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Differential Differences


Guest dapy
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I've learned a lot about engines and transmissions in my short old car dalliance...much of it here.

Next lesson is to help understand what goes on at the rear end. How do the ratios affect how the car goes. Is the final drive all we need to know? Are there different numbers for manual and automatic transmissions? Why would one change what came from the factory? If this gets too scientific and no one else cares, I'll go elsewhere.

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I'll try to answer this and keep it as short as possible. Question 1.) The lower the rear end gear, the quicker the engine will rev. Helps the low end performance, but hurts the top end performance. 2.& 3.) Most all final output drive for auto and manual trans. are 1-1, with the exception of OD. The trans gear ratios of 1st, 2nd, , etc. can be completely different from 1 trans to another. So your choice of rear gear ratios can get tricky. Rear gear ratio really depends on how the owner wants his car to react.

I used to drag race every weekend and I would change the gears depending if I was racing 1/8 mi. (5.13 ratio) or 1/4 mi. (4.11) .

Rear gear ratios effect everything, (fuel consumption, performance, drivability, engine life, etc.) Hope this helps.

Edited by bigdaddy
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Now I understand that the "differential" keeps back wheels from spinning in a turn. That I get.

The rest is a bit confusing. Depending on ratios through the gears (auto and manual) quicker pick up means higher revs in final drive (and poorer gas mileage)? Or slower pick up and less revs in final drive. And this can be changed somewhat depending on how my car will be used? OK, don't manufacturers try t optimize engine power and gearing to cover all driving. Do people mainly change "rear ends" in older cars?

When I investigate (old) car sale should I ask about this? And how do I evaluate the answer?

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My 1969 Avanti with the 327 Corvette engine has a 3.54 ratio. It's great in the quarter mile; but not so nice at 65mph. I'm considering changing it to a 3.31 or 3.07. The ratio is stamped on a tag on the differential. Another tag tells you if it has Twin Traction or not.

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  • 2 weeks later...

One of the ways to think about it too is that your final drive ratio dictates the RPM at which the motor turns at a given speed. This has RPM:Speed ratio has particular importance when laid over a graph of your torque and HP curves. If your engine makes all it's torque at high RPMs it will bog down when you punch it from a cruising speed if you have a numerically low rear end ratio (e.g. 2.87) where as if it were high numerically (e.g. 4.46) then your engine Is likely crushing in it's powerband and will feel like it has a lot more get up and go when you punch it. There is a fine line that one walks when building a streetcar to gain desirable performance, driveability, and practical fuel economy (OD transmissions help a lot with this).

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Just to give the reverse example if you have a big powerful torque monster 383 with a numerically low gear ratio (e.g. 2.87) then when you punch it your wheels will pretty much just spin as opposed to a numerically higher ratio (e.g. 3.31) where you have a lot more controllable conversion of power from the crank to the wheels.

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