Thoughts on changing the gear ratios ...

[SLR No.77]

Having just returned from Le Mans with the driving being a mixed route of Autoroute, A, B and some quite minor roads so essentially the complete mix, I'm absolutely convinced I need to improve my gear ratios to give a better balance for the driving I'm now doing. These days I virtually never spend time on track, or maybe if I'm honest with myself that should simply be never rather than virtually never, but I'm increasingly spending more time touring and often some quite big and high speed days.  I therefore need to canvass opinion and draw on the experiences of others who might have gone through the same.  Currently my top gear is such that I'm doing a touch over 80mph at 5,000rpm.

My car is a 1900cc Superlight R, the engine was rebuilt 10 years ago, it's running Caterham roller-barrels and is really sweet with around 160 lb/ft torque so it's no slouch when touring. The six-speed gearbox however could do with a rebuild, as could the AP Suretrac 3.62 diff.

But ... I have a spare Caterham origin 5-speed gearbox, plus a spare 3.38 open diff.  If the 6-speed and diff are going to be rebuilt I've effectively got a blank canvas and plenty of choice.

So what's going to work as a fast touring car:

• 6 speed with the 3.38 diff?
• 5 speed with the 3.62 diff?
• maybe even the 5 speed with the 3.38 diff?
• 5 speed with modified ratios and the 3.62 or 3.38?

The strong torque from the1900cc unit certainly gives plenty of scope for a higher first but I'm not sure where the limit would be. I'm also wondering whether to start with the diff before deciding which gearbox to spend any money on.

Thoughts appreciated, and no I'm not looking to change to a CSR!

Stu.

[Mcalvert]

Hi Stu - there are so many variable here, including tyre size and I'm not that familiar with the power delivery of the 1900 K-Series engine, but have you had a play with Gearcalc here - I would have though that with your knowledge of peak power and shift points for your engine, it would allow you to have a good play with options?

 

 

[TomB]

Fundamentally - is your noisy 1900cc SLR ever going to be a good touring platform?  If you turn your SLR into a relative limo, it might end up being neither fish nor fowl.  A CSR has a lot going for it in this space....

[Andrew 21 Edney]

How does this compare to what you have?

https://www.burtonpower.com/5-speed-close-ratio-full-helical-gear-kit-type-9-caterham-gbt9230c.html

[ScottR400D]

It might be worth you talking to Steve Perks of SPC. 

[Richard Price]

Stu,

is it a complete, ready to use, open 3.38diff?

if so, why not swap the diff and try the 3.38 ratio with your 6speed and see how you like it.

if you like it, drop it out again and have it rebuilt with the suretrack in it.

[7 wonders of the world]

Hi Stu,

After years of nagging Steve (SPC) relented a few year back and had another first gear option made for his Semi Heli kit

The 2.29 first set is superb, available with .87 or .82 fifth option, full alloy casing, I run mine with a 3.92 final drive with a Tracsport LSD (another great piece of kit which you wont better)

Drop me a BM with your email and I will send you a spread sheet with a raft of options for you to go through.

He also has some very neat upgrades for the 6 speed too if you decide to go that route.

[SLR No.77]

Thanks all for the responses.

#6  Richard, I've wondered whether to do just that as a first step, giving me an opportunity to get a feel for the gearing before I commit further. The current AP diff is mechanically sound but has probably done 30,000 miles hence my thought that it would benefit from a rebuild, however I'd probably leave it sealed if the 3.38 works and opt for either a Quaiffe ATB or as suggested, a Tracsport built into the 3.38.

A lot of good suggestions to speak to Steve P, maybe I need to throw the ratios into Gearcalc or a spread sheet and see what my available choices will yield.

Stu.

 

[Alan Reeves]

A 3.14 with a 6 speed box gives the same  ratio  in 6th as a 3.62 and a 5th in a 5 speed box

i went full bore with a 3.14 and a CSR and transformed long haul driving. so a 3.38 would give you a halfway house

and i would go for it with your 6 box

alan

[Alan Reeves]

A 3.14 with a 6 speed box gives the same  ratio  in 6th as a 3.62 and a 5th in a 5 speed box

i went full bore with a 3.14 and a CSR and transformed long haul driving. so a 3.38 would give you a halfway house

and i would go for it with your 6 box

alan

[anthonym]

I've read your o p several times wondering if and how to respond. I just swopped my 1987 BDR (Standard 169bhp max 7,000 rpm) five speed / 3.54 (I think) LS diff  for a CC standard sixspeed / 4:1 open diff. My use case being the opposite of yours, no touring, no motorways, no track days, no racing, trailer for any distances, to be used on high (that means often deserted) narrow mountain roads, usually Alpine. 

