Diff Ratios and drag racing
Discussion
My 1995 400 has a 3.73 BTR diff and I would like to go for a taller ratio. Mainly to lower cruising revs and also stop it bouncing on the limiter in 4th over the 1/4 mile.
What ratios were fitted to these as standard? I know where 3.08 gears are available, but is this too extreme a jump and is there something in the middle?
What ratios were fitted to these as standard? I know where 3.08 gears are available, but is this too extreme a jump and is there something in the middle?
So many variables.
Depends on your power output, torque spread, tyres, launch technique etc.
In my experience I found it made little to no difference in my 1/4 mile times and 60 foot times when I switched from the standard 3.45:1 to the longer 3.08:1.
For me it was a win win as the car was so much better on the road, cruising on the motorway under 3000rpm (90 mph) before the cam and the increased noise. MPG was much improved. With the 3.08:1 diff I even managed to run my fastest 1/4 mile time NA of 11.89 @ 114.
For a standard tune 400 with maybe 240 bhp then you may get a little better acceleration but I think it would minimal.
Depends on your power output, torque spread, tyres, launch technique etc.
In my experience I found it made little to no difference in my 1/4 mile times and 60 foot times when I switched from the standard 3.45:1 to the longer 3.08:1.
For me it was a win win as the car was so much better on the road, cruising on the motorway under 3000rpm (90 mph) before the cam and the increased noise. MPG was much improved. With the 3.08:1 diff I even managed to run my fastest 1/4 mile time NA of 11.89 @ 114.
For a standard tune 400 with maybe 240 bhp then you may get a little better acceleration but I think it would minimal.
I wonder why mine has such a low ratio then?!
Derek, mine is a 400 Turbo with around 350bhp (although dyno run on 11th May to find out exactly)
Best time so far is 12.5 at 118 but as said it was bouncing off the limiter in 4th over the line and 1st gear seems like a waste of time at launch, although 2nd is too tall.
https://youtu.be/TVZe2rTfPZ4
Maybe a 3.08 is the way to go, although 3.45 might be cheaper to try initially
Derek, mine is a 400 Turbo with around 350bhp (although dyno run on 11th May to find out exactly)
Best time so far is 12.5 at 118 but as said it was bouncing off the limiter in 4th over the line and 1st gear seems like a waste of time at launch, although 2nd is too tall.
https://youtu.be/TVZe2rTfPZ4
Maybe a 3.08 is the way to go, although 3.45 might be cheaper to try initially
Edited by AceOfHearts on Friday 26th April 09:08
AceOfHearts said:
I wonder why mine has such a low ratio then?!
Derek, mine is a 400 Turbo with around 350bhp (although dyno run on 11th May to find out exactly)
Best time so far is 12.5 at 118 but as said it was bouncing off the limiter in 4th over the line and 1st gear seems like a waste of time at launch, although 2nd is too tall.
https://youtu.be/TVZe2rTfPZ4
Maybe a 3.08 is the way to go, although 3.45 might be cheaper to try initially
By all means try a 3.45:1 it will certainly improve your car.Derek, mine is a 400 Turbo with around 350bhp (although dyno run on 11th May to find out exactly)
Best time so far is 12.5 at 118 but as said it was bouncing off the limiter in 4th over the line and 1st gear seems like a waste of time at launch, although 2nd is too tall.
https://youtu.be/TVZe2rTfPZ4
Maybe a 3.08 is the way to go, although 3.45 might be cheaper to try initially
Edited by AceOfHearts on Friday 26th April 09:08
If you do look to getting a 3.08 then you need the larger diameter option.
The 3.08:1 I had was the smaller and weaker and I soon broke it.
I currently have the old 3.45:1 fitted but would love to get a 3.08 again. Maybe a group buy and import from Aus
Going to 3.45 is less than 2 percent change, not worth bothering with.
