T25 Gearbox Question

[kyle_irl]

Hi all,

I have a T25 with a 1Y diesel fitted, when I got the van it had a 5 speed fitted, this gearbox gave up and I have recently replaced it with a 4 speed box. When I drove the van for the first time it seemed to rev very hard at around 50mph. I looked up the gearbox code and found some info relating to what RPM engines do with the different gearboxes, one is a DY code box which means that the engine is doing 3200RPM at 50mph! This seems very high, and makes the van unpleasant to drive!

My question related to various gearbox options! I had originally planned to fit a TDI engine with a flipped passat gearbox, however I began researching the gearbox options and found Aidan Talbots website, I'm not sure if you guys are familiar with his work but he is a gearbox specialist who offers a service whereby he can replace the top two gears of a gearbox with new ratio gears, meaning lower RPM at cruising speeds.

I know that if I go with the TDI and flipped passat box I will have more power and better gearing but I honestly don't mind the lack in power, the gearing is the main problem.

What are people running in their vans? Has anyone had the gear ratios adjusted in their vans?

Im basically trying to determine if I can run a standard gearbox or modified standard gearbox and get the cruising speeds I would have with a TDI conversion

Thanks in advance for any help or advice! Its greatly appreciated!

[mjslacky]

Hi, could anyone tell me how I post a question, many thanks Mike.

[greasemonkey]

Hi, could anyone tell me how I post a question, many thanks Mike.

You just have

Go to the section you want to post in and click on the button to create a new topic.

[Aidan]

irie Kyle

basically it's a balance of power torque and revs, gearing has to be appropriate to the engine, that's why there were nearly 100 different VW gearbox codes during the T3 life.

your 1.6 n/a diesel revs pretty happily and makes most of it's limited power fairly high up the revs, but has a good torque curve relative to it's power so it has a 5.5 or 5.83 final drive and it gets through the gears tidy and by 50 55 it's done. if you stick that on a 1Y it's still a 50-55 box as your ears are going oww that's a bit full on. The 5 speed gave you similar with a slightly longer final gear but closer gears.

You don't have masses of power or torque with the 1Y, it's roughly comparable to a 1.9 petrol carb engine (though not as nice as a good one) so can happily pull the ABD or similar boxes and it's good with a 0.82 fourth gear on top. That's my straight 4 speed for a 1Y recipe 4.83/0.82 for a camper. Much more than that and you will start to have to drop into 3rd on big high speed hills and rev the engine. Or go back to 5 speed to have more closer spread gears and a lower 1st but barely higher fifth eg ABN

If you want to pull 70mph at 3200 rpm or less you have to run a TDi and bespoke box, the 1Y just doesn't have the power to displace 8mcu of air at that speed. Or TDi and passat box of appropriate gearing. Or a petrol engine with some go like a scooby 2.5; the 2.1DJ 112bhp on AAP is pulling 3300 at 70 but the DJ will rev to 4000 happily still in the green band, though fuel flowing through the engine pretty freely at that sort of revs so not cheap but nice

[kyle_irl]

Hi Aidan,

Thanks very much for the response, I was actually looking at your website last week, very interesting stuff!

I was thinking of going back to the 5 speed for the short term to get the thing working and usable again, I had always planned to fit a TDI and passat gearbox, but would it be a long term possibility to fit an AAZ 1.9TD engine with the current 5 speed gearbox perhaps with the altered 4th and 5th gears to give a better cruising speed, what sort of outcome would this result is? Or would you recommend just going for a TDI and passat box seeing as these were designed to work together?

Thanks for the advice, it is greatly appreciated!

[Aidan]

AAZ is not a sensible long term engine choice imho it is very old technology , unless you have a stash of rebuilt engines, turbos and pumps to hand

There will only be a few more years of the current 1Z/AHU TDi engine availability window once they are all scrapped then you'll all be looking for the next generation of diesel engines, and none of them can be made to fit as cheaply; I'd be getting a second one and having that stripped ready to rebuild for the future if that was my engine of choice for the future as well if I had the space

IF you need to change engine then do it once, make a long term decision on wheeels and tyres and that then dictates the gearbox/gearing

[anaconda]

Hi Aiden , I emailed you a few days ago but you've answered most of my queries here. I fell into the trap of assuming 18 inch wheels would reduce the revs and give me a bit more cruising speed. I checked the Brickwerks calculator and realised my wheel diameter is actually less than the standard, due to low profiles! Consequently I'm looking to 'big up' the tyres, or at least as much as I can with a lowered van. I'm on a five speed originally a 1.7 1988 diesel and am looking to get the best comfortable cruising speed I can out of the 1Y. I may even change the springs and lift the van a bit if needed, again to maximise wheel diameter.

I've done 70 with the 1Y but it doesn't feel right at such high revs. With a wide wheel diameter (I'm going to increase circumference by about 12%) on this box, is 70 at safe revs still a no no?

Cheers, and apols for hijacking!