Today's Numpty Diff question

[Andy Lef]

OK so here goes.  I have a '92 reg ex 1.4ss K converted by Dave Andrews to a 1.8ss K, same cams, forged pistons etc.

I am trying to figure out what new gearbox might be suitable for the car to make it even better to drive. 

Anyone know what Diff I might have?

 

in anticipation 

[Jonathan Kay]

You can measure the final drive ratio by:

1 Jacking the car up and measuring the ratio of prop shaft turns to drive shaft turns, details available on request, or

2 Measuring the road speed (eg with GPS) at a known engine speed at a known gearbox ratio with known tyre circumference, ditto.

...

Which gearbox do you have?

...

Road, track or both? What are your current thoughts about:

1. The engine speed and noise when cruising?
2. The usefulness of first gear?

Jonathan

[Andy Lef]

Thanks Jonathan, 

Will try and jack up the car at some stage.

Gearbox is a Type 9 unmodified

• Keen to have something that is better for road (don't track the car);
• 1st gear is rubbish and I spend most of the time pulling away in 2nd - though that's a bit sloppy as it needs to have enough revs to make it worth the effort.
• I don't think a screaming 5th is what i want for cruising - am i getting old..................
• I've looked at BGH and they seem to have a nice compromise for utility vice cost;
• Clearly not too fussed about top speed if I'm only going to use it on the road;

Andy

[Wrightpayne]

Generally 5sp = 3.9 and 6sp = 3.6.

choices are change the ratios in the 5sp or go for 6sp and diff change.

I’ve not seen a 6sp for sale for ages but expect to pay 1.5 to 2k depending on recent refurbishment. I paid 1400 and it needed an 800 rebuild.

The 6sp ratios are sweet in a k series car esp on track where it keeps it in the power band all the time. For mainly road and touring I’d go with a modified 5sp keeping overdriven 5th and avoiding the diff costs.

Ian

[Petethediesel]

I have bought from BGH before and the quality/service is excellent. If you can provide them with an engine dyno curve of power/torque they will ratio the gearbox to match. You can also specify a "long"  5th for the motorway if that's your preference. They can fit stronger layshaft bearings and beef up the synchro too, depends on your budget.

    Pete

[7 wonders of the world]

The 2.66 1st gear that BGH offer is still way of the mark for a Caterham's meagre weight.

Take a look here for some superbly made gearkits with much more suitable ratios

https://www.tracsport.com/

Steve does like to chat though .....  

[Andy Lef]

Many thanks.

 

Not wishing to open a can of worms..........

Has anybody had experience of both BGH and tracsport gearboxes to offer comment on 7 WONDERS OF THE WORLD suggestion that "The 2.66 1st gear that BGH offer is still way of the mark for a Caterham's meager weight."

 

[david2]

I've been looking at BGH sporting close (2.66) and SPC touring (2.752)

My main concern is that the car is an SV and used only for touring 2 up, so the weight is around 750kg and we do have to drive in traffic through seaside towns.

being able to roll along at 10mph is important in these circumstances, particularly with a K series which has less torque and doesn't come onto cam until 2500 rpm. Too long a gear and you will have to dip the clutch constantly.

for my set up 1800 rpm is 9mph with the current 3.35 and would be 15mph at 2.48. So i think for me that 2.66 or the 2.752 is a better compromise but will still get me to 45mph in first and a 38% gear drop 1st to 2nd  (7500-4628) as opposed to the 46%!! of the standard box. It's the rev drop that I actually find more annoying than the shortness of gear as it bogs down at <4000rpm

I am 100% going to change the ratios this winter. The SPC is attractive as it seems to be completely manufactured from scratch where I assume the BGH to be remanufactured.

yours, confused Surrey

[Jonathan Kay]

Two points that should have been in my first response:

1. You can measure gearbox ratios as well as final drive ratio by jacking up or by GPS. But the former's going to require turning the engaged engine...
2. Calculators that will do the sums for the effects of changes are widely available, both online and as downloadable spreadsheets. (I mention the latter because there's one somewhere with common Caterham ratios already included.) Good ones also include the fractional speed drop on changing up, as described by david2 above. I think that's a very convenient metric for thinking about options.

Jonathan 

[david2]

this is the best i've found.

http://www.wallaceracing.com/calc-rpm-drop-shift.php

I'm going 2.66, 1.75, 1.26, 1.0, 0.86 BGH sporting close and adding a quaife ATB at the same time.

 

[Shaun_E]

This is the gearing calculator I've always used: https://locost7.info/gearcalc.php