I can't understand why these cars have anything lower than a 3.08 as std .. it's the same with the T cars / cerberas .. ridiculously under geared. Other than crawling in traffic I really don't understand it other than for the std 4 litre cars which might feel a bit sluggish initially.
Drive a car with a 3.08 in it and you will be converted I'm sure, especially if you do a lot of cruising miles.
Also, very few of us really punish the gearboxes even on a quarter mile dash (not like say autocar or whoever might when getting a 0-60 time) , so saving a gear change can have a good influence over a short distance. I haven't done the sums but would you need 4th over the quarter with a 3.08 .. I suspect maybe not.
I can't understand why these cars have anything lower than a 3.08 as std .. it's the same with the T cars / cerberas .. ridiculously under geared. Other than crawling in traffic I really don't understand it other than for the std 4 litre cars which might feel a bit sluggish initially.
Drive a car with a 3.08 in it and you will be converted I'm sure, especially if you do a lot of cruising miles.
Also, very few of us really punish the gearboxes even on a quarter mile dash (not like say autocar or whoever might when getting a 0-60 time) , so saving a gear change can have a good influence over a short distance. I haven't done the sums but would you need 4th over the quarter with a 3.08 .. I suspect maybe not.
spitfire4v8 said:
I haven't done the sums but would you need 4th over the quarter with a 3.08 .. I suspect maybe not.
That depends on the terminal speed you will reach which ultimately depends on bhp.Also if you have a modified engine to gain rpm lets say 6200+
I found I was changing to 4th with a 3.08 but only about 50 yards to the line but I was in the high 120's often on the nitrous.
I hope to be doing 1/2 mile racing at Elvington on the 11/12 th May running a turbo for the first time. I am aiming for 155+ in about 19 seconds.
So no doubt 5th will be getting some agro this time.
Are these still the ones to go for? I have had them saved in my watch list for years 
https://www.ebay.co.uk/itm/BORG-WARNER-DANA-M78-SE...
https://www.ebay.co.uk/itm/BORG-WARNER-DANA-M78-SE...
AceOfHearts said:
Are these still the ones to go for? I have had them saved in my watch list for years
https://www.ebay.co.uk/itm/BORG-WARNER-DANA-M78-SE...
Brilliant, maybe your friends could package a few and save a lot on postage too.https://www.ebay.co.uk/itm/BORG-WARNER-DANA-M78-SE...
I think that is the correct set but double check with John Reid at Readman Racing to be sure before we order.
I am definitely interested in buying 1.
I love all this I do,,, ain’t got no money to join in this year but f
k yeah,,,,, I want a 3:08 so I can get another 1/2 second out of mine too, especially for the cruising mode, Derek’s spot on as in the 3000 rev mark where things become louder. That alone is worth the cost. Hmmm, keep breaking em so I can get one next year boys 
ETA might even break the 32 mpg barrier too
You wanna try 1000 miles in less than 24 hrs at 3000 revs, that don’t half knock you about
k yeah,,,,, I want a 3:08 so I can get another 1/2 second out of mine too, especially for the cruising mode, Derek’s spot on as in the 3000 rev mark where things become louder. That alone is worth the cost. Hmmm, keep breaking em so I can get one next year boys ETA might even break the 32 mpg barrier too
You wanna try 1000 miles in less than 24 hrs at 3000 revs, that don’t half knock you about
Edited by Classic Chim on Friday 26th April 10:59
DangerousDerek said:
AceOfHearts said:
Are these still the ones to go for? I have had them saved in my watch list for years
https://www.ebay.co.uk/itm/BORG-WARNER-DANA-M78-SE...
Brilliant, maybe your friends could package a few and save a lot on postage too.https://www.ebay.co.uk/itm/BORG-WARNER-DANA-M78-SE...
I think that is the correct set but double check with John Reid at Readman Racing to be sure before we order.
I am definitely interested in buying 1.
and coming onto it at about 80 mph! 
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