Let's Talk Lowered GEARS, 5th Gear or REAR END
 

Who's got'em? Or are you thinking about getting them?

My car is a PLAY car only. Some track days (w/turns) and maybe some Drag just to put a few domestic friends in check. I am putting a ported motor with a single TO4e and if that's not enough going T-78. The trans and rear is out for suspension changes and trans rebuilt.

So lower 5th, I don't see myself going 200mph, or do the rear and with what gear?

How about the Kazz limited slip? Just for drag? or will it help for the track too?

Thanks, Jeff

I've got a 5th on order that's 0.806 : 1 ratio out of the Type R Bathurst RX-7 in Japan. Should be fun!

Brian

What would the top speed be with .806:1 5th?

I would think that the Kaaz diff would help in track as well since it will keep the car more predictable(you know both wheels are spinning vs. torsen).

Dragon has the 4.777 rear gears...says that the car seems to just stay in it's powerband.

^^^

Many 1st-gens have a .815 or .825 5th gear.

I rarely use 5th though :) For what it's worth, 7k in 4th (always 1:1) works out to about 5600 in 5th, which with 1st-gen tires and 3.91 rear gears is about 120mph.

I'd rather have the extra omph through all the gears, and use 5th more, than have a steeper 5th gear that I only get to use at super high speeds.

PaulC

With the stock 4.1:1 ring and pinion and the 0.806, I should top out at around 178 mph on stock sized tires. This is plenty for me. It should pull like nuts with that gear too. :)

Since I know someone will want to know, this will change my 5th gear RPM at 60 mph from about 2400 rpm to about 2700 rpm.

Fuel economy? Hahah. Who cares?

Brian

As for the shorter diffs... I have done a ton of calculations (yes, on a spreadsheet, not in real life driving)... and I can't justify a shorter diff for my car.

Think about it this way.... if you go to a 4.3 or 4.77, it's totally true that you will have more force at the wheels. But... you hit redline sooner and you have to shift to the next gear which has LESS multiplication of force in the transmission. It also depends on your car's powerband. So the gains might not be quite as much as one might think by just numerically comparing 4.1 vs 4.3 vs 4.77.

Using my 0.806 5th with a 4.77 rear would drop my top speed to 153 mph which isn't enough for me. Additionally, it lowers first gear by about 15% (and the others too)... First is already just right or maybe even a little too short in my opinion, so I don't want it to be even shorter. Also, how much time and money will it take to install the rear? Those things are more expensive and harder to install than just my new 5th will be.

Ideally what would I do? Well I'd pick new gears for more of them... make first slightly taller, 5th slightly shorter, and the remaining gears spread nicely in the middle. That's not practical money-wise though :)

Anyhow, that's my opinion on the matter... :)

B

who wants to lug the engine at 2400rpm at 60mph anyway? that's why i rarely use 5th... i don't like to load the engine in higher gears below 4k.

Check out guru motorsports they do fully redesigned 2nd & 3rd gen gearboxs that have a 1:1 5th gear.


There website can explain them better than i can:D

a year or so down the track i suspect i'll be grafting a supra 6 speed into my car with a 4.3:1 gear set, the 1-5 gears of a 6 spd supra box have close enough to the same overall spacing as the first 4 in a FC/FD box, so it'll be closer, also a 4.3:1 rear end will still allow approx 285kph (178mph) which i deem to be adequate.....

having 6 forwards gears will also be rather nifty. dont give me all the blah blah blah about how hard it will be to do, i am a new zealander and we have a hidden factor called kiwi engenuity. it'll be fairly straight fowards

I just put a tranny with 84 GSL internals in a 90 n/a case into my 90 GXL. The gear ratios, and % change are as follows:

90 ratios 84 ratios % shorter
3.475 3.674 6%
2.002 2.217 11%
1.366 1.432 5%
1.000 1.000 0%
.711 .825 16%


Overall, Im very impressed with the shorter ratios, especially the 5th gear. My car is a n/a, and 5th gear used to be useless for anything but cruising. But now I my car will get up and go in 5th like it should. Also, the shorter gears really help with my 8500 rpm shift points.

i believe the best way is to look at the type of events you are doing and work out what you deem to be an acceptable top speed then change your rear ratio to suit. That way you are making the most of having a 5 speed gearbox

I agree.

Ideally I try to set up my car so that I run out of RPM in 5th gear just before having to lift for the turn at the end of the fastest straight away. An ideal situation would be to have 5 usable gears so that you can stay in the powerband at all times.

