Gear Change - Your Thoughts?
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From: Brownsville Texas
Gear Change - Your Thoughts?
I have an 86 that I bought with a blown engine. Instead of replacing it with an L98, we replaced it with a regular short block 350. At the time I didn't know any better. So now I'm looking for a more "SOP" feel and they tell me the best way short of replacing the engine is to change the gears in the differential. It's an automatic and I currently have a GU2 RPO code which would be a 2.73 and would like to put in a 3.73. I know nothing about this but I'm told if I don't care about losing gas mileage, it will work just fine. I was originally just thinking of buying a rebuilt 3.73 differential from Eckler's but that's kind of expensive so now I'm just thinking about buying a good set of ring and pinion gears. I was wondering if anyone has any thoughts on this as to whether it would be a wise decision or not. Thanks. 
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St. Jude Donor '09
There seems to be a consensus here that the 3.07 is the best for the street. I changed mine from 2.59 to 3.07. Works for me but I have a stock L98.
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The cheapest way to do it, would be buy another center section with a 3.07 or better in it. Then swap them out. (Put a "WTB" post in the C4 parts section)You may need to have the chip in the computer re-tuned so the speedo reads correctly, also.
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From: Brownsville Texas
Thanks for the input. Is there that big a difference between a 2.73 and a 3.07? One thing I am concerned about if I put in a 3.73 is highway RPM. When I replaced the engine, all we did is buy a 350 short block. Everything else is the way it was originally including the tuned port. I just could have used some extra horses. Right now this is my approx. engine RPM with a 2.73 ... at least that's what the tach is showing: 55mph/1300rpm ... 60mph/1500rpm ... 65mph/1700rpm ... 70mph/1800rpm ... 75mph/1900rpm ... 80mph/2000-2100rpm ...
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Thanks for the input. Is there that big a difference between a 2.73 and a 3.07? One thing I am concerned about if I put in a 3.73 is highway RPM. When I replaced the engine, all we did is buy a 350 short block. Everything else is the way it was originally including the tuned port. I just could have used some extra horses. Right now this is my approx. engine RPM with a 2.73 ... at least that's what the tach is showing: 55mph/1300rpm ... 60mph/1500rpm ... 65mph/1700rpm ... 70mph/1800rpm ... 75mph/1900rpm ... 80mph/2000-2100rpm ...
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Keeping the D36, I wouldn't go higher than the 3:07. If you want a beefier rearend and plan on abusing it, get a D44 with the 3:54. Sure she'll suck down the fuel, but it'll be a blast to drive. I wouldn't put anything higher than a 3:54 behind an automatic that isn't trailered. Doing the D44 conversion isn't cheap though. It's certainly an investment (that you won't get a return on), but it'll be tough to break.
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From: Brownsville Texas
Keeping the D36, I wouldn't go higher than the 3:07. If you want a beefier rearend and plan on abusing it, get a D44 with the 3:54. Sure she'll suck down the fuel, but it'll be a blast to drive. I wouldn't put anything higher than a 3:54 behind an automatic that isn't trailered. Doing the D44 conversion isn't cheap though. It's certainly an investment (that you won't get a return on), but it'll be tough to break.
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Thanks for the input. Is there that big a difference between a 2.73 and a 3.07? One thing I am concerned about if I put in a 3.73 is highway RPM. When I replaced the engine, all we did is buy a 350 short block. Everything else is the way it was originally including the tuned port. I just could have used some extra horses. Right now this is my approx. engine RPM with a 2.73 ... at least that's what the tach is showing: 55mph/1300rpm ... 60mph/1500rpm ... 65mph/1700rpm ... 70mph/1800rpm ... 75mph/1900rpm ... 80mph/2000-2100rpm ...
Multiply the rpm with the 2.73 gear by 1.366 and you'll have your new rpm. AND, by the same token, the rear wheel torque will increase (with the 3.73 gear) by the same amount.
The engine rpm increase of approx 37% for a given speed would result to a fuel consumption increase, nearly in step with the rpm. And, you'd be topped out at around 129 mph at 4500 rpm (with the 3.73 gear).
The rw torque would jump from approx 350 to 478. From experience with 380 rw torque, grip is difficult even with 315x35s in low gear. So, there's that. You'd have to get it to the ground, or what's the point?
I'm thinking that if anywhere, that motor/gear combo might shine in a auto-X course, if nowhere else. Otherwise, cross-country cruising, or drag racing wouldn't be it's "thing". However, on "The Dragon's Tail" in the Smokey Mts. of N Carolina...? It might just be a hoot to drive!
I'm intrigued by the idea, I admit. But, my gut says normal drivability would suffer - not very practical. But, it certainly would operate well outside the normal window, and in a special application, it might be a lot of fun. You buy the tires and gas, and I'll find a niche for it!!
P.
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From: Macedon (near Rochester) NY
If you're using the original heads and intake, it's still pretty much an l98. Do you know anything about the cam in your replacement block? If it's horribly mismatched with everything else, you'd be better off changing that.
Racer
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Well someone has to vote for the 3.73 gears so I guess its up to me. I have 3.73 gears in my '92 with the 700r4. I now have a LS3 Hot Cam crate engine in it (GMPP LS376/480 Hot Cam) but ran the same gears when it just had the LT1. At 85mph I'm at approx 2900 rpms. However, I still get around 18 mpg overall. With the cam I'm glad we stayed with the 3.73 gears since the engine doesn't run smooth until at least 2500 rpm. So now the engine doesn't lope once I'm doing 75 mph on the freeway. If I had changed to 3.08 gears it would lope even at 85 mph. If you want real fun but still get reasonable mpg, go with the 3.73 gears. Just my opinion.
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Racer
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I'm not sure if it is "apples and oranges". Originally my car had 2.59 gears. The OP states that he currently has 2.73 gears, so why wouldn't the numbers I'm getting with my 3.73 gears be fairly similar to what the OP would get going from his 2.73 gears to 3.73 gears? Please educate me.
Le Mans Master
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St. Jude Donor '03 thru '26
Lts & L98s have different characteristics. You have a cammed LT motor, short runners more RPMs. He's got an L98 w/ long tubes, more torque oriented. Totally different animals.......this is a case in point of why the combos should match to maximize potential. They're both good motors, just in different ways.