View Full Version : Differential Gearing

01-20-2007, 09:38 PM
HI -
I'm a new owner of a '94 Indy Pace Car. Its been modified by the previous owners and I'm trying to find out some things that there isn't documentation for. So, I hope that some of the folks here can help me out. So heres the start of the questions:

- First question is the rear end gearing. I turn about 2200-2300 RPM at 60 mph in 5th gear. Is this about right?? What is the stock gear ratio for '94 Cobras?

- I just found out that the 302 has been stroked to 347. What is the stock stroke & the 347 stroke??

- I've noticed that the engine is rev limited - about 5600 rpm. I can feel the engine just wanting to keep on reving. Is the stock Cobra rev limited?

I guess thats it for now. Any help would be greatly appreciated.

01-21-2007, 10:28 AM
Welcome & Congrats on the new to you pace car... How many miles?
Ford only made 1000 of the pace cars, 1994 Cobra convertible only in Rio red with tan top & black N tan interior. No other options. There are a couple of us who have pace cars, Tony, Sherri (5K miles on hers), mine and possible some more. It should have a small plac on the center gear shifter cover stating what number of 1000 you have. You can also call Ford to get a STV certificate, a cost now & I don't remember the number, but I'll edit it later.

Stock gears on 94 Cobras is 3.08. Sounds like you might have 3.73 or 3.55. A easy way to find out is to raise the rear of the car, mark the drive shaft then have someone turn one of the rear wheels exactly 1 revolution. Count the number of times the drive shaft rotates and that will be the gear ratio. IE: almost 4 revolutions would be 3.73, 3.5 times would be 3.55 and around 3 would be 3.08.

A 347 motor is a 302 base block that has a different stroke crank, making the engine larger due to the longer stroke. It adds cubic inches with out increasing the external size of the block.
What heads on the car? Does the car have a cam? If so which one. Does the car have a chip?
The stock computers rev limitor is 5750 rpm (if memory serves me), should be red on the stock tac. The stock tacs are know for being off a little. The stock rev limitor pulls fuel out to keep the engine from reving higher, not good if you have a power adder, supercharger or NO2. If the motor is built with after market heads & cam, then it wants more rpm,s and can still rev higher. The stock heads (especially the valve springs) and cam are only good for about 5500 rpms or so.

Do you have a dyno sheet with the car? If not, it would be good to have it dyno tested with air fuel ratio to make sure you are not running lean or putting too much fuel & running rich. If it has not been tuned, then a dyno tune could free up some horse power for sure & cure any drive ablity issues it might or could have due to a after market cam.

Ask up, there are lots of knowledgeable people here who are glad to help.
One more thing: :ttiwwop: :)

01-21-2007, 11:23 AM
I put 3.73 gears in my '94 and it turns about 2500 RPM's at 70 MPH (and still gets 26.5 MPG on the highway!). I'd guess that your car has at least 3.73 gears or lower (higher numerically). One note worth mentioning - if someone did put gears in the car before you bought it, you can't be sure that they put in the correct speedo gear to match the differential gears. This means that you may not actually be travelling 60 MPH when you see 2200-2300 RPM's.

To be sure what rear gearing you have you should check the wheel vs. driveshaft rotation as Darrel mentioned. Or, if you plan to change the gear oil you can check the stamping on the ring gear or count the ring and pinion teeth and do the math.

01-21-2007, 09:31 PM
Thanks for the replies!!!

There's alot about this car I don't know yet, but I'll be "investigating" in the next couple of months (when she's in the garage!!). I will check the rear gearing via the method you described.

I have 2 dyno reports - one from '98 and the other from 2002 (I think). The work was done at Steeda Autosports. Does Steeda build and install engines?? I'm just familiar with their aftermarket parts. The dyno sheets have notes -1 5/8 headers, edelbrock heads, 65mm throttle body. I believe there is a 72 MAF (ok, I don't know what this is..). The later dyno report had a note about 8lbs of boost out of the Vortech supercharger. I also came across some notes about the crankcase having a 7 quart capacity. Also, the frame has been stiffened.

The motor is has bigger cam installed. Lopes pretty good at idle and idle is around 1000 rpm. I also saw some recent repair documents - according to them the car is tuned and tuned to run a little rich in order to prevent engine damage due to boost. Also, the cats are gone and new looking flowmaster exhaust system is installed - with X pipe. The motor sounds great and boy does she rev fast - especially when the vortech kicks in. I haven't had the chance to experiment on how to launch this thing yet - I start getting into it hard, the tires start (stock size) to spin and the backend fishtails - even in third. Starts to stumble - the rev limiter reducing fuel - about 5600-5700 rpm.

I plan on calling the repair shop that documented repairs and Steeda tomorrow to get some more info. I may end up calling the previous owner for more info. I have the kit from Ford SVT coming - hopefully it will come with the plaque. I need to get some paint work done and the top replaced and then just some plain and simple CRUZIN' this summer!!!

01-21-2007, 10:34 PM
1-800-FORD-SVT is their number and I'm not sure what kind of "kit" you are talking about but it won't have a plaque with it?? Ford will not send you a replacement plaque as that came with the car back in 1994. Your best bet is to find out who the previous owners are and see if they have the original paperwork with the car. Congrats on the Pace Car! :thumbsup:

01-22-2007, 12:32 PM
Congrats on your new-old car! I have close to the same setup on my 94. Be carefull with high rpm on a supercharged 347 (especially vortech). Sounds like the previous owner had it chipped and bumped the idle to a 1000rpm to keep it from surging up and down (problem with 94-95 computer).