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Gearing: Diff witchcraft

Old 03-29-14, 10:56 AM
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Gearing: Diff witchcraft

Continuing a discussion from another thread....

Originally Posted by DaleClark View Post
changing the rear gear will reduce the effort needed to move the car forward in any gear at any RPM all the time.

This makes driving just far more responsive.
Why is the above true? For example, in one instance say you have a 1:1 ratio using a shorter gear in the tranny but a taller rear end ratio. In a second example, same 1:1 ratio using a taller gear in the box but a shorter ratio in the pumpkin. How does this reduce the effort needed to move the car? Is this something we can measure via dyno? Can someone explain this to me?
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Old 03-29-14, 12:11 PM
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You're over complicating it. Think only of the differential first. Our ratio of 4.1:1 this means the pinion will spin 4.1 times to the ring gears 1 time. That's all it means. Changing the ratio to 4.3 or 4.44 which is popular on fds means the pinion will need to spin 4.3 times to the rings 1 time. This increases acceleration in any gear, not just your 1 to 1 gear.

In the other thread I recommended you go to Howstuffworks.com cool website
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Old 03-29-14, 01:20 PM
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I think I understand how it works just fine. I read the 8 pages anyway just to freshen up.

My point is that I think changing the diff ratio is EASIER but not magical vs changing every gear in the tranny.

Basically, if your ratio to engine RPM and tire RPM is the same, regardless of whether you achieved this via the diff or by changing all of the ratios in your gear box; the end result is the same.....correct?
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Old 03-29-14, 02:08 PM
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Should be. This isn't something that's commonly done, though, since it's just so much more expensive and tricky to do.

In the car world in general (esp. Mustangs, old muscle cars) changing diff gears is a VERY common upgrade.

It's all about mechanical advantage to the wheels. Like riding a 10 speed bike, starting in 1st gets you going easily and quickly, but you'll soon be maxed out on pedal speed and not going fast. Starting in 10th gear is a ton of work.

It's just all about changing the mechanical advantage between the engine and the rear wheels.

Dale
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Old 03-29-14, 03:52 PM
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Originally Posted by ItalynStylion View Post
I think I understand how it works just fine. I read the 8 pages anyway just to freshen up.

My point is that I think changing the diff ratio is EASIER but not magical vs changing every gear in the tranny.

Basically, if your ratio to engine RPM and tire RPM is the same, regardless of whether you achieved this via the diff or by changing all of the ratios in your gear box; the end result is the same.....correct?
Of course changing the rear end ratio is easier, as stated multiple times in this thread and the other thread. Most people don't give this as much thought as you because it costs almost as much as a Fd itself, Or at least half of one anyhow. But you do have it correct now.
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Old 03-29-14, 05:23 PM
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Let me simplify it.
Torque Multiplication.
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Old 03-29-14, 05:30 PM
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Sounds like my original understanding was indeed correct. Doesn't matter where the gearing is done, the end result is the same. As an avid mountain biker I fully understand the torque and gearing correlation. The only reason I asked was because the original statement (one quoted in the first post) made it sound like there was some sort of advantage (over cost and pita factor) to doing it at the diff vs the tranny.

I guess the biggest benefit by changing the diff ratio would be that it changes how close the spacing of the gears would be too. Making it almost like having a close ratio 6speed vs our 5 speed spacing; taking advantage of a more "peaky" powerband.
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Old 03-29-14, 11:07 PM
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The original Dale Clark statement only applies if like going from 4.1 to 4.3.
Not from 4.1 to 3.909. Applies more for close to stock engine power.

Our cars when modified have too much torque for the short lower gears.
More torque it just spends in the lower gears.

Being at over 400 rwhp, I would need large racing rear rubber for traction with even stock gears.
But mine is a street car running 255s, thus I went .806 5th gear and a 3.90 dif.
Makes 1-4 the taller for my increased power in lower gears and 5th a short ratio to 4th.
Even so, 1st and 2nd can not get traction when floored at 16 psi and higher boost with 255s.

Compare gear ratios for other higher powered cars.
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Old 03-30-14, 01:00 AM
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What do you guys prefer for a autocross and track use fd. 4.3 or 4.44? The car will also be driven on the street

I know it's close but any personal opinions? I spoke to ray at kaaz and he said 4.3 but others have said 4.3 doesn't make a huge difference and I should use the 4.44.

I have both right now. Debating on which one to put in.
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Old 03-30-14, 04:03 AM
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Originally Posted by ItalynStylion View Post
I guess the biggest benefit by changing the diff ratio would be that it changes how close the spacing of the gears would be too. Making it almost like having a close ratio 6speed vs our 5 speed spacing; taking advantage of a more "peaky" powerband.
Changing the diff doesn't change the closeness of the transmission ratios.
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Old 03-30-14, 01:19 PM
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How can we remedy the fact that most FD that make over 400rwhp have useless 1st and 2nd (other than stickier tires)? Can we play around the gearing for the 1st 2nd 3rd gears to get the most out of our more powerful cars?

