One of the few things that keeps me up late at night

Current motor 190hp/210tq and eventually an upgrade swap to 287hp/274tq.

Mostly for fun on weekends and World events, a few autoX. Obviously low would be better for drag racing but I think high would be more comfortable in 30-50mph city driving and it's also close to the Suby box gearing.

Opinions please.

Stock gears + H 5th
Click to view attachment

Taller 3rd & 4th to balance H 5th
Click to view attachment

Seems like you need to pick a role before you'll get sensible opinions: would you rather drag race or drive on the street?

What kind of motor, btw?

Its more about your torque band and how you drive it than anything.

If your going to do big track days or drag racing you can optimize your gears to JUST stay in the powerband. Remember, shifting slows you down.


However if you do autox or tight canyon runs you'll want a close ratio box so you can have a couple of gears in the powerband.

Close ratio box is fun.


Also another thing to note is if your running a turbo motor it takes SOME time to spool up (That time dependent on your turbo, tune, fuel) If its a tiny turbo its not much at all and in fact it will help your low end, a tight gearbox is nice for that. If its a big turbo like mine you can have a full 1-3 seconds of spool depending on what RPM you get on it. You'll need a slightly longer gearbox to be able to have room to spool and stay in powerband, but not to long that shifting puts you below your full spool RPM.

There is no perfect set-up.

I'm going with A,F,O,X,HA
HA (flipped) only because I had it, in fact I had them all except X which I had to buy. I forget what I paid for it, something like $150 for the set.
This is a good hot-street set-up for around the HP you are talking about.

Chris-I think your spreadsheet is calculating tractive force incorrectly; it appears to be missing the final drive ratio in the calculation.

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Stock with H looks better to me. It keeps more of a parabola shape (if you image connecting all of the dots) which is as close to an ideal power curve as you can achieve.

Does your torque curve really look like that? It's pretty flat. If you are down any torque at the low end (from expected) and go with the taller 3-5 you might find the car sluggish right after shifts.

No way I would gear a box for 200mph. No where to use it. It just becomes a tall 5th that is almost useless.

Pick a top speed where you think you will top out 5th at the end of your favorite straight and work backwards from there. Usually this will be somewhere around 140 - 145 mph.

I also hope you are going to be careful putting 270 ft#s through a 914 box. Make sure you install the usual strength upgrades and a cooler if you expect any life out of it. HP is not the problem. Its that relatively flat, high value torque curve in your calcs.

As far as a 5th gear H is concerned I think you are missing the point, it's used for a 3000rpm 80 mph highway economy gear with a big engine...not for going 200mph.

Again there's absolutely no such thing as a perfect transmission.
When choosing a gears (or cams, etc) for your 914 you have to be 100% honest with yourself and your intentions for the car.
A short geared over cammed car will be nothing but a nightmare as a street car. Just as sure as a mild cam, tall gear street cruiser will be a slug on the track.

The second chart definitely has better spacing between the gears than the first one. The rule of thumb for a racing gearbox is to have even mph (not RPM) spacing between your shift points. The second chart comes close to this. Since you're looking for a good cruising 5th gear, it looks like you've come up with a good compromise. A taller 2nd might be nice, but options are more limited there. GA is a good choice if you can find one at a decent price. I like that you are using a "PA" third (a backwards X) - I have that in my racecar!

Take the advice about beefing up your trans if you are planning to put close to 300 ft-lbs through it. As you probably know, many would say that's above the threshold of what a 901 will like. Treat it gently, especially in 1st, and no launches.

Scott

Understood. I would go with B first, GA or H second (904 mainshaft) and then restack if you want the rH 5th.

A will almost be useless with that much torque. An F second is pretty short. GA is not much better. My 911 has a GA second and its almost too short for 150Hp

For autoX, I suggest setting things up so that redline in 2nd gear with your tire size is right around 60 MPH. That is a common maximum speed for autoX events, and not having to shift up to 3rd to get there is a nice thing. Each shift costs you somewhere between a half-second and a second, depending on how quickly you are willing (and able!!) to shift. If your local events tend to run faster, it may be worth gearing things just a little taller or rather shorter so that you have a useful 3rd gear for events.

For a track car, you want to set up your top gear to redline at your top possible speed on the longest straight of the track you run the most. (Or just a bit over, in case you find more speed later.) Work backwards down to 2nd gear (or 3rd for 914s with their fixed 2nd gears) making sure the gaps are relatively even, or that some gears are set up for specific straights with the speed of the car in redline at the gear being the maximum for the particular straight.

For the street, you generally want nice tall gears. Very especially the top gear, because lower RPMs are almost always better for fuel economy, noise, and even engine wear. One semi-popular way to go is to set up the lower few gears relatively close for good acceleration out of slower corners, as tall a top gear as reasonably possible, then fiddle the almost-top gears (4th in the case of a 914 box, maybe 4th and 5th for a 6-speed box) to transition between the tighter low gear spacing and the taller top gear.

--DD

90% street car, 10%

Nissan VQ30de (Maxima) -upgrade VQ35de (350z w/FWD IM & cable TB & Nistune)


Bradhole created the file and posted it. I've just made changes as needed. Yes, what's that formula supposed to be?
That chart still references the original data on page 1, an 8000rpm -6. I'll post flat in a bit


Exactly.


Calculating the cost of the X gear + billet upgrades + at the limits makes engineering an adapter to the WRX trans seem smarter.

The seals are leaking pretty bad so I figured I'd look into my options while I'm there. Feels like the stock ratios would be the better fit with the 3.0L.

In your app 1st gear is almost useless, except for a be-low and hill starts.
2nd nothing (cheap) you can do, it's a fixed gear.
3/4 you can play with.
5th rH is your overdrive.

What I did is I built a custom box and a bone stock 914/4 box as a back-up.
My gearing A,F,O,X, rHA and was a Dr.Evil recommendation, except for my rHA gear, he did recommend H.
The rHA is a bit cheaper and is the 2nd tallest 5th gear combo, it's one tooth off of a H.

This combo is a hair taller than stock because the box will be treated basicly like a 4-speed, my 914 is 100% street.
My 914 engine is a 3.0 weber, 120/104 webcam (close to a GE60), twin plug 10:1, should be well over 200hp but still a hot street engine..

BTW my 1967 VW beetle is a 2600cc T4, 180hp with a 911-901 with bone stock 914 gearing, 195/65-15 tires and it's plenty fast enough to put a shit eating grin on your face.

That VQ35 is a nice motor Get the Uprev motor and get it professionally tuned by a nissan guy. They can make some good power with a free intake/exhaust and a pro tune.

That will be a fantastic motor in a 914


Go with the equal spacing gears. You wont need a close ratio gearbox. That motor has plenty of torque to lug around if you need to.

Uprev Osiris = Egas pedal and 350z/G35 gauges. Plus it's not really DIY tuning friendly.
Nistune is like MegaSquirt on steroids. I think with some component swapping it could run an EG33.

Happen to have a drawing of the Suby trans bolt pattern?