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Old 05-30-2008, 11:17 PM
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Transmission change

Just looking for input or knowledge. My engine is great for a street car. A built 350-425 hp sbc. But I am looking to improve its take off. Its running a 350 turbo trans, and a 9 inch 356 geared rear. I thought about going to a 411 . Then again, I was told by a neighbor that I should maybe look into leaving the rear alone and go with a 200R or a 700r transmission. He said going that going with a overdrive tranmission. It would give the feeling of a lower gear rear,but also when shifted to overdrive,it would be a lot higher
high gear for the highway. Anyone know what kind of diffrence I can expect. Just incase I am running 29" tires. Which would give a better or noticeable change.What would be better ?? Im not much into gear ratio numbers. Like when making the transmission change over how much of a lower gear would it have? Thanks

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Old 05-31-2008, 05:52 AM
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The 4:11 gears will give the most notable difference but after installing those ,you'll probably want the overdrive so you can cruise down the highway at a decent speed at lower revs.
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Old 05-31-2008, 07:46 AM
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What those guys miss on is that most OD transmissions have the same gearing in first through 3 as the normal transmission and the OD is added to the transmission..All changing to an OD tranny does is add the overdrive..for better hole shots one needs to change the rear gears..

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Old 05-31-2008, 08:38 AM
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Quote:
Originally Posted by OneMoreTime
What those guys miss on is that most OD transmissions have the same gearing in first through 3 as the normal transmission and the OD is added to the transmission..All changing to an OD tranny does is add the overdrive..for better hole shots one needs to change the rear gears..

Sam
Actually, with GM transmissions that's only true for the 4L80E (which uses TH400 gear sets for first through third).

Your TH350 has a 2.52:1 first gear.
The 200-4R has a 2.74:1 first gear.
The 700R4 has a 3.06:1 first gear.

If you keep your 3.56:1 gears, the 200-4R gives you a first gear ratio equivalent to using 3.91:1 gears with your TH350. The 700R4 would give you a first gear equivalent to 4.30:1 gears with your current trans.

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Old 05-31-2008, 10:09 AM
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What does the 2.52:1 of the TH350 work out to with the 4:11 gears? That will show the difference of changing the tranny or changing the rear gears.
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Old 05-31-2008, 12:26 PM
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Well thanks for the info. I though there was a difference in the 700r first gear compared to the 350.
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Old 05-31-2008, 12:36 PM
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Quote:
Originally Posted by 123pugsy
What does the 2.52:1 of the TH350 work out to with the 4:11 gears? That will show the difference of changing the tranny or changing the rear gears.
I guess I don't really understand your question. It really doesn't matter if the gears are in the trans or the axle, it's the overall ratio that the wheels see that matters. Just multiply the two ratios.

With the TH350 and 4.11 gears, 2.52 x 4.11 = 10.36 overall first gear ratio to the wheels.

With the TH350 and the original 3.56 gears you get 2.52 x 3.56 = 8.97 overall in first gear.

With the original gears and a 200-4R you get 2.74 x 3.56 = 9.75 in first. Divide 9.75 by the 2.52 in the TH350 and you see that this overall ratio is equivalent to using 3.91 gears with the TH350 (actually, the numbers come out to 3.87, but I rounded up to the nearest common gear ratio). Similarly, with the original gears and a 700R4 you get 3.06 x 3.56 = 10.89 in first, which is about equivalent to using 4.30 gears with the TH350.

If you're trying to optimize quarter mile times, then what also matters is the ratio drop at each shift and how that matches with the engine's torque curve, but the original question was about off-the-line feel. And yes, I'm ignoring things like friction losses in the trans vs. the axle. Those are second-order effects as far as this thread is concerned.
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Old 05-31-2008, 03:59 PM
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Looks like the 700r4 is the best way to go according to the numbers.
Thanks for the info. I just finished a 350 to 700r4 swap and my truck does seem to pull better.

Now if you used the 4:11's and the 700r4..........
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Old 05-31-2008, 07:29 PM
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Quote:
Originally Posted by 123pugsy
Looks like the 700r4 is the best way to go according to the numbers.
Thanks for the info. I just finished a 350 to 700r4 swap and my truck does seem to pull better.

Now if you used the 4:11's and the 700r4..........
Yup, the 700R4 has a great first gear ratio. What some people don't like is the big drop between first and second. If you have an engine with a fairly flat torque curve, this probably won't be an issue.
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