3.42 vs 3.72

[97formula]

ok im debating on geting 3.42 or 3.73 i have a 97 formula with moroso CAI , SLP loudmouth exhaust , and hypertech power programmer, what would you go with and what is the main difference. oh it will cost me 600-700 for parts and labor.

Thanks, eVan

[Hurley711]

A4 or M6?

[97formula]

haha sorry i forgot to post the most important thing ..its A4

[Hurley711]

3.73

[97formula]

can you give me a reason for that? Thanks

[koolaid_kid]

I take it that you currently have the 2.73, series 2 carrier?
I went with the 3.42 with my 97 Formula, and I have been quite pleased.
RPMs increased slightly at highway speeds (around 600-800, if memory serves me correctly).
Fuel economy dropped from 18 to 17 around town, and from 24 to 22 on the highway. Depending on right foot pressures, of course.
Performance increased from laying about 10 feet of rubber to about 40 feet, using the original tires.
If the car is strictly for quarter mile use, I would suggest the 3.73, as mine does not hit redline at the end of the quarter.
If you are seeking overall performance, I would recommend the 3.42. It has good overall performance, without having too high an RPM at highway speeds.
I would also suggest getting a diff girdle also, to decrease the load on the diff.
Just my 2 cents worth.

[97formula]

ya i appriciate that man thanks, but ya is there a huge performance diff from 3.42 to 3.73?

[koolaid_kid]

Not having driven a 3.73 to compare, I don't have the answer to that one.
Obviously, there will be one, but it will not be nearly as significant as the move from a 2.73 to a 3.42.
There are calculators out there that allow you to input variables such as your tire height, trans gear, diff ratio, etc., and tell you what RPM you would be running at any given speed. I think I have also seen ones that would calculate your engine speed at the end of the quarter.
Try Googling "engine speed calculator" for starters.
As far as overall performance increase, it will depend on the hp of your current setup. You are still on the mild side, comparatively speaking. Keep in mind, also, that you will (most likely) be making more mods in the future.
Good luck, and I assure you that you will be impressed with the move to either gear, compared to your current 2.73.

[lee1rbc]

3.73s all the way! I am getting ready myself to switch from 3.42s to 3.73s. In overdrive at 70mph with 3.73s you will be doing almost 2800rpms (very drivable). And at 120mph in 3rd you will be doing 5900rpms (perfect for the 1/4 mile!)
If you run a 13.9 at 99 with 3.42s, you will run a 13.75 at 101 with 3.73's IMO.

[hammertime]

With a lockup torque converter, the difference between 3.42's and 3.73's @ 70 mph is only 200 more rpm's (2200 vs. 2400). Should be plenty livable for all-around driving.

There are only a couple reasons to go with 3.42's in an auto:

1) You are never going to the track
2) You got a great deal on a take-out rear from a 6 speed car

[SLPLS1]

I have an a4 in my car with 373s and im only turning 2200 rpms at 70 and my hwy mpg actually increased when I put mine in because im not luging the engine like my original 273s

[koolaid_kid]

Go to:
http://www.csgnetwork.com/rpmcalc.html
There you may enter all the variables for both the street and the track.
There are multiple variables to consider, which this site allows you to calculate.