Author Topic: Rear Diff Ratios  (Read 8150 times)

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Offline lgee

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Rear Diff Ratios
« on: May 18, 2011, 09:48PM »
hello all i need to conduct a symposium regarding rear diff ratio's and how to change them / make them work in my U13.

1. What i want to do = change the finial drive or rear ratio of my car so that it accelerates faster for the hill climbing and track days

2. What i think i know = U12 =3:7 to 1, U13 3:37 to 1, Gtir = 4:12 to 1

3. What i think i know part 2. U12 uses long nose diff, U13-Gtir-Primera-Anveir use short nose diff and ratio's are interchangable but not with U12
 
4. What i want to do= i want to put the 4:12 ratio or higher in my car for more fasterness

5. Limiting factors= the transfer case issue of which one to use and the centre diff ratio is it different.

6. What it actually know= that this will probably create more questions than answers and that i really know fk all

Having said all that let the hypothesizing and postulating begin.

Offline lgee

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Re: Rear Diff Ratios
« Reply #1 on: May 19, 2011, 04:15PM »
Really i can believe no one has any info on this, cmon some one must know something, i need more fasterness

Offline Luke

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Re: Rear Diff Ratios
« Reply #2 on: May 19, 2011, 08:56PM »
Trust me Laurence when I say I know exactly what I'm talking about... your rear diff ratio matters squat. The final drive is in the gearbox.
Jason reffering to the OzVR4 Forum... "Even their forum is slow" "FJ20.com on the other hand, yep that S12 does Mono's"

Offline lgee

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Re: Rear Diff Ratios
« Reply #3 on: May 19, 2011, 09:00PM »
Trust me Laurence when I say I know exactly what I'm talking about... your rear diff ratio matters squat. The final drive is in the gearbox.

where luke?

Offline Luke

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Jason reffering to the OzVR4 Forum... "Even their forum is slow" "FJ20.com on the other hand, yep that S12 does Mono's"

Offline Luke

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Re: Rear Diff Ratios
« Reply #5 on: May 19, 2011, 09:09PM »
RS5F50A is the gearbox model... so any gearbox FWD or AWD will have the right parts inside it. Just go through a list of nissans and find the ratio you want.
Jason reffering to the OzVR4 Forum... "Even their forum is slow" "FJ20.com on the other hand, yep that S12 does Mono's"

Offline Budgie

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Re: Rear Diff Ratios
« Reply #6 on: May 19, 2011, 09:39PM »
Ah, I remember that thread. I don't know what would have a more appropriate final drive, maybe an awd Nissan van? Surely one of those would have a final drive better suited to acceleration?
dont piss me off with your pillarless shit captain snappy wrist

Offline SSS

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Re: Rear Diff Ratios
« Reply #7 on: May 20, 2011, 11:09AM »
Might pay to get something custom made laurence, i'm sure most of the awd guys here would go in for a group buy to bring the price down.

Offline Luke

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Re: Rear Diff Ratios
« Reply #8 on: May 21, 2011, 02:24PM »
After a rather in depth conversation with Laurence about this, Im still not confident that just changing the final drive is going to see an improvement in lap times, or pass times for a hill climb. You need to match your final drive to your torque band, making it shorter wont nessisarily make your car quicker, because you narrow your torque curve. Mine is actually taller than stock, and I have a nice phat torque curve in every gear.
Jason reffering to the OzVR4 Forum... "Even their forum is slow" "FJ20.com on the other hand, yep that S12 does Mono's"

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Re: Rear Diff Ratios
« Reply #9 on: May 21, 2011, 03:53PM »
So, an adjustment to tune maybe better? Something to broaden the torque spread of the motor he is running?
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Offline Luke

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Re: Rear Diff Ratios
« Reply #10 on: May 21, 2011, 05:19PM »
I don't know... I find it best to think of the gearbox and diff as torque multipliers. Depending on your ratios, the amount of torque at your wheels varies. It's a trade off, more torque for less time, or less torque for more time... if your cars not falling outside of it's torque curve between gears, I can't see that shortening the ratio will make much of a difference.
Jason reffering to the OzVR4 Forum... "Even their forum is slow" "FJ20.com on the other hand, yep that S12 does Mono's"

Offline dozer

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Re: Rear Diff Ratios
« Reply #11 on: May 21, 2011, 09:13PM »

1. What i want to do = change the finial drive or rear ratio of my car so that it accelerates faster for the hill climbing and track days



feed more boost into it!!  ;D

Offline chr1S

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Re: Rear Diff Ratios
« Reply #12 on: May 24, 2011, 11:40AM »
Luke you have the right thinking, but by shortening the ratios you're providing more thrust to the wheels, so the limiting factor here would be tyre traction.

