Is there a maximum highway cruising rpm for the 455? My 72 Riviera came with a TH400 peg leg stock rear end which I replaced with a 3:42 posi ( thank you Jim Mitschke). I immediately noticed an rpm increase on the highway, i.e. at 62 mph/third gear I'm turning 3000 rpm. I like to cruise at 70 mph, but do not want to cause damage or excessive wear and tear to the car engine. Thoughts? Thank you.
The way I look at it is if in high gear whether you go 2500 or 3500 same wear since at 3500 you get there sooner and motor not running as long. Plus life is too short to drive slow.....
You won’t hurt anything by cruising at 3000 rpm except maybe your wallet. Just keep an eye on engine temp and most importantly oil pressure. Minimum of 40 psi while cruising at 3000 rpm
I have a 455 in my 1985 GMC tilt-rollback truck. 3.73 gears and at 65 I'm running 2800 rpm. Exactly at torque peak.
I ran my 470 for hours at 3400 rpm with no issues. Hell on a pissed off trip with my 430 I let it sing at 3700 for about 10 hours.
Before I sold my X I drove it to BG one last time. 75-80mph (3.64 gears, not sure of rpm but around 3500) most of the way. It just ate up the mountains with the torque.... Then ran 13.0s (open stock manifolds at trans area, MK2 cam, stock intake and Qjet) in BG and drove it 12 hrs home.
No such thing as maximum cruising RPM. As mentioned, as long as oil pressure and cooling is good, the motor will just hum along indefinitely.
I wonder how fast my '68 350 is turning at 70 MPH in first gear? I was on a road trip and had installed a temporary CD player with some fairly loud speakers... After exiting a gas station in West Texas, I got up to 70 and was cruising along until the next break in the music, at which point i realized I had shifted into L rather than D. Car only has two speeds, and I picked the wrong one. Anyhow I shifted into D, and kept driving. No problems. It was howling though...
You can figure it out. The ST300 1st gear is 1.765. If your rear gear is 2.93, and you have stock tires, say 26.6" tall. 1.765 X 2.93 =5.17 336/26.6 X 5.17 X 70 = 4571 RPM without converter slip.
I’m just curious, the Op did not say whether he changed his speedometer gear. I’m pretty sure with stock tires, and the 342 rear gear, you should be doing 70 at 3000 RPM, or pretty close to that.
You are correct. 3k should be at least 70 unless the converter is way loose or the tires are 24" tall. Someone out there started a rumor that no engine can survive spinning over 3k for any amount of time. That person needs a dick punch...
No speedometer gear changes. My speeds were determined using GPS software on my iPhone. 3000 rpm consistently reflected 62 mph. I have the stock 15” Buick Rally wheels.
336/26.6 X 3.42 X 70 = 3024 RPM. Add in 150-200 for converter slip and you are up around 3200 RPM. A 72 Riviera should have 28" tires, so alter the above by that and you are right at 3000. 336/28 X 3.42 X 70 = 2873
I win a six pack of beer don’t I? When I was a kid my buddy owned a 68 GS 400 with a four speed. I could’ve sworn It did exactly 3000 RPM on the console tach at 70 mph. I agree with so many statements above that you can go way above cruising speed and never do any damage. My second engine build was my 1968 GS 350. Yes I paid for it to be balanced but I put all of it together by myself. I regularly cruise at 4000 RPM. It has 3:90 gears and I like to keep up with traffic. One time I had a buddy in the car and we were cruising with a bunch of other cars. We were running at 5000 RPM steady for a long ways. He was questioning whether my tachometer was correct, and whether my oil pressure was correct. I told him both gauges work perfect.
The engine in my motorcycle has the powerband start at 7K. Inboard boat engines routinely cruise at 4000+ RPM, for hours on end. If that boat has a "Jet" propulsion system, 2000 rpm is barely-moving-and-steering. There were a HUGE number of Berkeley Jet/Oldsmobile 455 powered ski-boats a few decades ago, and the Olds 455 is a long-stroke engine unlike the Buick 455. My own boat--454 Chevy powered and lake-water cooled--had adequate coolant temperature but excessive oil temperature when cruising over 3000 RPM, until I cleaned all the debris out of the engine water jackets. Once the rubber water-pump impeller bits and chunks were gone, I had to install a marine thermostat housing and thermostat to get the coolant temp above 120, and that ended the oil temp problem. The bigger problem is that you'll burn more fuel to spin 3000 rpm at highway speed than if the rpm was lower. The engine is likely to be less noisy, at lower rpm, too. You'd do well to get the RPM down. You'd gain in engine efficiency, engine longevity, NVH, and emissions.
My Wildcat with 3.42 rear and 28” tires runs about 2700@60 and 3000@70. The biggest issue is in between the engine drone is rather annoying and at 70 the engine makes a lot more noise. I take back roads when I can and take my time, freeways just get too noisy with the other cars and the exhaust. I’m getting the exhaust redone in a couple weeks and adding an x-pipe, so hopefully it’ll cut down on the drone a bit.
