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Old 10-08-2018, 05:56 PM   #1
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6 speed in 2.5

Iím a little surprised I canít find the answer to this but Iím thinking about installing a 6 speed in my 2.5l time trial car to increase my acceleration (my weight and hp are constrained). What all is involved in this? Do the half shafts, flywheel, etc need to be swapped out?
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Old 10-08-2018, 06:57 PM   #2
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Follow up question. Can a 2.5l final drove be installed in a 6 speed or can the triptronic final drive be used?
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Last edited by truegearhead; 10-08-2018 at 07:04 PM.
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Old 10-08-2018, 09:59 PM   #3
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final drive is the wheels - change your sidewall height?
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Old 10-09-2018, 12:58 PM   #4
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final drive is the wheels - change your sidewall height?
Iíve already choked down on the diameter for this reason but am thinking about going to 16Ē wheels. How about this option, looking at the ratios a 2.7 transmission with a 2.5ís final drive would give me a 6% lower gear in 4th and 5th. Anyone know of a 2.5 final drove can be installed in a 2.7 transmission?
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Old 10-09-2018, 03:09 PM   #5
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Iíve already choked down on the diameter for this reason but am thinking about going to 16Ē wheels. How about this option, looking at the ratios a 2.7 transmission with a 2.5ís final drive would give me a 6% lower gear in 4th and 5th. Anyone know of a 2.5 final drove can be installed in a 2.7 transmission?
Since ring and pinion have to be matched I'd think you'd have to tear down the trans to change the pinion. Even though both trans use a 9 tooth pinion gear the pitch difference would change from a 32 tooth to a 35 tooth ring gear.
Why not just install the 2.5 trans? You get the better gearing and can easily go back.
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Old 10-09-2018, 03:38 PM   #6
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I have a 2.5 trans now, the 2.7 trans has shorter 4th and 5th gears but a taller finally drive
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Old 10-09-2018, 03:42 PM   #7
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Well perhaps this option.

Audi Swap

Ok never mind I looked up in the Bently. You already have a 3.56 final drive ratio.......

Too bad you're not closer I'd give you a set of 16" wheels just to get out of my garage...

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Old 10-09-2018, 04:01 PM   #8
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Well perhaps this option.

Audi Swap

Ok never mind I looked up in the Bently. You already have a 3.56 final drive ratio.......
I should have a 3.889 (same as Audi), the 2.7 has the 3.55.
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Old 10-09-2018, 04:30 PM   #9
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perhaps gbox or cms can fab you a ring and pinion to your specs. expensive, but probably cheaper than a transmission swap.
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Old 10-09-2018, 04:37 PM   #10
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Probably should have mentioned this rule. The gear set has to be from a boxster

Any internal gear(s) from the original manufacturer (or donor vehicle manufacturer in the case of an engine/transmission swap) may be used, provided they originate from a model that is eligible for competition in ST5, and they are not straight cut/dog ring gears
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Old 10-10-2018, 10:16 AM   #11
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I don't think you'll gain too much from a swap like that. The 6speed is practically geared like the 97-99 5spd plus a 6th gear. It would basically just give you a better cruising gear on the highway.

I'm fairly sure a 2.7 trans with a 2.5 ring and pinion swap is possible. My real question is does the pinion shaft have to come out of the transmission or does the gear possibly bolt on and and can be done through the differential opening.

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Old 10-10-2018, 08:38 PM   #12
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To me, Iíd rather keep 5-speed tranny with 2.5 engine and upgrade the engine (even to 2.7) to gain acceleration. Or reduce weight... which Iím in mission now, so far ditched 35 lbs and shooting for 100lbs incl. spare tire/ tool kit. Stereo is gone but I had a cassette player with only 4 tapes left in my music library which are stretching out so no real loss. It feels better and dash without center console/ shelves adds spartan old schoolness from 70ís 911/ 914.
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Old 10-11-2018, 03:15 AM   #13
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I don't think you'll gain too much from a swap like that. The 6speed is practically geared like the 97-99 5spd plus a 6th gear. It would basically just give you a better cruising gear on the highway.

I'm fairly sure a 2.7 trans with a 2.5 ring and pinion swap is possible. My real question is does the pinion shaft have to come out of the transmission or does the gear possibly bolt on and and can be done through the differential opening.
My question as well. It would be great if I didnít have to brake the whole thing apart. As far as the gain, Iím not expecting a lot but itís a time trial car. Iím looking for tenths of seconds on 2 minute lap out of this. 300-500rpm would do wonders on different parts of the track.
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Old 10-11-2018, 09:17 AM   #14
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My question as well. It would be great if I didn’t have to brake the whole thing apart. As far as the gain, I’m not expecting a lot but it’s a time trial car. I’m looking for tenths of seconds on 2 minute lap out of this. 300-500rpm would do wonders on different parts of the track.
Sorry for the senior moment previously. My reading comprehension is usually fairly good, but I was definitely confused.

To replace the pinion shaft it requires a complete transmission teardown and Ring gear and pinion shaft are sold as matched sets. Part #28 is the pinion shaft.



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Old 10-26-2018, 02:11 PM   #15
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It'd be really cool if the end of the shaft was keyed and the pinion gear was able to be swapped by removing a nut. There's probably a reason they don't do that.
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Old 10-26-2018, 04:46 PM   #16
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You are right, there is a reason why that isn't done. You'd also have to change the ring gear if you change the pinion gear (they have to mesh to each other; if you change the ratio, you change the number of teeth, and no more meshing of gears).
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Old 11-02-2018, 03:52 PM   #17
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You are right, there is a reason why that isn't done. You'd also have to change the ring gear if you change the pinion gear (they have to mesh to each other; if you change the ratio, you change the number of teeth, and no more meshing of gears).
Changing the ring gear is a piece of cake. I think it's pretty much understood that if you change the pinion gear you also have to change the ring gear. That in itself is not a reason why the pinion gear isn't easily swappable.
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