 MPH out of it.

Now 3000 RPM doesn't sound very high by modern engine standards, but what can I expect from this relatively long stroke flattie?

Merle

[Gwellman]

It partly depends on which engine you have. The 218s have a shorter stroke than the 230s. And the long blocks with reasonable strokes (237, maybe even 251 but not 265s) are probably a bit safer than the short block engines. I drove Power Wagon with a 251 about 1000 miles at 3000 rpm with no failures.

With a 218 or 237/251, I think you could drive it at 3000 rpm. But I would probably opt for 2800 to stay a bit safer.

I did have a 218 in a 1950 Plymouth that I drag raced as a kid. I ran this to 5000 rpm and shifted full throttle. It never broke. I used to drive it to and from the dragstrip. I was probably living on borrowed time.

[55 Fargo]

Merle, not sure if these old engines would like 3000 rpms sustained for hours on end.

In the factory they underwent 50 hours of 3600 rpm, or until the engine blew.

Max HP is at 3600 rpm.................Fred

[oldmopar]

Have you thought about trying to find a set of 16" wheels ?

[Merle Coggins]

I was just browsing through my service manual and found the specs for Max RPM on governed engines. The spec is 3200. I guess I'll use that as my "Redline RPM" and go from there.

As for the 16" wheels... I don't think they would be correct for my 3/4 ton truck, but that was a thought too.

Thanks,

Merle

[wayfarer]

Even if your rig is 100% stock I doubt that anyone who actually drives theirs will give you any grief for putting on larger diameter tires so that you can drive yours.....hell, I'd look for some 17's...keep those rpm under control.

[TodFitch]

With decent oil and cooling the engine should run at 3200 RPM pretty much forever.

My 1933 is geared more like a truck (4.375:1 rear end) than the newer (late 1930s and 1940s) Plymouths. I use 3300 RPM as my max for long distance driving on the freeway. With my tires that works out to about 63 MPH. And at that speed I end up being more worried about the skinny bias ply tires, suspension and tiny brakes than my engine.

If you are upgrading any part of the drive train for higher sustained speeds then you really ought to look into upgrading the steering and the brakes too. But then you might just end up with a modern car with an old body which is a different hobby than I am into.

[greg g]

Well thaere is that 80% rule, that says a motor should be fine at 80% of its HP rated RPM. So 3600 X .80 is 2880. The difference between 2880 and 3000 is pretty negligable.

As for personal experience, I am running 225 75 15 on my 230 flathead car with 4.11's. I ran those calculators a few times and last summer I drove home from Charlotte NC, running at 65 on the speedo, which the calc told me was 3100 RPM. It was fine. The measured mile showed me the odo and the speedo are about dead nuts on.

As for wheels, I'm running 5 inch wide wheel vintique deals I believe the stock rims are 4.5. The 235's may be pushing the limit a bit ont he stock rims.