Lowering Gearbox Ratios

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??

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

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

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

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

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

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.

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.

Anyone???

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

I've already said to Aidy at AH motorsports so i guess i'll quizz him some more.