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What diff do I want?


wvumtnbkr
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I am thinking about swapping the rear diff on my RX7 to get a better gear ratio.

 

Right now, it is a 4.10 gear.  I was thinking something like a 3.73 would be much better.

 

The rear of the car is IRS.  I know there are a few kits out there to use a ford 8.8.  I was wondering what other diff might be usable.

 

I need the diff to be strong, light weight, have good choice in ratios, cheapish, easy to find.  Gotta work with the stock RX7 suspension.

 

 

Do the BMW diffs work well? 

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I'm confused....there are kits to swap in a Ford 8.8 diffs to the RX7 housing?  Or swap in the whole housing?  Sounds like a great option to me.  I don't think you'll get much cheaper, more durable or have more ratio options than the 8.8, honestly.

 

The Nissan R200 (S13-15, J30, Z32) diff has a OE viscous (meh) and several aftermarket options available (os giken, Tomei, etc).  I've even heard the ebay torsen diff holds up pretty well if you swap out the hardware and make sure its assembled correctly.  Tons of people putting well over 500lb-ft through the R200 and rarely hear about ring/pinion failures....and those are mostly drag launches.

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What's wrong with the 4.10?  Are you running out of gear?

 

If you want to choose the right ratio semi-scientifically, you need a power (or torque) curve for your engine, the trans gear ratios, and a speed trace for the track you're interested in optimizing for.  Generate a tractive effort curve, scale it by final drive ratio, and then tweak FDR to trade off avg power vs # of shifts.

 

If not optimizing for a particular track, set final drive so that the car has just a few rpm left in the top of your highest gear at the max speed you expect to achieve during the season.  The goal is to maximize time spent near peak power, and you do that by keeping engine rpm in a narrow band around peak power.  The change in rpm from a shift is smaller in the higher gears, so you generally want to set your final drive so that you're using the highest gears possible without running out of revs.

Edited by Hi_Im_Will
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I am not racing Daytona this year, so I think the top speed should be okay in the top of 5th.  If I did have a dyno chart, could ya walk me through the permutations to figure out the proper ratio "semi-scientifically"?

 

 

Here are the top speeds achievable in each gear with the current setup....

 

26.35
45.13
70.09
98.83

137.26

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3 minutes ago, wvumtnbkr said:

I am not racing Daytona this year, so I think the top speed should be okay in the top of 5th.  If I did have a dyno chart, could ya walk me through the permutations to figure out the proper ratio "semi-scientifically"?

 

 

Here are the top speeds achievable in each gear with the current setup....

 

26.35
45.13
70.09
98.83

137.26

 

Yeah, I could show you how to do it.  At first glance I wouldn't change FDR based on those speeds in gear, but need a power curve and actual ratios (and tire size, forgot that) to get a better feel or it.

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3400_dyno.jpg

 

Tire size is 245 40 15 for this example.  That is the size that I WANT to run.  I went with 245 40 17 for Mid O until I saw how much higher the car needs to sit with those larger tires on it.

 

1st                  2nd         3rd             4th          5th

3.75 2.19 1.41 1

0.72

2 minutes ago, mhr650 said:

Early first gens and automatic second gens used a 3.90 which is not a big change from the 4.10 but if you wanted to try one I have a couple laying around that I can send  your way.

A 2nd gen 3.9?  If so, YES!  I WANT!

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1 hour ago, Hi_Im_Will said:

What's wrong with the 4.10?  Are you running out of gear?

 

If you want to choose the right ratio semi-scientifically, you need a power (or torque) curve for your engine, the trans gear ratios, and a speed trace for the track you're interested in optimizing for.  Generate a tractive effort curve, scale it by final drive ratio, and then tweak FDR to trade off avg power vs # of shifts.

 

If not optimizing for a particular track, set final drive so that the car has just a few rpm left in the top of your highest gear at the max speed you expect to achieve during the season.  The goal is to maximize time spent near peak power, and you do that by keeping engine rpm in a narrow band around peak power.  The change in rpm from a shift is smaller in the higher gears, so you generally want to set your final drive so that you're using the highest gears possible without running out of revs.

 

I think somehow an IMSA forum bled over into the CC forum.  

 

Holy crap.  

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1 hour ago, wvumtnbkr said:


A 2nd gen 3.9?  If so, YES!  I WANT!

 

The ones that I have are first gen, so you would have to swap the ring and pinion into your housing which would involve setting pinion depth and backlash. Those things are not really difficult but do require some special tools and specific knowledge to get it right. I use a 4.78 Kia housing which already has the pinion depth setup from the factory, I just swap the open diff for an LSD from the RX and set the bearing preload and backlash.

 

You should be able to find an automatic second gen housing pretty easily, good thing about using one of them is that it would save you 25 points. The loophole to get a 75point gift  by claiming your car started out as an automatic is closed now, but you are already paying the swap points for the transmission so it would make using the auto diff ratio a freebie.

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1 hour ago, wvumtnbkr said:

3400_dyno.jpg

 

Tire size is 245 40 15 for this example.  That is the size that I WANT to run.  I went with 245 40 17 for Mid O until I saw how much higher the car needs to sit with those larger tires on it.

 

1st                  2nd         3rd             4th          5th

3.75 2.19 1.41 1

0.72

A 2nd gen 3.9?  If so, YES!  I WANT!

 

 

Here's your tractive effort curves with a 245/40R15:

000Capture.thumb.PNG.c253bb71373113d24640369340296ec3.PNG

 

Original is 4.10 FDR, New is 3.90.  Note the giant torque holes at the top of each gear - you need a higher rev limit.  Also of note, in the 55-125 range, you will average 177.1hp with the 4.10, vs 176.6 with the 3.90.  Not really significant - they both have giant torque holes.  Just a question of whether you want the hole at 90-98mph, or 95-105, and whether or not going faster than 125mph is important.

