Engine/gearbox/final drive suggestions
Thread Starter
Intermediate
Joined: Nov 2016
Posts: 26
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Engine/gearbox/final drive suggestions
So I’ve a got 1976 l48 350 with a 4 speed manual box. Engine is mostly standard, it’s got headers/intake and carb. The engine is a bit tired. So I’m thinking of getting a Chevrolet performace 350/290 crate. I use my car on the highway quite a bit so want to be able to cruise at 70/80 at the lowest rpm I reasonably can. I’m guessing I have a Borg Warner super t10. I don’t know what diff/final drive I have. From the research I’ve done would a 3.08 give me what I’m after or would it be a dog in first gear? At the moment it revs very high at 70. Or do I need to consider changing the gearbox. I do quite like the 4 speed for the old school aspect and would rather not change my box for cost purposes. What’s my best options?
Le Mans Master
Joined: Mar 2001
Posts: 6,240
Likes: 1,013
From: Watkinsville, GA and Glen Cove, NY
A T5 would be a good swap for you and you could probably keep your rear gears. There is a recent thread here somewhere detailing the swap. Probably your most cost effective solution to get manageable cruising rpms.
Here you go.
https://www.corvetteforum.com/forums...d-upgrade.html
Here you go.
https://www.corvetteforum.com/forums...d-upgrade.html
Last edited by 69ttop502; Dec 13, 2017 at 06:36 PM.
Burning Brakes
Joined: Oct 2012
Posts: 1,152
Likes: 252
There is a code stamped on the flange bottom of the rear telling what ratio you have. Could be 3.08 (OA), 3.36 (LR), 3.55 (OB), or 3.70 (OC).
A 3.08 kills acceleration from a standing start but would get you the lowest highway revs.
Since you have a stick the acceleration might not be affected as bad as an auto.
A 5-speed swap gets best of both worlds but there are cost and fabrication impacts.
A 3.08 kills acceleration from a standing start but would get you the lowest highway revs.
Since you have a stick the acceleration might not be affected as bad as an auto.
A 5-speed swap gets best of both worlds but there are cost and fabrication impacts.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
My vote is for putting in a 5-speed.
Knowing that you drive a lot on the interstate...and plan on putting in a new engine...I would do what I can to get the RPM's down at highway cruising speeds to save wear and tear on my new engine.
If keeping it a four speed because that is your thing. Then I guess 'it is what it is'...but your are willing to put a new engine in it.
I have installed the TREMECS and also put 700R4 automatics in and so far...not one customer has regretted it.
IF you plan on doing what you are doing (3.08 gear and such)....you might want to make sure that you calculate for the correct speedometer gear that goes on your speedometer fitting so your speedometer reads correctly...if that is important to you.
DUB
Knowing that you drive a lot on the interstate...and plan on putting in a new engine...I would do what I can to get the RPM's down at highway cruising speeds to save wear and tear on my new engine.
If keeping it a four speed because that is your thing. Then I guess 'it is what it is'...but your are willing to put a new engine in it.
I have installed the TREMECS and also put 700R4 automatics in and so far...not one customer has regretted it.
IF you plan on doing what you are doing (3.08 gear and such)....you might want to make sure that you calculate for the correct speedometer gear that goes on your speedometer fitting so your speedometer reads correctly...if that is important to you.
DUB
Thread Starter
Intermediate
Joined: Nov 2016
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It looks like I might have a 3.08 diff in it? Which seems odd that I’m not getting low revs at highway speeds. How do I find out what my box is? I’m guessing my car was originally an auto because it has a bolt in crossmember and has been cut out in the tunnel. Didn’t even know that when I bought it 😬
Burning Brakes
Joined: Oct 2012
Posts: 1,152
Likes: 252
It looks like I might have a 3.08 diff in it? Which seems odd that I’m not getting low revs at highway speeds. How do I find out what my box is? I’m guessing my car was originally an auto because it has a bolt in crossmember and has been cut out in the tunnel. Didn’t even know that when I bought it ��
Last edited by Dynra Rockets; Dec 14, 2017 at 06:27 AM.
Le Mans Master
Joined: Oct 2007
Posts: 7,453
Likes: 976
From: RI, Now Franklin/Nashville TN
Just for a reference starting point, the 76 base engine L-48 came either with a wide ratio 4 speed with standard 3.36 gears or the optional gear of 3.08. OR the 76 L-48 automatic came only with 3.08 gears from the factory.
Melting Slicks
Joined: Dec 2009
Posts: 3,233
Likes: 657
From: Thunder Bay
Clearly your simplest solution is to swap in a Tremec 5 speed transmission. There are several models with different gear ratios available. When calculating an appropriate trans/rear end ratio, multiply the first gear ratio by the rear end ratio. You should come out with a total final first gear drive ratio of about 10-11:1 That will give you a driveable first gear.
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Thread Starter
Intermediate
Joined: Nov 2016
Posts: 26
Likes: 1
Ok thanks guys, I would consider the wct5 gearbox. I’ve done a bit of searching on them, are they strong enough for a 350 ft/lb motor? How do I workout the tyre turn to driveshaft turn to find out what final drive I have?
Burning Brakes
Joined: Oct 2012
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Racer
Joined: Oct 2001
Posts: 309
Likes: 12
From: Pleasant Prairie WI
The easiest solution would be change the ST-10 you have to a 2.88 1st ST-10, and the rear end to a 3.08...
Everything un-bolts and bolts back together, no modifications...
Hope this helps... Eric
Everything un-bolts and bolts back together, no modifications...
Hope this helps... Eric
Race Director
Joined: Oct 2008
Posts: 19,220
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From: Long Island
Use your phone for the speed and record the RPM at 60. Go on line and look up RPM vs speed and it will give you the gear ratio. Figure rear tires are 26.75".
I would go with a 5 speed kit and you will have the best of both worlds.
Quick off the line and cruse at low RPM.
With a 3:55 rear I cruse at approximately 2000 at 80. Silver Sport 5 speed kit.
I would go with a 5 speed kit and you will have the best of both worlds.
Quick off the line and cruse at low RPM.
With a 3:55 rear I cruse at approximately 2000 at 80. Silver Sport 5 speed kit.
Last edited by BLUE1972; Dec 14, 2017 at 07:01 PM.
Thread Starter
Intermediate
Joined: Nov 2016
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So I jacked the car up and did the wheel turn to drive shaft calculation. In his video he turns the hub twice. If I do mine twice I get about 6.5 turns! But with one rotation of the wheel I get 3 and tiny bit, you can see in the markings in the picture. I think I’m gonna have to do a rpm at 60mph test because I can’t understand why I’m getting high revs if I’ve got a 3.08 rear.
Burning Brakes
Joined: Oct 2012
Posts: 1,152
Likes: 252
So I jacked the car up and did the wheel turn to drive shaft calculation. In his video he turns the hub twice. If I do mine twice I get about 6.5 turns! But with one rotation of the wheel I get 3 and tiny bit, you can see in the markings in the picture. I think I’m gonna have to do a rpm at 60mph test because I can’t understand why I’m getting high revs if I’ve got a 3.08 rear.
Le Mans Master
Joined: Oct 2007
Posts: 7,453
Likes: 976
From: RI, Now Franklin/Nashville TN
Thread Starter
Intermediate
Joined: Nov 2016
Posts: 26
Likes: 1
I’ve done a speed to rpm test. So I guessing my tach could be out a bit. My tyres are about 26.75” at 60mph I’m at 2750rpm , 65mph I’m at 3000rpm 80 I’m at 3700. Is there any 4 speed bolt in box that could give me better highway speed with my 3.08 rear?