wich diff ratio would better suit me & my car?

[Miura]

I know it's been done to death over the years but I just want to canvass views for my car and my driving style/usage of the car.

 

Car is a 1.6 on SS ECU, piper 633s and race 4-1 exhaust (so more or less a 1.6SS with a bias towards no torque down the range and power in the last quarter of the rev counter)

 

I use the car a lot for touring so would appreciate to retain sensible revs around 70 mph

 

I currently run the 5 speed gear box with 185/60/14 tyres.

 

I am about to go for the 6 speed and 13 inches wheels on 185/70/13 tyres

 

my questions are :

 

1/ should I go for a 3.6 or 3.3 diff or retain the 3.9?

2/ what sort of characteritics should I expect in terms of driveability in trafic, enjoyment of the car and revs at a stabilised 70 mph for each of the 3 ratios?

3/ market value of second hand 3.6, 3.3 diffs and source?

 

 

[Shaun_E]

Thomas - I would stick with what you have and see how you like it. As you know most of the noise at motorway speeds is wind and road noise - the revs don't make that much difference. The combination of 6 speed and 3.92 diff will give nice acceleration! Plenty of sprinters running that combination.

According to gearcalc 70mph = 3976 rpm

With the 3.62 diff it would reduce to 3671 rpm

So not a huge difference.

 

Yellow SL #32 - member of Drowned Rat Racing

[F355GTS]

Thomas

 

My 1st 7 was a supersport with 3.92 and 6 Speed, it was great for track, I didn't really used it on the road but Rob Grigsby drove it back from Imola via Monte Carlo and I seem to recall him swearing a lot about the noise/ highly strung nature when trying to cruise, I don't have the gear calculator to hand but IIRC 85mph is something like 6k revs in 6th due to it's 1:1 ratio . I would go for a 3.62 to make it a bit more relaxed, non LSD diffs seem to be quite cheap, I think I paid Nifty £50 last Year

 

Mark

[Powderpuff]

My 7 is running 6 speed and 3.92 and I have to admit it's not the nicesy car when cruising.

 

I notice it far more now I'm running roller barrel induction, the noise is very tiring (even with earplug in) when doing 80(ish) on the motorway ☹️ With the old 1.6ss engine I didn't find it a problem, doing 3500 miles on the USA2005 tour was fine.

[Andy Best]

I did the 5 - 6 speed upgrade in 2002 on my then 1.6ss and kept the 3.92 diff, there was a plan to upgrade the diff to LSD at a later stage that I have not done yet spending the money on engine mnods first. After three trips to lemans and many 300 mile blats I will be keeping the 3.92 when I evetually upgrade. My engine probably makes more noise than yours now (VHPD on Jeveys) but to be quite honest I don't notice it and the extra acceleration puts a big 😬 on my face.

 

 

Andy

[Bricol]

Same here - 6spd and 3.9 - fantastic! And done lots of motorway miles abroad to get to more exciting bits in and around the Alps - where it really comes into it's own.

 

Bri

[Beelzebub]

4K rpm = 70 mph give or take. 3.92, six speed & AO32 shod 13 inch wheels.

[Alex Wong1697456877]

3.3 will be too tall for your engine. 3.3 with your engine and a 5 speed would be way way too tall. I have a 3.38 with 6 speed mated to a JPE engine and it's great, but it needs that grunt and power to make use of the gearing and the 6 speed doesn't have the overdrive that your 5 speed does.

[Peter Carmichael]

3.62 diff would work best. Just my opinion. I may be wrong.

[david nelson]

I have a 1.8ss 6 speed. Last year it had a 3.62 diff this year a 3.92.

 

I thought the 3.62 was fine and would have been happy with it but changed as mainly use the 7 for sprints/hillclimbs. I mainly do a and b roads so the 3.92 is great. I did go to Leeds and did not feel that the diff was the problem. I was traviling v.fast up the M1. As for noise well the music though the autcom helps out.

 

If i were you keep your current diff and see what you think. its not a big job to change the diff if you dislike it.

[Miura]

Thanks everyone, the logical path would be to try the 6 speed on my current diff, see what I think and move to 3.62 if 3.92 too low for my liking (which I suspect will be the case) simple really 😬 thanks guys

[Malc Dudley]

Mine is a six speed with a 3.92 diff and found it high reving while touring.Reduced the problem by changing the rear tyres from 185s to 205s.

 

dragged along by 195 horses

[Kevsta]

Just a thought, but to see what the cruising is like (as long as your car is still on the road) go for a drive on the motorway and leave it in 4th...that is 1:1 and will give you an indication what it will be like to drive like...

 

 

[Miura]

Kevsta, I will be doing just that

[Sheds Moderator]

T(h?)omas, how about a BGH modification of your existing 5 speed box to retain the existing top gear for cruising and bring the others closer together?

[Miura]

BOSS: because I already have bought the box 😬

[patneale]

Thomas.

I have a 6 speed 140 HP 1.8 Xpower SV with 16 inch 205 tyres. Gearing is 19.2 MPH/1000 revs in 6th. Standard Axle ratio. This set up is really totally wrong for my use as I don't go on the track and use it for touring and a lot of it... 32000 miles since March 2003. Acceleration is excellent, but cruising long distances is a slow business as motorways are a pain both on the wallet and the ears. If you are prepared to go fast then you are for ever stopping for fuel even with the SV's extra tank capacity. The result is that I tend to potter along at 65 MPH on Motorways and get about 35 to a gallon, at 90 it is about 20 to the gallon or least to seems to be and at 120 (abroad) i reckoned on under 10!! So B roads are my choice! What Caterham needs to do in view of the current Green mood is to have a wide range 6 speed box which will pull at least 25 MPH/ 1000 revs in 6th for the "ROADSPORT" models. The problem with just changing the axle ratio to get high gearing on the present 6 speed is that 1st gear becomes rather high and the clutch will not like the slipping to get a smooth start. A proper wider ratio 6 speed is the answer. My 1.3 hatchback has 24.3 MPH/1000 revs in 5th and does 40+ to the gallon and my large diesel automatic (5 speed with lock up) has 34.3 per 1000 revs and does never less than 45 to the gallon and over 50 on longer trips. With petrol now at £1 per litre we all need higher gearing !!! Caterham 0-60 5.3secs Diesel Auto 11.5sec and 1.3 12.5 secs Top speed Caterham 120MPH Diesel Auto 120MPH ...1.3 96 MPH.. official figures. Speedo figures are 130 on Caterham and diesel and 100 on 1.3 !!!