RPM at 70 MPH

[GAZRNR]

I trying to pick the right combination of parts at the start of my project to minimize my "redos". A hard lesson learned searching for HP in the 6.

 

Can anyone tell me what their RPM is at 70 mph running a carberated 350 300-350hp and a 5 speed out a camaro? I'm tring to match the trans/rear gear for life on the road and strong off the line properties. I know there is a program to project the RPM but I want real life confirmation and your comments on drivability.

 

3.54

3.70

3.90

4.11

 

Is anyone who has run the above combinations with different rear end ratios?

 

Thanks

[fl327]

you would have to match your cam, intake, manifold, gears, diff gears to get the max potential

out of your project, go 3.90 jor 4.11 with a lumpy cam that makes high end power in order

to get it out the gate, i am not sure what your rpm would be with those gears, but i do know that my friend who has a mustang with 3.73's and a t5 does 80mph at 2900 or so, and they are good range for street and freeway.

[Guest greimann]

I'm running a T5 from a '92 Camaro with a 3.7 rear gear and 225-60R15 tires. The RPM at 70 is about 2100. My motor is a Chev 350 with about 350 HP. I think the ratios are about right. The first 4 gears do most of the work and 5th gives you a great freeway cruise. For grocery getting and putting around town I find I can save a few shifts and start in second gear. For raw acceleration 1st gets you out of the hole pretty quick. I usually don't grab 5th unless I'm doing 60 mph or better.

 

Just as a refresher, the ratios in a '92 T5 are : 2.95, 1.94, 1.37. 1.0, 0.62.

 

The programs that calculate RPM at speed are pretty accurate. My favorite that does that plus much much more is Cartest2000. This program is more of a performance simulator that allows you to predict acceleration times base on parameters you enter about your car. To answer your question about which ratio is best for your situation, you could enter all of the different gear ratios and run the simulator to find out which gives you the best 1/4 mile or 0-60 or top speed. It is extremely vesatile and fun. You can even race other cars in the database, either on a road course or on a drag strip. Look for Cartest2000 at:

 

http://home.earthlink.net/~patglenn/

 

If you want I can run the sim for you if you give me:

 

engine displacement

compression ratio

max hp at rpm

max torque at rpm

weight

gear ratios

tire size

 

Some of those paramaters you can guess at if you don't know as long as you keep them the same while changing just one variable to determine the performance difference.

[GAZRNR]

Thanks for the the feed back

 

Greimann: My plan is to use a GM 350 crate motor with 10:1 compression, 355 HP, 405 torque, weight of 78 Z coupe @ 3200 lbs, T5 trans, 225/50/16 tire/wheel size

 

I have an open R200 with a 3:90 ratio that is in need of a rebuild. With my plans for the above engine trans combination I only want to build 1 rearend. At the cost of a limited slip from MSA at $780, plus ring & pinion at @$450, add another $200 for bearings & seals and then labor to get is assembled: this is money I want to spend once.

 

Knowing what the RPM at highway speed will be will help me pick the correct combination.

 

If you could run numbers for 3:54, 3:70, 3:90 & 4:11 and let me know what you get that would be great.

 

Thanks

[Guest greimann]

Ok, here are the results from Cartest 2000. These numbers are based on the criteria given above with the assumption of a 6000 rpm redline and the ZZ4 Hp 355@5250 and Tq 405@3500. Forgive the lame tabulation.

 

Rear....0-60....1/4....RPM@70 MPH

 

3.54....4.6....13.29....2055

3.7.....4.59...13.29....2100

3.9.....5.18...13.28....2250

4.11....5.12...13.28....2420

 

Note that the 3.9 and 4.11 rear gear requires a shift to 3rd in the 0-60 run, hence the longer time. The program also factors in traction limitations. The 1/4 mile times show this. A one legged rear end cannot go any faster than low 13's without slicks or other traction help.