JoeD

now we all know that swapping out the stock final drive gears for 4.30s or 4.33s will improve off-the-line acceleration, but what about top end? what effects does the shorter final drive gearing have on 40-120 acceleration, for example? or 60-140? ive heard some people say it will be slower, and some say it will be faster. which one is it? are you better off with the stock rear-end if you want highway acceleration?

and Jim, CarTest numbers comparing two identical cars with the only difference being final drive would be really helpfull.

TIA

Flybye

Hwy acceleration will improve until you reach your new (lower) top speed which will now be redline limited. Well, it was before, too, but if your gear change is drastic, then your previous top speed was most likely hp limited if still close to stock.

That's my theory, anyways. Let's see if I am right

RX7Elmo

the higher number final drive will increase acceleration across the board, but more drastically off the line. The disadvantage of the 4.33 is probally fuel consumption and revving higher than usual on the highway

rpm_pwr

Yup they're right. Final drive is just a torque multiplier. So long as you can control wheelspin (not an issue in most cars at highway speeds) then in gear acceleration is increased accross the board in direct proportion.

-pete

NickSimcheck

JoeD

well, these numbers jimlab posted from CarTest of his car shows that higher rear end ratio does not always equal better top end acceleration (scroll down):

https://www.rx7club.com/forum/showth...ht=ring+pinion

rpm_pwr

The figures to watch are:

0-100 mph (seconds):

2.80 - 7.38
2.91 - 7.37
3.00 - 7.31
3.33 - 7.24
3.45 - 7.22
3.70 - 7.10
3.89 - 7.01
4.11 - 6.94

Note that the 4.11 was the fastest. I couldn't be bothered figuring out the details but basically here's the breakdown:

0-60 mph (seconds):
2.80 - 3.15
2.91 - 3.04
3.00 - 2.96
3.33 - 3.33
3.45 - 3.35
3.70 - 3.38
3.89 - 3.36
4.11 - 3.25

The figures are a bit ratty I suspect it's to do with how the program guesses wheelspin. Notice the higher ratios are a jump ahead? That's the extra shift. If he was to be making more power, he would have more pull in the extra gear and you'd see the 3.33 - 4.11 times tumble while the 2.80 - 3.00 times would jump because of the extra shift required.

Quarter mile (seconds, mph):
2.80 - 11.30 @ 137.03
2.91 - 11.26 @ 137.63
3.00 - 11.22 @ 137.98
3.33 - 11.16 @ 132.86
3.45 - 11.18 @ 133.15
3.70 - 11.19 @ 134.52
3.89 - 11.18 @ 135.82
4.11 - 11.18 @ 135.71

There's nothing in it. Obviously the lower ratios are worse because they have to hold gear or shift early. The 3.33 seems like an anomaly of the wheelspin stuff again, but maybe just Jim's cut-and-paste skills

0-200 mph (seconds):
Top Speed (mph, drag limited):

Powerband vs drag. There's nothing in it. It's bullshit anyway because a 0-200 / top speed run is hardly a regular occurance for anybody.

40-70 mph (seconds):
2.80 - 1.91
2.91 - 1.82
3.00 - 1.75
3.33 - 1.56
3.45 - 1.51
3.70 - 1.43
3.89 - 2.21
4.11 - 2.05

Extra shift for 3.89+

40-100 mph (seconds):
2.80 - 4.01
2.91 - 4.64
3.00 - 4.59
3.33 - 4.61
3.45 - 4.61
3.70 - 4.56
3.89 - 4.35
4.11 - 4.15

Extra shift for 2.91+

70-140 mph (seconds):
2.80 - 7.12
2.91 - 7.07
3.00 - 7.69
3.33 - 7.52
3.45 - 7.37
3.70 - 7.12
3.89 - 7.22
4.11 - 7.92

Extra shift for 2.91+
2 extra shifts for 4.11 Numbers look wrong for 3.89,3.70

The 0-100mph is the most fair comparason because the all gears start at the same RPM and road speed. Notice that despite the extra wheelspin the 4.11 wins out.

In summary:
The 2.91 is better for fuel economy and short in-gear blasts. It's also pretty handy if you happen to live at Bonneville.

The 4.11+ is better for off-the mark acceleration and longer in gear runs. Ideal for track racing for example. Yeah the 70-140 times look like the lower the better but you can bet that as soon as that 2.91 geared car has to grab the next gear it's all over.

How's that
-pete

rpm_pwr

Oh yeah and Car Test IS JUST A PROGRAM. It's not often you see someon put on higher gears and run SLOWER is it? Unless they are then forced to shift just before they hit the traps.

