Hello everyone –

I’ve got a ’75 Olds 455 which I intend to rebuild and install in my ’70 Cutlass S. This will be a weekend cruiser with some giddyup, not a drag strip racer. I’d like to know what a “safe” engine RPM would be for extended highway driving. This will help me define the rebuild, and define the rest of the drivetrain. I’ve been using 3200 as a max-sustained RPM and 5500 as a do-not-exceed RPM, but I’ve seen numbers like 2800 and 4500 (respectively) on other threads. I want to be able to safely scoot across town on the interstate, maybe go a little longer on occasion, but I’d also like to improve the low ratio rearend currently in the car. Gas mileage is not a concern, I just don’t want the thing to blow up. Any experiences or recommendations would be greatly appreciated.

 

From my personal experience, I spun a stock one up to 5300 at the track and it spun a bearing.

However the replacement motor has been spun up to 5300 a handful of times and runs good.

I suppose it depends on the camshaft, rotating assembly weight, and age of the bearings.

I ran my car at the track shifting at 5000 and it ran the same trap time as shifting at 5300.
If you have a healthy motor, I'd say 5k is the limit.
Ask me again in the fall when the racing season is over and I'll let you know if the motor is still together.

 

Bigger is not always better. why are you swapping motors? smog motor is not a great alternative, is the car missing the engine? here is why I ask. years back i rebuilt my 350 and turbo tranny, with few mods and alot of tweaks, needless to say with highway gears she clicked off 13.90's at the track, and got 25 mpg at 55-60 here on the highway. Bigger is not always better.

 

Assuming fuel consumption and sound pressure levels are not factors:

With a proper rebuild, a 3500 cruise should be no problem. Your max RPM (highest shift point) will be determined by the engines power output.

Norm

 

OP said it will be rebuilt. If it is done properly, it will no longer be a "smog motor".

Norm

 

With stock type J heads (2 inch valves) on it, you will be beyond the engines power band at 4500 rpm. That short block cannot get enough air in it for more power at higher engine speeds. NO need to rev it any more than that. If you WERE to race it, it would actually slow the car down.I have seen race cars that shift in drive, because going to 5000+ lost 2 tenths. One in particular, I tried driving it, and slowed it from it's 13.30 pace, to a 13.50 because I thought it would go faster if I held it in first and second longer.That car normally shifted 4300-4400 rpm, had an aftermarket cam, 3.90 gears, and was extremely consistent. Live and learn.
3.23-3.55 gears will be more than enough.
JMO
Jim

 

Thank you all for the replies – this gives me RPM numbers I can use with confidence.

I hadn’t considered the sound level during cruise Norm, thanks for pointing that out.