4.30 or 4.56 that is the question???
 

Alright over the winter up here in Michigan the weather isn't to nice, so my car will be stored. While she's away I will be guting the whole rear end and installing 31 spline moser axles, SVO cover, the posi unit is still up in the air as well as the gear ratio. I do a lot of city driving and when I do drive on the free way I drive about 80 mph. So my question is, is the 4.56 to much? Also what would be a wise posi unit to buy?

Thanks, Will

Just guessing here by I'd say you would be right around 3200 RPM at 80MPH with 4.56s, since I'm at 3050 - 3100 with 4.30 gears in my car. I drive mine in city traffic with no problems but really what it comes down to is how tolerable you are to shifting.

For the differential it really depends on how much track use your car will see. The 31 spline ford unit is a step up from the stock 28 spline unit but not really worth it. Auburns should be burned when it comes to any drag racins. True tracs I've heard are pretty good but not unbreakable. C locker or detroit lockers I've heard are very solid but make lots of noise and can be tricky to drive with. So as you can see there is no easy choice. If you were to stick with DRs and maybe slicks once in a while the True trac would probably be the best bet. For gears 4.56s are the perfect gear for a NA Cobra.

Bill

Thanks for the input Bill, one posi unit you didn't mention was the new Eaton. J Miers a fellow Cobra owner in this forum said he blew one up with four passes on slicks. Then I talked to Paul at Paul's High Performance in Jackson, Michigan and he said his Eaton lasted four seasons of NMHRA racing. I just don't want to blow one of these damn things in every couple of months.

I will be going to the track hopefully every other weekend and doing a lot off street racing both with drag radials so I really don't want a detroit locker.

Thanks, Will

That is exactly why I didn't bring it up. Fourcam281 on the corral grenaded his as well on 7K RPM launches. Seems they are just as bad if not worse than the ford unit. There are also some people that will tell you the Auburns work great throw a few 7K RPM launches at them and we'll see.

Bill

What about the auburn Pro Series?

So Bill is the Tru Trac the same as the Trac Tech that Steeda sells? If so than I think i'll get that one along with the moser 31 spline axles and 4.56 rear end.

Will

All the Auburns are garbage unless you want a road handling car, they are not for drag racing.

Bill

Yes same thing.

Bill

You should be 3300 rpm at 80 mph with the 4.56's. More like 3100 with the 4.30's.

:rolleyes:

I run the Auburn PRO Series in my 750hp truck, and I couldn't be more happy with it, and most of the drag racers around here use them too, running anywhere from 12's to 9s, etc...I don't understand why your saying they are not for drag racing?

:rolleyes:

Because I've seen piles of them at a local shop whos replaced them because another local shop swears by them. Just curious but is your 750HP truck an auto? Manuals are a little harder on launch. I'm talking from experience I've seen so many of those things grenede from high RPM launches it's not funny. A friend of mine swore up and down how good they were but never hit the track. Then I got him out there and after about 20 passes blew his Auburn. He was running slicks and went to the Auburn Pro and about 15 passes later than went as well. 6500RPM launches are not easy on rear ends.

In fact go to the Auburn web site look for anywhere that states their differentials are for drag racing. They even tell you that is not what they were made for. This is from their web site in the FAQ...

6.) What differential should be used for drag racing?

If the vehicle is strictly used for drag racing, a spool would be the answer. Do not use a spool if the vehicle is street driven. It could cause broken axles and other problems, and is not street legal.

Instead of saying on the street use the Auburn differential they just tell you spool or no spool.

Maybe the Auburn works for you but based on my years of experience at the track I would never recommend them to anyone.

Bill

Yeah right after I replied I scrolled up and saw 7000k launches:eek: , so disregard my comment. Yes my truck is an auto. But I've had good luck with it.

That was why I was asking I know of two people at our track that run them (I'm sure there are more) but they in particular have had no problems but they run 9 and 10 second cars with C4s and leave the line at 3500 RPMs. Neither of them have any issues either.

Bill