Whats the top speed possible for my car with the 700r4 and 3.73's? What rpm will i run doing 60mph? Thanks

 

http://www.f-body.org/gears/

175 MPH in top gear w/ 3.73's at 6000rpm.

60 MPH in top gear is 2100rpm.

This is assuming a stock 245/50/16 tire size.

 

are you sure 175? These cars i hear top at 140mph top (maxed maxed) and with smaller ratio rear how can it go faster?

 

Take a look at that calculator I linked above, 4th gear w/ a 700R4 and 3.73 rear goes 175mph at 6000rpm. Thats quite high for a stock motor, if you were at 5000rpm that would be 146mph. And of course, these calculations are assuming you'd actually have the power to go that fast. My stock 350TPI IROC with the 3.42 rear end would go 150mph, I tested that. My 305 car might go that fast too, but I that tranny doesn't have the WOT 3-4 shift, so I can't get into 4th w/ my foot mashed.

 

IIRC, GM advertised the L69/3.73 combo at 138.

JamesC

 

I just LOVE it when people say stuff like that.

Looks great on paper; but in the REAL WORLD, a car's top speed is that speed at which the horsepower the car consumes in going at that speed, equals the horsepower output of the engine. Once you get to that point, the engine is "maxed out" so to speak, and cannot push the car any faster.

The HP requirements of a body travelling through a fluid or gas, once a certain point in its speed is reached (usually somewhere around 40-50 mph in a street car) is determined by its coefficient of drag, and from there, increases with the CUBE of the speed. In other words, to go twice as fast, you need 8 times as much power.

With these cars and their Cd, the HP limit of the engine starts being a factor at around 100ish mph. So let's say, just to put a rough number on it, a 6-cyl car with 120 HP or whatever those make, will go 110 mph.

Past that point, an otherwise identical car with 2 times as much power, which would be 240 HP (a car with a L98, say) will go 120 * the cube root of 2 as fast. That works out to 138 mph; assuming of course, that whatever the engine RPM at that point is, matches the point at which the engine makes its 240 HP (4400 RPM or whatever the peak HP RPM is for that motor). Any mismatch there, and the car won't get up to that speed.

So no, in the real world, gearing has VERY LITTLE effect, ultimately, on the car's "top speed", except to the extent that it permits the engine's RPM to reach its HP peak at the same time as the body reaches that same HP of drag. Meaning, it's entirely possible for a car with a HIGHER number gear such as 3.42 or 3.73, to have a higher "top speed" than THE SAME car, identical in every other way, except with 2.73 or 3.08 gears.

Very very few of these cars will go much faster than about 145 mph. The Firebirds will go a bit faster than the Camaros, because they have a lower Cd due to their hood line among other things (with their headlights down, anyway). On the Camaros, adding headlight covers makes a measurable Cd improvement, but of course in some cars like the L69, that blocks the engine's air intake, so it's counter-productive. But the TPI Camaros with their air intake, can be helped that way.

Note that this has NOTHING to do with safety; like, bump oversteer (making the car start fishtailing and become impossible to control), or tires exploding, or the brakes functioning, or anything like that.

 

I assumed the question was theoretical, as in, if I go from a 2.73 to 3.73 rear-end, will my top speed be reduced.

 

Again in theory the 2:73 gears will give you more top end then the 3:73 gears. This for identical cars.

 

If I was the kind that would bet on anything, I'd bet that you could take, let's say, that 138 mph L69 car with 3.73s; and put 2.73s in it; and the top speed would be LESS than with the 3.73s.

If the theory doesn't account for the observed facts, then it's time to throw it out and get a new theory.

We can talk about "theoretical" all we want; but if we do, it's probably a good idea to use ALL the theories (as well as including the known FACTS that the theories must operate within) rather than picking out the ONE theory we happen to like, and ignoring THE FACTS as well as all the others. Oversimplifications of that kind are not a good way to plan our lives.

 

Sounds good. So, v10viper04, whats the build on the motor in your car?

Lets looks at that with some numbers. Lets take a mild 350 car, and lets assume it makes 260hp @ 5000rpm, which is enough power to drive the car to 160mph with the correct gear ratio.

With a 2.73 rear-end and a 700r4, its possible that it would get to the top of third gear at 5000rpm, which would be 140mph. Shifting into 4th would drop you back down to 3500rpm, and its unlikely it would make enough power to pull any mph further, its more likely start to lose speed.

Now, change out to a 3.73 rear end, top gear would hit 102mph at 5000rpm, going down to 3500rpm in 4th gear, that would continue to pull, and run 4th gear up to 5000rpm at 146mph. It might make enough power over its peak to pull beyond that - it might make it as far a 150mph before the power falls off to much and prevents further acceleration.

However, the best rearend choice would be a 3.42, 5000rpm in 3rd would go to 112mph, then going into 4th the car would continue to pull to 5000rpm, which would be 159mph. Going with a lower rear end wouldn't make the car any faster because it wouldn't keep the car in the power, going with a higher rear end would cause the car to reach its peak power too soon.

So, I get what you guys are saying about theoretical vs. the real world - I just went with the short answer, obviously if you're building a car for top speed your gear selection is critical. I assumed the original question was posed as theory, when the asker rebutted with are you sure, I clarified, of course you've gotta have the power, I guess I assumed that as obvious.

 

170mph?? Right. Maybe if you could lock up the converter.

I cant get above 115mph because the engine is hitting the 5800 redline. Of course that's because of the 3000rpm stall.

 

Yea, and you're still in third gear... 700R4's don't go into 4th at WOT unless they have been modded to do so. I can do about 110mph with my 2500rpm stall at 5500rpm in third, so 115mph at 5800rpm sound right on the money.

 

I used to let up the throttle just enough so the transmission will hit 4th gear. Got up to around 140 in my iroc that way. (long time ago and a decently built HSR combo) Don't know yet what my new combo is capable of.

 

well my new gear choice has been made for me

 

my 700r4 goes into 4th at wot...but i put in in OD not D

 

I guess that would explain why I was doing 112 in third gear...

Mike