The Stovebolt Site Forums The Shop Area General Truck Talk Comfortable cruising RPM for 261 eng.

What is an appropriate sustained cruising RPM for a 261? The engine is in good condition. Thanks.

Mike

Between 2400 and 2800 RPM. You can go higher without harm, but it gets loud and your gas mileage will suffer.

Most of the high RPM noise comes from the fan. Stovebolt engines in general were designed to run all day, every day at 3K+ RPM, and the 261 has a heavier duty lower end than either a 235 or a 216. It was intended to power medium duty trucks with heavy loads, usually running up against a governor for hours at a time. Most people are too timid to work a stovebolt the way it was intended to be used. 3200-3600 RPM was where they developed their maximum horsepower, and they will still do that if the rod bearing clearance and oil pressure is right.
Jerry

Seems like your truck will tell you where it's happiest at. For me the 235 likes around 2800 - 3000 RPM at cruising speed. Any less and it feels like it's lugging, any more and it feels like it's wound a little tight. You need to consider your gearing and any load as well. I have changed the rear gear on my '58 1.5 ton to a 4.88 (from a 6.17) and added a 0.73 overdrive (NV4500). I shift around 3000 - 3200 RPM. The Truck will do 65 MPH or better but feels comfortable at 60 - 63 MPH, that's close to 3000 RPM in overdrive..

Thanks everyone.
I am building a 1938 Suburban with a 261 and Power Glide. I am trying to do the math to find an appropriate gear ratio for all purpose use. I was shooting for 3.08 because try-five passenger cars with 235 and Power Glide came with 3.08 as the standard ratio. I figured if it was good for them, it would be good for me. I was thinking that I should keep my RPM to around 2200.
It's good to know that I can safely run at a higher engine speed. It increases the rear axle choices. There is a 3.42 axle out there that would give me 62 mph at about 2500 RPM or 69 mph at about 2800. That is starting to sound reasonable to me.
Thanks again.

Mike

I have '59 with a 327, 3.08, 4 speed Muncie. I wish I had gone with the 3.42.

I put a LOT of highway miles on a 58 Chevy Delray coupe with a 3.70 rear end and a 235. 14" wheels and "3 on the tree". It was very comfortable at 65-70 MPH, and 80 wasn't much of a problem.
Jerry

Mike, I don't know about the 261 but in 1959 GM changed the 235 in a move to help reduce bumper to bumper or slow moving traffic smog. They changed the camshaft and the change produced more torque at lower rpms...nearly a 20% improvement. There used to be a GM letter sent to all dealerships explaining this on the GM Heritage pages but I haven't been able to find it since they "cleaned up" the site years ago. I have a T5 (fifth gear of .86) and at 60 I'm about 2000 rpm...which is the max torque rpm of the 1959 235. I feel safest at 60~62mph. What it means for me is the first 3 gears match the OEM final drive ratios of the 1952 AD truck as GM designed it, fourth gear goes to 1:1 and that gear drops rpms by 25% and then fifth drops another 14%. Is it perfect? No, in oncoming wind or notably hilly areas I will need to drop back to 4th gear and I lose that 14% fifth gear bonus but it was the best compromise I could make. I can still load 1000 pounds of "stuff" in the bed and drive without damaging the clutch or overstraining anything and the reduction in rpms at 60 mph is dramatic and the engine doesn't mind it. If you look at the torque curve for the 1959-1962 235 you'll see at 1600 rpm you're pretty much at max torque and you remain there on a flat line until around 2800 rpm before you start falling off. My suggestion is to stand on the shoulders of the GM engineers. If you had bought an El Camino with a 235 you could have also had a 3 speed with overdrive (a 30% reduction in 3rd and OD). It is about 300 pounds heavier in curb weight than the AD pickup. You would have had a 3.70 rear end and 3.70 x .7 is around 2.60. Depending on the year of production you might have had a 4.11 rear end and that final drive ratio would have been around 2.87. A friend of mine owned a 1960 and had OD with a 3.70 rear end and the high payload option (I think it allowed him to haul up to 1100 pounds). Somewhere between those two final drive ratio numbers will be okay as the GM engineers saw it. Mine is around 2.65 or so. Good luck!

Jon G and anyone else interested, the information you mentioned on the 1959 235 engine changes can be found on pages 28-29 of this 1959 Chevrolet Truck Engineering Features publication, courtesy of the web site were it resides.

Don't ask me how I remembered where to find these tidbits - LOL!

Great information, Dan. Are there other books for more years on the website?

Yes sir! Here's the main link to Dezo's Garage. Hover over the "North America" sub-header to begin going through the drop down menu, then slide your mouse arrow over "GM" and then further slide to the "Chevrolet" or "GMC" columns to select the year group.


Once in the year group chosen, select and click on the specific year, then pick from the pictured documents and pamphlets and read to your hearts content. Although everything Chevy and GMC produced isn't there, there is still a lot that can be found.

It's all free to view and even save copies if you want. I've found a ton of info to help answer many of my own questions as well as helping answer fellow Bolters questions from time to time.

Sorry for the slide slightly off topic, but thought it worth the effort to share the information.

Back to 35mike's "Comfortable cruising rpm..." question.