 

Now, let's say you fill those torque holes by raising your rev limit to 6000rpm, and assume that power falls off pretty sharply (to 170 at 6k RPM):

000Capture2.thumb.PNG.0a2a9606923305e00d8001cb006d8e2f.PNG

Original is 4.10 with a raised rev limit, new is 3.90 stock limiter.  Torque holes got significantly smaller, top speed went up (above what you could get with the 3.90), and average HP over the 55-125mph range went up to 178.8 HP.

 

Obviously, just averaging HP over the range isn't quite accurate - ideally you'd use a speed histogram from the actual track and do a weighted average, but it's illustrative here.  This analysis also shows that for your car, 3.90 vs 4.10 is very track dependent - where on the track you hit that torque hole will determine the correct ratio for you.  But the really illustrative part is that in a perfect world, you would spin that engine to 6000RPM, then figure out how to make it live so you can go REAL FAST!

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6K it is.  Lets see what breaks!

 

I mostly understand the chart.  I think I will try to make one so I can fully understand it.

 

I probably should have made one BEFORE I picked an engine and trans!

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Wouldn't think you would have to explain to a rotary guy that more RPM is always the answer LOL

 

I need to do a chart like this to see what I get on the new engine when I raise my limit from 9000 to 10000. Getting some parts made from sand in this new engine should really help…

Edited by mhr650
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Sorry Rob, I was confusing your diff post about ratio to the other thread about LSD.  Cool that you can swap in the 8.8 (I did a quick google on it), but I would think unless you're using big power the weight penalty and points hit might not be worthwhile.  That 8.8 IRS pumpkin is big female-dog, even the aluminum one.

 

Will, I am curious.....how does the overdrive 5th play into determining what FDR to run? Seems to me you would want to focus on the gears up to 1:1 and try to stay out of overdrive.

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42 minutes ago, Bremsen said:

Will, I am curious.....how does the overdrive 5th play into determining what FDR to run? Seems to me you would want to focus on the gears up to 1:1 and try to stay out of overdrive.

 

 

It's a check-and-see, totally depends on what the ratio is.  In Rob's case, the engine RPM change from 3rd to 4th is almost exactly the same as from 4th to 5th.  If the rpm change going into 5th was much larger than 3-4 or even 2-3, it may be worthwhile to set FDR so you never use that top gear.

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11 minutes ago, Huggy said:

dont get too caught up in the theoretical calculations...

 

More right pedal, less middle pedal...

 

Graphs are cool, but you have to remember not to get carried away.

 

Thats what I would have thought too. 

 

Wouldnt it it be very track dependant anyways? As in, if you are consistently needing to choose between bouncing of rev limiter or shifting then you want more gear, but that would change so much from track to track?

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16 minutes ago, thewheelerZ said:

Wouldnt it it be very track dependant anyways? As in, if you are consistently needing to choose between bouncing of rev limiter or shifting then you want more gear, but that would change so much from track to track?

 

 

Absolutely.  My posts above were demonstrating the general case, for when you just want to buy a diff and run it for the season, instead of making changes between events.

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Personally, I'd like to see how you apply the histogram and weighted average at a specific track.  I've always wondered what those engineers are poring over on their laptops trackside.

Edited by Bremsen
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I wanna take the stock RX7 engine and gearing and tire size and see how that plays out.  say 7K rpm redline

 

455798d1328580296-s4-213-000-miles-dyno-

 

Tractive effort is torque at the wheels through the drivetrain, right?  So I just take the dyno chart and figure out the wheel torque at that particular vehicle speed AT the rear wheels?

 

245/40/15

 

3.48 (1st)
2.00(2nd)
1.37(3rd)
1.00(4th)

0.70(5th)

 

 

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16 minutes ago, wvumtnbkr said:

I wanna take the stock RX7 engine and gearing and tire size and see how that plays out.  say 7K rpm redline

 

455798d1328580296-s4-213-000-miles-dyno-

 

Tractive effort is torque at the wheels through the drivetrain, right?  So I just take the dyno chart and figure out the wheel torque at that particular vehicle speed AT the rear wheels?

 

245/40/15

 

3.48 (1st)
2.00(2nd)
1.37(3rd)
1.00(4th)

0.70(5th)

 

 

 

Tractive effort is force at the ground, and includes the tire size (you can adjust tire size to effectively change your FDR).  Otherwise yup, use the dyno chart, your trans ratios, fdr, and tire size to find force at the ground and speed for each rpm in each gear. 

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soo, I think I am getting weird numbers....

 

My tractive force equation is = TQ (at each RPM) * trans gear ratio * rearend ratio / tire circumference. 

 

I am getting numbers for 4th gear of less than 100.

 

say 105 lbft of torque x 1.00 (4th gear) x 4.1 (rearend ratio) / 5.9 (tire circumference) = 72.4

Edited by wvumtnbkr
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I feel like an idiot right now....

 

I have all of the data and can NOT figure out how to get it all on 1 chart....

 

This is maddening!! 

 

Myself and another engineer are both working on this and feeling like idiots....

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Any chances you can tell me how to trendline 3 different plots on the graph?

 

Also, how are you able to tell the average HP from the graph? 

 

Or, is it not from the graph?  Is it from the data and knowing the HP at each vehicle speed and averaging them together.... 

 

I am gonna need to buy you a beer or two at WGI....

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