The Stovebolt Site Forums The Shop Area Engine Shop 235 rpm

Quick question... what is considered the optimum cruising rpm for a fresh 235?
My 54 3100 is about ready for a road trip and I would rather not overrev the engine trying to "keep up with the Joneses." Thanks.
Jethro.

Don't overthink stuff.

I've been driving my 1950 3100 non-rebuilt 216 on the freeway at 50-55mph after a long dormancy. I've put over 3000 miles on it in the last year - 2000 just in the last 6 months.

You'll be able to tell what the truck is comfortable doing while you're driving it.

I ran my 216 at 3,000 rpm’s for a few hours on trips. Never had an issue. I wouldn’t worry about it! 50-55 all day long with stock driveline.

Otto... I figure as fast as I can hold it in the road comfortably is about 60/65. 2800/3000 rpm or so.
Back in the late 70's with my 51 3100, 216 3sp, I drove like it wasn't mine. Matter of fact, I drove everything like it wasn't mine. Blew a couple of engines, too. Been there, done that, don't want to do it again.

Right, but you know better now. You'll be able to tell whether the engine is stressing or not.

I believe the stovebolt sixes were horsepower rated at around 3600 RPM, so I'm pretty sure the factory wouldn't have done dyno testing at engine speeds that would destroy them. They're pretty much self-limiting on top speed with the carburetor, intake manifold, and port design in the head, so people who get a case of the vapors when the speed exceeds 2500 or so have another thousand RPM to work with. The big rigs with governors on them would run wide open all day at 3K+ RPM, hauling heavy loads. That's something few, if any stovebolters will ever do. Keep the rod bearing clearances in the ball park and change the oil regularly, and these old engines will take a huge amount of that kind of treatment. I ran a 1958 DelRay 2-door coupe with a 235, a 3 speed, and a 3.70 rear end and 14" tires 80 MPH frequently. The engine finally developed a rod knock, but it took a couple of years of abuse to do it.
Jerry

Later 261s were rated at 145 HP @4000 RPM. Maximum torque was @2000 RPM. Some of this difference may have been the marketing dept. leaning on engineering to run the engine up to speed to get the HP figures, but even the early 235s were HP rated at 3300. My 1-ton with original gearing would turn 3300 @ 60MPH. It got about 8MPG doing that but otherwise didn't complain. I now run the 261 in overdrive at 2500 at about 2" vacuum for hours on end and my only complaint is the gas mileage. If you hold RPM near the torque peak, the specific fuel consumption goes way way down. On mine a hotter cam with no other changes improved fuel consumption by about 30% @ 3,000RPM. So you can either turn it slower or use a hotter cam to get reasonable fuel consumption.

With the wide original transmission ratios, bringing the engine up to the HP peak before shifting will keep it well inside its torque range in the next gear.

Jethro, do you have new shock absorbers, wheels and tires on your truck?

Installing those three things made a night and day difference in how my truck handles at all speeds. I can let go of the wheel at 60mph on a freeway and the truck tracks straight as a die. If your white knuckle experience is due to wandering or jumping around, I'd suggest looking at those three things (assuming your steering gear and linkage isn't worn out).

I was just looking at your tires. What is the date code on them? My brother just had a belt separate on a 10 year old tire that had treads that looked nearly new. His car shook and wobbled all over the place.

The tires also look more like off road tires, not really conducive to smooth highway travel.

Quick answer: no more than 2400 rpm.

Longer answer (including the "why?" answer to the statement above): The torque/horsepower curves were different for different years of the 235 and they're interesting and well worth studying. Your sweet spot (1954) is going to be somewhere around 2000 to 2400 rpm. At 2400 rpm your torque line begins a steep incline downward and intersects the horsepower line around 2550~2600 rpm. Beyond this rpm point, efficiency simply won't be what you might wish it was. So I'd say keep your cruising speed at 50 mph if you have the stock rear (4.11) and 28 inch diameter tires (that will be apx 2400 rpm). And this will match what driving one of these trucks (in stock condition) for over 50 years has proven to me time and time and time again. Hills and wind will be a problem for gas mileage, naturally. But if gas mileage isn't a concern, then drive at whatever speed you consider safe. There is (or used to be) one GM internal memo on the Heritage Page site showing the AD truck was designed for a safe highway speed of 50 to 55 mph. I had it bookmarked, however GM changed the Heritage Pages and yesterday's bookmarks don't all work any longer plus some data disappeared. I wish I had downloaded and saved it, but I didn't

Here is a link to the Heritage Pages for your truck:
https://www.gm.com/content/dam/comp...hevrolet-trucks/1954-Chevrolet-Truck.pdf

And if you're curious to see what the torque curve for the 1959 235 looks like (it is much different---is a long flat line instead of a peaked one), here is a link to that data:
https://www.gm.com/content/dam/comp...hevrolet-trucks/1959-Chevrolet-Truck.pdf
GM managed to get this long flat line by altering the camshaft (that data and the explanation was once also on the GM site, but I haven't been able to find it now). This was done to try to help reduce smog as I understand...produce more power at a lower rpm range and keep that power band consistent through several hundred rpm. The 1959 235 hits close to max torque at 1600 rpm and doesn't lose it until around 2800rpm. Study the facts regarding the camshafts used and you can see the differences.

Good luck on your trip!

Pic from 1954 Truck Data Book.

Note that the torque and HP curves are on different axes in the GM documentation.

By definition, torque and hp are equal at 5252RPM. If both were plotted on the same axis the torque curve would be completely above the HP curve.

I run my original 54 235 at 50-55 all day long. It will do 65 but like Otto said, it just doesn't feel right. I wouldn't want to run at that speed very long.

I've found that both my original 62 235 and current 58 261 both come alive at around 2400 rpm and will start to fade around 3000 rpm.
Recently I replaced the 235 Carb I had on my 261 with a 261 Carb and it will climb past 3000 a bit easier in the lower gears.
A few years back I drove a 660 mile trip in 12 hours at 3000 rpm at 60 mph and got better gas mileage than usual with complaints from the motor.

John

I always appreciate being told when I'm wrong.

Panic, I think Jerry already covered that. Thanks.

Jethro As others said you'll be able to tell when it's happy. 55 or 60 should be fine aspecially if it's broke in.

panic Aggressive bolters don't know everything,they just think they do !! They give there professional opinion on stuff they've never tried !! So don't sweat about the gouges.

I have a 79 Nova rear end in my 54 with 2.73 gears and run all day long at 70 mph, 9 trips to Kc. and 3 trips to the west coast. Gas mileage is around 15 mpg.