Best gearing for 915/3.6

[Mike the mechanic]

Hey guys, does anyone know what would be optimum gearing for a 915 mated to a 993 3.6 in an SC nearing 300bhp? The car is a street car and will be tracked on weekends. Top speed is not a concern, but wouldn't want it to rev too high at 65mph.

Also, where could we buy a rebuilt 915 with our gear choices and perhaps some mods such as billet side cover, steel syncros, etc...
Thanks!

[Moses]

Mike,

We all struggle with this a bit. Todd (Embs) and I would like a taller first gear because we get to the redline so fast with the present gearing. Jack Olsen wants even shorter gearing to take advantage of the torque curve on the track. Some folks want a close-set gear ratio for matching revs during a shift.

First, decide what you want. When the tranny is out, dropping in a new gear set is not too difficult or expensive. Bill Verberg is one of the local experts on gear ratios. If you let him know what changes you are looking for, he will offer good advice.

[Jack Olsen]

Optimum is a relative term. Ideally, you'd dyno the engine, and match each gear to the particularities of your torque curve. In my case, I was most concerened with close ratios for 2-5, since that's what I use on my local tracks. My car is not optimized for 0-60 times, and has a very low top speed (136 mph, with current tires).

For a mostly-street car, my gut feeling would be to go for a strong, aluminum 915 box with stock gearing. 300 hp in an SC is not going to leave you craving a whole lot more power -- unless you're into drag racing.

[Bill Verburg]

Several things to consider are tires and use.
here' mine 915/67 w/ 25.7" tire, 8:31

first is way too low(high numerical), tires can be spun uselessly w/ ease, The rest works great for mostly street use w/ a lot of interstate use.

The gearing looks like this in use(compared to a proposed 265/35x17 tire


If this was a track car, I would make 4^th 5^th, get a a taller 1^st and 2^nd and put 4^th inbtween the old 3^rd and 4^th

[Mike the mechanic]

Guys, thanks for the replies. What I had in mind was to keep the stock r/p, and first gear, and install slightly closer ratio 2-5th. Jack you are right about not craving too much more power out of a 300hp sc, as this engine has gobs of torque everywhere. The thing is that the tranny has to come out for a rebuild pretty soon, and the owner of the car would like to make a few upgrades where he can.
Bill, thanks for taking the time to post the chart and all the numbers, greatly appreciated. The tire size is presently 265/35/18, but he'll probably go with 315/30/18 this spring. I don't know the diameter, so i'll check it up on Tire rack's website.
Thanks guys!
Is Powerhaus II a good place to shop for gears (or rebuilt tranny with our gear choices)?

[ischmitz]

I really like my early 915 7:31 r/p in my 74 Targa with 964 3.6 The fifth is long enough to be able to cruise at comfortable at around 3000RPM and 70mph while the lower gears are short enough to cause some real damage. Almost feels like a closed ratio box up to 4th. ... I know there has been alot of talk about the early 915's being too weak for the 3.6 power but I am happily driving mine for about 6 month now and 5000miles without any sign of deterioration. I guess the fact that I remind myself that it's not a drag racer every once in a while and don't launch it like an idiot helpedWe'll see how it'll hold up at Willow Springs on Wednesday

Ingo

[k9handler]

the only part of my 915 to show wear is the main bearing, it's shot and I am considering how to either repair or replace. I have adjusted the clutch so that it eliminated the vibrations caused by the bearing...but I am in need of a solid fix real soon. My first thought was to simply purchase a used tranny and replace the one in it now, and then with time to work I could tweek the tranny and get a better understanding of how it all works.

As for gearing...1st gear is just to get the car in motion, 2nd gear is where I make the jump and hit appx 40 mph, 3rd is the bread and butter (best gear) as it runs all the way to 90 mph! 4th gear is great and will pull strong all the way to redline at 120 mph, while 5th allows me to drive at 85 mph (3500 rpm) and will pull strong all the way to redline and a top speed of appx 160+ (guage stops at 150).

As for the speeds I mention, the guage may be off a bit but when I used a GPS it was very close....hmmm I guess I could do a top speed run with the GPS! Tranny fix first!

[Bill Verburg]

We've bought some of the holey gears from them, nice pieces!. Paul Guard is another good choice.

[widebody911]

While you're in there, I strongly suggest that you install some sort of a cooling system for the transmission, as well as the heavy duty side plate and bearing carrier.

While were at it - when did they go to the aluminum transmission case?

[Bill Verburg]

when did they go to the aluminum transmission case?

'78 SC

If you've got 300 hp or more importantly the wide torque band that probably comes w/ it(3.8?) then the taller tires and gearing are a good match. I liked the stock mg '76 915/49 that came w/ my car w/ 245/45x16(24.5") tires and stock 964 engine.

[widebody911]

How detrimental is it to continue using the older mag case?

I ask because I have a lot of bearing noise 'already' - it's only been a year or so since my last rebuild. This is with the HD upgrades, SWEPCO fluid, and a cooler. It just seems that I'm wearing this baby out faster than I'd hoped, and I don't track mine nearly as much as Jack does.

[wevoid]

Thom

Using the old mag case is not really a problem - if it is in good condition.
Your rebuild might have been into already significantly worn cases, as a result the issues that lead to the original wear have been exacerbated by the additional torque of your 3.6l motor.

The HD upgrades are generally only going to add a margin of safety to an otherwise sound transmission, none of the upgrades I am aware of will actually prolong the life of an ailing or worn transmission.

Mag cases have an iron insert in the pinion bulkhead bearing that prolongs the interference fit of the key pinion roller bearing, but even those are eventually going to stretch.

In answer to the ratios question, I dont ever recommend changing second for a street car (rarely for a track car too). The first gear is such a compromise, often people look at changing second, purely to reduce the compromise of first gear.....
Then the compromise tends to move upstream to effect all the gears, without creating an advantage that warrants the extra cost of the new 2nd gear.

Changing 3rd, 4th and 5th will achieve nearly everything you could ever want, if one of the stock ratio packages is not close enough.

Aluminum cases started with the 915/61 version circa 1977, where there were both aluminum and magnesium versions of the 915/61.

Regards

Hayden