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?

You know, the way I read the post you are talking about was not as a claim that he could go 178MPH. I understood that he thought this was a more appropriate theoretical top end than was 153. As for the tracks you mentioned, I certainly am going nowhere near 178 on any of them (I've drive about half of them), but I can see 153 easily being an issue. I hit speeds right about there at the Glen, Pocono, VIR, and even Summit Point. I'm sure I would easily be up there at Road Atlanta if I were driving my car. And keep in mind that my car is pretty much all stock, other than a downpipe - at least it was at the time.

My point is that I didn't consider 178 to be a speed that he planned to acheive, more that 153 was too limiting. Just my $0.02 :).

RICE RACING actually has a good formula using Cd numbers which will tell you how fast (aerodynamically) your certain car can go with X HP

in a 2nd gen 390Hp at the flywheel (it think) is enough to propel you to 300KPH

this formula says a 1st gen should be able to do 330ish KPH with the HP RICE RACING'S motor is making and he is running outta gearing at 8800rpm in 5th gear at 320ish KPH

Cheers
Shane

I was helping Daryl with his, but he needs 100 posts before he can use it.. I'm going to use a new pic for my avatar, just haven't gotten around to uploading it :)

PaulC

Here is all I'm saying...

153 mph would not be enough for me on the street. Frown if you will, but any close race with a high powered car will not be settled at 100; the winner might not emerge until well up into the speed range. I raced another RX-7 back when my car was still twin turbo, and we were dead even up to the top of 4th at which point the other guy lifted and coasted. That was about 145 mph, so you can see that if we had kept in it, we would have been past 153 pretty fast.

I will probably never need to do 178, but I can assure you the car will do it even with the shitty stock 5th. I've had it up to 160 about a dozen times and it wasn't showing any signs of petering out. :)

Anyhow, I agree that for track use, 178 may not be realistic, but hey I drive other places besides the track. I'm just trying to improve my 5th to make it a useful gear on the highway.

B

A friend of one of my employees was killed this week in a street race during the middle of the day!

And then I have to go and read this:

and....

and...







:wtf1:

Is there no common sense here? When will everyone learn? You guys must all have jobs with decent income, requiring a level of intelligence that should lift you beyond the lunacy of street racing, right?

Take it to the track! [/rant]

I have to second that motion. Exibitions of speed on the streets is not racing. unfortunately these acts on the roads bring a bad image to legitimate racing and racers. I would hope that everyone could see the stupidity of running a car on public roads in excess of 160MPH. a pothole could cause one hell of a wreck that could kill more than just the "racers". racing should be done on the track with proper safety precautions taken. this helps all the people who must work to support their racing get to work on time and home from work with enough daylight left to fix their race cars. keep it on the track.

Hey I do my high speed stuff by myself with nobody around... usually very late at night.

If there was a track closer than an hour away, I'd never take my RX-7 out on the street.... But there isn't one.

B

SORRY TO HEAR OF YOUR FRIEND CHRIS, THAT BITES, BEEN THERE.

High speed driving at night? That makes lots of sense also. :confused:

Well then how do you qualify your earlier statement about racing other high-powered cars? Here it is:

153 mph would not be enough for me on the street. Frown if you will, but any close race with a high powered car will not be settled at 100; the winner might not emerge until well up into the speed range.

Sorry, but it doesn't sound like late night cruising alone to me.

Sorry, but that is lazy, rationalizing bullshit. The two closest race tracks to me are 1 and 2 hours away. You don't see me jetting around town in the middle of the night at 140 mph, just because I'm too lazy and undisciplined to wait for a track day. Sorry if this sounds harsh, it's not directed entirely at you. I hear the same line of crap from my customers who buy race gas for their street cars, and it's always the same.

No one is around when I do it, so what's the harm?

I never put anyone else in danger.

The track only runs Weds. and Sat.

The track is too far away.

Then I have to buy a helmet too!

The overriding fact is that the street racing crowd is too self centered and egotistical to have to play by anyone else's rules. Maybe they think of themselves as "outlaws". :rolleyes:

That's not really street racing if you're going that fast... that's more like "open highway open-air dyno pulls", no racing required, just floor it and see who has a better power/aero ratio.

Basically, more dyno queen nonsense.

Only 1 hour to a race track sounds like a good deal. closest track to me is more than an hour and a half towing. regularly travel 2-3 hours to alternate race track and I have towed more than 8 hours before to get to a race. I hope that you never encounter a regular citizen on his way to work while you are out by yourself "racing".