Gear Ratios

[m_2975]

Hi Guys,

Have been looking at doing a mod to my petrol BX by putting the top gear from a diesel gearbox in it, thus reving lower on the highways. I know that it mucks up the ratio a little in terms of acceleration but for touring it makes it more relaxing. I have seen it done before and it works well on the GTI engine.

Can someone please tell me how fast the engine in a BX TZD revs in top gear at 60mph and at 50mph?

I'd like to make a comparison in engine speed.

Thanks

[jeremy]

Diesel turbos - about 26 MPH per 1000 revs - ordinary diesel about 23, estates may be a bit slower.

[AndersDK]

TBH I believe you would be disappointed with such a mod.
Any petrol engine would have their max torque (pulling power) at much higher rpm than any diesel.
The torque figure is exactly what determines how the engine performs at low rpm.

Petrols 3500-4500 rpm
Diesels 1800-2100 rpm

Nothing to do with the Bhp's as any petrol usually would have lots more Bhp than any diesel.

[Jaba]

This mod is well worth doing. You will be running at 500 rpm less which makes the car a lot quieter and a bit more economical too.

Performance in top does suffer a bit but you are trading noise level,comfort and economy for performance. How much performance loss ? Well in acceleration terms from 40 to 80 mph in top gear in a standard car - then a modded car over the same distance will be doing 75 mph so you lose the time it takes to accelerate that extra 5 mph.

You need to use the 5th gear from a turbo diesel box for most models except the 8v GTi. For this gearbox the fifth gear from almost any other model will do the job. The 8v GTi has a much higher final drive ratio than the other models so lower 5th gear ratios will work.

The calculation for road speed per 1000 revs is as follows (for 8v GTi) :

5th gearset sizes, 37/32 times final drive ratio, 17/61, times 1.8m, the
rolling circumference of the wheels, times 60 to get the kmh At 1000
rpm.

Just substitute another 5th gearset ratio to calculate the difference.

Fitting TD 5th gears to a GTi would not be a good idea @ 45.8kph per 1000 revs.

[DavidRutherford]

to answer the original question directly: Every BE3/XUDT (be it a 9 or a 7) combination I've had has 2250rpm for 60mph in top. Which is why I like to drive them at 60mph as much as I can, as at 2250rpm the engine is also at it's highest point of thermal efficiency. (Peak torque and peak thermal efficiency are one and the same on a mechanical injection diesel engine)

[m_2975]

Thanks for the input guys. I will be looking into it in more detail over the coming weeks. Now that I have that information i'll draw up a graph for the 5th gear out of the petrol and the diesel and see how much difference it will make in terms of performance.

I have noticed, based on the infor you have all given me that the 5th gear out of a diesel box would be useless under 50mph (80kph). I will also do some more driving to see if my driving style uses 5th under 80kph much. I know that the engine would have to work harder in 4th to bring it up to a comfortable speed for diesel box 5th but I tend to work the engine hard anyway through the lower gears.


Will let you know of my final decision.

Thanks

[Mike E (uk)]

I did this mod to my BX16V a few years ago.

For this I used the Turbo D 5th gear cog set, and needed to have the bearing turned down a bit to make it fit the cog.
I had 20mph for 100rpm, now it is 23mph.

The ex 16V cog set is what you want for an 8V GTI BX, no machining required this time.
From 21.6mph for 1000rpm to 24.7mph.

The mod makes my car far more civilised on motorways,
the reduction in performance is not significant.

You use 3rd or 4th for overtaking anyway.

But the noise level is far lower.

If anyone wants the cogs for their GTI, let me know.

I have the ratios/tooth numbers in an Excel spread sheet John B gave me.

Mike