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school me: gear ratios...

trev

*****istrator
Admin
November 11, 2007
13,982
1,699
Co. Laois
My query is specific to my Jetta but is relevant to all drag cars I think so keep your hair on ;)

I was talking to Dave at the weekend about this drag racing lark. I have plans for more power this year but I think my current gearbox is a limiting factor in how fast I can get up the strip. It's from a TDI mk3 Golf. I sought it out specifically when I had the engine I'm racing in my daily B4 Passat. It has the best gears for low-revving at cruising speed of all the 02A boxes, giving the best fuel economy. The flip-side of that is that it's slower to accelerate, being high-geared. Which is NOT what you want if you're drag racing :ROFLMAO:

dave suggested looking for an equivalent gearbox from a petrol engined car. However, after doing a bit of research and number crunching, I'm not sure if the petrol gearbox is going to be the solution either.

the problem is the pretty short rev-range of a diesel engine. For example, running 165/70R14 equivalent motorsports tyres. I was flying across the finish line at 92MPH at 3900RPM in 4th gear. I haven't had the car dyno'd (yet!) but I can tell myself the power drops off dramatically beyond 4000RPM so it wasn't far off the ideal spot in the rev range for crossing the line. 5th gear is way too long in this gearbox to even bother reaching for it before the finish line. I tried a couple of times but it felt like I was at the bottom of the power curve all over again and I saw no appreciable increase in top-end speed or decrease in ET so I for the remainder of the runs I just used the first four gears.

now, looking at the petrol gearbox options, I can compare the predicted vehicle speeds for similar RPM and tyre size.

The TDI which is in the car gives these speeds at a sample engine RPM of 3900

1st: 25.7
2nd: 43.6
3rd: 64.9
4th: 92.6
5th: 118.3

The typical petrol gearboxes available give these speeds at 3900RPM:

1st: 19.3
2nd: 34.3
3rd: 50.9
4th: 70.7
5th: 86.9

you can see that at 3900RPM I'd actually be going SLOWER across the line in 5th than I was in 4th with the current gearbox. Plus I'd have the delay of an extra gear change along the way. If I had a much longer rev range they might be the way to go but the diesel just doesn't want to go far beyond 4000 RPM like I said.

I could use a taller tyre, for example, a 195/65R15 equivalent tyre would get me 94MPH at 3900RPM with any of those gearboxes.

has anyone got any advice? maybe a box from an earlier IDI diesel engine like the B3 Passat 1.6td or the B4 1.9td would be the middle-ground I need, I haven't checked those numbers yet...
 

wackerwacker

Club Member
Club Member
November 24, 2009
923
213
waterford
Trev I know where there is a 6 speed box out of a 150 gti tdi golf.

Just a thought �� �� �� �� �� �� �� �� ��
 

trev

*****istrator
Admin
November 11, 2007
13,982
1,699
Co. Laois
yeah I think a trip to the dyno is on the cards. Need it remapped anyway so I'll get them both done together. I'll see where I need to go from there.

I would never have thought of changing gear early to use more of 5th. Cheers :)