I've decided to ditch the Porsche 741 tranny in my turbo diesel powered '65 SC coupe. The gearing is just way too short for this engine (70mph=3600) and there is very little in the way of available gear sets for this box. There is no choice in ring & pinions at all, and only a tall 4th out there. In case you hadn't heard, Porsche stuff is expensive -- that's over $700 just for 4th set, and it still doesn't get me where I need to go ratio-wise. So I'm thinking it's time for a custom type 1 VW box, I know there will be some fitting issues, but none that should be insurmountable.
When looking into what is available for type 1 boxes I find myself overwhelmed by choices
Get this -- 37 1st and 2nd main gear sets, 14 3rd gears, and 21 4ths! Ring and pinions? A selection of 5 different ratios. Yikes!
I've been busily charting away using this cool Excel program I found
http://www.glenn-ring.com/tech/tranny_calc.xls and I've pretty much decided on 3.11/1.86/1.14/0.7. A little larger step between 2 and 3 than I'd like, but still pretty good. It's the ring and pinion that has me guessing though. The engine output curves that I have (for a stock 1.6TD) give maximum torque at about 2700, and best BSFC at just about 2 grand. My engine is quite heavily modified (GTD nozzles, Giles pump, VNT15, ARP studs, metal HG, 944 turbo intercooler, long runner GTI gasser intake manifold, and a "no muffler" muffler
) but I'm thinking that though the values may be different, the peaks should be in about the same range.
So maybe I should be looking at about 2200 to 2600 for an engine cruising speed? The 3.88 R&P would give about 2500 at 70, and the 3.44 would be about 2300 at the same speed. That lower number is just about the same as 5th gear in the 9Q Quantum box (the engine came from a Quantum) in a car weighing about 400# less and with much better aeros. It's also just about the same as the gearing in our old 524 BMW, a more powerful car, but a much larger and heavier one as well. I don't want to give up too much acceleration though, and the 3.44 is a special aftermarket gear set that will cost an extra $300+.
What does anyone here think is an ideal cruising RPM for a light (2000#) and aerodynamic 1.6TD powered car, and would that extra margin (-200rpm @ 70mph) be worth the extra money?
Topic: choosing gear ratios (Read 1902 times)
