GTI transmission mod for fuel economy thread

[bhtooefr]

Our cars have either the ACH (1985 diesels), AGS/AOP/4S (1986-1990 diesels), AWY (1990 diesels, if the Bentley can be believed - I'd lean towards 1991-1992 - it looks like a misprint), or AVX (ecodiesels) transmissions.

Us ACHers can get an upgrade by going to the AGS/AOP/4S tranny, but not much of one, and not in 5th gear. AWYers can go to ACH or AGS/AOP/4S for an upgrade. AVXers can go to any other diesel tranny for an upgrade.

However, there's an even better upgrade for lower RPMs. The ACN/AON/ASF/ACL tranny. It was used on 8V GTIs, and at 70MPH, gives a 200RPM drop from the ACH and AGS/AOP/4S, and more from AWY and AVX. This is a big deal.

This was discovered in both the TD top speed and the Jetta/Passat NA swap threads. I felt that it deserved it's own thread, however.

I think that the ACN/AON/ASF/ACL is our version of TDIClub's beloved .681 5th.

So, how much would it cost to do this swap? FWIW, I can see reasons why a GTI owner would want one of our trannies - faster acceleration, without much top speed sacrifice. So, it could be a straight across swap, and it might not be.

[ricosuave]

a guy is selling his acn for $125 in calgary

[firestorm13666]

sould a 5th gear swap a better way to go?

[bhtooefr]

To what 5th gear would I go?

Of course, could the final drive be swapped? ALL of the gassers use the 3.667 final drive, so swapping to that would make my ACH in effect an ACN/AON/ASF/ACL.

[firestorm13666]

so what would be the best 5th gear to swap in?Also as far as gearing what would be better gearing wise a whole trans swap or just a 5th gear swap?Thanks

[bhtooefr]

Well, let me do some calculations here...

Of course, acceleration would be much better (than a GTI tranny) if first through fourth remained the same. A final drive change would change EVERY gear.

Here's the numbers for 70MPH in 5th gear, with 175/70-13 tires...

3.941 FD, 0.745 5th: 3050 RPMs
3.667 FD, 0.745 5th: 2838 RPMs
3.941 FD, theoretical 0.693 5th: 2837 RPMs

And then, I got curious. What about 0.681, everyone's favorite swap over at TDIClub?

3.941 FD, 0.681 5th: 2788 RPMs

These charts will come in very handy, FWIW.


(NA diesel)


(turbodiesel)

[ricosuave]

anyone have the same charts/info for the 1.9 aaz?

[firestorm13666]

Myself i do not knwo much about gearing.So if some was to swap their 5th gear to eather one of the two you are talking about you could drive 70mph and still be grate fuel milage because your engine is turning less rpms then with a stock 5th gear?Also would be safer on the high way now that you could do 70mph without worryign about mile.

[bhtooefr]

Exactly. By going to taller gears (smaller number), you reduce the RPMs, increasing the fuel economy.

The point of those graphs is to figure out how much power is available at a certain RPM value. With the stock gears, the NA diesel is putting out a maximum of approximately 41.5 horsepower. With a .681, the NA diesel would be putting out approximately 37.5 max. IIRC, 30 is needed to maintain 70MPH on level ground with no headwind.

[firestorm13666]

Thanks.So which gear do you feel si the best swap and what trans can i find it in thanks.

[RabbitJockey]

http://www.scirocco.org/gears/


theres a nice site

and my friend has an 81 caddy that has a turbo diesel from a 1985 jetta, the transmission is an FF though, is this the original tranny and not the 85 tranny?

[mortskeg]

Hey bhtooefr,
Just wanted to clarify some stuff that I was thinking was a bit different than the above data.  Correct me if I'm wrong though. :wink:   I can't speak for the A2 gti, but my A1 gti had a transmission that was not great for highway cruising, and would be not much better than a GC 4-speed on a diesel.  3500 rpm in 5th gear produced only 63mph on this 8V gti.  I thought that the A2 gti's were also close ratio'd but perhaps not the the extent of the A1's.

I think that the ACN etc. transmissions were used in the "normal" 1.8 cars and not the gti's but not 100% on that.  Also don't forget that alot of the diesel cars came with the FO, FF, and FN transmissions.

I always thought that the lower the RPM in 5th gear, the better, but that is only true to an extent.  For those of us with lowly NA's I wouldn't want to venture any steeper than what a ACH gives you.  Any steeper and with any bit of a headwind or slight incline, 5th becomes unusable and the fuel economy benefit is negated.  Also it seems that the engine starts building up heat with the higher load on it and lower RPM's.

When I had my gti it was great in that there was a gear for every corner, but you're right, I would have been much happier to get my hands on a "diesel" transmission, not because of faster acceleration, but because of the lower highway RPM's that the diesel's have.  That said, until I have a td or a larger displacement NA, I don't think I'll be trying to go much steeper.  I think the best fuel economy I've had in my pickup (upper 40's) has been with a 1.5NA mated to a FO tranny.

I know that alot of the trans charts on the net have inaccuracies, but here's another one I like:
http://www.4crawler.com/Diesel/VW_020_transmission.shtml
Hope this helps.

[zyewdall]

I think that the ACN etc. transmissions were used in the "normal" 1.8 cars and not the gti's but not 100% on that.  Also don't forget that alot of the diesel cars came with the FO, FF, and FN transmissions.

The charts are just for A2 5 speeds I think.  FO, FF, and FN were for A1 right?  My '91 GTI had an ACN transmission in it.  Don't know what else came with it.

[bhtooefr]

Yes, that chart that I posted is intended to be a full listing of all 5-speeds offered on A2s sold in the US and Canada.

[BlackTieTD]

i heard '0.681' fifth! :?: more info? what transmission did that come in? i only know of .71