Modifying Gearbox Ratios

[Bally123 0]

Hi

 

After our first shakedown on track at mallory today I am left wondering about gearbox ratios....

 

Currently running a 1.6 gti, the gearbox has been modified by the last owner. Basically a LSD and modified 3rd gear. This was done simply to suit hillclimb events. However after driving hard today the ratio change from 4th to 5th is huge, and to be honest caused some issues. I appreciate this will make no difference to a hillclimb event but can't help thinking I would have enjoyed the day / car a little more today if it could be solved/improved.

 

I still have to figure out exactley what is in there at the moment so sorry for lack of details regarding current ratio's

 

Right now I want to know if any other options exist for getting 5th gear closer to 4th gear.

 

Any common mods?

 

Cheers Bally

[OlberJ 1]

Sounds like you've got a diesel 5th in there maybe, if it's a big jump.

 

Going back to standard 5th is probably what you want.

[petert 588]

You said it's modified, so who knows what combinations are in it. What I can tell you however, is that there are three possible 5th gear ratios that I know of:

 

0.865:1

0.81:1

0.76:1

 

Also, 3rd and 4th must be changed as a pair, so there are only two combinations there:

 

1.360:1 and 1.069:1 (standard GTi set)

 

1.280:1 and 0.969:1

 

I'm not sure how that will help you, other than pull off the side cover and start counting teeth!

[Tom Fenton 1,542]

I'm wondering what final drive you have fitted. As my track day car will only see 5th gear at the circuits with real long straights, Silverstone, back straight at Donington, and if you are really getting a move on, up Avon Rise at Castle Combe. Certainly at Mallory I wouldn't use 5th. This is with a 1600 GTI gearbox fitted, 7200rpm rev limit on an Mi16 engine.

[Miles 331 1 Cars]

5th gear isn;t that easy either, As there are 2 types of bush and shaft size to cope with

[Galifrey 1]

5th gear isn;t that easy either, As there are 2 types of bush and shaft size to cope with

 

Might go for a taller 5th in mine Miles, 4k rpm at m'way speeds is a bit much

 

 

Taller final drive ratio might be better for road use.

Edited December 7, 2009 by Galifrey

[Bally123 0]

Thanks for the comments so far guys.

 

Petert: Regards ratios for 5th, the Haynes manual only lists 0.865 and 0.757, where does the ratio 0.81 come from?

 

Tom: I am currently running a modified 1.6 motor and a 6K limiter, I found that Gerards was flat in 4th and running out but changing to 5th the car just would not pull through, felt and sounded like it dropped a lot of revs between the changes. From memory I think it dropped back to about 4500 or less. Kinda hard to tell as with a damp track things were getting a little exciting

 

I have done a quick excel plot of the std ratios and I think I may have found a clue! If the gear box was originally the ratios listed in the Haynes manual as close ratio then changing 3rd/4th from 1.28/0.969 to 1.36/1.069 would make sense for hill-climbing, however leaving 5th gear at 0.757 would then give a problem on track. The rpm differences seem to calculate out quite well and would show that this set up would mean the revs dropping back to around 4250 when changing to 5th

 

So can anyone confirm that the gti would use the close ratio specs?

 

Assuming I am on the right track (subject to some more testing!) can i swap out 5th for a 0.865 gear pair, or as Miles suggested could this be more complicated?

 

I have made friends with a local breakers yard so getting parts should be easy.

 

I'll keep ya posted....

 

Bally

[petert 588]

The 0.81:1 is found in the Mi16x4 and some diesels I think. Every GTi box I've had, whether 1.6 or 1.9, has the 1.360:1 and 1.069:1 3th/4th set. The other set is found in Mi16's and lower spec. models. You can certainly swap 5th gear. You just need to find the correct type for your box.

Edited December 7, 2009 by petert

[Anthony 1,003]

Tom: I am currently running a modified 1.6 motor and a 6K limiter,

Why only a 6k limiter? As standard 1.6 GTi's are limited to ~6750rpm, and that would give you enough revs not to need 5th at all...

[Tom Fenton 1,542]

Its a good point, especially if its modified surely it should rev past 6000rpm?

[Bally123 0]

Fair point about the 6K rev limit, basically it's just running the set up as purchased, I need to get it on a dyno next and see what the curve/power is like. It's quite possible that I can raise the rev limit but felt it was best to start conservative for its first outing.

 

Plan is now to get on a dyno before xmas with some luck which should also help me to define the actual gearbox ratios at the same time. In the mean tme I'll ask my man at the breakers if he has any boxes kicking about.

 

Cheers Chaps...

 

Bally