A kind fellow club member (whose daily driver is also a 7) drove the car to Switzerland from CC and to my surprise was not critical of the motorways experience and also wasn't deaf on arrival here (the Powerspeed exhaust is quieter to my surprise) . So I thought that was very interesting given the sixspeed/4:1 combination generally is seen as likely to be horrible for motorways. I also use a sixspeed/3.62 (I think) in my K series and again I avoid motorways because.. because I like air conditioning and relaxed travel for long distances, and peace and quiet, and what I call "self rescue". 

Now then, others near here do vast distances by Seven only, usually S.V. These have five speeds and normal diffs and I can observe in the aforementioned mountains, can pick up their heels when so desired. I.e. they are fast (enough, given the use case). 

So my K engine revving to nearly 9,000 is on the cam for not a very wide rev range, so that's where the sixspeed fun is, staying on-the-cam. That does not sound like a touring way of being. Fun though, I love it :-)

My BDR is new to me in this guise, but in the little I have done so far, good grief she goes.  

I don't know if all this helps at all, not being scientific at all, but it is what comes to mind reading your opening thoughts.

What would I do I wonder. Five speed. 1st as long as possible. Long legged diff. LSD or open makes no difference to me as yet, except open means no graunching noises. Otherwise I view the fivespeed as a three speed box, 1st is for towing caravans and 5th is overdrive for motorways, leaving three to play with. Make 1st useful. That means getting the sixspeed 1st ratio in to the fivespeed box. Whatever that is. 

This has been an interesting journey of a post evaluating several or many years in some ways.

Edit: keep both 5 and sixspeeds and diffs available to mix and match according to your immediate purpose. Lots of spanner activity :-)

Anthony

 

[7 wonders of the world]

Stu,

Spreadsheet here with the work done for you....... ping me an address 

 

[SLR No.77]

#12. PM sent.

I've run the 6 speed ratios through GearCalc with 215/55-13 ZZS tyres and the results are interesting ...

3.92:

 

3.62:

 

3.38:

 

So unless GearCalc is wildly inaccurate, my installed diff is actually a 3.92 despite having 3.62 written on it (in marker pen) and 3.62 being the diff it should have been specified with when new!
My speedo under-reads slightly at higher speeds compared to the GPS but it clearly shows around 82mph at 5000rpm, although the rev counter could of course be inaccurate.

If my current diff is a 3.92, swapping to a 3.38 might be a bigger step in the right direction than I thought.

Sorry Anthony, not too many Alpine passes near here!

Stu.

[anthonym]

no indeed, I will have a look at all those calcs with great interest to see how they match my uses

3.38 matches where my thoughts were going in your case

[anthonym]

BDR

[SLR No.77]

Anthony, if my diff is indeed a 3.92 then your gearing isn't a great amount shorter than mine, around 100rpm difference at 85mph in top gear.

Stu.

[anthonym]

Glad you said that Stu as I was wondering if it was my imagination, so my gearing isn't as outlandish as I had thought. Good to know and explains why it doesn't seem too bad at all in use. So if anything is a bit different it's simply that older 7s don't usually have sixspeeds in them. - anthony

[Richard Price]

although the rev counter could of course be inaccurate.

Stu,

My standard tacho seems to over read throughout the rev range when compared to what my data logger and shift lights say. 

[Jonathan Kay]

"My standard tacho seems to over read throughout the rev range when compared to what my data logger and shift lights say. "

Would you like to make them consistent?

Jonathan

[anthonym]

Same here, how does one fix that?

edit: 1987 BDR

[Mechanical Moz]

I had my early 90's rev counter refurbished and calibrated and it still under reads.  The ECU will show 7200 via the OBD port and the rev counter reads ~6500.

If touring comfort is your priority I would avoid the Tracsport semi helical kit.  The racket out of it in 2nd and 3rd is enough to drown out both the exhaust and diff chatter in mine, to the extent it's tiresome.  Stu, you're welcome to experience it for yourself next time I see you.

The full helical kit linked above wasn't available when I had mine done, I'd like to know how quiet it is.

[7 wonders of the world]

Full helical ratios are not ideal for a 7 though....

The gear ratios of this kit are:
1st: 2.752
2nd: 1.698
3rd: 1.237
4th: 1:1
5th: 0.815

My semi heli is pretty much half way between a stock set and SC gears in terms of noise, perfectly acceptable, at once the reins are slackened the TB's drown everything out ...