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speed vs. rpm

S

stick

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i just put a rebuilt 350 into my blazer which formerly had a 400 in it. with the old set up it was a 55mph vehicle. here is what i have now;
sb350
th350
np203
3.73's
31" tires
i put a tach into the truck when i put in the new engine and at 55mph it will sit at about 2500 rpm's. the question that i have is what is an acceptable rpm for sustained highway driving?
 
With a 350 I would say you could prolly run as high as 3000-3500. The SBC can handle a lot more RPM than that for sustained periods. 2500 is pretty good. I turn 1700 at 75 but I have 3.08 and 700r4. It is also a dog :o .

Ira
 
i turn 2000 rpms or so at about 60 or 65 mph in my 89 jimmy.iv got a 350,3.75 gears and 33's, thats usually towing my boat to the beach or back but its about the same ether way.
 
Just how fast did we ever get that 3cyl car of your going......and how long did it take to get there?
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I think it was about 100 and it started to float over bumps :eek1: . I took a good 3-4 miles to get that high what with no 2nd gear and all :doah: .

We did make the trip for Orlando to Ft. Meade in 1 hour.

Ira
 
I see no problem running 3500rpm as long as there is a load on the motor. If there is no load then it can be hard on it. I have run mine at 3700rpm for extended periods of time on the expressway...
 
That's a little high if you want to see some fuel mileage.....1800-2800 rpm at 60 mph is the range for a street-driven SBC, your combo runs 2283 rpm at 55 mph and 2906 rpm at 70 mph
I don't think you drive around at 55 mph so i would suggest 33" tires or 3.55 gears which will turn the engine at roughly 2750 rpm at 70 mph :bow:
 
just wondering. where do these figures come from. is 3000 okay ? or is it asking too much out of an engine that i don't want to have to rebuild again for another 100K?
 
i try to keep it under 3000 rpm for sustained long driving.im about 65 at 3000 and i think that if good enough.anything over 3000 is just pushing the engine.2200 to 2800 is optimum for gas milage and better for the engine.you keep it under 3000 and youll be fine.
 
CyberSniper said:
I see no problem running 3500rpm as long as there is a load on the motor. If there is no load then it can be hard on it. I have run mine at 3700rpm for extended periods of time on the expressway...
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What is hard on the motor above 3000 RPM? If oil pressure is good, I don't see why more RPM can hurt anything but gas mileage. Also, my dd probe gt is spinning 3800 or 3900 @ 80 mph, and the engine in it (2.5 Mazda V-6) is considered an easy 200K motor.
 
chulisohombre said:
i try to keep it under 3000 rpm for sustained long driving.im about 65 at 3000 and i think that if good enough.anything over 3000 is just pushing the engine.2200 to 2800 is optimum for gas milage and better for the engine.you keep it under 3000 and youll be fine.
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really?? I went down a highway the other day at 74mph.. turnin 3k... so that isnt bad??? phew.. I thought that would be gas hoggin it... but guess not... I dont expect to do 74 often... more like 65-70 tops.. I was debating a nv4500... but dont wanna bother... :bow:
 
i do have a set of axles with 308's in them. will they kill my towing ability? i know that i could get more highway speed out of them. but sometimes i like to take trash to the dump without putting it into the blazer.
 
3.08s and towing now that is funny. I have towed a car with my 3.08s and boy was it a dog. I also have 31's and coming over the pass through Shasta in Northern Cali I was in firt gear all the way to the top.

Ira
 
38377k5 said:
What is hard on the motor above 3000 RPM? If oil pressure is good, I don't see why more RPM can hurt anything but gas mileage. Also, my dd probe gt is spinning 3800 or 3900 @ 80 mph, and the engine in it (2.5 Mazda V-6) is considered an easy 200K motor.
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If there is no load on the motor then the reciprocating mass can oscillate. This can cause wrist pin wear and rod bearing wear. It can also cause valvetrain wear. If you notice, most modern vehicles won't let you go past 3500rpm in neutral or park.

I run my 305 at high rpm all the time. Towing I was pushing 4,000rpm in third gear at 75mph at times. Most of the time, I get the same gas mileage running in third gear or overdrive... the only difference is how hard the motor works at the RPMs. It's all about how efficient the motor is.
 
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