# Safe Cruising RPM



## khinton (Jun 22, 2008)

I have a 68 gto with 400 H0 with 3.55 rear-- car runs 3200 rps at 70mph--My question is is this rpm safe to cruise at or will it hurt the engine to drive for sustained periods of time at this speed/rpm? I realize I could change to a 3.36 in my present carrier but I did have a 4.33 when I bought the car..Could not locate a new 3.36 but found the 3.55. Don't want to change it again


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## geeteeohguy (Feb 2, 2008)

It depends on your valve seats. If you have the original type, non hardened valve seats, sustained driving at or above 3000 rpm will cause valve seat recession. If they are the hardened seats made for use with unleaded fuels, you'll be fine. The rest of the engine, 3200rpm, no problem. If you do have the stock seats, vary the speed from 60mph to 70-75mph as you go. That may help......


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## 66tempestGT (Nov 28, 2009)

unless you plan to drive 250 miles per day i wouldnt think twice about it. if you are like most gto owners it is a weekend/special occasion car. i challenge you to drive it enough to wear it out. good luck:cool


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## khinton (Jun 22, 2008)

I do have the hardened valve seats--Is there a rule about how many long distance cruising rpms an engine can sustain with no damage--like a percent of redline rpms,etc?:confused


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## nOObiE do0 (Jan 30, 2010)

I drove my 72 daily 12 years ago.It has 3:90 gears and it was 3000 at around 55 or so.
I just made sure i didn't stay at a constant rpm.I was told to do this till the engine broke in.But i developed a habit of doing it.Hell i still do it with all my cars.


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## geeteeohguy (Feb 2, 2008)

With the hardened seats, no worries. Let your wallet and the speed limit dictate your cruise speed.


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## 646904GTO (Feb 10, 2008)

One issue no one addressed was oil pressure. If the engine has a stock or 'hi volume' oil pump it can pump most of the oil 'upstairs' and cause oil starvation with continued hi RPM cruising. That is really rare but it did happen to a friend of mine. He spun a rod bearing. Usually one would not see the starvation damage until the crankshaft was opened up.


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## jetstang (Nov 5, 2008)

As stated, RPM's don't kill a motor, oil starvation does. The 90 454 SS trucks came with 3.73s and no OD, 10 MPG. The problem they had was the accesory brackets would brake from spinning the AC and Alternator at the RPMs.
My 3.36 geared car pulls 3000 at 70, so not enough dif to change gears over from 3.55s.
Gotta remember, until 1980?, no cars had overdrive, so this was very normal. My 78 Corvette topped out at redline at 105 MPH, where just spoiled now with the new cars cruising 90 MPH at 2000 RPMs.


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## geeteeohguy (Feb 2, 2008)

Jetstang is right. We're spoiled with overdrives and lock-up converters. It used to be the norm to cruise at 3,000-4,000 RPM all day long. Gas was cheap, America was a productive country, not an importer of everything, etc. I believe that 3-4k rpms all day long will NOT cause the oil to stay upstairs and cause damage., all else being equal. 5000RPM may, though!!!


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## xcmac26 (Dec 1, 2008)

Correct me if i'm wrong, but my understanding of IC engines in general is the closer you are to 85-90% of the engines capability (red line), the happier the engine is. More power per cylinder, better combustion, better lubrication etc. etc. So long as your oil pump is getting the juice to the heads, you're golden. Run Lucas if you're concerned about oil starvation.

I know we had a 79 caddy with no overdrive and more than 300,000 miles on the motor with no rebuild. Thing ran like a top because it cruised to and from florida twice a year. higher RPM, good oiling and well maintained. good to go.


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## jetstang (Nov 5, 2008)

xcmac26 said:


> Correct me if i'm wrong, but my understanding of IC engines in general is the closer you are to 85-90% of the engines capability (red line), the happier the engine is. More power per cylinder, better combustion, better lubrication etc. etc. So long as your oil pump is getting the juice to the heads, you're golden. Run Lucas if you're concerned about oil starvation.
> 
> I know we had a 79 caddy with no overdrive and more than 300,000 miles on the motor with no rebuild. Thing ran like a top because it cruised to and from florida twice a year. higher RPM, good oiling and well maintained. good to go.


For engine efficiency, higher RPM make more horse power, because of higher cylinder pressure. The engine is more responsive, but not happier. It's like sprinting or jogging, I'd rather walk or sit-like your engine.

High mileage motors are either overdesigned or well maintained motors. There are motors that crack heads and puke timing belts every 50K miles. The 70's ultra low compression motors can live forever as they haven't worked a day in there lives at 7.0 compression ratio.


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## geeteeohguy (Feb 2, 2008)

The higher you rev an engine, the more heat and wear you generate. Still, over-revving is better than lugging. That said, the worse thing for an IC engine is cold start-ups, and short drives that do not let the acids, by-products of combustion and water condensation vaporize out of the oil. The best kind of driving to do for longevity is moderate rpms (cruising rpm) over long distances, such as highway travel. Stop and go driving, over -revving, and especially lugging an engine are all bad. Keep the RPM's reasonable, the trip at least 10 miles long, the oil changed and with ZDDP additive installed, and a clean air filter, make sure you have your wallet and sunglasses, and turned off the oven, and you'll be fine.... In my opinion, if you were to tear down two identical engine 's from two identical cars, after 100,000 miles of identical driving conditions, but one car had a 2.73 rear gear and the other a 4.33, I think you'd find the 2.73 geared engine would be in much better condition. It would have turned a lot fewer rpms over the 100k miles.


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