C3 BBC Gearbox's
Thread Starter
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
C3 BBC Gearbox's
Thinking long term about replacement of M21 on my 68' BB Vette. With a 'stroker' engine and 3.08 rear end the stock M21 ratio's work really well for me. Had the engine built with plenty of low end torque so some the 5 and 6 speed transmissions all seem to have an ultra low 1st gear which (for my car) would result in too much wheelspin and wasted motion. Ideal scenario would be something like a 5 speed with first 3 ratio's same as M21 but with perhaps a 50% to 70% overdrive 5th gear. Need to have TQ rating in excess of 600 ft/lbs. Considered a Gearvendors add-on overdrive, but the cost is close to a new gearbox - a 60's Hone unit would be a consideration but used units seem to be close to the new cost of a GV unit.
Pro
Joined: May 2011
Posts: 722
Likes: 58
From: Karlsruhe (Germany)
Have a TKO 600 with a zz502 and 3.70 rear end.
Don´t think the first gear is too low.
When your engine has more power, why don´t you just launch in 2nd?
Previously I had the M21 too, and the first gear was always to long to drive slowly with the car. Well this doesn´t matter if you use it only on the track and not in normal traffic.
Don´t think the first gear is too low.
When your engine has more power, why don´t you just launch in 2nd?
Previously I had the M21 too, and the first gear was always to long to drive slowly with the car. Well this doesn´t matter if you use it only on the track and not in normal traffic.
Drifting
Joined: Mar 2011
Posts: 1,347
Likes: 121
From: Kernersville NC
I think you would be unhappy with a 3.08 and a TKO .64 OD. I have the .64 and 3.55 rear gear with a healthy big block. It runs 50 mph just above idle rpm. Check the ratio/speed/rpm calculator on American Powertrain site to see what you would be turning with your setup. You may opt for the .80 OD with that rear gear.
Thread Starter
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
I think you would be unhappy with a 3.08 and a TKO .64 OD. I have the .64 and 3.55 rear gear with a healthy big block. It runs 50 mph just above idle rpm. Check the ratio/speed/rpm calculator on American Powertrain site to see what you would be turning with your setup. You may opt for the .80 OD with that rear gear.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
Thread Starter
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
Good point - I have been running M21 with this combo for a few years and it has held together so far (although I am careful with it - so no 6000 rpm/clutch dropping drag starts!)
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
When I had the engine and trans in my signature in my 80 Vette with 3.07 gears, it ran best and got it's best mileage above 80 MPH.
Thread Starter
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
If your engine is that beefed up and has much cam in it, 1500/1800 RPM will be way below the efficient cruise RPM. 2500 RPM is not that high a cruise RPM for a large cam. What are the specs on this engine and the cam?
When I had the engine and trans in my signature in my 80 Vette with 3.07 gears, it ran best and got it's best mileage above 80 MPH.
When I had the engine and trans in my signature in my 80 Vette with 3.07 gears, it ran best and got it's best mileage above 80 MPH.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Leave it alone or go Richmond 5 speed with 1 to 1 5th gear. If you go with overdrive you will be below the operating range of your cam at highway speeds.
Thread Starter
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
Ordinarily (and with my own past experience with other cars) I would agree with you - however with the dual plane inlet and relatively small oval port heads (for the engine capacity) port velocity I believe is quite high allowing good clean operation from 1500 rpm upwards.
Le Mans Master
Joined: Mar 2001
Posts: 6,240
Likes: 1,013
From: Watkinsville, GA and Glen Cove, NY
I agree! Nothing worse than trying to run "below the cam" with all the bucking and jacking going on. I mean 1500-1800 is where the OEM 6 speeds run with way less cam. I like the the idea as well of tall rear gears and a 1 to 1 fifth. More efficient than overdrive. Nice car by the way.
Instructor
Joined: Aug 2003
Posts: 167
Likes: 2
From: BLACKSBURG VA
C3 BBC gearbox
I have a 73 496 full roller with a rebuilt Muncie backed by a Gear Vendors Overdrive and 3:36 rear. Motor dynoed 632 hp. It runs like a rocket and the drive line is extremely solid. Overdrive in 2nd, 3rd, & 4th is awesome. I will probably switch to 3:55 or 3:73 sometime in the future to help take off with this motor.
I love the GVO and am doing it again on my new 73 BB project.
I love the GVO and am doing it again on my new 73 BB project.