-pete

TireSmokin7

Perhaps this will help. Below is a set of colums. First colum shows the drive ratio. Second shows the RPM for that ratio at 60 MPH in 5th. the next five colums show the maximum MPH at 7500 RPM for each of the gears.
3.90 2375rpm 39mph 68mph 98mph 137mph 191mph
4.10 2500rpm 37mph 65mph 94mph 131mph 182mph
4.30 2625rpm 36mph 62mph 89mph 124mph 173mph
4.777 2900rpm 32mph 56mph 81mph 112mph 155mph

jimlab

That depends, Pete... ever ridden in a 2,750 lb. car with a 12.17:1 final drive in first gear with an engine that makes ~300 lb-ft. of torque at idle?

My first through fourth ratios are the same as the Z06, and with only a 3.42:1 transaxle, the Corvette is damn near helpless in 1st gear after a 550-585 horsepower MTI Z07 engine swap, even with P345s in back. Now bump horsepower to ~670 and drop 300 lbs. of curb weight, and contemplate what a car with a 4.10:1 diff would be like in first gear.

Shorter gears are ideal for the rotary, because of its lack of low-end power. With the 3.48:1 first gear and 4.10:1 differential, final drive in first is 14.28:1. However, more torque allows you to use less gear. Final drive in a Z06 is 10.15:1 in first. Final drive in the F40 is 8.03:1. Final drive in the McLaren is 7.65:1. Apparently you don't always need "towing gears" to be very quick. What is infinitely more important is having the gearing to keep the engine in its power band in each gear. A close ratio transmission like the McLaren's, coupled with low weight and lots of torque allows it to run a 2.37:1 differential, for example. In other words, the requirements of my car are not the same as a "standard" RX-7's. I don't need a 4.10+ differential for quick acceleration.

That said, sticking with the 4.10 ring and pinion in the differential is certainly cheaper, and both Car Test and professionals in the industry have suggested starting in second gear. Again, remember the engine makes 300+ lb-ft. of torque at idle, and it never drops that low again, even at redline. So here are some more numbers to contemplate, or pick apart, if you're in that sort of mood...

G92 T56, 4.10:1 differential, 1st gear start (12.17:1)

0-60 - 3.19 sec.
0-100 - 7.36
60 ft. - 1.75
1/8th ET - 7.30
1/8th mph - 99.32
1/4 ET - 11.09
1/4 mph - 137.40
0-150 - 13.68
40-70 - 1.85
20-100 - 5.91
60-150 - 9.68

G92 T56, 4.10:1 differential, 2nd gear start (8.49:1)

0-60 - 2.80 sec.
0-100 - 6.97
60 ft. - 2.00
1/8th ET - 7.24
1/8th mph - 102.71
1/4 ET - 10.94
1/4 mph - 138.97
0-150 - 13.27
40-70 - same
20-100 - same
60-150 - same

I don't believe the first set of numbers for an instant. A WOT launch in 1st gear would simply turn the tires to sludge, even with drag radials, I'm betting. Car Test, interestingly enough, calculates the same amount of wheel spin for all the gear ratios I've run... about 0.5-0.6 seconds at launch, which is obviously highly suspect. It is just software, as you pointed out, and it's only as accurate as the formulas used and coding skills of its creator.

The second set of numbers is more believable, in my opinion. Dropping final drive to 8.5:1 gives the car a much better chance of hooking up, and the improved performance in the quarter and 0-150 is due to the car being nearly at redline in 4th at the end of the quarter mile. BTW, I included 0-150 numbers as a more real-world indication of acceleration, hopefully that meets with your approval.

Here are 0-150 Car Test numbers for a few other cars...

Stock 3rd gen. - 39.67 sec.
Stock MKIV Supra - 39.41
1994 ZR1 Corvette - 26.75
1998 Viper GTS-R - 24.94
2002 Z06 Corvette - 23.72
2002 Porsche 911 GT2 - 22.90
BPU MKIV Supra - 22.35
360 RWHP 3rd gen. - 20.55
1991 Ferrari F40 - 18.44
Pettit Banzai 20B - 14.81
1997 McLaren F1 - 13.55

I don't need Car Test to tell me that 1st gear with a 4.10 differential is a lost cause in my car without some form of traction control. I also don't need Car Test to tell me that it's going to be fast, regardless of what I put in the back for gearing. Any car with a roughly 4.1:1 power to weight ratio would be.

So back to the question... gear limited top speed in the RX-7 is as follows (P275/40-17 tire)...

4.10 - 207.2 mph
4.33 - 196.2
4.50 - 188.8
4.77 - 178.1

In a ~360 RWHP RX-7, 60-150 (highway acceleration) is as follows...

4.10 - 16.21 sec.
4.33 - 15.99
4.50 - 15.77
4.77 - 15.48

So yes, a shorter differential gear will most likely increase highway performance in a standard RX-7. By comparison, however, the best 60-150 mph time I was able to come up with for my car (9.09 sec.) was with a 3.73 differential.

