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Lowering Gearbox Ratios
#859626
02/07/2009 10:32
02/07/2009 10:32
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Alfamale
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Alfamale
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Whats the best approach to take? I'm building a track only 20VT (first one in Ireland!!) and I'm looking to lower the gear ratios. the reason being the track in question is Mondello Park, very very tight track with only a short main straight, so the emphases will be on handling and mid range acceleration. The car is currently being torn apart in order to add some lightness to it, and will be running around the 300bhp mark. The car in question is a 1997 20VT 5 speed Suggestions??
Last edited by Alfamale; 02/07/2009 10:33.
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Re: Lowering Gearbox Ratios
[Re: ]
#859727
02/07/2009 12:54
02/07/2009 12:54
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doug20vt
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doug20vt
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alfa internals from the 166, 156 and gtv are interchangeable with the coupe internals, you could swap the final drive ratios, if you could get a hold of a second hand 6 speed box you could put the internals or just the final drive from either the alfa gtv, 156 3l or 2.5l in to the box, the final drive from the gtv is 3.563, from the 3l its 3.733 and from the 2.5l its 3.93 in 6 speed boxes, you should be able to use these final drives in the 5 speed as well http://locost7.info/gearcalc.php if you download the gearcalc from that you can put the different final drives in to see what effect it has on the speeds in gear, ratios for the 5 speed coupe box are final drive 3.111 1st gear 3.800 2nd gear 2.235 3rd gear 1.520 4th gear 1.156 5th gear 0.914 6th gear 0.816 on the 6 speed boxes, these ratios are the same for all the alfa 6 speed boxes as well other than 5th which is 0.971 as oppose to 0.914 on the coupe, alfa altered the gearing on its 6 speed boxes by changed the final drive
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Re: Lowering Gearbox Ratios
[Re: ]
#859758
02/07/2009 14:03
02/07/2009 14:03
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nyssa7
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nyssa7
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not entirely true - the 6 speed box in the 2.5 V6 156 has following ratios combined with a 3.938 diff
1st 3.5:1 2nd 2.235:1 3rd 1.52:1 4th 1.161:1 5th 0.971:1 6th 0.816:1 so 1st, 4th and 5th are different as well as the much lower final drive
my understanding is you cannot just swap the final drive with the Coupe leaving the Coupe gear ratios intact. We were unable to do this in my swap over
I've set my gearing spreadsheet assuming 235/635x17 slicks, so not EXACTLY correctly in a Coupe application, but the differences will be of the same order
Speed in gears at 7500rpm: Gear Coupe 6sp Alfa 6sp 1st 42mph 37mph 2nd 77mph 58mph 3rd 106mph 85mph 4th 139mph 112mph 5th 176mph 134mph 6th 197mph !! 159mph
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Re: Lowering Gearbox Ratios
[Re: ]
#859784
02/07/2009 14:59
02/07/2009 14:59
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doug20vt
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doug20vt
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there are slight variations in the gearing as trevor says, i have however just had dave from rusper alfa build me a box and he was able to use the gears from the coupe and just use the final drive from the alfa, i asked him to swap 5th gear for the alfa cog for better spacing between the ratios which he was able to do
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Re: Lowering Gearbox Ratios
[Re: ]
#859822
02/07/2009 16:08
02/07/2009 16:08
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nyssa7
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nyssa7
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fair enough, he must have disassembled more. TBH I was happier with the entire Alfa gear set for my application anyway as they are more evenly spaced
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Re: Lowering Gearbox Ratios
[Re: ]
#859856
02/07/2009 16:55
02/07/2009 16:55
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doug20vt
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doug20vt
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there is virtually nothing in it to be honest, 1st is slightly shorter on the alfa and i think the difference in 4th gear equates to about 0.1mph per 1000rpm, i did however ask him to swap the coupe 5th gear for the alfa 5th as it made the gears more evenly spaced, if i was doing it on a 5 speed box would probably keep the coupe 5th as it slightly longer than the alfa 5th, that would be on a road application though, for the track you might be better with the shorter top gear
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Re: Lowering Gearbox Ratios
[Re: ]
#860113
03/07/2009 08:13
03/07/2009 08:13
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Joined: Dec 2005
Posts: 3,158 Near Reading
JohnS
I need some sleep
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I need some sleep
Joined: Dec 2005
Posts: 3,158
Near Reading
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There are two gearbox families that the coupe can use. ALl the above info is based on the late 5 and 6 speed
But...if you have an early 5 speed box (in a pre MY98 coupe) there are a wide range of ratios available out there as the lancia delta boxes are compatable.
Former low boost hero - 616BHP@1.5 bar. 2.4 20VT RIP
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Re: Lowering Gearbox Ratios
[Re: JohnS]
#863296
08/07/2009 15:05
08/07/2009 15:05
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sparco
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sparco
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So guys i have a spare 5 speed box and an Alfa 2.5 V6 6 spd with the 3.9 final drive. Can i get the alfa 6 speed internals into the 5 speed casing which means i could get my gearbox modded and sitting ready for my rebuild???
Failing that i will have to wait until i pull my engine out and do it then but it seems plausible if you can change the final drive ratios.
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Re: Lowering Gearbox Ratios
[Re: ]
#864393
10/07/2009 14:52
10/07/2009 14:52
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sparco
Unregistered
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sparco
Unregistered
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Re: Lowering Gearbox Ratios
[Re: ]
#864406
10/07/2009 15:09
10/07/2009 15:09
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doug20vt
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doug20vt
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i asked a couple of folk about this and most said they didnt think it could be done but weren't entirely sure, you could trying phoning dave at rusperalfa and asking him, he is really helpful and might know http://www.rusperalfaromeo.co.uk/ he is also someone i would recommend to do the work as the labour charges were very reasonable
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Re: Lowering Gearbox Ratios
[Re: ]
#864430
10/07/2009 16:13
10/07/2009 16:13
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sparco
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sparco
Unregistered
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I've already said to Aidy at AH motorsports so i guess i'll quizz him some more.
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