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I have a '51, 3100 with a '58 235 and SM 420. I pull a 775 pound Teardrop Trailer with about 400 pounds in the bed of the truck.

I used the Engine RPM calculater ( http://www.csgnetwork.com/multirpmcalc.html ) and using 1.71 as a final drive ratio, at 40 mph, I get an engine speed of 2914 rpm.

2 questions: Is the 3rd gear ration for this tranny 1.71?

Can I go as high as 3200 rpm for short periods if NEEDED for slowing on hills?

Thanks for any input.

Final drive ratio out of the tranny in high gear should be 1 to 1.

...but I may need to downshift to 3rd gear. The ratio for 3rd gear is 1.71...I THINK.

Thruworkin

Sm-420 ratios-
1st...7.06
2nd...3.58
3rd...1.71
fourth...1 to 1
reverse...6.78

Thanks Folks. Is 3200 rpm safe for short periods? I have no idea what the max rpm is for this engine...and I used the Search function.

Thruwurkin

I don't think that 3200 will hurt it, I ran some numbers (bore/stroke etc) through a formula I found in an auto math manual, and come up with about 4500 rpm as a max redline for a 235. I've had mine up to 3700 rpm with no ill effects.

3200 should be fine on a full pressure 235 in good condition. I've run my 261 up there several times with no trouble.

Thanks very much for all your responses. I spent a lot of time bloos sweat tears and $$$ building this 235 and sure don't want it coming apart because I "abused" it. Your answers were just what I was hoping for.

Thanks again.

Thruwurkin

I regularly spin around 3000rpm for hours at a time on the freeway with my daily late 235.
On 2500 rpm you could go for ever without even a degree raise in temperature.
My block rebuilt was at least 20 years ago and I still get high compression and oil pressure.
- 235s just keep running without any problems, if you always stay under 3500.

I've had my 235 spinning 4,500 rpm on several occaisions. But the stock cam/intake setup won't let you go that high. Usually you can't get a stock up to 4,000 rpm. What really kills these engines is lugging them low enough to knock, it really pounds the mains.

Just for grins, my friend has had his 261 over 6,000 rpm. But it has been balanced and ported so it will breath.

It has always been my opinion that the cams and breathing used were the limiting factor in the 235-261 engines. They just sound like they are in the seat with you at high speed but can safely be run above 4000 RPM for a lot longer than you might think if in good condition and have been modified to let the air in/out. Race engines run a great deal faster.

I don't think I'll ever have to run this at a rpm over 3600 amd that will be for going down hills (to help the brakes).

Thanks again for all this very useful info.

Thruwurkin

Hey 53 Money Pit, Do you have the specs on a 261 as far as hp and torque? I think I know where there is one and its green in color. In a old chevy water truck. Was in another truck before. Still supposed to run well. Just a thought. H.

On the subject of max. rpm's.....I think you have to take it on an engine by engine basis. I just got my rotating assembly back from the balancer. He concluded that the rods in my 60-235 engine, were so far out of whack, that he replaced them with a good set. The weight variations were as high as 13 grams off. Fortunately, he had a set of balanced rods that he graciously donated to the cause.

This engine had been rebuilt by the PO and had been running with this unbalanced situation. I'd venture to guess, that it would have had a substantially lower red line than one that was closer to being in-balance. Can't even imagine what was happening inside the engine at even 3000 rpms!! Generally speaking, the standards in the 50's and 60's weren't quite as tight from the factory....let alone, when some backyard mechanic slapped a bunch of mismatched parts together, and made an engine.

I think that anyone who is rebuilding a vintage 4 main engine, would be wise to invest in getting a balance job, even if they aren't intending to max. out the revs on a regular basis. It would be a shame, to spend big bucks on a bore job, cam, bearings and everything else that goes in, without insuring that the whole package will be as smooth as silk all the way through the power band. Mine will probably never see 3500 rpm's, but it won't be trying to self destruct with each rotation of the crank.

I was a bit skeptical when my builder strongly suggested getting it balanced, but am thankful I took his advice. I think it will turn out to be money well spent!!!

Dave