Topic: changing rearends
in Forum: C3 Driveline Components
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Lifetime MemberPERRYSBURG, OH - USA
Joined: 4/23/2004
Posts: 317
Vette(s): 1975 T-Tops,Lite Custom work,Many engine mod. 700R4 trans. body is shaved,17" wheels bf goodrich g-force tires,heavy sway bars front&rear rack&pinion steering,550 slolom springs with gas shocks.
I have to change the rear end in my 75. It is presently a 308 gear tied to a 700R4. Well my question is a 370 to much? I was just told by a supplier that my car would be sceeming on the highway, and that I shouldn't go any bigger than a 355 gear? Is this good advise or is it because all he had was 355 rearends to sell. Any help anybody????
Also on the other hand I don't want to give up the drivability of the car either. Any 370 gearheads out there? Thanks,George
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With the overdrive it would not be bad on the highway, but that trans has a 3.08:1 first gear and you would be thru first gear in a heartbeat. I have 3.55:1 gears in mine with the 700R4 and it has plenty of snap off the line and at 70mph it's reving at about 2200rpm.
I think you're better off with the 3.55 gears, just my .02 cents.
Dave
Lifetime Member #116
Dave's '82 
Lifetime MemberPERRYSBURG, OH - USA
Joined: 4/23/2004
Posts: 317
Vette(s): 1975 T-Tops,Lite Custom work,Many engine mod. 700R4 trans. body is shaved,17" wheels bf goodrich g-force tires,heavy sway bars front&rear rack&pinion steering,550 slolom springs with gas shocks.
Ya my 308 gear dosn't have much of a first gear with the 700R4. But anything would be better than what I got for take off. With 400+ hp I can hardly turn the tires. But it cruises at 1500rpm at 60mph. Thanks Dave for the input. George
Lifetime MemberHot Springs, AR - USA
Joined: 4/24/2004
Posts: 3236
Vette(s): 69 Conv #'s match 427, TKO-600/.64, 3.36HD-Posi, HT, T/T, PS, PB, PW, SP, Leather, Comp XE264HR & Roller Rockers & Lifters, Air-Gap RPM intake, Holley St Av 770 VS, MSD 6AL+Dist+Blaster SS, K&N, Jet-Hot Hooker Side-Pipes, Steeroids, Al Rad, Spal Fans
the ideal first gear ratio is 10.0 but between 9-11 are good streetable gears. Under 9 you dont get any good romps on start over 11 and you just smoke tires.
3.06 * 3.08 = 9.42 so you really arent that far off with your current differential.
3.06 * 3.36 = 10.28
3.06 * 3.55 = 10.86 (the highest I would go)
3.06 * 3.70 = 11.322
as for why you cant turn the tires with 400hp may have more to do with your engine setup than gearing. as even a relatively mild engine can turn tires with a 9.42 final drive.
You may have a high horsepower enigne that doesnt get its peak torque until very late.
Torque is often a much better measure of a streeet car and unless you are driving on the audobon or drag racing torque is far more important.
lets take the three small block engines for 1970.
350/300 380 ft lbs
350/350 380 ft lbs
350/370 380 ft lbs
The 300 horse gets it peak at a much lower rpm than the 370.
May people are so worried about that peak horsepower they overlook a much more usefull metric "area under torque curve"
What type of cam and heads do you have?
Lifetime MemberPERRYSBURG, OH - USA
Joined: 4/23/2004
Posts: 317
Vette(s): 1975 T-Tops,Lite Custom work,Many engine mod. 700R4 trans. body is shaved,17" wheels bf goodrich g-force tires,heavy sway bars front&rear rack&pinion steering,550 slolom springs with gas shocks.
Thanks for the info. That realy breaks things down. As for my lowend torque. My 350 has late mod.center bolt heads with 62cc comb.chambers,pocket ported,as well as matched to intake and headers. Comp-cam recomended a TPI grind for lowend torque with edelbrock intake. I have been scca racing this car for five years now and it performs well. Please don't get me wrong. I just want a little more in the lower end take off and yes I think the 355 will work well for what I want. Thank you very much for all of your help. This why C3vr is the best. Thanks again,George
Lifetime MemberApache Junction, AZ - USA
Joined: 10/15/2006
Posts: 285
Vette(s): 1982,150K,Renegade CFI,Hooker Side Pipes,85 fuel pump,Bilsteins,HD suspension, Aluminum radiator, SharkBar/w 5-Point Restraints. 3:73 gears coming soon.
I have a 3:73 rear end I will be installing in my 82 soon...I have been told the same thing - that it will be to much, especially on the freeway. If I don't like, I'll have the stock rear rebuilt with something lower. I've been told that there isn't much difference between 3:55 and 3:73??
Be sure to post your results and I will do the same.
Be sure to post your results and I will do the same.
Lifetime MemberHot Springs, AR - USA
Joined: 4/24/2004
Posts: 3236
Vette(s): 69 Conv #'s match 427, TKO-600/.64, 3.36HD-Posi, HT, T/T, PS, PB, PW, SP, Leather, Comp XE264HR & Roller Rockers & Lifters, Air-Gap RPM intake, Holley St Av 770 VS, MSD 6AL+Dist+Blaster SS, K&N, Jet-Hot Hooker Side-Pipes, Steeroids, Al Rad, Spal Fans
true the gap from 3.55 to 3.73 is not as large as 3.08 to 3.36 or from 3.36 to 3.55
there is a 3.42 rear end ratio that was never facotry for a corvette but would probably bee an even better ratio to use in this case.
Lifetime MemberPERRYSBURG, OH - USA
Joined: 4/23/2004
Posts: 317
Vette(s): 1975 T-Tops,Lite Custom work,Many engine mod. 700R4 trans. body is shaved,17" wheels bf goodrich g-force tires,heavy sway bars front&rear rack&pinion steering,550 slolom springs with gas shocks.
Thanks again!!! alot of good info as well as ideas. The 355 seems to be the pick.....I'll let everone know how it works out....it's going to be a while untill the install but I'll post asap.
See yas and keep posting!!!!!!!!!!!
See yas and keep posting!!!!!!!!!!!Former Member
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Mounds View, MN - USA
Joined: 5/24/2007
Posts: 1031
Vette(s): 70 LT1 coupe, 69 350 HP coupe, 69 390HP 427 coupe, 71 LS5 convert, 85 coupe, 93 coupe
That '82 is pretty underpowered, so the 3.73s will really help off the line, but you will be turning about 2400 on the highway. That might be too high a price to pay.
Larry
Larry
in Forum: C3 Driveline Components
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