V8 MAX RPMs @ highway speeds?

I hate to keep buggin you guys here but here is another question. What is a comfortable RPM for my 401 for long range highway speeds? Mine is running aprox 3000 RPM @ 65 MPH in 4th gear, around 2500 @ 55, if I recall. My CJ3B would turn purple at 50MPH. Dont want to tare up my engine and MPG is not much of an issue since I drive it about 10 miles to work and back.
Just info I would like to know if I feel like driving to the coast in it. People out here in California are on your bumper if your doing less than 90 on the freeway it seems.:driving:

 

3000 @ 65 is a common speed. If the engine is stock redline is 5500, but I have had mine to 6000 in short bursts with out problem.

Jay

 

3000 rpm should not be a problem at all. My 401 would turn around 3200 in my old J-truck at 65.

 

I don't like keeping a V8 over 2600 to 2700 rpm steady cruise. I think that any faster than that just accelerates wear and uses more fuel. 6's and 4's can run faster, but I like to keep the rpm down on an 8.

FWIW, most automotive applications run a V8 from about 1800 to 2400 rpm at highway speeds. There are various reasons for this, but longevity is one of them.

 

In theory, I'd agree, but I'm not sure the difference will be very significant. I know that the modern V8s run at 'slow' speeds like 1800-2400, but I am skeptical that you can compare those V8s with carbureted V8s from the 60s, 70s and 80s. Modern engines have much more sophisticated fuel mixture control, and the geometry and mechanics of the engines have been optimized to run efficiently at those 'slow' speeds.

I also realize that modern cars require much less power to move at highway speeds, so I think it's likely that a Jeep engine cannot run efficiently at 'slow' speeds in real life.

Just opinion though - it would take lots of tests to find out. I know that most drivers get acceptably long engine life from their classic V8s regularly running at over 3000 rpm on the highway. Too much optimization is going to be uneconomic... a 401 will go many, many miles turning 3000 rpm on the highway. I wouldn't worry about it. Just drive it, change the oil often, use an oil that's intended for flat tappet engines, and keep an eye on the hot idle oil pressure.

 

If you want to bring the RPM's down a little, swapping up in tire size will help some.

 

My oil pressure runs a consistant 59 to 60 lbs at normal temperture @ high RPM, about 25 lbs @ idle hot, Shell Rotella T synthetic oil . The cooling system is right on, new coolant, good radiator and I'm running a littel cooler than factory 160 deg thermosdat . The factory specks call for a 180 deg, that I will install during the winter time up here which can get into the low 10's. The Summer season here gets into the high 90's and the Jeep still runs in the green on the engine temp.

Again fuel econemy is not so much an issue as engine life. I will probabley go through the motor at some point when I restore the whole truck but this will be at some time in the distant future as it serves as my daily driver for now. The wife has the new truck. My JEEP is about 90% unmolested working original except for the 304 to 401 swap, front seats, T18 and some evil spirits under the dash.

 

Well modern v8's may run at lower rpm but so did older models. My old Plymouth (1969) with a 318 run 2200rpm at 60 mph, my dads 77 Cutlass was even slower. I would not want to run a v8 for 1000 miles at 3500rpm but 2750-3000rpm for a couple of hours at 60-65mph would be ok.

 

This isn't the best comparison but I thought I'd add my 2 cents. We run 4 older carbureted style Chevy 454's and two 350's as irrigation motors. We've only rebuilt the top end on one of those motors in the last 10 years and they run at 2600-3300 RPM 25 days a month for 6-7 months. If u assume that they're running at ~ 60 MPH that's a lot of miles lol. I wouldn't worry about running ~3000 Rpm and I wouldn't invest any money to get ur motor to run slower since ur not concerned about mpg. The 401 might or might not crap out on u in a couple of years, regardless of a 800 RPM change during highway speeds.

 

This is my recollection as well. 70 Charger RT, 68 Chevelle, 72 Chevelle, 63 Rambler, 69 AMC Ambassador, 73 Dart, 74 Charger, and on and on. All ran 2200 to 2500 at 60mph or so with stock gearing.

 

Am I missing something but just about any truck that came with a tow package, before OD was common, ran 4.10 gears and a 30"ish tire from the factory. With this tire and gear combo they ran 3000+ RPM all day without any problem. Yes, I'm sure they wear a little faster (not enough to worry about over the life of the engine) than one running 2200 but it won't hurt the engine running that RPM all day long.

 

Yep, heartily agree. I think the engine noise is annoying and there's more friction from the rings moving faster, but IMO you can run 3000-4000 RPM all day long with no ill effects. At worst, the life of the rings will be scaled with the distance they travel on the cylinder wall, so comparing 3000 to 2200 give 2200/3000 * 100% = 73% of the ring life you might have with the engine traveling slower. I'm not convinced that the speed of the engine will have much impact on the fuel economy... the only loss of fuel might be to the higher friction of the engine at higher speeds, but I'd guess that will be a very small fraction of the total, especially for a vehicle that spends so much fuel overcoming the resistance of the air.

It's possible that modern cars have pared away at rolling and air resistance to the degree that engine friction is a significant loss, and lower speeds contribute significantly to fuel economy ... don't know.

Regarding older passenger cars that turn at slower speeds, sure they would, but they also had passing gears that would upshift at 75 mph and WOT. They were meant to be smooth and quiet at highway speeds. Plus their power requirements at highway speeds would always be very much lower than light truck's could be. Apples to oranges.

 

saying running 3000 rpm in a "v8" as opposed to a v6 or I4 would be any different has nothing to do with the size of a motor.

the main factors that determine a safe speed to run a motor at are: stroke, engine block metal composition, and valvetrain integrity.

for example, an AMC 150 2.5L 4 banger is happy running all day long at 3500-4500 rpm with no exessive engine wear, where as try and take a Willys L- or F-head motor up to those sustained speeds and you're going to see a piston shaped hole in your hood. the main difference speed limiting factor between those two motors is their stroke. the same goes for a pushrod driven valvetrain when compared to the more modern SOHC and DOHC motors, where you can take the later up to much greater speeds because you don't have to worry about bending a pushrod.

when we're talking about the older V8s from the 60s and 70s the same thing applies. For instance, if you had two AMC 401s on stands and ran them for 100 hours. The first motor at 2500 rpm and the second at 4500 rpm, then you took them apart for inspection you'd be hard pressed to find any greater wear in the motor running at 4500 rpm. It's largely due to the fact that most of the V8s of that era were relativly short stroke and had adequate valvetrains to maintain those speeds.

so getting back to the original point, go for it. run the motor at 3000 rpm all day and don't bat an eye. if you're motors in good shape you should have no problem doing that for many years to come.

- evan