Sebring transmission, What gears for me? Options

[Jetsetsurfshop]

Ok Guys, what do you think....
Want to get a racing tranny for my track car with Sebring in mind. What's the best set-up? Is a LSD a good idea? Quaife? How about a cooler? What's an intermediate plate?
Thanks for the all the help, I'm really green with trannys. (IMG:style_emoticons/default/biggrin.gif)

[carr914]

You want Tall Gears for Sebring. My 2.5 TP had a 901 Box with Shortened Gears ( I think 5th was an X Gear - I woulld run out of Gear on the Back Straight going into 17.

A Quaife is a LSD, if you go with a LSD, might as well do a Cooler w/Spray Bars. An Intermediate Plate is just that - the about 2.5 Plate between the Main Transmission and the Back Half. A lot of people use a Billet Aluminum although Dr Evil does not like them

[Matt Romanowski]

To choose gearing, we would need to know your motor size, power curve, tire diameter, and budget. Do you have the budget and desire to change second gear?

If you have the budget, go with a Guard LSD and a billet intermediate plate. For a serious track car, you need a good cooler too.

[brant]

What redline.
I'm not sure a cooler is needed for a 4 cylinder

[wndsrfr]

Ok Guys, what do you think....
Want to get a racing tranny for my track car with Sebring in mind. What's the best set-up? Is a LSD a good idea? Quaife? How about a cooler? What's an intermediate plate?
Thanks for the all the help, I'm really green with trannys. (IMG:style_emoticons/default/biggrin.gif)

There's an excel worksheet titled "914 901 gear ratio sheet" by Greg Braun you can find the link here:
http://www.914world.com/specs/regearing_901.php

It'll really help you noodle out the options.

FWIW, I'm running 2 cars at VIR which has a 4000' back straight with some uphill in it but a slow entry (Oak tree, about 40mph).
I re-geared my /4 which has a Raby kit 2316 weighs 2200# and puts down 165rwhp redlined at 6200 running Star Spec 225/50/16's to the budget build of A/F/KA/S/Z by flipping 5th and 3rd and buying an S for 4th (thanks Dr. Evil) and don't run out of revs on the straights.
My /6 which has a 2.7 RS spec engine weighs 2000# and puts down 185rwhp redlined at 6800 running 225/45/15 Toyo RA1's is using stock gearing but the tire diameter is about 5% smaller and don't run out of revs on the straights either.
Before I re-geared the /4 I found 5th gear too tall to provide any real acceleration--now it's a perfect match.
If you can grab a spare tranny, send the gear stack to Mike (Dr. Evil) to have him do the budget re-gearing & try it out....I think your car will like it!

[Jetsetsurfshop]

Thanks guys-
I don't mind the current set-up as it stands. Just seems that 5th will never redline. Ill look at my footage and update my post later today.

[carr914]

Why would you want 5th to Redline? You don't have a 6th!

[Jetsetsurfshop]

Why would you want 5th to Redline? You don't have a 6th!

I guess I don't. I just feel like 5th isn't geared low enough. Maybe more horsepower is the answer!

[Matt Romanowski]

You want to be hitting redline in top gear at the end of your fastest straight. If you don't, you're geared too high.

[carr914]

Yeah, til you get to the next Track & don't have enough gear

[brant]

Yeah, til you get to the next Track & don't have enough gear

optimum is still optimum...
even if you have to change boxes for each track

thats why formula cars use hewlands and re gear for every track
(pro's too)


we build a duel purpose box for the same reason
it has a useable "F" first gear
some tracks are 1st-4th
others are 2nd - 5th

[carr914]

You want to be hitting redline in top gear at the end of your fastest straight. If you don't, you're geared too high.

But you are Not hitting Peak Power at Redline, you've already past it and are the down slope

Yeah, til you get to the next Track & don't have enough gear

optimum is still optimum...
even if you have to change boxes for each track

thats why formula cars use hewlands and re gear for every track
(pro's too)


we build a duel purpose box for the same reason
it has a useable "F" first gear
some tracks are 1st-4th
others are 2nd - 5th

Very few people on this Forum have the $$ for Multiple Gearbox's for a DE Car

[stownsen914]

You want to be hitting redline in top gear at the end of your fastest straight. If you don't, you're geared too high.

But you are Not hitting Peak Power at Redline, you've already past it and are the down slope

If you're optimizing for best power at a given track, you want to hit redline at the end of the longest straight. Yes, redline is past the power peak. Racers optimize maximum power "under the curve" (to borrow a calculus reference) by revving past the power peak so that the engine spends the most time near the power peak. This is true even for fifth gear.

I suppose you could make an argument to have top speed right at the power peak for a track with extrememly long straights like Lemans.

If you don't want to change gearsets for each track, you can gear to redline in fifth gear at the end of the longest straight at the fastest track you normally drive, and you're probably in good shape.

