915 tech, calling experts

[Levi_Harris]

Hello,
I need some guidance from the experts. What I am looking for is advice on whether or not a 915 can or should be used for my application and what upgrades would be considered necessary. My car is a glass bodied tube frame kit car that is intended to be a streetable track day car and a couple of times a year, off to a hill climb.
It's mostly Porsche and VW parts with some Audi thrown in as well, the suspension is all 944 turbo but the car still retains the VW torsion. The 915 is the largest transaxle that will fit without cutting the center section out of the torsion so it's my first pick. The cars finished weight should be around 1800lbs.
The power plant, which is also under construction, is a turbo charged type 5 wasserboxer putting out just shy of 400hp at 7000rpm. This estimate is based on results of an engine that was already built by an English tuner, check out the dyno sheet. Although it has, a lot of hp, it's small displacement at 2.1L and doesn't make a whole lot of torque. The peak torque is right at 300ft,lbs at 6800 rpm with the engine only producing about 200ft,lbs at 5500rpm. At very sane (streetable) boost levels…This engine went much, much further and at ludicrous boost levels pulled over 630hp. I will not be turning it up that high… ain’t no way… just too damn dangerous.
I do not plan on drag racing but hillclimb launches aren't much nicer... No clutch dumping though and yes, I know how to drive... the pedals are really close together for a reason.

Couple of things that are a must;
A quality LSD.
A heavy duty side cover.
Wevo gated shifter guts, just cause I want it…

Couple of things I’m not sure about;
Could I get away with a late mag case like a mag 915/61?
Would it be real stupid to use 7.31 gears? The aftermarket 8.35 gears cost way more than I want to spend... This isn't going to be cheap allready. An extra 3k is over my budget.
Is the two pulses per rev at these torque levels going to be a problem?
Do I need to have the case machined for bearing machined in bridles or will the drop in bearing support be enough?
Should I replace the first gear and mainshaft and the second gear with stronger parts? Guard?



For those who are interested, it's called an Amante GT. This one is, to the best of my knowledge, one of the few factory built cars. Along the line somewhere, it was seperated from it's original chassis. This car was last registed in 1972 in the San Jose, CA area.
As far as I know, there is only one still on the road today and it's corvair powered.



Thank you for your attention.

[Levi_Harris]

Nobody?
Oh well, I did have some correspondence with Hayden at Wevo, super cool guy who was extremely helpful.
I'd still very much like to hear some of your opinions as well.

Thank you much.

[winders]

What did Hayden have to say?

Scott

[Levi_Harris]

In a nutshell; Go for it. Late mag case, Wevo bearing support, heavy duty side cover and if the budget allows, shifter gate. Stick with the stock ratios at this point and stock 8.31.
Just good parts and care with a refresh and see how the car behaves. If it needs different gearing after the car is going address it then.
Said my torque figures where well suited for a 915.

[quattrorunner]

I run the 915/61 with 930 engine. Good ratios other than first being crazy. I'm sure I'm higher than 300lb.ft. and it works. I'm told to avoid drag racing launches and not to use full throttle in first. Your lower torque numbers would seem fine with the harder treatment. I've used other cars with 915 and launched them with 200lbft and the cars held up well.

[Elombard]

What is a wasser boxer?

I Think a 915 will be fine based on driving a few with 3.0s and 3.2s and reading about others exploits with larger more powerful motors.

[Levi_Harris]

A wasserboxer is lingo for a type 5 Vw engine. It's a watercooled horizontally opposed four cylinder. Most reside in late model vanagons but they have been proven a time or two as very potent turbo motors. The heads are pretty good out of the box and they are very light. A long block is only about 260lbs.

[Elombard]

Wow - I did not realize that VW ever put water to the opposed motors. Thanks

[Porsche84]

Levi

You should consider special gears for 3, 4 and 5 instead of a lower R/P. There are out there used so you don't need to spend a ton. I did that and the rpm drops 800 to 1000 between shifts which keeps the engine in the power band, better than a lower R/P. I'm using a 915 with a 3.6 pushing near 300 hp. I used the Wevo bearing retainer and the Wevo gate inside the trans. I'm using the later factory side cover with no issues. They are quite a bit stronger than the early ones (79 and older I think).

[Elombard]

Hey Jerry,

If I wanted to re gear as you describe, what gears woudl I look for? Are they all aftermarket or are you mix and matching from different years?

I guess I should do some searching.

[Porsche84]

Hi Erik

Porsche made a lot of special gears when they introduced the 915. You could order a "hill climb" or an "airport" set for instance. Gearing depends on what you want to do with the car. Taller gears if you want to save fuel, closer ratios if you are doing track. I built a spreadsheet and decided what I needed top speed to be (Watkins Glen back straight) then picked that gear. Then I picked gears that stayed within 800 to 1000 rpm after a shift. The chart below was from the Carquip website and is what I use to see what is available. However, I have been able to find used Porsche gears except for one gear I ordered new.

[Matt Monson]

We manufacture almost every gear that Jerry has listed there. But they will be 2-3 times the price of used factory race gears. And about the same price as NOS factory gears, but much better made than what they produced 25-30 years ago...

Regards,

Matt Monson
Guard Transmission LLC

[Porsche84]

I second Matt's comment. Guard gears are the best.