I need a 6th gear...

[dd74]

So I'm driving home on the freeway, and I'm in my top gear (5th) at around 4,000 (80 mph). At this rate, torque and horsepower are fairly in the sweet spot of a stock 3.0 with a 7.31 r/p and 50-series tires. I can pretty much accelerate with an eye's blink.

Here's the sitch: I start wondering if I can adapt a 6th gear to my 915 box. Is that possible? Has anyone done such an adaptation? I'm not complaining about my set up - I love it, actually. I'm just wondering about the possibilities.

If so, I'd keep the gears as they are, while leaving 6th as a true cruise gear - i.e. nice and low.

What do you guys think?

[Adam]

I've heard that you can piggy-back an extra gear on 4-speed 915s, for just that reason, but I don't know if the 5-speed has any room left.

Pity they don't have a tail/propshaft, you could add a Laycock-style electric overdrive!

[ChrisBennet]

You already have a 6th gear - it's just in the wrong direction.
-Chris

[dd74]

Originally posted by ChrisBennet
You already have a 6th gear - it's just in the wrong direction.
-Chris

Nuyk! Nuyk! Nuyk!

[dd74]

I suffer from one of those unfortunate scenarios where I lose phone numbers. Nonetheless, at one time, I once had the home phone of the daughter of the old man who created Hewland transmissions. Great woman, but then I didn't need a tranny then.

Wish I had her phone number now.

[cstreit]

I've heard that Helands were mainly for lower horsepower cars...

Why not just change out 5th gear for something taller?

[sithot]

Two choices:

Go to a higher aspect ratio.

Get new gears for your box.


First choice is cheap, second is not. Figure on $5,000 for a nicely done "stacked" box or just buy an over-drive 5th for your car. Tons of choices which may or may not cost tons of money!

Good luck,

Tom

[dd74]

Hewland makes one box for 140 hp max single seat race cars. The rest look to be for GT, GTS, LMP, etc race cars.

According to their web site, they don't suggest their transmissions be used in street cars. Straight-cut gears and all, just aren't streetable, I guess.

I could swap out fifth, but because of my torque, fifth has become almost a power gear now, and at about 90-100 mph, it's got a lot of punch, probably attributed to my car's weight and the small tires. I indeed like that as do most who've driven my set up.

Someone in Excellence advertises modified 915 and G50 transmissions allowing for sixth gearing, but I just can't remember who.

I would go with Hewland, but the cheapest (and weakest) tranny they have is about $20K!

[dd74]

Originally posted by sithot
Two choices:

Go to a higher aspect ratio.

Get new gears for your box.


First choice is cheap, second is not. Figure on $5,000 for a nicely done "stacked" box or just buy an over-drive 5th for your car. Tons of choices which may or may not cost tons of money!

Good luck,

Tom

Thanks for the suggestion, Tom. But I do want to avoid higher aspect ratios. I went lower specifically to guarantee my car's responsiveness within the powerband I'm most likely to drive in. I'm usually at 4,000 to 5,500 RPMs with my engine, and I think 50-series tires truly help that out.

As far as a 5th overdrive, 5th is already overdrive for the car. But the way the car is set up now, 5th no longer acts like an overdrive gear.

[Bill Verburg]

993 6 spd can be installed w/ some ..ummm.. mods

or

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1st 2nd 3rd 4th 5th 6th

3.400 2.000 1.276 0.941 0.717 0.612 Helical

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[ruf-porsche]

Originally posted by sithot
Two choices:

Go to a higher aspect ratio.

Get new gears for your box.


First choice is cheap, second is not. Figure on $5,000 for a nicely done "stacked" box or just buy an over-drive 5th for your car. Tons of choices which may or may not cost tons of money!

Good luck,

Tom

Third choice buy a 996 with a six speed trans!

[Por_sha911]

Originally posted by ruf-porsche
Third choice buy a 996 with a six speed trans!

I like your thinking. When in doubt, find an excuse to buy yet another P-car. Then, sit back with a cup of good coffee and admire your accomplishments in life.

[Tyson Schmidt]

Your only option is a taller 5th. But you already have the taller 29/21 5th gear in your '74 trans. (Versus the 29/22 of the earlier 915's) But if you could find the 5th gear from a 912E, it is taller still.

I used to think my 5th gear was too short, until I rebuilt my 2.4 to E/Solex spec and raised the rev limit to 7,100. Now it's just right!

[wevoid]

Originally posted by dd74


According to their web site, they don't suggest their transmissions be used in street cars. Straight-cut gears and all, just aren't streetable, I guess.

dd74,

The straight cut gears are not the problem, many street cars have had staight cut gears, none recently I would guess, noise is pretty high. Mini, for one I can recall, Austin Healy etc.

Main problem in the Hewland transmissions you were looking at is the absence of synchronizers.

I spent about 4 months with an XT_915, with dog gear engagement, non synchronized, in my road car when we were doing some product development. Cool fun doing clutchless, close ratio, near full throttle upshifts. Makes a CIS 2.7 feel like 300 hp.

The noise and the inconvenience in traffic was woeful, once the R&D was over, it came straight back out.

It is possible to do a 6 speed 915 with sequential shifting, but again with dog gears intended for racing. We have a layout for this solution, but have not taken any steps to build a test unit.

Regards

Hayden

[Tyson Schmidt]

Yeah, they are NOISY!

I've driven racecars with straight-cut gears, and the open exhaust seemed pleasant and calming by comparison.

Gotta love dog-engagement though! Who needs a clutch? Reminds me of my dirt bike days, shifting a dog-box.