Gearboxes

[Mr Whippy]

Ok, I know very little about them inside. Apart from making ones from Lego in the olden days, I have little real metal gearbox experience.

My car is revving at 3500rpm @ 100mph, which is quite short. Stock my car is 90bhp and at that the gears seem fine.

At 125bhp they are feeling a little short, especially when cruising.

With a projected 150-160bhp with some future changes (and those a friend has already made) the gearing is almost unbearable in top gear.

http://www.205gti.com/techgearboxUK.htm

As far as I can gather, the BE3R gearbox on my car has a 70/19 final drive (3.68:1?!) and using that and the above numbers for a 1.9 GR 405, they get my speeds per gear almost perfectly.

Problem is, the final drive seems to be the longest one, and the 5th gear appears to be the longest one too.

So ultimately we can't make the car have longer gearing with off the shelf bits.

Does anyone know of the viability of making what I guess is the 'crown' wheel in the final drive smaller and reduce less (I guess the 70 is the tooth count on the crown, and 19 the teeth on the input cog wheel) so maybe go for say a 60 wheel instead?

Is this a common thing to have to do? Do people make these? And on a fwd, would this be all part and parcel of say a limited slip differential assembly?

What kind of costs would you be looking at?

Thanks

Dave

[Sam_68]

Mr Whippy said:

My car is revving at 3500rpm @ 100mph, which is quite short.

That works out at 28.5mph per 1000 rpm overall gearing, which is actually quite tall.

For example, with that gearing, at 6,500rpm (if the engine could be made to produce enough power to overcome the aerodynamic drag), you'd be doing 185mph, which ought to be plenty in a Pug hatchback!!

In terms of final drive ratios, the bigger the number, the shorter the gearing (ie. top speed reduced but acceleration increased). So, for relaxed cruising and good economy, you need a smaller number (taller gearing); so the 3.471 ratio listed on the link you gave would do the job, but the acceleration would be a blunted slightly.

If I've misunderstood you, and what you are actually after is better performance (both acceleration and, realistically, top speed, since I'm sure your car can't pull maximum revs in top gear at the moment?!), then you need a bigger number for the final drive ratio; the 4.429 ratio listed on your link is the 'shortest', so would give best performance at the cost of reduced fuel economy and higher motorway cruising revs.

I wouldn't have thought that having special CWP combinations manufactured would be cost effective. Even a standard CWP would cost several hundred pounds, plus a gearbox rebuild to install it, so having something designed and cut from scratch is going to cost you a fortune... cheaper to buy a more suitable car - but you seem to have alternatives either side of the ratio you are currently using, simply by choosing a suitable final drive ratio from those available on the link you gave.

To answer the other bit of your question, though, CWP's are normally not included with a limited slip diff; the diff is a separate item.

Edited by Sam_68 on Monday 3rd September 13:20

[Mr Whippy]

Sorry, car is a diesel, so max rpm's are ~4000rpm, and it's developing peak torque at 2500rpm, so at a cruise on the motorway at say 80mph it's at almost 3000rpm, when in theory it could probably pull the same at 2500rpm.

The 3.471 is a better bet, just not sure if that needs the swap over of the crown wheel and the drive wheel into that (I believe that is 62/14 from what I can see on my parts CD (final drives only unfortunately))

Just that is only ~ 6% longer, and I have reservations about torque loading. Again, will all these be much the same torque loading potential, or is it likely some gearbox components are stronger than others even within the same family (BE3 in this case)

Just looking at some VAG stuff, and the newer Peugeot's, with diesels 6th taking the cars to around 100mph at 3000rpm, much better for motorway work/economy. I can already set off in 2nd with a little slip day to day at the other end too, so even 15% ontop of 1st wouldn't be an issue at all, infact it'd make it perfect (maybe not for hillstarts with a caravan on, but I don't do that anyway )

Dave

[stevieturbo]

Diesels do already tend to have the tallest gears.....


Maybe try some 22" wheels ?

[GavinPearson]

To achieve what you are looking for you probably need a new output shaft as they tend to have the final drive gear cut into them, plus a new crownwheel. I would talk to Quaife about this, as it is well within their capabilities.

The hard part is choosing the ratio, just remember that although you have said that 2nd is now your starting gear, and it might be a simple gase of upping the drive by that percentage, the reality is that when you start to load the car up and need to go up hills you may need closer to 1 and a halfth if you see what I mean. Also do some calculations on what it will do to top speed, if you get it wrong you will top out in 4th and not be able to use 5th to achieve the theoretically possible top speed.

For the power you are quoting I should imagine that the torque you have is very high, so I would think an LSD would be a smart investment.

[Mr Whippy]

Yeah, the one in a million hillstart on a 1 in 5 gradient with passengers and luggage might be amusing, so maybe 15% longer overall would be as far as I'd want to go... it'd just make 1st feel that much longer, as it is now it's quite a hard gear to use. 2nd can often be smoother for setting off in.

Anyway, I digress.

A longer 5th would be better, but I have no idea how you do that, hence the idea that a longer final drive is probably more viable, and if the LSD is part and parcel of a replacement crown wheel then better still.
Just alot of people are now tuning these earlier 306 Hdi's to ~ 150bhp, and the 5th gear really is silly, nearly 3000rpm at 80mph or so. The gearing is relatively short and with ~ 250lbft of torque 1st and 2nd are almost always a bit playful as the turbo gets going, so an LSD option would be nice too, and with longer gearing the high torque wouldn't rear it's head with wheelspin as often in the lower gears.

Just the price, but I'll go on the proviso there would be five as I rekon there are enough punters out there for that number, and see where it goes from there.

Need to go learn more about gearboxes and how they work (eek)

Cheers for the help and ideas so far everyone.

Dave

[anonymous-user]

Quaife's website has a nice little utility you can download, then plug in gear and final drive ratios and get the overall ratios, speeds in each gear, etc. so you can play with different ratios to see what effect it has.

[love machine]

Good ones here as well for everything automotive

http://www.csgnetwork.com/automotiveconverters.htm...