Swapping 3.92 to 3.62 CWP

[Smithy77]

I'm on my 3rd Caterham, and with each car acquisition, I've gradually got closer to my ideal spec. Started out with an ex-megagrad with 140bhp K in Roadsport trim and race dampers, and now I have a Superlight with 230bhp K, 6-speed box, Quantums, Tillets, roadsport cage etc, and it is spot on for what I want - mixture of fast road use when warm and dry with about 6 trackdays a year. Absolutely loving it!

The previous owner, however, wanted it set up for the odd bit of sprinting/hill climbing. He therefore got the LSD set up with a 3.92 CWP. As you can imagine, the acceleration is absolutely bonkers, and I do love this about the car. What this does mean though is c.4300rpm at 70mph on the motorway, and on my first (and only) trackday at Combe in October, I could hit the limiter in 6th with ease well before the end of the straight.

I have acquired a 3.62 CWP with the intention of dropping the diff out over winter and taking it to Road and Race to make the swap. My worry is, will this take a noticeable edge off the bonkers acceleration which I have grown to love?

As you can probably tell, I have generally not worried about making track focused compromises when it comes to car spec, but this would (or could?) represent a move in the opposite direction.

I would welcome thoughts and opinions, particularly from those that have done similar, before I make the switch just to convince myself I'm doing the right thing.

[rj]

I would say no, it wouldn't.

 

[Ian Mackenzie]

Most of the cars with the 6 speed box run a 3.6 ratio diff.

Cheers

Ian

[Smithy77]

Most of the cars with the 6 speed box run a 3.6 ratio diff.

Ian, this I am aware of, but this being the only 6 speed Caterham I have driven, I have no benchmark to compare against. I'm am just slightly concerned that the 3.62 diff with take away some of the bonkers acceleration which I love most about the car.

In theory, the gaps/rev drops between gears will not change, but each gear will become higher geared/longer. I should probably just stop worrying and get on with it... 

[John Vine]

I suspect you'll notice it but I doubt the effect will be significant.  The plus will be a slightly less frantic road manner.

My R400D (at 220 bhp, similar to yours) has the 3.62 and I find it provides a good compromise on the road.  Like you, I love the performance but some would say it's still too low-geared!

JV
 

[Wrightpayne]

You could always stick the diff in as is, try it out then swap the LSD if you're happy with it. Few hours effort with minimal cost to help you decide.

Ian

[Jonathan Kay]

Have you calculated road speed in each gear with the current and proposed ratio?

In theory, the gaps/rev drops between gears will not change, but each gear will become higher geared/longer. 

And there'll be fewer shifts in reaching any given speed. (There are stories of cars being rigged to optimise this to record good times for arbitrary measures such as 0 to 60 mph.)

I should probably just stop worrying and get on with it... 

;-)

Jonathan

[Krismuss]

*thumbup*Smithy...don't do it!!!!!

My last 7 was circa 170BHP and came with a 3.9 diff I then changed to 3.6 and whilst it made every gear longer it did take away the 'bonkers' element that Caterhams are all about!

My current 7 (SLR) is somewhere between 210-230BHP and runs a 3.9 diff (on a six speed box) and I wouldn't have it any other way. Fat Arnie's is circa 250BHP and runs a 3.9 too. We've both done European track days and Arnie went on tour to Italy earlier this year, no problem (though he did run 15" wheels to help with the gearing).

If you really want to know what bonkers acceleration is...when I bought my car it came with a 4.1 diff!!!!!!!

Why don't you join the No Trailer Trash group on FB and come with us to Abbeville next May?!

Chris

 

 

[Alan Reeves]

I'm with JK on this....it's only the very first tear away from the stationary point that the minimal higher ratio comes into play. And if you run out of revs on a circuit you loose out and surely need to address 

ive gone much higher but don't do tracks and decided that 3 , 4th around the Lanes and things is a delight....horses for courses

 

 

[Smithy77]

I suspect you'll notice it but I doubt the effect will be significant.  The plus will be a slightly less frantic road manner.

I don't mind the frantic road manner - you can tone that down by skipping 3rd and 5th quite happily, but then use all the gears when you want to have fun - it's just the high revving cruising that I wouldn't mind toning down. The other main issue is not having enough top end for track days. Not sure how Castle Combe compares to other circuits such as Snetterton or Bedford.

You could always stick the diff in as is, try it out then swap the LSD if you're happy with it. Few hours effort with minimal cost to help you decide.

I don't actually have a spare diff, just a loose 3.62 CWP, so this isn't an option unfortunately. 

Have you calculated road speed in each gear with the current and proposed ratio?

Yes. As an example, using 215/55/13 ZZRs:

- Current 3.92 diff gives me 4160rpm at 70mph in 6th and at 8000rpm a top speed of 50mph in 1st, 67mph in 2nd, 85mph in 3rd, 103mph in 4th, 120mph in 5th and 136mph in 6th.

- By contrast a 3.62 diff gives me 3830rpm at 70mph in 6th and at 8000rpm a top speed of 55mph in 1st, 73mph in 2nd, 92mph in 3rd, 111mph in 4th, 130mph in 5th and 147mph in 6th.

Krismuss - great, I'm now having second thoughts again!  I'll have to have a look into Abbeville. What's the deal with trailers?

[Krismuss]

Smithy, the No Trailer Trash Trackday  started about 5 years ago and we've been doing 1 or 2 track days a year in France. All generally within 100miles from Calais and everyone drives their 7 there and back (or not if you have a mishap like me at Croix several years back). 