I remember you mentioned you want to run a bigger turbo later as well? (Lgee) you need to make sure you're not accelerating the engine too fast (by gearing) for the turbo to even spool up properly, otherwise if you find yourself off boost it will ramp through the rpms way too fast to account for the inertial losses of the turbine..

I suggest increase the final drive first, then look at playing with your turbo once you can safely rev the engine harder to make up for the better ratios.

I'm working on a spreadsheet at the moment that calculates the thrust force required for a given set of gears, i will model it into tyre data later, but analysing tyre data is no easy task...

Food for thought, rally cars run a restrictor, which effectively "flattens" the torque curve as they choke it, they still run mega close gears with a taller drive, watch how fast they mash through the gears whilst keeping the turbo happy.
« Last Edit: May 24, 2011, 11:42AM by chr1S »
Don't let the opinions of the average man sway you. Dream, and he thinks you're crazy. Succeed, and he thinks you're lucky. Acquire wealth, and he thinks you're greedy. Pay no attention. He simply doesn't understand.

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Offline lgee

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Re: Rear Diff Ratios
« Reply #13 on: May 24, 2011, 02:54PM »
I remember you mentioned you want to run a bigger turbo later as well? (Lgee) you need to make sure you're not accelerating the engine too fast (by gearing) for the turbo to even spool up properly, otherwise if you find yourself off boost it will ramp through the rpms way too fast to account for the inertial losses of the turbine.

Smaller turbo the one i have takes a long time to spool 4300rpm i need to get that down, i want to downgrade to a gt2871r with.64 rear housing or get my rear housing changed to a smaller one, that is probably where i will head.

Offline lgee

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Re: Rear Diff Ratios
« Reply #14 on: May 24, 2011, 04:56PM »
i just got off the phone to the turbo guy talking about going to a smaller rear housing for better response and he tell me i have the wrong turbo on my car must have been packaged incorrectly it is far to big for my car it runs a gt3070 rear blade with a .86 rear housing, he is going to do me a deal on a new super response turbo, which will make teh car accelerate better.

Offline Budgie

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Re: Rear Diff Ratios
« Reply #15 on: May 24, 2011, 05:33PM »
shit thats massive... mine is .64 rear housing and a .70 intake housing, its really getting going around 3000rpm.
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Offline chr1S

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Re: Rear Diff Ratios
« Reply #16 on: May 24, 2011, 06:46PM »
Do your revs drop much below 4300rpm on the track?
Don't let the opinions of the average man sway you. Dream, and he thinks you're crazy. Succeed, and he thinks you're lucky. Acquire wealth, and he thinks you're greedy. Pay no attention. He simply doesn't understand.

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to be fair, it wouldn't be ANTRX if we stayed on topic.

and if someone wasn't cranky at Chr1s for something he said...

Offline lgee

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Re: Rear Diff Ratios
« Reply #17 on: May 24, 2011, 07:20PM »
Do your revs drop much below 4300rpm on the track?

its all about the take off, when im doing a track day yes it does on the slower corners like honda and lukey, its a pig at winton with the slow speed stuff.

Offline chr1S

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Re: Rear Diff Ratios
« Reply #18 on: May 24, 2011, 07:46PM »
Do you have launch control?

Honestly, you can get around that issue if you had a good ECU to control the engine. Even the low speed corner stuff, you can make math channels to have an ORB cut based for anti-lag.

I just have this feeling you will find the smaller turbo will be not enough in the future.. you do have alot of drivetrain losses and the car isn't exactly light. The Honda boys with prepped K series engines would eat you down the straights if you go back to a smaller turbo I say..
Don't let the opinions of the average man sway you. Dream, and he thinks you're crazy. Succeed, and he thinks you're lucky. Acquire wealth, and he thinks you're greedy. Pay no attention. He simply doesn't understand.

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to be fair, it wouldn't be ANTRX if we stayed on topic.

and if someone wasn't cranky at Chr1s for something he said...

