The Stovebolt Site Forums The Shop Area General Truck Talk loooooow RPM engine?

I've been reading many comments about what you guys are revving the 216's to. I see numbers like 2000, 2200 2500, heck my BS lawn mower revs more than that. My great big old 320ci air-cooled Lycoming with its pistons the size of water buckets cruses at 2800. Why are you all keeping the rpm so low on these engines? Is there a directive or a red line published somewhere that limits them to very low RPM. I would think that even the splash oilers wouldn’t have a problem running 3000 to 3500.
Denny Graham
Sandwich, IL

Why are you all keeping the rpm so low on these engines


we aren't ,. they are built that way...low compression eng.

racing engines can go up to 10,000 plus, these can't, I think about 4200 is top for a 235 and even then might take you awhile to get there....you still would not want to drive it that high for very long

Generally the longer the stroke, the lower the RPMs. A long stroke straight 6 cylinder has great torque needed for hauling loads.

Short strokes will generally have higher RPMs and get more of their horsepower/torque from the RPMs, and have much lower torque at similar RPMs to long stroke engines.

I have seen folks indicate that the combination of heavy cast pistons and babbitt rod bearings require a light foot in these old engines.

I don't know if this is smoke and mirrors or actual engineering fact. :confused:

Stuart

The red line on a babbit-pounding 216 is somewhere between 3000 RPM and 3500 RPM, as you mentioned. (GM says that Max bhp is 92@3300 RPM.)

The main reason is the babbited bearings. When you wind it way out, the babbit isn't supposed to be able to handle the weight of the cast iron pistons coupled with the heat from the friction, and softens up a little too much. If you were to swap out to aluminum pistons (and insert bearings), you could get away with winding it out further.

For comparison, The red line on a 235 ('54-'62) is 4500, IINM.

That number can be upped quite a bit by using aluminum pistons. The lighter pistons take a lot of stress off of the babbett. I've conversed with an owner of a 39 sedan with the 4.11 who said he cruises at freeway speeds in his aluminum piston 216.

Well gosh... an unstyled John Deere G don't even rev up to bout 800 rpm. You and yer fancy 6 seeelender ingines, revving up so high.

Denny
Tell us more about your Lycoming engine. Is that an aircraft motor?

The fact that there are only four main bearings adds into the mix also. Higher rpm's are more likely to start inducing flexing and vibration into the relatively long crankshaft than in an engine with a shorter crank or one with more main bearings.

Yep. Oh, sorry I'm not Denny.

Yep, Tiny took care of it, four banger opposed.
Interesting opinions as to what this engine should turn.

At 3500 RPM, I feel like my engine is yelling at me. Runs very nice at 1000 - 1500, especially since going with HEI.

Not sure where I come into this one but I think it's all in your head. I can hear when I need to shift my truck. I have never had a tach in a vehicle that was a manual transmission, two auto's with tachs (go figure). I am not sure where I shift my engine but I would be suprised if it was over 3000 by much. Seems to me that shifting by ear was the preferred method. I say rev til the power stops and then shift, and 60mph is your top without reworking some things.

Kyle

Brendan probably is the closest, to what I would conceder the RPM that this (216) engine could safely run at. The torque and horsepower curves cross at around 2750; this is where you will feel the power begin to drop off. The gross horsepower peaks at 90 at 3300, however it won’t be pulling as hard at that RPM as it would be if it were in the torque curve. If I was hauling a load I think I would operate it in the 1200 and 2000 rpm range. I cannot see any problem with pistons or valves operating an engine at 4000 rpm.

Of course once your out on the autobahn and up to cruising speed you don’t need all that much torque and running the engine up to 4000 rpm should not hurt it. Now all that being said, one has to conceder that this would only be applicable to a new engine (rebuilt is not new) with everything broken in and in proper adjustment.

One other significant thing I did notice in looking through the Chevrolet Truck Data Book is that none of the charts go above 50 mph. This would strongly indicate that none of these trucks were designed to operate over that speed from the factory. Also as we all are aware if is, the higher the rpm, the shorter the life of the engine, so we have a balancing act going on here.
Denny Graham
Sandwich, IL

Does anyone kinow what the peak torque rpm is on, say, a '51 216? I would think that, like a diesel, the engine's full-power operating range would be between peak torque and peak horsepower speeds.

Stuart said,"I have seen folks indicate that the combination of heavy cast pistons and babbitt rod bearings require a light foot in these old engines."

As a truck engine is clearly designed to be run frequently at open throttle, I can't see that statement being correct, and the one time I've had 3000# of timber in Epic's box, she seemed very very pleased indeed to be pulling hard. In fact, she seemed happy at 45+mph, and when she's empty, 40 is plenty.

Lower rpm's are better for fuel economy, provided that the engine is not allowed to lug. A lug condition exists when the engine is operating at a speed less than peak torque rpm at full throttle. If the engine is allowed to lug for too long, heat will build up in the top of the combustion chamber, and the risk of burning exhaust valves is sharply increased. The cure is to back off the throttle, or downshift.

What is the redline on a '66 250? And what is the max rpm? I've heard 6000+ on them because they have short strokes and 7 main bearings.

The torque peak on a 216 is at about 2200 rpm. The torque peak on the bigger sixes is at lower rpm, the 261 peaks at about 1500 and the GMC 302 at about 1300.

The 7 main bearing engines really can stand much higher RPM's, but you have to be careful with that, because the harmonic balancers separate after a while and slip, then become ineffective. Bad harmonics make the bearings wear funny.

The redline on my BMW six is at 7000, they have the harmonics pretty well controlled and the pistons and rods are really light.

Of course, none of it even comes close to my rotary- that thing's power peak is at 9000 rpm. Mazda put a rev limiter on it because the clutch and alternator blow up when you go above that. It's as smooth as a turbine.


Lugging an engine is also bad for the bearings- the low rpm knocking makes the rod bearings slap metal against the crank.

Josiah, many years ago, in a far off land,I used to run midgets. Chevy II's were common, a few Sesco's (1/2 of a Chevy V8), a Falcon dominated the track but one of the local builders put a Mazda rotory in a car. there wasn't anything that could touch it, that is when it was running. Way back then Mazda didn't have the apex seals worked out yet and the racers were over reved it and chewing up the gears pretty quick. But when it ran, it sure did put out a lot of power for such a tiny engine. Of course that was from the high revs, it was a little weak coming out of the corners but caught up in the straight-of-way.
Denny Graham
Sandwich, IL

The Honda 120 hp per liter (naturally aspirated)inline 4 cyl. is another 9K redliner. It's been holding up well for an engine that begins to make power after 6K. I know of several with over 100 thousand miles on them. Missed downshifts have grenaded more than a few though....
Kim

Looking at the HP and torque curves for a 49 216 http://chevy.tocmp.com/chevyresto/4982.htm the NET torque peaks at about 1100 rpm (168 ft-lb) and drops by about only 4 ft-lbs at 2000 rpm then the curve drops off sharply. The gross torque is nearly flat at 174 from 1200 to 2000.