Just having a think about what gear box to use on the Polo. GT gear ratios would be spot on as they are nice and short. Won't really be hitting speeds in excess of 80-90mph I wouldn't have thought and this leaves 5th gear redundant. There might be some gains in packaging size/weight/transmission losses if we could fit the first 4 gears and differential from the GT gear box into a 4 speed casing. Do people think this would be possible? If so then it might be worth picking and choosing some different ratios as well?

The performance gains might be minimal but every little helps, especially if it can be done cheaply. Added bonus is no miss-shifts up to 5th.

Just get a 1 litre 5 speed box (code AHZ or CEM) which are the same gears as a GT box, but a shorter final drive diff ratio. These are the closest ratio 5 speed boxes, ideal for short sprints. Go for the CEM version should you ever want to fit an LSD into it.

I reckon it'd work, the input & output shafts in an 084 are different but as far as I'm aware only because they are shorter. It should be possible to put the gearset onto 084 shafts and use everything else from the 084 box.

Although I can't imagine you'd get any noticable gains over just putting the 085 box on. In terms of weight I imagine having a shit before getting into the car would have more benefit :)

It'd be a minimal saving true...but no constantly meshed 5th gear either. If I stumble across an 084 it might be worth a punt. Nothing an afternoon in the garage with some pullers and other tools wouldn't sort.

Nick I pressume CEM has bolt on final drive then? That's good news!

Edited 6 October, 200916 yr by Tav

The CEM will have a steel rivited on final drive from the factory. You can drill these out easily enough and replace with nuts and bolts.

There are quite a few 084 box codes too, so i wouldn't be surprised if there is already one out there with GT ratios.

I had a look in the spread sheet but nothing struck me in the 4 speed section...some low FD's but the gears are longer given the smaller choice in ratios. Also couldn't find a mention of CEM. Going to look on ETKA for it just now.

Correction, got my wires slighty crossed. AHZ and CEM (same gear set as GT's) are both pressed on final drive 4.26 ratio crown wheels.

The AYZ and CMC (same gear set as an 8P) have a final drive ratio of 4.26 Its the CMC gearbox which has the rivited on final drive crown wheel!

Ahhh ok, CMC is missing from the spread sheet as well, will have a look on ETKA. So CMC + GT gear set = short ratio w/potential for LSD.

Yep.

Only snarl up there is that I ain't seen a CMC box before. I had a CEM on my Terra engine and i had an AYZ (currently in bits for spares) on my 1 litre mk3 saloon.

So CMC + GT gear set = short ratio w/potential for LSD. CMC is missing from the spread sheet as well, will have a look on ETKA, wonder what car it came from? Wonder if VW sell/stock individual gears? I doubt they'd be cheap, great if something could be put together from used boxes...working to a tight budget

Looking at ETKA to try and understand what gears would be interchangeable, looks like the shaft has a different diameter for 5th and thus the gear has a smaller 'hole' through the center.

The 1L box does look like a good shout for a stock gear box. 175/50 tyres and 7500rpm limiter puts the GT top speed in 5th at almost 130mph which is useless. I think the 1L is nearer to 115mph with the other gears sitting pretty much spot on for 30/50/70/90MPH.

Edited 6 October, 200916 yr by Tav

I would get hold of an AHZ first of all, simply becaause they are easier to get hold of and then look out for a CMC. Late mk3 1 litre 5 speed cars are likely candidates for a CMC gearbox

Here is the performance of an AHZ plugged into the Hunter spreadsheet.

AHZ

Final Drive ratio 4.267

Gear // Ratio //Engine Revs(rpm) // Actual Speed with 175/60/13 tyres

1st // 3.45 //6500 //27.87 mph

2nd // 2.087 //6500 // 46.13 mph

3rd // 1.469 // 6500 // 65.54 mph

4th // 1.098 // 6500 // 87.69 mph

5th // 0.851 // 6500 //113.14 mph

I know a chap with an AHZ gearbox in his garage who might part with it.

I have the DCD from a mk4 1.0. Riveted crownwheel. Easy to drill and convert to bolts.

4.27 diff.

1st 3.45

2nd 2.1

3rd 1.45

4th 1.1

5th 0.89

It requires an engine that revs.

Fuel economy will be terrible.

Is this a track/sprint car? If not,then..

I fitted a 1.0 4-speed to the 1.6 and it could scream at 90 for a moment but its top speed was really 70mph - where it'd shout for another gear,as it drank fuel.

Not exactly ideal for any road longer than an eighth of a mile.

An alternative is a GT style 5-speed box with short ratios,3.8 diff and a very long fifth (My own is a medium geared 5-speed with 3.5 diff and long fifth,as i wanted the same but longer geared with an overdrive),so you have your short four plus an overdrive.

Food for thought,because with your plan,you'll get frustrated very quickly if this is for the road.

I dont see how there can be an issue putting it into the wrong gear either,unless the linkage is poorly adjusted or you cant drive - I've never done that,is all.

Hillclimb & sprinting, so fuel economy isn't too much of an issue. As for the wrong gear comment it's not something I've ever done either but completely eliminating the chance of it happening isn't a bad idea, it is after all one of the main benefits of a sequential box, no driver error.

It's good to bounce ideas about as might come up with something useful. Our class has a 130+ BHP Corolla GTi 1600cc in it. They didn't look like the quickest of drivers last year but they've got a significant power advantage and being an old car not too much of a weight penalty. A few little cheap modifications could be enough to help the Polo get close to/surpass their times. The 1400cc Saxo is audibly disadvantaged by it's current economy gearing, not something the Polo should have issues with.

Edited 7 October, 200916 yr by Tav

If your main aim is to avoid mis-selecting 5th, then just adjust the gearlinkage accordingly, and/or bolt a bracket onto the linkage to prevent 5th from ever being selected.

Weight saving and decidedly marginal transmission loss benefits aren't really matched by the effort/cost involved to build a custom 4-speed.