whats a diff ratio worth

[stockstandard]

I am up for a new diff soon and I am wondering how much time you can drop with a diff ratio. I am thinking about time in the high 14's.

I have a stock 3.08 diff, and am considering 3.45, 3.7 and 3.9 ratios. The car pulls damm hard in 1st and 2nd but seems to die a little in 3rd. Traction is not yet a major issue. I hit limiter in 3rd just before crossing the line at the moment.

Any ideas appreciated

[SpoolMan]

Originally Posted by stockstandard

I am up for a new diff soon and I am wondering how much time you can drop with a diff ratio. I am thinking about time in the high 14's.

I have a stock 3.08 diff, and am considering 3.45, 3.7 and 3.9 ratios.

Any ideas appreciated

Hamo's AU6 had a PB of 15.05 had a diff change from 3.08 to 3.9 went 14.86 but it also had a air intake pipe changed at the same time... it's a auto, my thought's are with a manual it could be a little better as you can rev it harder than a auto of the line...
he used 16 inch tyres..
remember with the EEC-V you may need the speed limiter removed when changing to a lower ratio...

[jonbays]

You want to pick the diff that will have you just hitting peak power in third as you cross the line.

Now a flat 15 second should be around 95mph and so you want third to peak at around 100 or 160k. if your engine has peak power at 5000rpm that means a 32kmh/100 rpm third which would be about a 3.7 diff depending on your tyes.

I think hamo overdid it with the 3.9 uless he is running big 18" wheels and tyres.

[EFFalcon]

my car went from a 16.6 to a 16.1 with a diff gear change and some fresher injectors.
but it was completely stock apart from the diff gears.and its an auto :P

[SpoolMan]

Originally Posted by jonbays

I think hamo overdid it with the 3.9 uless he is running big 18" wheels and tyres.

Can’t agree, Rod done the maths and the tyre roll out numbers and he is just into 4th over the line, its spot on, we would like to try 15's if we can get something that sticks, most drag/street tyres in 15 have a side wall that high its like running a 19''....
We have some 17's to try, and maybe some 16'' toyo's for a run or 2....
It’s actually having changes done this week, trying a smaller cam, because we can’t run a hi-stall big enough to get the power down with the current Cam. Shocking 60 footers...
The cam he has now would be a animal in a manual..

[stockstandard]

Thanks for the replies, sounds like the gains are better than I expected.

Originally Posted by XRFPV8

Hamo's AU6 had a PB of 15.05 had a diff change from 3.08 to 3.9 went 14.86 but it also had a air intake pipe changed at the same time... it's a auto, my thought's are with a manual it could be a little better as you can rev it harder than a auto of the line...
he used 16 inch tyres..
remember with the EEC-V you may need the speed limiter removed when changing to a lower ratio...

Speed limiter already gone

Originally Posted by jonbays

You want to pick the diff that will have you just hitting peak power in third as you cross the line.

That is what I thought, but I am doing that now with the 3.08 - I hit 6000rpm just before I hit 400m. I lose time if I short shift so I need a ratio that will take me deeper into 4th to make up for the shift.

Originally Posted by EFFalcon

my car went from a 16.6 to a 16.1 with a diff gear change and some fresher injectors.
but it was completely stock apart from the diff gears.and its an auto :P

Thats a decent drop, what ratio do you have now?