Longer Diff, slower times?

[dbv8]

The BTR diff in my TVR is tired and will be going to Jon Reid at Readman racing for a much needed rebuild.

The current ratio is 3.45:1 but I have a crown and pinion set to give me 3.08:1
I like the idea of a lower cruising rpm and maybe better mpg on the motorway etc and more importantly want to try a high speed event such as vmax next year.

Speeds currently with 3.45 on 255/50/16 Mickey Thompsons and 5800 rpm changes…
1st = 45
2nd = 67
3rd = 98
4th = 130
5th = 168 (has easily reached 155+ NA)

With the 3.08 diff
1st = 50
2nd = 76
3rd = 110
4th = 146
5th would be revving about 5550 at 175 which is my ultimate target.

The car currently runs 11.6 @ 125 on only a 65 shot of gas but I will be using up to a 150 shot next year.
I have 308 bhp NA and the car weighs about 1150kg with me in it

So the question is if I do go ahead and fit the longer diff how much do you guys reckon it will affect my ¼ mile times?
I am hoping a little more gas will make up the difference.

Edited by dbv8 on Thursday 8th December 11:19

[redvictor]

more power = less leverage needed(gear).That will equal less tyre spin if you're on street tyres.

[dbv8]

On MT streets. I will be getting much more heat into them next year. There is at least a tenth to be had to 60ft also. Previous best of only 1.83

[SILICONEKID340HP]

How do you know its a 3.45 ? it could be a 3.77..

[dbv8]

SILICONEKID340HP said:

How do you know its a 3.45 ? it could be a 3.77..

I think you sometimes underestimate me Daz

Isnt your diff a GKN? Then yours may well be 3.77.
Mines a BTR and is a 3.45.... I know becuase i lifted the rear wheels and they rurned 10 times to 34.5 turns of the prop.

[SILICONEKID340HP]

So mine could be eather 3.31 or 3.77, im a little confused how do i check which one it is ?if it is the 3.77 what is involved to change it to 3.31..Cheers Derek

Edited by SILICONEKID340HP on Monday 19th December 20:40

[dbv8]

SILICONEKID340HP said:

So mine could be eather 3.31 or 3.77, im a little confused how do i check which one it is ?if it is the 3.77 what is involved to change it to 3.31..Cheers Derek

Edited by SILICONEKID340HP on Monday 19th December 20:40

Jack the rear wheels off the ground Daz. Get someone to count the prop revoltions as you turn a rear wheel 10 times. 33 = 3.31 and nearly 38 = 3.77.

If you really want a longer diff then either search for a second hand one or find out who sells the crown and pinions for GKNs.
My 3.08 BTR gears came from Aus.

[Stan Weiss]

dbv8 said:

The BTR diff in my TVR is tired and will be going to Jon Reid at Readman racing for a much needed rebuild.

The current ratio is 3.45:1 but I have a crown and pinion set to give me 3.08:1
I like the idea of a lower cruising rpm and maybe better mpg on the motorway etc and more importantly want to try a high speed event such as vmax next year.

Speeds currently with 3.45 on 255/50/16 Mickey Thompsons and 5800 rpm changes…
1st = 45
2nd = 67
3rd = 98
4th = 130
5th = 168 (has easily reached 155+ NA)

With the 3.08 diff
1st = 50
2nd = 76
3rd = 110
4th = 146
5th would be revving about 5550 at 175 which is my ultimate target.

The car currently runs 11.6 @ 125 on only a 65 shot of gas but I will be using up to a 150 shot next year.
I have 308 bhp NA and the car weighs about 1150kg with me in it

So the question is if I do go ahead and fit the longer diff how much do you guys reckon it will affect my ¼ mile times?
I am hoping a little more gas will make up the difference.

Edited by dbv8 on Thursday 8th December 11:19

Do you know what your trans gear ratios are? Are you listing speed in each gear in MPH or KPH?

Stan

[dbv8]

Stan Weiss said:

Do you know what your trans gear ratios are? Are you listing speed in each gear in MPH or KPH?

Stan

Speeds are in MPH.
Gears are
1st = 2.915
2nd = 1.932
3rd = 1.323
4th = 1
5th = 0.8

[SILICONEKID340HP]

Have you sorted the diff out ?

[Stan Weiss]

I think that the gear change at most will cost .1 in your et. Are you running the 65 shot of gas for the whole 1/4 or using a progressive controller? You et seem high for the MPH that you are running.

Stan

[dbv8]

Stan Weiss said:

I think that the gear change at most will cost .1 in your et. Are you running the 65 shot of gas for the whole 1/4 or using a progressive controller? You et seem high for the MPH that you are running.

Stan

I can live with that

That 65 shot was on a direct hit at full throttle, so just a blip in 1st gear with a short shift to 2nd and fully on the gas apart from the gearchanges.

I now have a genuine 100 shot and a progressive controller to play with.

You are right about the ET and MPH. I should be running quicker but traction is the problem i need to fix.
The car with me in it will weigh about 1150kg. I run on decent 255/50/16 Mickey Thompson Street ETs but only get mid 1.8's to 60 foot. My best to the 1/8th is 7.59 @ 98 and my best 2nd 1/8th is just under 4 seconds.
I think the biggest problem is that the car runs IRS. Ideally a live axle with a multilink would give me the launch i need but it would be too much of a sacrifice i think.
I am confident i can tweak the rear 3-way shocks and suspension settings, and with more heat in the tyres due to a linelock should take a tenth off to 60 foot.

The ultimate aim would be to hit 10.99 @ 130 with a car that returns 30mpg and can hit 170+ with a little chemical assistance

[Stan Weiss]

Have you run the car N/A and if so what was your 60 foot times?

Your MPH shows you have the power to run much quicker with good traction.

Stan

[dbv8]

The car runs 12.4 @ 113 NA with similar 60 foots. I find I can't launch any higher than about 2500 rpm. Traction is definitely the shortcoming