CSR Diff Choices

[Jmracing66]

Hi all,

My CSR currently has a 3.14 open diff with a six speed box.
I am planning on using the car for more trackdays than the previous owner so am looking into having a Titan LSD fitted to make it a little more sure footed on a circuit.

Whilst this is going in I'm wondering whether it's worth switiching to a 3.62 or 3.38 ratio diff to give the car a little more acceleration out the corners. The 3.14 seems like it may be a little long for tracks.

Anyone got a particular view on this ?

Cheers

[sdio]

If it is track only why not go crazy short and try 3.92...

[nicemd]

The diff ratio doesn't affect acceleration except from a standing start. (as long you can keep the engine in the power band, which you probably can with a 2.3 Duratec)

[DCL]

I've been using a 3.92 with the Titan diff and I can say it works very well, but you'll run out of RPM very quickly on the standard CSR engine. I require 8600 RPM to get to 130 MPH with a 1:1 top gear, and Kumho V70 tyres.

My advice would be to look at the circuits you will be using and work out the speeds you want to achieve, then work backwards from that. I suspect upgrading your 3.14 diff with the Titan would be the cheapest/easiest, and probably the fastest with a manual gearbox (for all but the most serious competition driving anyway).

[shrink1061]

nicemd said:

The diff ratio doesn't affect acceleration except from a standing start. (as long you can keep the engine in the power band, which you probably can with a 2.3 Duratec)

not sure i understand your logic. If the diff is longer, it makes every single gear a longer gear, which of course will affect acceleration.

[bcr5784]

shrink1061 said:

not sure i understand your logic. If the diff is longer, it makes every single gear a longer gear, which of course will affect acceleration.

Yes but it means with the longer diff you will at times be in first when you would have been in second (and therefore accelerating faster) , second when you would have been in third etc. Take an imaginary case with equal spacing between the gears and a diff change that put first where 2nd was, 2nd where 3rd was etc. Acceleration would be identical once you had got beyond the range of the original first gear.

Of course in this case it's more complicated and with the shorter diff you will be doing more gearchanges which cost time (1/2 second plus in plane, 1 second plus cross plane). What net gain or loss in lap times is a complex calculation and track dependant.

[shrink1061]

I think in most cases the difference in ratios isn't too huge a problem, and depends really on engine power more than anything.

That said, Blyton park in a 420R with 6 speed and 3.62 diff we were mostly in 3rd and 4th with one nip up to 5th on Lancaster.

In my 1.6K with standard 5 speed, it's all 3rd and 4th with 4th being long enough to make 5th unnecessary.

In that instance, I could probably make better progress with shorter gears.

[nicemd]

shrink1061 said:

not sure i understand your logic. If the diff is longer, it makes every single gear a longer gear, which of course will affect acceleration.

Another way to think of it is that power is what accelerates the car, and the power output won't change with different diff ratios. (still, only valid as long as you are able to keep the engine at the peak power rpm)

However, as already mentioned there are other reasons for changing diff and gear ratios but they are often depend on what track you intend to drive.

[DCL]

Having run several different ratios, all I can say is that it's a lot more complicated than you might think. But basically, the longer diff ratios tend to have less gear changes and that seems to be the biggest advantage if your engine has a flat torque curve - and the Duratec usually does.

Getting full power is important, but (by definition) you seldom use peak power in the parts of the circuit that you have the potential to go faster, so more gears is not what's going to get quicker lap times, unless you're up there with the best.

[Jmracing66]

I use a variety of UK tracks, Mainly Brands Indy, Snetterton or Silverstone. The car is driven to circuits and used as a weekend road car so spends some time on longer journeys.

I have a choice of building up one LSD...using either the original 3.62 or the more recent 3.14.

[bcr5784]

Jmracing66 said:

I use a variety of UK tracks, Mainly Brands Indy, Snetterton or Silverstone. The car is driven to circuits and used as a weekend road car so spends some time on longer journeys.

I have a choice of building up one LSD...using either the original 3.62 or the more recent 3.14.

I'd agree with DCL - I'm doubtfull if you will gain in lap times by going to a shorter diff - even if you are very quick and prepared to make pretty brutal gearchanges. If your car was a 1.4 K series it might well be a different matter.

[jimhcat]

I had the 3.62 LSD in my CSR 260 and found it to be way too low geared in 6th. I was running out of revs before half way down the hanger straight at Silverstone. It was also a pain on the road as the revs were high at motorway speeds. I was planning to change the diff but instead sold the car to make way for a 620R

[Jmracing66]

jimhcat said:

I had the 3.62 LSD in my CSR 260 and found it to be way too low geared in 6th. I was running out of revs before half way down the hanger straight at Silverstone. It was also a pain on the road as the revs were high at motorway speeds. I was planning to change the diff but instead sold the car to make way for a 620R

Right, that's the answer, thanks very much. I'll hang on to the 3.14 and put an LSD in.