Scott

[brant]

Shane,

I didn't do a good job at providing you an answer
like everyone replied, there are lots of ways to do what you are after
and also diminishing returns at some point..

there is perfect, optimum, expensive

and option #2, less expensive, not perfect and better than stock


picking out gears often becomes a compromise
for example the chart might show that a "W" gear would be perfect, but that is an $800 gear and you can often get away with a different ratio that is not quite as good, but only $100

so for my first race trans, I think I was similar to you and trying to learn about transmissions/gearing without breaking the bank

take your longest track or most frequent track.. it sounds like sebring is already the track you've picked. gear for red line at sebring in 5th

I'm guessing that sebring may be your longest track also?

then depending on your current motor state of wear.. you may want to pick ratio's for your current motor redline and power, or for your not yet built.... next motor redline and hp.

usually the 4cylinder has a much lower redline than a 6, which gives you cheaper gear selection usually.

also you can probably get away with only changing the top 3 gears that you mostly use on the track.. that can save you a few dimes too. you can pick 2nd gear also but there are only 3 ways to do it... stock, GA not too bad, and 904= very expensive
So if stock F will work great.. and if not try to see if a GA 2nd will be acceptable for your needs?

you need to pick your 5th gear first and then plan the lower ratio's from there
Ideally the rpm drop between gears will get lower as you shift up... as the torque of the next gear is reduced due to higher speed and wind resistance you want the rpm drops to tighten

so off the top of my head,... my 4th to 5th gear rpm drop is currently only 900rpm
but my 3rd to 4th is slightly more... maybe 1200rpm (I can't remember)

the 4cylinder redline makes this less touchy than a high rpm of a -6. The pro's dyno each motor and pick gears using a dyno result... We aren't pro's so pick redline in 5th and go from there.

once you pick your 5th gear optimum.. then see if there is an affordable alternative and keep doing the same for 4th and 3rd.

I don't know sebring, but people commonly use M-S-X for 4cylinders with close to stock redlines... and a cheap version of MSX is Ka-S-X

my first race box was Ka-S-X with a locked diff, no cooler.

of course differentials, and cooling are more money and a whole-nother topic
and add a lot of money to the basic race transmission
you can likely get away with out them unless you want to spend that much money all at once. differentials add heat... (a locked diff wears out your tires but does not add heat)


brant

[Jetsetsurfshop]

Heres some pictures of my extra transmission. The shift linkage is completely different then mine. What do I have here? (IMG:style_emoticons/default/WTF.gif)

[Jetsetsurfshop]

(IMG:style_emoticons/default/WTF.gif)

[ThePaintedMan]

That, my friend is a tailshifter, which came on the 70-72 models. 73 is the year that they switched to side-shift. A tailshifter can be converted to side-shift, but for the amount of money and work involved, I never could understand why anyone would. There are still plenty of good side shifters out there that can be rebuilt.

[carr914]

A tailshifter can be converted to side-shift, but for the amount of money and work involved, I never could understand why anyone would.

The Only reason to do one is if it is a 914-6 Tail Shifter - you want to keep the Correct Case Numbers & the different Gearing

[stownsen914]

To comment on the option that Brant mentioned to use a locker diff ... I have one in my 914/6 racecar. I have been happy with it, and plan to continue using it instead of a differential. There are a couple reasons to not use a locker though:
1. If you ever intend to drive the car on the street or autocross it - cars with lockers don't like to make sharp turns AT ALL. I'm talking about the kind to pull in your driveway, park, etc. This is not an issue on the track.
2. For lower horsepower applications, the scrubbing of the inside tires that you get with a locker probably eats horsepower. For a momentum / lower horsepower car, you wouldn't want to give up the horsepower.

[brant]

To comment on the option that Brant mentioned to use a locker diff ... I have one in my 914/6 racecar. I have been happy with it, and plan to continue using it instead of a differential. There are a couple reasons to not use a locker though:
1. If you ever intend to drive the car on the street or autocross it - cars with lockers don't like to make sharp turns AT ALL. I'm talking about the kind to pull in your driveway, park, etc. This is not an issue on the track.
2. For lower horsepower applications, the scrubbing of the inside tires that you get with a locker probably eats horsepower. For a momentum / lower horsepower car, you wouldn't want to give up the horsepower.

those are good points
and they do eat horsepower..

however interestingly enough
we have a local talent, and my builder, that did back to back tests with a locked diff in a -4 cylinder car

the car was 2 seconds a lap faster with the locked differential


I loved the driving style of the locked differential. Especially in braking..
but I am currently running a TB differential due to a discussion I had with Paul Guard and my builder both...

still a locked diff is a very economical way to make a car faster if you are willing to live with the trade offs.

they aren't easy to drive until you learn them
and I doubt most people would like them based on first impressions
they are slower at first until you learn to adapt to them