Safe RPM on a Flat Head?

[Merle Coggins]

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

[Reg Evans]

As mentioned on the car side find some 16"ers and get a tall tire. I have some 750 x 16" tubeless radials on my Coupe and they measure 31.5" tall.

Those will get ya going a mile a minute with a comfortable RPM of 2390.

[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

[norrism1]

My 51 had 16 on it.

[Young Ed]

Merle the 39-47 3/4 ton trucks have a unique bolt pattern on their 15s. Like yours they are the only size with 15s. If that is true for the pilot house trucks you would probably be talking custom wheels to go to 16s.

[IanT_Qld]

The VIN plate says max power is at 3600RPM, I guess this is less than Red Line, but I would hate to go harder. Assume 80% (? ) as save operating Revs. Giving 2880RPM and on 29inch tyres a speed of 100 Km/h (62MPH) making me happy, still I only drive at 80kmh as I dont like my unboosted drum brakes.

http://www.4lo.com/4LoCalc.htm This is a calculator someone on here posted for tyre size, revs and other cool things.

[oldmopar]

Merle the 39-47 3/4 ton trucks have a unique bolt pattern on their 15s. Like yours they are the only size with 15s. If that is true for the pilot house trucks you would probably be talking custom wheels to go to 16s.

That is strange so they had 3 different set ups in 39-47 as my 1 ton has six lug 16" wheels but that matches up with the 60's dodge w300 which is where I got my 19.5 wheels

Ed

[Merle Coggins]

Thanks Ian. My VIN tag is missing. That's good info. If the max horsepower is rated at 3600, I guess it should be capable of spinning at least that fast.

[48Dodger]

Maximum torque is what you want for best engine performance. This occurs at a much lower rpm than when you hit max HP. Mine 1950 tag reads 82 hp @ 3600 rpm for the 218 ci. My original tire/rims are 16-7.5, 8ply, standing about 26.5 inches tall. Sounds like your on the right track if you can hit 56 at 2500 rpm.

48D

[Young Ed]

That is strange so they had 3 different set ups in 39-47 as my 1 ton has six lug 16" wheels but that matches up with the 60's dodge w300 which is where I got my 19.5 wheels

Ed

Right 1/2 3/4 and 1ton all had different wheels although I think 1/2 and 1ton were both 16s with optional 20s.

[MBF]

I've never had my 52 1 ton w the 16" budd duals over 55. At 52 she sounds like she's getting ready to leave the ground-amazing amount of fan noise. I try to keep it around 40-47 where she sounds comfortable. I just have to keep a window scraper in the vehicle to scrape the bugs off the back window after a long run! I don't like pushing older stuff which is why I also don't pull my antique tractors. Maybe its a sympathy thing-I'm getting older too and don't want anyone pushing me to the point where I break either. Mike

[greg g]

I wonder how much of the frightening noises we think our flatheads are making is from the fan? We have gotten used to well insulated, well sealed cars with electric fans. So our old cars and trucks probably more so transmit more noise than we are used to. wind noise, tire noise, engine noise tranny noise, are all pretty much taken out of the equation on our modern cars.

As far as real life rpms go, most contemporary cars are gearde to be turning just over 2000 rpm at higway cruising speed. My curent car a VW Jetta with a5pseed, is HP rated at 3750 rpms. It has a 4.25 diffy and only a .86 OD in 5th gear, at 70mph its cranking out 3400 rpm. Pretty close to the 85% deal of HP rpm. Seams Happy, gets about 35/6 mpg on trips.

I know its much shorter stroke than our old 6's but our old engines should live at 3000-3200 rpm cruise for a long time.