Offline lgee

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Re: Rear Diff Ratios
« Reply #19 on: May 24, 2011, 07:56PM »
i intend to get the smaller turbo and have it set up with its own dump pipe on a manifold, sell the one i have and buy a bigger one but smaller than the current one, and have that set up as well, it only takes couple hours to swap them over, and then all i have to do is load the different maps in nistune and im away

Offline Luke

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Re: Rear Diff Ratios
« Reply #20 on: May 25, 2011, 07:09PM »
Chris, WTF is thrust force? You mean torque? If you don't know what your talking about, don't tell me I don't.
Jason reffering to the OzVR4 Forum... "Even their forum is slow" "FJ20.com on the other hand, yep that S12 does Mono's"

Offline Luke

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Re: Rear Diff Ratios
« Reply #21 on: May 25, 2011, 11:00PM »
There's a fair difference between a close ratio gearbox and a short final drive... let's not confuse the two.
Jason reffering to the OzVR4 Forum... "Even their forum is slow" "FJ20.com on the other hand, yep that S12 does Mono's"

Offline chr1S

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Re: Rear Diff Ratios
« Reply #22 on: May 25, 2011, 11:46PM »
Yeah, I obviously don't know what I'm talking about lol. Please, enlighten me....

Thrust? It's a force... Torque? Which is a moment.... "Thrust" in this case is the force applied to the ground by the tyres assuming no loss of traction. It is a FUNCTION of torque, how can one not exist? If there is no torque, there is no "thrust". Seriously, If you're an engineer of some sort, since you claim you know what you're talking about, god help the company you work for. I don't like things turning personal, but you poked the stick first.

Enough talk, lets get to numbers - that's what everybody cares about, even the media.

Here, I've gone from one extreme to the other, with respect to final drives only. I plotted the force required to pull 2g of acceleration, which is represented by the "ideal" line.





It's pretty damn obvious that the thrust has increased substantially...if  you could have pure traction off the line, the plot with the greater thrust would eat the longer final drive by a long shot and I'm actually modelling that right now. It's also obvious that i'm missing another 2-3 gears so the plots are not completed but the initial results are there. If you can't understand what these plots are telling you and how to manipulate the inputs, well sorry, I can't really help you there.. you're better off paying through you're arse to a "glorified performance i can't tune for shit but i say i can and make your car stick to the road like shit can to a blanket" mechanic.
Don't let the opinions of the average man sway you. Dream, and he thinks you're crazy. Succeed, and he thinks you're lucky. Acquire wealth, and he thinks you're greedy. Pay no attention. He simply doesn't understand.

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to be fair, it wouldn't be ANTRX if we stayed on topic.

and if someone wasn't cranky at Chr1s for something he said...

Offline Luke

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Re: Rear Diff Ratios
« Reply #23 on: May 26, 2011, 12:45PM »
My bad Chris... I'll eat my words on this one, you clearly do know what your talking about, sorry mate. I'm not an engineer, I'm just a sparky with an avid interest. So I'm looking at your graphs, particularly the thrust at the wheels one, there's clearly more thrust with the shorter final drive. I'm not arguing the fact that this will happen, It's obvious with a shorter final drive you generate more torque. What I'm seeing now though is that with the shorter final drive, your clicking third gear at around 42km\h, and second gear is used from roughly 30 - 42km\h. I understand you have taken it from 1 extreme to the other, could you do a comparison where it's not so extreme?

I would love to see another graph that could take in the time taken to make a gear change. It's all good to plot this stuff in a perfect world, but on a race track where you could fall outside the ideal speed for a gear in a certain corner, short shift, or ride the limiter, the deal changes.

On a drag strip, no doubt, if you can change the gears fast enough, shortening the final drive will get you there quicker, but on a race track, I'm not convinced it's a sure thing.
Jason reffering to the OzVR4 Forum... "Even their forum is slow" "FJ20.com on the other hand, yep that S12 does Mono's"

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Re: Rear Diff Ratios
« Reply #24 on: May 26, 2011, 03:55PM »
Best thread on antrx for a while, I'm actually learning something for once :P Keep it up guys! :) haha!
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Offline lgee

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Re: Rear Diff Ratios
« Reply #25 on: Jun 7, 2011, 06:43PM »
and then it stops.