The Stovebolt Site Forums The Parking Lot Big Bolts Gas burner truck speed/RPM in days past

Just kind of curious. For those that ran medium/heavy duty gasser trucks when they were new, and time was money; how hard did you run them? And what, if any problems did you have from sustained high engine speeds?

In particular I'm interested to hear how the 4.120" stroke 292 I-6 held up compared to the 283's and 327's.

Hey,66Submarine! i have a '55 Chevy T.F. 6500/261/SM420/2-speed rear,(not sure of gear ratio's). Engine tag states XXX H.P.@ XXXX R.P.M. I know that I achieve peak horse-power @ 3500 R.P.M.,which I believe is the "norm",for medium/heavy trucks of the '40's and '50's. Once up-to speed,I can drive 55,all day long,without having to down-shift either the trans,or rear-end. I still have about a 1/2-inch of pedal before "She" hits the floor,but I know the sound of a "Happy" engine. Don't expect more than 6-8,(if you're lucky),M.P.G. I pulled the data-plate from an old WHITE WB22T semi-tractor,(about a '48),and that engine peaked at 3500 R.P.M.,as well,but for the size of that flathead 6,it only achieved 122 H.P. It was rated at 40,000 G.V.W.,(tractor/trailer comb.) It had a 5-speed main box,with a 3-speed auxy.,to help it achieve highway speeds,without over-stressing the engine. Average M.P.G.,(as I was told),was 3-4 M.P.G. The 292's didn't come on the scene,until about '63,and I'm not familiar with the data on these engines. Most engines built in those days had a life-span of 100,000 miles,on average. If not worn too badly,they could be re-built,with an expectancy of another 50,000 miles. Hope this helps.

Thanks for the input, Bill. I think the standard two speed was 6.40/8.72, so with a 9.00-20 tire 55MPH should be about 3,000RPM. Good to know I can spin a 235/261 up to that if I ever get one.

My question I guess, is more aimed at what it takes to actually cause damage. SBC's for example can take 3,500+RPM at cruise, and that can be seen in boats; but a 292 was a one year deal as a marine engine. Normally I'm very easy on stuff, but I really like knowing the limits beforehand so I can determine when I'm actually beating up on it.

If you are interested in 292 specifications, that information is here, in the 1966 specifications manual at the GM Heritage Center (that link is a large download).

As I recall, 3,000 RPM is at the net maximum horsepower "plateau", but the net torque increases at higher RPM?

Love that site. Max HP/TQ as per GM were 3,600RPM/2,400RPM net, gross 4,000RPM/1,600RPM.

What I'm more interested in is how they (292's) take sustained high speeds in the real world. Everyone (today anyway) seems to be afraid of the longer stroke vs., say, a 250.

But, the way I see it, the rod/stroke ratio is actually slightly better, and you get 3/8" rod bolts. Pistons are 24.90oz, 327 was 23.46oz. Seven main bearings to keep everything stable, and later engines like mine had 12 counterweight cranks.

I just don't see what makes it so intrinsically hard on itself at high speeds. Obviously it's going to suck fuel at 3,500RPM and not really make any more power, but I don't see why a 250/350/454, etc. can do it and a 292 will self destruct like some seem to think. It was designed for heavy trucks with deep gears, after all.

Horsepower and torque lines cross at 5250rpm, I can't see how horsepower can plateau and torque increase as rpm increases, it is impossible.

Hey, 66 Submarine! was the life-span of the marine 292 caused by linking the prop directly to the flywheel,without a trans? Water has quite a bit of resistance ,so it may have been harder on the engine trying to get up-to-speed,than it was to run it at a constant high speed. Just a thought. B.T.W... I had the head off of my truck just last August,and the previous owner KNOWS that my 261 has sat more than 10,out of the last 15-years that he and his neighbor owned the truck. i's never been re-built,or tore-down as far as anyone knows. MY point to this matter is that there is virtually no ridge on the cylinder walls,compression tests within 10#/cylinder,and it uses virtually no oil! Odometer reads 98,000 miles,and NO sign of smoke. The way mine runs,it should be good for at least another 50,000 miles. I know my truck hauled some heavy loads of cinder-bloc,as well as a back-hoe towed behind it!! I think as long as you maintain any vehicle,it will respond accordingly. I change the oil every 1,000 miles,since my truck doesn't have an oil filter,(S.C.O. engine.)I use Valvoline ZR 5W50 Racing oil,as well as 4 oz. of ZDD additive per oil change. It's recommended for use in solid-lifter engines,since the gov't. put regulations on zinc in oil,which our 216,235,and 261's were designed to operate with,un-less a passenger car equipped with an automatic-(Hydraulic lifters).

Oh, I didn't mean a "one year deal" in that they blew up in a year - I meant they only put 292's in boats a couple years, and information is scarce. My bad, ha ha.

Interesting to hear that your 261 has relatively little wear after 98,000 in a two ton truck at fairly high RPM. Kind of reinforces what I'm thinking.

I personally use Delo 400 15w40 diesel oil in my stuff. High quality, readily available, inexpensive. Also, later hydraulic lifter engines still need the EP zinc, it's roller tappet lifters that don't really need all the EP stuff. I'm all-too-well versed in the self changing oil engines, they're real common around my place. Too common, really... I need to work on that!

Anyone have anything to say about the 292 in particular? There's got to be someone on here that's beat on one in the fifty years they've been around.

I never owned a 292, but there were a bunch of them around here hauling rock with end dumps when they built IH10 around here in the 60's. They ran them wide open all day long and claimed they were the best engines they had ever run. I was hauling grain and feed ingredients during this time with a 550 GMC with a 370 cu. in. engine that was actually an Olds 371 block. I stuck a set of rings and bearings in it about every 100k miles. My shift points were 3800 and 4200. I sometimes over ran the 4200 down hill but I never let it get below 3600. It was still a strong running engine when I wrecked the truck in 1968.

Thanks Crenwelge! That's what I was looking for. Good to hear, that's what I figured. Funny to think that a lot of people nowadays feel uneasy running one at 3,000 for some reason.

3,800RPM would have been about 70MPH, I figure? It's also interesting that even screaming along like that you got 100K out of it before you needed to freshen it up. Again, I kind of have to laugh thinking how many today will very authoritively tell you a 60's half ton wasn't made to go over 55MPH, ha ha. Always interesting to hear from guys like you who found out what worked and didn't in the real world.

Hi,66Sub! I had a '65 G.M.C. 3/4-ton Camper special,equipped with a 305E V-6,and a 2-speed "Slushbox" Tranny. It also had a 4:11 posi. rear-end. Even with the 16.5 tires,top speed was 55,and that was FLOORED. Bigger trucks could achieve a little better speed with the use of a 5-speed O.D.,or an auxy. transmission.

That's VERY interesting, Bill. My daily driver right now is my '65 C-30, it has 4.10's, an SM420, and a two barrel 283. I'm running slightly taller 7.50-17's on the back, but it will run 60-65MPH really easy. If I dropped the hammer I'd be surprised if it wouldn't run 90MPH or so.

4.10 rear with 17" rubber probably would run 90.

Thanks,Crenwelge! I blame the 55 M.P.H. top-speed,on the 2-speed auto. My best friend's dad,at the time,had a '66 3/4-ton,same truck,but had the SM420. We've had that truck up to 90,several times! Better hit a gas station quick,though! Ha Ha!!

Don't know what the problem is they got more main bearings than a V/8, seen a lot of them running dirt tracks, balance them and spin them 6500 allllll night long.