Hey all,

I have a question about my rear end. Firstly, the car is an 01 base, A4 with original 2.73's. So far I have a tune from Diablo, a ported TB, a Borla Stinger cat-back, a Dewitt's radiator and cooling fan setup, and a Callaway CAI. I recently purchased a 3.42 diff from someone on Ebay and had a shop rebuild it with c6 guts based on information from a thread on our forum.

I also installed a mild 2500 stall by Phoenix Transmissions.

Now to my issue: It doesn't exactly drive the way I expected. It feels a little sluggish when I punch it at speeds between 40 and 60 and sometimes it feels as if the rpm's are lower than they should be at a given TP. Almost as if it should've down shifted already, that sort of thing. It doesn't seem to have that punch I've learned to associate with stall converters and shorter gears in my muscle cars in the past. Also, the take off speed seems inconsistent. Sometimes it pulls hard enough to chirp to 2nd and other times it's, well, kinda slow, lol.

I changed the axle ratio in my T1000 Diablo tuner to 3.42's before driving it. Well, actually that isn't true. The trans shop, which is reputable, for some reason drove it from one of its locations to another without telling me before I got the tune in. They said it ran fine, but I'm skeptical.

How can I verify the gears are 3.42's? Also, How can I tell if the stall is working correctly? I asked several people, including the trans shop and Phoenix, about whether or not the stall would require tuning, but got mixed responses.

Any help would be appreciated.

 

The easiest and most accurate is to count the teeth on the pinion and on the ring gear. Divide the number of pinion teeth into the number of teeth on the ring gear. For instance if you have the Getrag 3.42 differential, you should have 41 teeth on the ring gear and 12 teeth on the pinion. 41/12=3.41666 and is rounded up to 3.42.

If you want to check by jacking the car up and checking here's a link: http://www.how-to-build-hotrods.com/gear-ratio.html

And if you want to do a calculation for the "advertised" diff ratio using tire height and speed use one of these links: https://www.randysworldwide.com/calculators/rpm/ or https://spicerparts.com/calculators/engine-rpm-calculator.

I'm also running 3.42 gears with a 3600rpm stall. My car does feel a little lazy if I take off gradually from a stop, but if I hit the gas hard enough the engines flashes to the trans stall point it is an exciting take off.
I'm not quite sure what you meant about punching it between 40 and 60mph and the rpm's feel lower. Can you feel whether it has dropped into lower gears? The tach ought to give you a good indication of that.

You can tune shift points, degree of firmness in the shift, and converter lock up speed.

Best wishes for an easy resolution.

 

Thanks for the links Route99, I'm going to check them out. I was canyon carving a couple weeks ago and was on a gentle upgrade. I punched it to get in front of a slower car (we were cruising at about 35-40 mph) and was surprised at the lack of pick up. A few weeks before that, with the stock gears and stall converter, I did the same thing in the same canyon and passed the car in front of me like a bat out of hell.

Other times, when I'm just cruising down the street, it almost feels like it goes into over-drive too early.

I don't know why it feels like that. D=

 

You have to a lot more than change the ratio with a Diablo when you change to 3.42s. Several transmission parameters need to be changed in the pcm for it to function properly. You need to find a good tuner who has experience with the C5 and you'll be amazed at the difference.

 

Like BigGun said, you need a good tune up of the transmission/PCM, the shift points, and also someone who knows how to program the DBW lag for an auto.