Gearbox ratios for performance engines

[Haveacamper]

Just wondered if any if those with performance engines around the in 180+BHP have upgraded their gearbox ratios to improve 1st and 2nd.

My 091 box 1st gear is pretty much non existent when accelerating off the line.

I've had my 3rd and 4th changed but I'm now thinking that 1st and 2nd would be better off closer to the Porsche standard box ratios.

1st and 2nd are ok if pootling around but as I've changed third to span the gap to my longer 4th gear I now have a bit of a gap from 2nd to third.

Not sure I fancy the work or cost involved going 5speed especially as I've sunk x amount im to rebuilding the 091 with possibly less than ideal ratios.

I appreciate that 1st and 2nd will mean a new main shaft and islets rather than just gearsets

 

[K@rlos]

I’ve had my 091 built by a John Walkett for my bus which is now putting out about 200hp and he recommended putting a longer 3rd & 4th in but leaving 1st & 2nd stock ratio. That’s as far as my help can go I’m afraid as I’ve not yet driven the bus with this box or revised motor.

However I built a bus for someone about 3 years ago which had a stock 091 bolted to a 1955cc which was about 145hp and that pulled hard with the 091 ratios. But naturally you may well need those gears to be longer due to your engine. I also suspect I may require the same in the fullness of time lol. I’d quite fancy a Berg 5 speed but the price is horrendous and I’ve heard tails of them being weak too

 

[Haveacamper]

Thanks mate, Thats interesting about the feedback from John. I had similar when I had Bears motorsport rebuild my gearbox. I think the reason being is the cost of replacing 1st and 2nd gear. 3rd and 4th are simply gearsets you change where as to 1st and 2nd gear are all part of the main shaft, so you have to purchase specific aftermarket main shafts.

In my case, I appreciate that the buses are heavier than the original porsches, and that the buses were designed to be loaded up and used as a work horse, but that it not what the majority of us use it for.

I have now found that with the available torque from my engine, and kind of acceleration off the line (and I'm not talking about red lining it here) 1st gear becomes pointless. Comparing the original 091 and the 915 box that would have come with my 911 SC engine you can see the disparity:

(I've included the Final drive in the following overall gear ratios)

091
1st - 17.37
2nd - 9.41
3rd - 5.76
4th - 4.06

915
1st - 12.33
2nd - 7.10
3rd - 4.89
4th - 3.88
5th - 3.08

Here is the saw tooth curve comparing the two gearboxes when changing gear at 3000RPM:

The dashed lines show my taller 4th gear and the taller 3rd gear I choose to try and span the gap.

 

[Tofufi]

In my case, I appreciate that the buses are heavier than the original porsches, and that the buses were designed to be loaded up and used as a work horse, but that it not what the majority of us use it for.

I have now found that with the available torque from my engine, and kind of acceleration off the line (and I'm not talking about red lining it here) 1st gear becomes pointless. Comparing the original 091 and the 915 box that would have come with my 911 SC engine you can see the disparity:

(I've included the Final drive in the following overall gear ratios)

091
1st - 17.37
2nd - 9.41
3rd - 5.76
4th - 4.06

915
1st - 12.33
2nd - 7.10
3rd - 4.89
4th - 3.88
5th - 3.08

I don't have quite the power you have, but I've got the following ratios:
(numbers in brackets are including the diff to compare with yours)

1st gear: 3.091 (13.69)
2nd gear: 1.889 (8.37)
3rd gear: 1.318 (5.84)
4th gear: 1.040 (4.60)
5th gear: 0.857 (3.80)

It's lovely to drive as the gear ratios just flow into each other. I'd imagine the large gaps between the ratios on your 091 make it a lot more awkward if you're not pressing on. I now have 3 gears in which I can do 60mph...

My 1st gear is a good ratio with just under 100bhp, so I can imagine how useless the 17.37 is with 180bhp!

With your additional power I'm sure it could easily pull the 3.08 5th gear for very relaxed cruising, just make sure the engine stays cool

 

[K@rlos]

I imagine you could do with a longer 1st and maybe 2nd as a Porsche flat 6 has a considerable amount of low down torque. It was a case of shooting in the dark a little for me as I had this box built I’d not driven the bus with this hotter spec motor. Before I had a pro street box with stroker motor in a milder guise. Hopefully the ratios went for will work well but it’s something only time will tell. Although that said I spoke to mates who have similar engines and asked them about gear ratios combos like you have.

It’s a pain in the arse but maybe it’s worth getting a longer 1st and 2nd fitted.

 

[Haveacamper]

Thanks Tofufi. I was actually reading your 5-speed conversion the other day before I wrote my thread. A 5-speed would certainly be beneficial!

It seems that Weddle do a first and second gear combination mainshaft with 3.11 and 1.93. Which would give the overall ratios (with my already longer 3rd and 4th) as:

1st - 14.22
2nd - 8.82
3rd - 5.37
4th - 3.47

Again, 1st is still shorter than the box the engine was originally paired to, but dam sight closer than the 17.37 from stock 091 box. regarding maintaining cooling, the 915 ratios are the standard ratios for the porsche engine, so as long as I dont go taller than that, then I should be good. I'll be ensuring my cruising speeds at 100, 80, 60 kph etc are reasonable.

@Karlos, I would be very interested to hear how you go with the new box paired onto the new engine

 

[Tofufi]

To be fair, with the amount of power and torque you have it's possible that the extra gear ratio won't be needed as much as for someone with a less stout engine. I'd not spotted you had a Porsche motor rather than VW.

To my mind, the 5-speed is a much nicer option - the only thing is whether the cost can be justified, especially for a 915 gearbox as they are daft money now. If I were you I'd keep it as it is until you find a cheap Porsche box, then sell on the current one to offset the cost