New gears vs. engine mods

[Sully]

Okay, new guy here...and I'm sure this will open a "can of worms". BUT, what do you all think is the best way to go for an initial investment in performance upgrades:

1. Replace the 2:73 differential with a 3:15 (or maybe a 3:42) and do a dyno-tune.

OR...

2. Add an cold air intake, cat-back exhaust and then dyno-tune.

I want to keep the car pretty "tame and sane" as a driver, don't plan on racing it...mostly cruising, trips, etc.
thanks in advance for the opinions!

Sully

[speedmaster4]

3.42 gears...3.42 gears...3.42gears

unless 90%+ of your driving is flat turnpike, or you are general motors-worrying about tiny mpg for CAFE

[sharkcar71]

If you have a six speed go with 3.90's.They will be no problem with that .50 sixth gear.

[Sully]

Sorry all, should have mentioned that it has the A4 tranny...
I'm basically looking for the best performance for under $2,000.

[sharkcar71]

3.42's will be perfect with the A4.They would be your best bang for the buck for under 2k.

[Mike Mercury]

Originally Posted by Sully

New gears vs. engine mods

Head & cam gives the measureable HP improvement, but that only occurs at a narrow RPM range.

I think one notices the improvement of gears more because the benefit is felt at all RPM's and in any gear (even overdrive).

But for racing and 1/4 mile times... I'm sure the H&C & headers are the better route.

But for all-around SOTP feel, gears are tough to beat.

http://mikemercury.home.att.net/342.htm

[Sully]

Originally Posted by Mike Mercury

Head & cam gives the measureable HP improvement, but that only occurs at a narrow RPM range.

I think one notices the improvement of gears more because the benefit is felt at all RPM's and in any gear (even overdrive).

But for racing and 1/4 mile times... I'm sure the H&C & headers are the better route.

But for all-around SOTP feel, gears are tough to beat.

http://mikemercury.home.att.net/342.htm

this forum is great stuff!
That is what I've come to understand...that the gear change will apply across all RPM's and enhance any future upgrades I do, like intake/exhaust, etc.
Seems like everyone is recommending the 3:42 instead of the 3:15...any "drive-ability" issues that you've encountered? What about recommended vendors for the differential?
Sully

[Mike Mercury]

rear gear choice is personal.

But many will tell you that with the A4 C5, the 3.42 is the best "all around" ratio.
If you frequent the dragstrip weekly and a timeslip rules your life (and that is a great life to live)... then the 3.73 or 3.90 ratio and a high stall converter may be the better choices.

When going from the 2.73 to any other ratio, you'll be forced to get a PCM reprogram... there's no way around it. Because of that, you might as well go to (at least) the 3.42

About the only benefit going from the 2.73 to the 3.15 would be that a Chevy dealer can do the reprog for you using their GM Tech Tool 2 device. My local dealer will do this reprog for $65.

You may ask then... why not go to the 3.42, 3.73, 3.90, or 4.10 ratrio and get this cheap dealer reprog. That's because the GM Tech Tool 2 programer won't allow the selection of non-factory rear ratios. So for the A4, the only ratios the Tech Tool 2 will allow selection of is the 2.73 or 3.15 (since they were the only two A4 ratios available from the factory).
After market programers (LS1Edit, Predator, HP Tuners...) will allow gear rescalling for any of the ratios.

When I upgraded my A4 to the 3.42, my 2002 (at that time) was only 6 months old and had 3k miles. I wasn't about to put a used carrier in such a new car. So I purchased a brand new 3.42 carrier. But these days, there are a couple of places that can retrofit ring & pinion gears into your existing carrier case (even in the 2 series case); DTE in Indiana is one of them.

But since the 3.42 ratio was the only one available for the 6 speed C5's.. there are a lot of used low-mileage 3.42 carriers being sold. They go for anywhere from $400 to $600. brand new 3.42 carrier can be purchased for about $1100.

if you haven't yet, here is a video if a girl driving my 2002 after the 3.42 upgrade (and that was the only mod at the time):

http://hometheater.home.att.net/ohiogirl1year.mpg

:cheers:

[Sully]

Mike,
Thanks so much for all the info on the gearing. You've given me some good info to mull over. I will start looking for price and availability on carriers. I assume that a 2-series case belongs to a 2:73? My car has about 19K miles on it, but the factory warranty is still in effect, so would also be a factor.
Then I've got to find someone reputable in Reno or Northern California to do the work...not sure I have the time nor expertise to tackle that myself!
thanks again!

Sully

[gcmlear]

How much difference at cruise with the 342'S? Say at 75 mph, about 1600 rpm right now, what would new rpm be? This with a six speed. Does the speedometer have to be adjusted? What would top end speed be now. Thanks

[bradfordsvettes]

gear changes are usually the best bang for the buck Steve

[DRTH VTR]

Originally Posted by gcmlear

How much difference at cruise with the 342'S? Say at 75 mph, about 1600 rpm right now, what would new rpm be? This with a six speed. Does the speedometer have to be adjusted? What would top end speed be now. Thanks

1600 X 3.42/2.73 = 2005 RPM

[Sully]

Originally Posted by DRTH VTR

1600 X 3.42/2.73 = 2005 RPM

Which ratio would you go with in a A4 coupe, driven daily (weather permitting), trips, etc. NOT raced. Any recommendations on new vs. rebuilt vs. used?
thanks
Sully

[DRTH VTR]

Originally Posted by Sully

Which ratio would you go with in a A4 coupe, driven daily (weather permitting), trips, etc. NOT raced. Any recommendations on new vs. rebuilt vs. used?
thanks
Sully

I once had a '97 A4 with the stock gears. I would have liked lower gears, but I cannot make any informed recommendations. I am sure that others will chime in.

[Mike Mercury]

Originally Posted by gcmlear

How much difference at cruise with the 342'S?

hear's a quick-n-dirty way to calculate C5 rpm changes when upgrading rear ratios:

from 2.73 to 3.15 (add 15% to any given RPM)
from 2.73 to 3.42 (add 25%)
from 2.73 to 3.73 (add 37%)
from 2.73 to 3.90 (add 43%)
from 2.73 to 4.10 (add 50%)

from 3.15 to 3.42 (add 8.6% to any given RPM)
from 3.15 to 3.73 (add 18.5%)
from 3.15 to 3.90 (add 24%)
from 3.15 to 4.10 (add 30%)

for example, if your cruising the highway in 4th OD (with an A4) with 2.73 gears - and the RPM is 2100... then if you upgrade to 3.42 gears, the new RPM at this same MPH will be (times 25%) 2625. If you had upgraded to 4.10 gears, the RPM will now be (times 50%) 3150.

I did the 3.15 to 3.42 upgrade - which is the least amount of change one can make in the C5 (just 8.6%). And I felt it in every gear... even in overdrive.