3.91 Diff going in! Performance measurement?
Discussion
My 2006 Sag will be having its 3.46 diff replaced by 3.91 next week. Does anyone live around the Edinburgh area with a 'Racelogic' performance meter or similar?
I thought it would be good to do some runs before and afterwards to see what the effect is, and post the results on the forum. May help some of you decide if it is worth fitting it to your own cars.
FYI, cost all in for diff swap, courier, vat and fitting about 1200 quid.
I live in east lothian, 20 miles east of Edinburgh.
I thought it would be good to do some runs before and afterwards to see what the effect is, and post the results on the forum. May help some of you decide if it is worth fitting it to your own cars.
FYI, cost all in for diff swap, courier, vat and fitting about 1200 quid.
I live in east lothian, 20 miles east of Edinburgh.
I had my 3.73 diff changed to a 3.91 last week, Couldn't really exploit it on the way back as the roads were a bit greasy,It certainly seems livlier! The main thing you will notice is it is reving a lot higher in top, I kept going to put it into 5th only to realise it already was in 5th, You should notice a massive step up from a 3.46 to a 3.91, Enjoy.
We've had the 3.91 diff in ours for about 2 years, it does make it a bit more nippy, but its not like outrageously different other than revs/speed ratio. Its probably like adding 20-30bhp at the wheels if you worked out the gearing/cascade curves...which of course is noticable, but its not like bolting a blower on there 
To be honest, I think thats how the factory demo cars were that were figured by magazines and such like...and they were the higher spec engines (406 vs 380bhp at launch)) so the 3.91 diff just really takes you a little bit closer to being able to get anywhere near the claimed performance figures ....
in reality on a Racelogic P box (sold mine last year due to lack of use!) I think you will struggle to get near 4secs to 60 and 9secs to a ton...in fact that would be a good run in my experience... thats achiveable by a mere mortal owner, whilst having some mechanical sympathy ..... rather than a magazine tester doing dozens of runs, trashing a clutch, just to see what the car would do
I never got better than 9secs in the Sag on the newer 3.91 diff, did an 8.5secs to a ton a few times in my Elise, and 12secs easy enough in the Vantage. The Elise had a bit of an unfair advantage as it had sticky rubber and traction/launch control for sprint launches
To be honest, I think thats how the factory demo cars were that were figured by magazines and such like...and they were the higher spec engines (406 vs 380bhp at launch)) so the 3.91 diff just really takes you a little bit closer to being able to get anywhere near the claimed performance figures ....
in reality on a Racelogic P box (sold mine last year due to lack of use!) I think you will struggle to get near 4secs to 60 and 9secs to a ton...in fact that would be a good run in my experience... thats achiveable by a mere mortal owner, whilst having some mechanical sympathy ..... rather than a magazine tester doing dozens of runs, trashing a clutch, just to see what the car would do
I never got better than 9secs in the Sag on the newer 3.91 diff, did an 8.5secs to a ton a few times in my Elise, and 12secs easy enough in the Vantage. The Elise had a bit of an unfair advantage as it had sticky rubber and traction/launch control for sprint launches
All
I had my Sag serviced in February, added 3.91diff and C/R gearbox, it feels more planted on the road, and a lot better when accelerating when say dropping down gears to overtake
Im not sure how much diff it has made but my smile is just the same everytime Im out in her
which was today, and prob tomorrow!
Rgds
Derrick
I had my Sag serviced in February, added 3.91diff and C/R gearbox, it feels more planted on the road, and a lot better when accelerating when say dropping down gears to overtake
Im not sure how much diff it has made but my smile is just the same everytime Im out in her
which was today, and prob tomorrow!
Rgds
Derrick
blackiepaul said:
I had my 3.4 changed to a 3.7 diff. I was told you should not run a 3.9 unless you have the close ratio box? Was this rubbish then?
3.7 over the 3.4 was a great change.
Yes, it was rubbish. Difference between the CR and normal boxes is 1st and 2nd gears. Maybe 3rd too. Certainly not 4th and 5th. 3.91 worked just fine with my 'standard' box ratios. Saying that, I'm sure a close ratio box would have made things even more rapid.3.7 over the 3.4 was a great change.
blackiepaul said:
I had my 3.4 changed to a 3.7 diff. I was told you should not run a 3.9 unless you have the close ratio box? Was this rubbish then?
3.7 over the 3.4 was a great change.
Yes it was tosh. The only to be avoided at all costs combination is a 3,46 diff and a CR box.3.7 over the 3.4 was a great change.
As Targ says, the CR is required to get real rewards.
bogie said:
sidewayz said:
Nice garage bogie!
cheers 
only just spotted this ! ...Sag is up for sale when it comes back from its service this week, Im afraid I still cant drive (nor walk unaided!) and we have too many cars sat around doing nothing
Didn't know you were not able to drive and very sorry to hear this mate.YHM.
Steve
dickieandjulie said:
Interesting...my 3.73 requires replacement and I was considering swapping for a 3.91. Is it worth it? Only real concern was the higher revs/noise on longer runs.
It's a nice upgrade and I agree with the comments on the performance gain. It makes a standard 3.6 Six appreciably nippier but you do tend to use higher revs all the time and therefore,depending on your exhaust it can make a longer run noisier. The CR box with the 3.91 is probably the best compromise if you want to avoid this. I have a standard box with a TVR Sports exhaust and do long journeys and it's OK. Mind you,the wife refuses to do long distances in the t girl so maybe I am going deaf?dickieandjulie said:
Interesting...my 3.73 requires replacement and I was considering swapping for a 3.91. Is it worth it? Only real concern was the higher revs/noise on longer runs.
Richard,At 80mph, the car will be doing 150 more revs than before - you'll hardly notice the difference in noise or fuel useage.
All IMO of course and I'm probably deafer than most haveing spent too many years between TVRs and Ducatis...
sidewayz said:
dickieandjulie said:
Interesting...my 3.73 requires replacement and I was considering swapping for a 3.91. Is it worth it? Only real concern was the higher revs/noise on longer runs.
It's a nice upgrade and I agree with the comments on the performance gain. It makes a standard 3.6 Six appreciably nippier but you do tend to use higher revs all the time and therefore,depending on your exhaust it can make a longer run noisier. The CR box with the 3.91 is probably the best compromise if you want to avoid this. I have a standard box with a TVR Sports exhaust and do long journeys and it's OK. Mind you,the wife refuses to do long distances in the t girl so maybe I am going deaf?
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