I just set my car up for a race in a couple of weeks. My gears are as follows:

1st 1.89:1
2nd 1.61:1
3rd 1.34:1
4th 1.12:1
5th 1:1
ring and pinion 4.44:1

I will be able to use all 5 gears in the race with this setup.

Another thing to consider is having a direct drive (1:1) top gear is an advantage because you will have less parasitic HP loss from your drivetrain when you need it most.

Yes, this may be true, but the narrow power band of the rotary is nearly always helped by the shorter gear.

That's why I said nearly always. :)

and

I'm not ready to raise the BS flag yet, but exactly where does this theoretical (IOW aero drag notwithstanding) speed occur? On the street? I pray you'll say no. On the race track? Lessee, you're in MD.

Pocono? Maybe, just maybe depending on the layout.
Lime Rock? Nope.
NHIS? Nope again.
Summit Point? Not likely.
VIR? Questionable, because the turns that lead to the long straights are pretty slow.
Watkins Glen? Doubtful.
Charlotte? Uh-uhh.

Courses you may be able to achieve those speeds (assuming you travel):

Road America, Brainerd, Daytona, Topeka (maybe), Road Atlanta (big maybe), Texas World, Michigan, Mosport.

You don't list your mods, so I'm not saying it's impossible. I have just seen a lot of guys on this forum declare that they have run some rediculous speeds without having a clue as to what it takes to actually attain a speed that high. It's kinda like when guys try to figure out their HP numbers. :)

Lots of guys list these top speeds without realizing that A) on a race track, eventually you will run into a corner and have to slow down, and B) on the street, it's likely you will kill yourself or others.

Just my opinion, of course. :)

I've got 400 hp at the wheels... So it hauls ass :) I'm going to Summit Point on the 20th, I hope to be able to top 160 at the end of the main straight even with my still-stock 5th ratio. I might only hit around 145 because shifting to 5th might not be worth it if I'm on the brakes in like 2 seconds anyhow. We'll see. I will have video to back up my claims! ;)

Yeah I do some crazy crap on the street too, but no need to upset everyone in this nice thread. :)

Brian

Hey Buttlips, where'd you get a trans with those nice ratio's? Dog box? If you don't mind me asking, how much $ did it cost?

What class u race in?

Thanks
-bp-

I race in SCCA E Production.

The gear box is built by Roger Mandeville. It is a set of Leeson gears that have been put into a stock 1st gen RX-7 case. He started building them originally for an IMSA class where the rules said you could do anything you want to your tranny, but it must stay in the stock housing.

It is not a dog ring box. The gears are "involuted spline" gears, which is a true crash box (no synchronizing mechanisms at all). Roger sells them for just over $5,000, but they can be found used for under to $3k.

The advantages are the nice ratios, and the fact that it is a bolt in unit (no drive shaft, clutch, or other modification necessary).

The disadvantage is that it is difficult to learn how to drive. Since it's a true crash box, the RPM must match perfectly on shifts (particularly down shifts) or it won't go into a gear.

Hello, Just wanted to report my experiences today at Summit Point.

I was able to hit redline in 4th every time down the straight running only 11 PSI. I didn't feel that I had the cooling to run my street setup of 14 PSI. I could have even maybe picked up a few mph in 5th, but I didn't see the point in shifting to 5th since the braking zone was coming up pretty fast at that point.

If I had been at 14 PSI, I should have been able to get into 5th and actually use it for a short bit. :)

Regards,
Brian

so 4th with 4.10 gears, 225/50/16 tires, and a 1.0 4th gear, that equates to ~144mph, if you redline at 8K..

PaulC

my 1st gen hits a aerodynamic brick wall at 7800 with 4:88's in 5th, able to use 3rd 4th and 5th all around track instead of just 3rd 4th with taller gears.

....and you race in a vacuum! :) Aerodynamic forces will lower that number a bit. I still don't see where 178 mph would come from except illegally (way illegally) on a stretch of highway.

Hey, i'm not arguing Chris :) He says that's what he can do, that's what the calculator says he's at, based on the numbers..

I think Wargasm underestimates the effort required to accelerate those last 30-40mph.

PaulC

Right, the aero forces go up exponentially. So if 400 hp will get you to 150 mph, it might take another 200 hp or more to get you the last 30 mph to reach 180 mph.

I think that once you get above 130 mph or so, it's really hard to rely on the accuracy of the speed calculators. But unless you actually do it, that's all most of us have to go on. :)

Paul, what happened to your avatar?