Gearing choice depends on A) engine powerband, B) transmission gear ratios, C) what your application for the car is, (drag racing, highway racing, road course, autocross...) and perhaps most importantly in this case, D) availability of gearsets. ~$2,500 for a 3.73:1 differential isn't worth 0.6 seconds to me. Not when it's highly unlikely that I'll ever run into anything that could match that level of acceleration.

rpm_pwr

Jim I never said that the higher the better is for you, I just meant that you will have better acceleration fom a given RPM in each gear with a higher ratio assuming ideal traction. That's the catch IDEAL TRACTION.

In your car, eventually and anything else with a fat powerband, then yes 1st gear is a write off. But I was under the impression that you were fabbing your own box so I assume you could play with ratios. If so, then yes I do beleive you should try and keep the higher ratio rear because chances are you won't be pulling 200+ on the front straight of Le Mans all day. So I would have though it would have been better just to be lower 1 and 2 ratios? No?

Which brings me to my next point - there's no point qutoing the McLaren gears. They're built for constant high speed so they need close ratios at high speed. As a result they can pick a lower rear ratio because they couldn't care less about low speed acceleration. That's why F1 (and F40 etc) 1/4 mile times are oddly high - because they don't want to win drag races they want to win high speed circuit races.

Now if you're not in a position to pick gear ratios then I completely understand your decision, but, I'd be trying to keep it as high as you can without turning the car into a 5speed

Who knows maybe I've misjudged what you want from the car but I get the impression you're not after high speed circuit racing, rather street/strip/ sensible highway stuff.

Either way JoeD will never get 650HP out of his Dad's car or his ever-coming S2000 so he doesn't need to hear about traction issues now does he?

Can it tell you which gears were used for the runs? That'll will tell you why you come out with a 3.73.

-pete

jimlab

Yep... 60-150 mph:

Ratio - Start gear (start rpm), n gears, End gear (end rpm), seconds

2.73:1 - 2nd (~4,400 rpm), 3rd, 4th (~5,300 rpm), 9.83
2.80:1 - 2nd (~4,500 rpm), 3rd, 4th (~5,400 rpm), 9.86
2.90:1 - 2nd (~4,700 rpm), 3rd, 4th (~5,600 rpm), 9.86
3.00:1 - 2nd (~4,900 rpm), 3rd, 4th (~5,800 rpm), 9.89
3.23:1 - 3rd (~3,600 rpm), 4th (~6,300 rpm), 9.68
3.42:1 - 3rd (~3,800 rpm), 4th (~6,600 rpm), 9.47
3.73:1 - 3rd (~4,200 rpm), 4th (~7,200 rpm), 9.09
3.90:1 - 3rd (~4,400 rpm), 4th, 5th (~6,000 rpm), 9.74
4.10:1 - 3rd (~4,600 rpm), 4th, 5th (~6,400 rpm), 9.68

3.73:1 is the quickest because it's a 2 gear shot, and ends just past peak power.

Unfortunately, I don't have the luxury of hand selecting gear ratios for the transmission unless I want to put a lot more money into the project, so my gearing choices are pretty limited. With the other readily available gearset for the T56, performance suffers in nearly all areas. There's a reason why Chevy chose the same ratios for the Z06.

My choices in back are either 3.90 or 4.10, unless I want to have a ring and pinion custom made, and that appears to be both expensive and time consuming. The 3.90 gearset isn't really much of an advantage from the analysis series I've run. A bit better from 0-100 and 20-100, but that's about it.

3.90, 2nd gear start...

0-60 - 2.94 sec.
0-100 - 6.42
60 ft. - 2.05
1/8th ET - 7.27
1/8th mph - 103.38
1/4 ET - 11.01
1/4 mph - 138.64
0-150 - 13.47
40-70 - 1.39
20-100 - 5.30
60-150 - 9.74

rpm_pwr

Just what I thought. Every rear end ratio has it's optimum start and finish speeds. If you change that to finish at, say 160 or 170 then the 4.10 would win.

If you can't select ratios then I couldn't agree more with your choice. Might as well have a vaguely useable 1st if you're going to be on street tires most of the time! Just keep the 4.10 in the back of your mind if you ever change yor mind about what sort of use the car will have.

-pete

jimlab

Actually, I'm keeping the 4.10 because A) I already have it, B) the 3.90 isn't worth the expense, and C) an "ideal" ring and pinion ratio is prohibitively expensive. More than I want to spend, anyway. I'm tired of spending money on this car.

I'll have a more or less unusable 1st gear (with the fringe benefit of being able to do nasty John Force burnouts at will), but at least 6th gear will be a "pulling gear", and I can short-shift 1st to get the car rolling hard and then hammer 2nd, or start in 2nd. Car Test recommends a 1,000-1,300 rpm "launch", either way.

rpm_pwr

Jim, you're admitting you have something less than ideal? OMFG! This should go straight to the archives

-pete