Rpms

Route666 · Dec 4, 2003

These questions are for a V8 engine.

What is a good rpm for cruising at 62MPH (100km/h)?

What is the lowest rpm a motor should be turning cruising at 37MPH (60km/h)?

Thanks guys

DarkBuddha · Dec 4, 2003

The correct answer is: depends on the engine. I tend to think of 60mph as a cruising speed, so I'd think you'd want the engine rev'ing at the rpm that it is at its most efficient (based on VE maybe?). I've always used a vacuum guage as a good way to determine a good curise rpm... just try to maintain the highest vacuum possible. My 302 always liked right around 2300 rpm, which pulled 18 on the vacuum guage, and was about 57mph with the 3.0 gears and super tall 215/75/15s...

88 Fox GT · Dec 4, 2003

It depends on what you mean by a "good rpm" Good for gas mileage, or power? My 302 is at about 2300 rpm at 55-60 mph. I get good gas mileage, about 15-17 mpg. It has good power there too, I don't need to downshift to pass unless I really want to haul arse! I can get from 55-65 or 70 pretty quick without downshifting.

jcp123 · Dec 4, 2003

mine hovers right around 2000rpm at 60, gets over 20mpg, and gets up to 70 in a snap. most engines should cruise that speed anywhere from 1800-2300rpm for ideal mileage; but as dark buddha says, every engine is a bit different and you should take a vacuum reading to determine your most efficient RPM.

RGS0907 · Dec 5, 2003

It’s totally dependent on your rear axle ratio and tire size. If you have a 4:11 rear, you’ll be screaming at least 3000 RPMs at 60. I have a 3:26 to 1 & I run about 2500 RPM at 60 with 15” wheels. The vacuum guage depends on how much you put your foot into it. At cruising speed (60 or so) if you barely touch the gas to keep the speed up on a flat road, your vacuum reading will be way up.

Route666 · Dec 5, 2003

Thanks for the input, I meant best rpm for efficiency, and I want to tune the car around the cruise rpm as well as around-town rpms.

Edbert · Dec 5, 2003

RGS0907 said:
It’s totally dependent on your rear axle ratio and tire size. If you have a 4:11 rear, you’ll be screaming at least 3000 RPMs at 60. I have a 3:26 to 1 & I run about 2500 RPM at 60 with 15” wheels.

I also depends on the power curve of your motor. That is what is so cool about an overdrive gear to me.

At 60 MPH in overdrive my car is going to be running about 1,800RPM where I only have about 360 ft/lbs of torque. A simple downshift to 3rd where I am at 1:1 and I jump to 2,500 RPM where I am close to 400 ft/lbs and the converter is about to its stall speed (2800).

Route666 · Dec 5, 2003

I was considering building the gearbox/diff combo so that with max engine rpm in top gear it only goes the vehicles maximum speed. By maximum I mean the aerodynamic maximum. When I do this, the cruise at 60 rpms are at about 2500 or so, so should I not worry about out-gearing the car? ie should I gear the car to do 350km/h when it will only do 265 so that the cruising gear rpms are good?

I think cruising is more important, and I suppose obtaining the maximum vehicle speed in a lower, STRONGER gear is probably a better thing to do anyway.

What do you guys think?

RGS0907 · Dec 6, 2003

Route666 said:
I was considering building the gearbox/diff combo so that with max engine rpm in top gear it only goes the vehicles maximum speed. By maximum I mean the aerodynamic maximum. When I do this, the cruise at 60 rpms are at about 2500 or so, so should I not worry about out-gearing the car? ie should I gear the car to do 350km/h when it will only do 265 so that the cruising gear rpms are good?

I think cruising is more important, and I suppose obtaining the maximum vehicle speed in a lower, STRONGER gear is probably a better thing to do anyway.

What do you guys think?

I doubt that you will be able to get the 265 Kmh cruising speed on a 65-73. That's 165 Mph! What car/engine are you thinking about? Is it an automatic? Big blocks don't like to rev as much as small blocks, I like to keep mine below 3000 rpm unless I really floor it. If you gear it to max out at your top speed, you will need to go to a 4:11 or higher rear. That's not a great cruising gear. Why don't you put in an OD trans so that you can have the best of both worlds?

Route666 · Dec 6, 2003

I've done the aero and gear calculations. 69 fastback, coefficient of friction of 0.65, pretty crappy, so the top speed isn't all THAT high, with 500 horsies at the fw. With the power, I should be able to get 165mph, using a 5.0L DOHC modular motor with billet rods, and forged crank and pistons, and some slippery coatings on certain bits. I've been told by the engine builder that it will go "9000+ rpms". I was planning on a TKO II, hopefully a TKO, with the better overdrive. I won't build it to rev that high, I'll use a very streetable cam, so max power probably 6500 or a little higher, limited to 7500 or something. My goal is a car that sounds like an insane asylum at WOT. Anyways, I think cruising is more important than gearing to max speed, and it seems smarter to reach top speed in 4th, not OD, for strength reasons. It just seemed a bit silly to have a potential (max rpm) speed of like 350km/h or more, when the car won't actually go that fast due to wind resistance.

88 Fox GT · Dec 6, 2003

It can go that fast, if your engine makes enough power to overcome the air resistance. But, your whole car itself may not be build to handle that stress of those speeds.

Edbert · Dec 6, 2003

Route666 said:
Anyways, I think cruising is more important than gearing to max speed, and it seems smarter to reach top speed in 4th, not OD, for strength reasons. It just seemed a bit silly to have a potential (max rpm) speed of like 350km/h or more, when the car won't actually go that fast due to wind resistance.

Exactly! Top speed should never be considered in OD anyhow. What I was looking for is being NEAR redline in final 1:1 gear at the then of the 1/4 mile or about 120MPH. Overdrive is for cruising only, so you can do 65MPH on the highway without going deaf from RPMs or wearing out the motor and passengers

Route666 · Dec 6, 2003

Speed_Demon1965 said:
It can go that fast, if your engine makes enough power to overcome the air resistance. But, your whole car itself may not be build to handle that stress of those speeds.

Understood, and I doubt I would ever drive it that fast. Maybe 200 (km/h) just to say I've done it. Depends on how fast I can take it on circuit track I suppose. I'll be welding all the seams, using subframe connectors and modern suspension, and putting in a 6-point rollbar. I'm not building it to race, but would like to take it circuit and drag racing several times per year.

But yeh, thanks all for working out that question for me. DOH DOH DOH. I don't really know why I cared about gearing it to max speed rather than what its main usage will be.

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