07-30-2018, 06:43 AM
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#1
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Registered User
Join Date: Jul 2018
Location: England
Posts: 3
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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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07-30-2018, 05:14 PM
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#2
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Registered User
Join Date: Jul 2013
Location: Bastrop, Tx
Posts: 2,644
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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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Woody
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07-30-2018, 05:26 PM
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#3
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Racer Boy
Join Date: Sep 2015
Location: Seattle, WA
Posts: 946
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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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07-30-2018, 07:54 PM
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#4
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Certified Boxster Addict
Join Date: Nov 2010
Location: Los Angeles
Posts: 7,669
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Quote:
Originally Posted by Racer Boy
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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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
__________________
1999 996 C2 - sold - bought back - sold for more
1997 Spec Boxster BSR #254
1979 911 SC
POC Licensed DE/TT Instructor
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07-30-2018, 08:20 PM
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#5
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Motorist & Coffee Drinker
Join Date: Jul 2014
Location: Oklahoma
Posts: 3,778
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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.
__________________
I am not an attorney, mechanic, or member of the clergy. Following any advice given in my posts is done at your own peril.
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07-31-2018, 12:21 AM
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#6
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Registered User
Join Date: Jul 2018
Location: England
Posts: 3
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Quote:
Originally Posted by Racer Boy
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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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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07-31-2018, 01:44 AM
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#7
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Registered User
Join Date: Jul 2013
Location: Bastrop, Tx
Posts: 2,644
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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.
__________________
Woody
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07-31-2018, 02:05 AM
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#8
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Registered User
Join Date: Jul 2018
Location: England
Posts: 3
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Quote:
Originally Posted by itsnotanova
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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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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07-31-2018, 07:02 AM
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#9
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550 Anniversary
Join Date: Jul 2012
Location: Surrey, UK
Posts: 747
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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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07-31-2018, 08:06 AM
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#10
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Registered User
Join Date: Oct 2014
Location: California Central Coast
Posts: 1,476
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Quote:
Originally Posted by edc
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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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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07-31-2018, 08:39 AM
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#11
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Registered User
Join Date: Apr 2010
Location: Canada
Posts: 3,127
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Quote:
Originally Posted by 911monty
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.
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which can be modified also by changing the sidewall height of your tires ...
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07-31-2018, 08:54 AM
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#12
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Registered User
Join Date: Oct 2014
Location: California Central Coast
Posts: 1,476
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Quote:
Originally Posted by The Radium King
which can be modified also by changing the sidewall height of your tires ...
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Exactly. I modified my original post to reflect tire circumference with some math.
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