Diff ratio

[AshG]

Originally Posted by snapper

The type 9 whilst better ratios than an MT75 is still not ideal
1st to low as is second 5th to long
The ideal is a V6 box with a long first and a 3.9 diff
Tyres add to the ratio mix but in essence you want to get 1st and 4th close enough to give 40+ mph in 1st and about 125 in 4th
5th can be for long cruises
Extra rev range for the engine helps close the gap

125 in 4th yeah good luck. anything less than 200bhp in a 7 and you want 4th geared for 90-95 and 5 geard for about 125-130. look at the ratios in a caterham for reference, their ratios are about as spot on as it gets for a 7. aero is a huge limiting factor on a 7. mine is a fire breathing 2.0 turbo running over 200bhp and it will only do 145 flat out which in a 7 is bloomin quick and a total assault on your body and senses, the other issue is that around 125ish the steering gets very light an twitchy on the mass majority of 7's

[AshG]

oh and to answer the initial question. forget the diff. take the box to bgh and get them to rebuild it with their uprated gear set which has better ratios.

[robo]

Woooooooo chaps, he is just pointing out the fact that he finds 1st gear a bit on the short side. I dont suppose for one minute he cares if it does 145 mph or not. The first suggestion about the v6 box is valid so I put the numbers in the gear calc and it makes about 5mph difference, its a start, Better diff ratio would also help even if the engine wont pull it to max revs in top. loads of options really. It sounds to me like he would prefer it to be purring along at 2000rpm at about 70 and thats a bit of me

Another poss way of calculating diff ratio http://www.bimmerhaus.com/tech/diff.html

Bob

[AshG]

Forget that 7's are all about instant acceleration as I have discovered driving mine. Ideally you want 3rd and 4th sitting right in the power band

[Not Anumber]

Ive found out from the last owner it's a 3.62 ratio diff. Is this fairly average ?

[alga]

Yep, it's the "bog standard Sierra diff" ratio. I have the same. Some really weak ones had ~3.3, some stronger engines have 3.92, but 3.62 is the most common.

[snapper]

I've done a lot of research on gear ratios and have talked to serious race car builders, the difference in aero between a 7 and a Fury with exactly the same engine, gearbox, diff and tyres is 10mph down the long straight at Snets
125 Vs 135
I have seen over 115 with a Pinto 2.1 135 ft/ibs my next engine is projected at over 155ft/ibs 170+ bhp and an 8000rpm red line

My 3rd gear gives 90+ and I get well over 60 in second, I'm happy with that
80 in 5th is still a bit of a drag 4000rpm and first still runs out to early
At sometimes will get the 2.7 first but I'll have to sell some stuff first