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Old 07-30-2018, 06:43 AM   #1
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986 gearbox failure

Hi all,

I've created an account on here just to ask this, but I've got a 2002 2.7 and second gear exploded on me the other day so I'm looking to get a replacement gearbox. I've heard from a few people that the 2.5 gearbox will fit my car, and has shorted ratios which I'd like as my main criticism has always been the gear ratios are just too long. If anyone has any input, or if anyone has done the swap please enlighten me on if it is possible, and if it is what I'd need to make it work.

Cheers,

Liam.

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Old 07-30-2018, 05:14 PM   #2
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It's a direct swap. I've only known people to do that swap on race cars so I'm not sure if your speedometer will be off or not.
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Old 07-30-2018, 05:26 PM   #3
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Is there a big difference in the gearing? I don't like the gearing in my '02, the lower gears are too tall, and the upper gears (at least 5th) is too short.
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Old 07-30-2018, 07:54 PM   #4
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Is there a big difference in the gearing? I don't like the gearing in my '02, the lower gears are too tall, and the upper gears (at least 5th) is too short.
Too tall or too short is hard to quantify so here are the gear ratios so you can make your own determination:

G8600 (97-99)
1st gear 3.50
2nd gear 2.118
3rd gear 1.429
4th gear 1.029
5th gear 0.79
Final Drive 3.89

G8601 (00-04 Base)
1st gear 3.50
2nd gear 2.118
3rd gear 1.429
4th gear 1.09
5th gear 0.838
Final Drive 3.56

G8620 (00-04 S)
1st gear 3.818
2nd gear 2.20
3rd gear 1.516
4th gear 1.216
5th gear 1.024
6th gear 0.841
Final drive 3.444
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Old 07-30-2018, 08:20 PM   #5
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Audi/VW have a similar transmission with some other possibilities for gear ratio. I don't have first hand knowledge, but here's an old thread:
Audi Swap

Looks like its not quite a 'direct' swap, but the parts needed can be swapped from the old transmission.
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Old 07-31-2018, 12:21 AM   #6
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Is there a big difference in the gearing? I don't like the gearing in my '02, the lower gears are too tall, and the upper gears (at least 5th) is too short.
My entirely unscientific research (watching acceleration videos of 2.5's and 2.7's back to back) leads me to believe that the lower gears are in fact shorter, no idea about the higer gears though.
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Old 07-31-2018, 01:44 AM   #7
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I've seen guys go both ways with the transmission and even put a five speed behind a 3.2 or 3.4. The guys who either put a 2.5 5 speed behind a 2.7 or a 2.7 5 speed behind a 2.5 don't really notice a difference. Some spec boxster guys have caught with 2.7 5 speed transmissions and didn't even know until they got inspected by the scrutineers at a race. The spec guys are burning through 2.5 transmissions so quickly that you might have a hard time finding a good used one. I'd stick with the 2.7 transmission because of that. Of course you're in England so finding a 2.5 transmission won't be so hard.
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Old 07-31-2018, 02:05 AM   #8
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I've seen guys go both ways with the transmission and even put a five speed behind a 3.2 or 3.4. The guys who either put a 2.5 5 speed behind a 2.7 or a 2.7 5 speed behind a 2.5 don't really notice a difference. Some spec boxster guys have caught with 2.7 5 speed transmissions and didn't even know until they got inspected by the scrutineers at a race. The spec guys are burning through 2.5 transmissions so quickly that you might have a hard time finding a good used one. I'd stick with the 2.7 transmission because of that. Of course you're in England so finding a 2.5 transmission won't be so hard.
One of the driving factors behind the decision is that the majority of the gearboxes on ebay seem to be from either a 2.5, or a 130k+ miles 2.7, and all the 2.5 boxes are within a reasonable distance to me
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Old 07-31-2018, 07:02 AM   #9
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Looking at the 2.5 Vs 2.7 box ratios you won't notice any difference as the first 3 ratios are the same.
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Old 07-31-2018, 08:06 AM   #10
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Looking at the 2.5 Vs 2.7 box ratios you won't notice any difference as the first 3 ratios are the same.
Not true. The final drive ratio is ultimately the determining factor. This is the ring and pinion gear ratio in the differential section of the transaxle. 3.89 revolutions of the pinion to rotate the ring gear 1 revolution, which rotates the axles, is significantly lower than 3.52 to 1. doesn't sound like much but it is. i.e at 4000 engine rpm 4000/ 3.89= 1028 axle revolutions and 4000/ 3.52 =1136 axle revolutions, (assuming direct drive). So you can see the 3.52 has travelled much further. (additional 108 revolutions) you then multiply that by tire circumference to determine distance travelled. So while the first three gear ratios are the same the 2.5 final output is lower. where the last 2 gears in the 2.7 are much taller.

Last edited by 911monty; 07-31-2018 at 08:48 AM.
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Old 07-31-2018, 08:39 AM   #11
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Not true. The final drive ratio is really the determining factor. This is the ring and pinion gear ratio in the differential section of the transaxle. 3.89 revolutions of the pinion to rotate the ring gear 1 revolution, which rotates the axles, is significantly lower than 3.52 to 1. doesn't sound like much but it is. I won't go into the math here but it is easy to look up. So while the first three gear ratios are the same the 2.5 output is lower. where the last 2 gears in the 2.7 are much taller.
which can be modified also by changing the sidewall height of your tires ...
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Old 07-31-2018, 08:54 AM   #12
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which can be modified also by changing the sidewall height of your tires ...
Exactly. I modified my original post to reflect tire circumference with some math.

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