Topic: Rear end gear choice??
in Forum: C3 Driveline Components
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Adam's - Thanks for mentioning the speedo, thats one detail I hadn't thought of.
Rick D - Whats it like driving with 4.11's at 60mph?? I've always heard about guys that had them but never knew anyone that did.
I should have mentioned in the original post, changing the trans isn't an option. If I'm going to do that I might as well change out the engine to an LS-5 and computer or at least an FI 383 stroker. I'm trying to stay somewhat original as its a nice mostly unrestored car.
Anyone out there with an M21 and 3.73 rear end?? I'd love to hear from ya!
Thanks all!!
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Lifetime MemberSHELBYVILLE, TN - USA
Joined: 7/5/2002
Posts: 3942
Vette(s): 1976 L48 auto
1978 L82 4speed
1994 LT1 6speed
1978 L82 4speed
1994 LT1 6speed
Dad's 76 has a 3.70 in it, and at around 60mph it turns right around 2800 rpm in 3rd gear(turbo 350)... my 78 has a 4.11 and in 4th gear 60 mph was about 3400... now with the Tremmec overdrive its aroun 2200-2300 rpm...
StingrayJim 
NCM Lifetime Member #1936
Stingray's '76���������������������������� StingrayJr's '78���������������������������Lil Red '94 
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Hipo69,,with the m22 and 4'11s im turning about 3200-3400 at 60 mph, I have a differant cam in her so she is just starting to hit her torque curve about there,its fun to drive around locally but I wouldnt want to strike out cross country with the 411s,good luck rd
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
[QUOTE=Hipo69]Ok, I know I'm new here but here is my question. I'm planning on having the rear end rebuilt this spring and I'm thinking about doing away with the original 3:36 gears. Here are the particulars:
1969 350/350 M21 4spd. Convertible (Convertible=Heavy)
Rebuilt Original Engine:
Still has 11:1 compression
LT-1 intake
Holley 650 carb
Cam is slightly bigger than '70 LT-1 with solid lifters
MSD ignition and distributor
Headers
2.5" Dual exhaust with H pipe
Trans is original
Needless to say its built for the higher end and not for low end torque. It's a dog out of the hole but really screams over 3500rpm. It's all high octane fuel but then we don't drive these cars for economic reasons.
Any thoughts on gear ratios? I'm thinking about 3:55's or 3:73's. I'm not concerned about fuel economy so much but I'd like to be able to drive 70 mph without feeling like the engine is screaming. We live in the city and we don't take it on long trips (rarely over 60mph for more than 30 minutes) so we don't worry about overheating or the effects of living with high performance cars on long road trips.
Any input from those that have experimented with gear ratios would be appreciated. Especially those that have gone from 3:36 to 3:55 gears. Will I even notice a difference? Or will I have to go to 3.73's (and their inherent disadvantages) to notice anything?
[/QUOTE]
cthulhu 2008-01-27 19:39:07
1969 350/350 M21 4spd. Convertible (Convertible=Heavy)
Rebuilt Original Engine:
Still has 11:1 compression
LT-1 intake
Holley 650 carb
Cam is slightly bigger than '70 LT-1 with solid lifters
MSD ignition and distributor
Headers
2.5" Dual exhaust with H pipe
Trans is original
Needless to say its built for the higher end and not for low end torque. It's a dog out of the hole but really screams over 3500rpm. It's all high octane fuel but then we don't drive these cars for economic reasons.
Any thoughts on gear ratios? I'm thinking about 3:55's or 3:73's. I'm not concerned about fuel economy so much but I'd like to be able to drive 70 mph without feeling like the engine is screaming. We live in the city and we don't take it on long trips (rarely over 60mph for more than 30 minutes) so we don't worry about overheating or the effects of living with high performance cars on long road trips.
Any input from those that have experimented with gear ratios would be appreciated. Especially those that have gone from 3:36 to 3:55 gears. Will I even notice a difference? Or will I have to go to 3.73's (and their inherent disadvantages) to notice anything?
[/QUOTE]
you would be better off leaving the gears 3.36 and putting in a tko-600
best of both both worlds
10:1 launch ratio and 1800 rpm at 70mph in OD
your m-21 has a 2.20:1 first gear (giving you 7.392:1) even 3.73 * 2.20 = 8.206:1
still not very impressive. you want to be between 9 and 11 (i prefer 9.5 to 10.5) even with a 4.11 * 2.20 = 9.042:1 youre barely in the sweet spot.
unless you plan to drop the clutch at 3500 rpm everytime that trans isnt going to give you decent performance off the line without going to a rear end gear that is going to require ear plugs to run at 55mph.
work this out mathematically and not anecdotally and youll see that your only option to have a drivable car with a top and bottom end with that m-21 is 3500 rpm clutch dumps and that isnt much of an option.
I have a truck with 3.73 and no OD. it sucks... it can pull a house (it has pulled over a few garages) off its foundation but driving it the 8 miles or so to get to lowes is like riding in a dump truck. Loud as hell, sucks gas like a vacuum.
I have explained this quite a few times in great detail. search for my posts and the words gear ratio and you'll find some highly detailed mathematical analysis.
in Forum: C3 Driveline Components
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