Correct me if I am wrong, but you only change gearing for better acceleration if your car is underpowered (yes I am a noob at this)

I would like to know the opinion on this matter since I am at the point of being able to make the gears at any ratio I can't think of (I am getting them made from scratch with a stronger alloy)

Andrew
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Old 03-30-14, 03:15 PM
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Some single turbo guys use the 3.90:1 ratio R&P from the automatic FD. The lower ratio gives you more of an opportunity to hook up in lower gears.

Dale
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Old 03-30-14, 10:07 PM
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I really want to try 4.44 gears with my low 300rwhp 99 twin setup i have now.

Jason
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Old 03-31-14, 07:27 AM
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if you have a shorter rear end .

What that means is the ratio is higher .

so for our stock differentials 4.10 for every 4.1 turns of the driveshaft it turns the wheels around 1 full rotation

now if you get a shorter rear end or a higher ratio like the rx8

4.44:1 it takes 4.44 rotations of the driveshaft to turn the wheels around 1 time .

because our cars make the most power up high in the RPM we want it to be up high in the RPM aslong as possible . If you have a shorter / higher geared differential you achieve that by putting less load / work on the engine to rev up because its working less to turn the wheels since 4.44 rotations of the driveshaft is 1 rotation of the wheel VS 4.1 essentialy its a gain of 3.3 rotations of the driveshaft . so the engine is working less to reach its idea powerband

Just like rally cars they have short gearing in order to stay in the powerband always

Now the downside is top speed / cruising rpm because once you upp the differential ratio say if you are at 3k rpms in 5th gear . Now you will be at 3300 because the wheels are turning slower compared to the other differential

This increases motor acceleration and thus car acceleration

Basically motor RPM is then multiplied By the transmissions to the driveshaft depending on each gear ratio .

then The driveshaft RPM is multiplied by the Differential into the wheel RPM .

and that results in the speed you are going .
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Old 03-31-14, 08:49 AM
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Originally Posted by ItalynStylion View Post
I think I understand how it works just fine. I read the 8 pages anyway just to freshen up.

My point is that I think changing the diff ratio is EASIER but not magical vs changing every gear in the tranny.

Basically, if your ratio to engine RPM and tire RPM is the same, regardless of whether you achieved this via the diff or by changing all of the ratios in your gear box; the end result is the same.....correct?
no it is similar , but not the same .

with a transmission gearing change say when you shift at redline your RPM's drop to 5000 on the FD transmission .

if you swap the transmission ratios for a shorter gearing . instead of dropping to 5000 it will drop to 5500 <-- example not actual facts .

for a transmission you can control using the ratios at what RPM the next gear starts .

VS when you swap the diff you will still shift form redline to 5k BUT it will take less effort to make it to redline because the diff's ratios are spinning the wheels at a lower speed .

you basically achieve similar results . But different ways .

swapping the diff is a quick . Cheap way of bypassing our transmissions that were made with really tall ratios in order to give some illusion of fuel economy.


Mazda knew this , the RZ's and the spirit R's in japan Came with 4.33 differentials to bypass the tall gearing .
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Old 03-31-14, 09:10 AM
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So a custom gearbox/rear + differential would give you the best of both worlds, right?

A 6-speed (manufacturers seem to be producing 7 and 8-speed transmissions now) with an ideal ratio spread (depending on your application) would give the FD excellent acceleration and reasonable cruising RPM and top speed.

I know a certain 600+ hp time attack FD with a 20B that could really take advantage of a custom gearbox.
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Old 03-31-14, 09:33 AM
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Originally Posted by HiWire View Post
So a custom gearbox/rear + differential would give you the best of both worlds, right?

A 6-speed (manufacturers seem to be producing 7 and 8-speed transmissions now) with an ideal ratio spread (depending on your application) would give the FD excellent acceleration and reasonable cruising RPM and top speed.

I know a certain 600+ hp time attack FD with a 20B that could really take advantage of a custom gearbox.
well after you need a mix of both . depends how fast you want the car to go .

for the stock FD trans , a 4.44 is a very good alternative

sorta just shortens the gearing enough that your time out of the powerband is limited .
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Old 03-31-14, 11:53 AM
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Originally Posted by HiWire View Post
So a custom gearbox/rear + differential would give you the best of both worlds, right?

A 6-speed (manufacturers seem to be producing 7 and 8-speed transmissions now) with an ideal ratio spread (depending on your application) would give the FD excellent acceleration and reasonable cruising RPM and top speed.

I know a certain 600+ hp time attack FD with a 20B that could really take advantage of a custom gearbox.


Yep. That would be the ideal. I'd love an overdrive 6th gear for highway cruising at low revs with closer 1-5 gears. Would need to be combined with the appropriate rear-end ratio.
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