J Chen

Yes I agree, a big bore should
be the way to go but has anyone
driven a properly set-up 2.5L engine
with a 6 speeder ? If all out power
is put out of the equation, I would
think that a 6 speed 2.5L would be
compatible to a 2.8 or 3L 5 speed.

glen2002

If you are looking for a performance improvement I would go with the S2 ring and pinion which also gives the performance improvement but for less money. I have this on my other car. Mind you I do like the 6 speed, just wouldn't part with 10 large ones for it before other priorities

glen2002

JChen, you are right the lower gearing is probably worth 50-100hp

CPTdooberhead

This could be resolved by taking the ABS components from the 968 when installing the tranny in a 951, which is where speed is drawn from on the 968... I don't see why everyone thinks having the tranny AND use of the speedo on the 951 is so impossible...

- Randy

rage2

After getting the 2.8L big bore block, I'm no longer wanting a 6 speed. The gear ratios are perfect now, even with a big turbo... huge powerband!

I opted for a 2.8L all bore instead of a 3.0L+ bore and stroke because I wanted to keep reving high and not move my torque band. Not sure how well a stroked motor would like 7000rpm track day duty.

J Chen

Thanks guys for sharing your views.

Darius Juca

I was not sure about the strength...what about the 1 st gear ratio? would it be too short? I have allways prefered less gears..my favourite was my 930..4 speed....

J Chen

Less gears would be good only
if you have mega torque to
compensate for the wide gear
spacing.

Laust Pedersen

Because of the typical torque drop-off as a function of rpm, the HP curve is almost flat between 4000 and 6000 rpm (or 5000 and 6500 rpm depending on the turbo). If the gear ratios are designed so the engine can be kept between those rpm values then more gears will not increase acceleration. In essence if you can keep the rpm between those values it does not matter which gear you are in.

Looking at the ratios for our 5-speed gearbox, it will drop the rpm’s from 6000 to 3500, 4100, 4500 and 4800 rpm when going up through the gears. So only the step from 1st to 2nd is not fully optimal (for acceleration purpose), assuming constant power between 4000 and 6000 rpm.

J Chen

Hence the reason for my thread Laust. If it were you, which approach
would you take ? I figure that a properly
set-up 2.5L in combination with a 6 speed
should be compatible with a big bore with
5 speed.

Duke

Are you sure? I actually thought about the rpm drop between 1st and 2nd today while driving to work and I saw a 1500rpm drop, not 2500rpm.
Or perhaps I was just too tired!?

hosrom_951

More power = faster accelration, generally. By madding to a big bore, you would be looking at 500+hp right? it would be better to sinatll an 89 turbo S gearbox. Known for harder first gears and reloiablity overall. If this is a matter of speed. You can always replace the fifth gear with a longer one. Would be pricey, but that what you do for speed.

As acceleration is concerned, if you would install a 968 tranny on a 951 (whichcould be done) then it should be from a 968 turbo car (they came with 305hp stock).

But again, the ratios have to be reviewed and compared with the 951 tranny.

I would personally go for a bigger engine, turbo ect. because its much less of a hassle and lots of people done it, so experience is always a better hand.

Transmissions are delicate, not many people messed with them. Even Powerhaus who claim to have a 500+ STREET 951, their driveshaft system only saw a better and stronger clutch. Stock tranny.........

tommo951

A friend of mine has a 968 turbo, yes an original factory one and that has the late 944 Turbo 5 speed unit so the answer is there.
Go for the 3 litre instead it will allow you to do even more in the future. The 6 speed is not really required if you get your torque curve right. Everybody seems over concerned with pure BHP but it is torque that will get you off the line and keep you accelerating through the gears.

ljd-924SE

my vote would be to sell the 951, purchase a high mileage 968 for cheap, and do the powerhaus 968 turbo conversion.

J Chen

Would you sell or trade your
beautiful wife for a ugly one ?