Relatively cheap weekend all things considered, we stay two nights, have plenty of beer and a great open pit track day courtesy of Lotus on Track (exclusive for 7s generally).

If you're in Hampshire do you know Steve Carrol & Andy Morgan? They'll give you the 'low down' on how good the trips are!

Chris

[Jonathan Kay]

- Current 3.92 diff gives me 4160rpm at 70mph in 6th and at 8000rpm a top speed of 50mph in 1st, 67mph in 2nd, 85mph in 3rd, 103mph in 4th, 120mph in 5th and 136mph in 6th.

- By contrast a 3.62 diff gives me 3830rpm at 70mph in 6th and at 8000rpm a top speed of 55mph in 1st, 73mph in 2nd, 92mph in 3rd, 111mph in 4th, 130mph in 5th and 147mph in 6th.

i don't think you're going to lose much objective acceleration on the road. That leaves the advantage of lower engine speed when cruising and higher maximum speed on the track, which is what you were looking for. But there is another aspect: do you enjoy changing gear, and would you miss doing quite so much?

Jonathan

[Shaun_E]

I run a 6 speed + 3.92 diff for sprinting and racing. I'm on 235/45R13 Kumho V70A so even lower geared than you. Max speed is 131@8500 rpm in 6th but the only place this is a major problem is Goodwood where I'm on the limiter before Fordwater and likewise before the kink on Lavant straight. I can see that at Combe you'd be on the limiter through Folly as well although not for as long (I've not done a track day or race there, only a sprint with this engine and car set-up so don't know for sure). At Brands Hatch Indy I can just hit the limiter before Paddock Hill.

If you fitted the 3.62 diff you'd not reach the limiter in 6th with 230bhp. I've got a 260bhp Duratec and at the top end of 6th, even though power is still climbing, the rate of acceleration has dropped hugely and any gains in top speed with a higher diff ratio would likely be offset by the loss of acceleration on all the other parts of the circuit.

I have to confess to hardly using the car on the road these days but ear-plugs have always been the best solution  - my bigger problem is fuel range - about 100 miles  - due to thirsty engine and low gearing.

[Smithy77]

Well I'm glad I started this thread and thanks for all the input. It's given a good mix of perspective. I think I will hold off from making the switch this winter as I have a few other jobs that need doing - rear shocks need a service; rear chassis members need re-painting; rear pads to change; coolant circuit mods to do; low OP warning light to fit - so the diff ratio really is at the bottom of the list in terms of priority, and I could do with the extra time and money to be honest. This will also give me the chance to try different circuits next year to see how much of a problem the top speed is; if it is a problem, this can be a job for next winter 

Krismuss - I will PM you about French trackdays

[Grim Reaper]

I changed from a 3.62 to 3.14 a couple of years ago, best thing I ever did, it makes long distance trips a breeze and I can't say I've noticed much loss of acceleration.

1.8 DVA K series 210bhp 6 speed box

[Alan Reeves]

Grim, that's what I did also 6  speed and 260 CSR ....

[OliverSedlacek]

I've just changed from a 3.9 to a 3.6 diff and initially I thought I had made a mistake. After a couple of hundred miles though I got it through my thick skull that I just need to be in a lower gear some of the time!

With 6 gears to play with, there should always be an ideal gear for any situation, so the diff ratio only really affects what happens in top and in bottom. The 3.9 is clearly sub-optimal as you are running into the rev limiter in top. Assuming you can still break traction in first (and with 230 BHP I'm assuming you can) then I don't really see a problem.

[John Vine]

...so the diff ratio only really affects what happens in top and in bottom.

I'm not sure that's strictly true.  I'd certainly agree that with a 6-sp there'll be a suitable gear available at all speeds.  But the rate of acceleration at any particular point is going to depend (other things being equal) on the speed/revs ratio.

Smithy77's speed/revs graphs look like this:

3.62:

3.92:

The rate of acceleration is indicated by the slope of each line -- the flatter the line, the greater the rate.

JV

[OliverSedlacek]

Well it might not be strictly true, but my in practice it is.

[Smithy77]

John Vine has a point, and it is a fact the rate of acceleration, in all gears, will be less with a 3.62 diff. The car is still new to me so I think I'm making the right decision to give it a full season/summer to evaluate and make a decision. Thanks again for the input

[Smithy77]

John Vine has a point, and it is a fact the rate of acceleration, in all gears, will be less with a 3.62 diff. The car is still new to me so I think I'm making the right decision to give it a full season/summer to evaluate and make a decision. Thanks again for the input

[Jonathan Kay]

The rate of acceleration is indicated by the slope of each line -- the flatter the line, the greater the rate.

That doesn't sound right to me... more, please.

... and it is a fact the rate of acceleration, in all gears, will be less with a 3.62 diff.

That may be true overall, but it isn't at all points. Imagine, for example positive flat-out acceleration at a road speed where you have just had to change up in the lower geared car because of the rev limit or being over the top of the power curve. In the higher geared car where you hadn't yet had to change up you'd be at higher engine speed with more excess power and greater acceleration.

Jonathan

[John Vine]

The rate of acceleration is indicated by the slope of each line -- the flatter the line, the greater the rate.

That doesn't sound right to me... more, please.

Well, it's just another way of saying that the lower the gear the greater the acceleration.  Compare sixth gear with third at, say, 30 mph.  Clearly, you need to select the appropriate lower gear  -- dropping into first at 40-50 isn't going to get you far.

Re the original post, I think it all boils down to the OP's reluctance to forgo the overall "bonkers" nature of his 3.92.

JV