Factory Five Racing Forum banner
1 - 13 of 13 Posts

·
Senior Member
Joined
·
5,559 Posts
Discussion Starter · #1 ·
I'm going to be doing an IRS retrofit this winter and I need some opinions on the gear ratio.

With the live axle setup that I'm currently running, I have 3.55's. I'm very pleased with the overall performance, but 5th is really not effective unless I'm over 80 mph. First is a little short with the T-5 (3.35 first gear), but I'm used to it so that's not a big problem.

My new pumpkin (aluminum '99 Cobra) has 3.27 gears. I know alot of guys run those and like them, but I'm going to be rebuilding the pumpkin anyway, so I'm considering a gear swap while it's apart. Since I like the 3.55's, that is my first choice, but since 5th seems too high for the type of driving that I do (very little highway), I've recently been thinking about trying 3.73's.

Anyone using 3.73 and happy? Has anyone driven FFR's with both 3.55 and 3.73 to compare? Is there a huge difference in driveability?

thanks, Brian
 

·
Charter Member
Joined
·
1,103 Posts
I have only driven with 3.55s. That is what I see the most of. I know some with 3.73s. From what I understand they are very close. The 3.73s are a little faster off the line, and a little lower in top end. What I here the most about the difference is that on SOME TRACKS the 3.73s may cause you to need to shift more.
Also if you have gobs of HP then you don't need to think about the higher gear ratios.

Robert
 

·
Senior Member
Joined
·
985 Posts
My IRS came with 3.27s so I went ahead and drove it like that for a year. I recently put in 3.73s and I like them a lot better. I wish I had never wasted my time with the 3.27s.
 

·
Registered
Joined
·
3,154 Posts
I pulled the 3.27s out last week. Like others that read the assembly manual I found 3.55s in my original live quadrabind 8.8 will let a 302 get to 125 mph on a short straight in 4th. Not on a public road. I don't care about saving gas. I just wanted the performance back I enjoyed for the first 6 years in 1225. My original donor world class T5 is doing great.

The 99 Cobra aluminum center will be ready tomorrow with 3.55s. It was easy to remove. I pumped the oil out first. Tipped the pinion straight up, rotated it 90 degrees to port and down it came.
 

·
Senior Member
Joined
·
5,559 Posts
Discussion Starter · #5 ·
So far it seems like the consensus is that I won't be happy with the 3.27's anyway...

Now, since I will be getting new gears, I just need to choose the correct set! I have a 351W that will eventually be stroked, which will replace my 302. So with the added hp and torque, the 3.73's might be too much gear? It seems they would work great with my present engine combo, though...

Man, this is almost as tough as picking the "right" color!

Brian
 

·
Registered
Joined
·
1,297 Posts
If you already have 355's, I would use them. There is not that much difference between the 373's and 355's. I have 373's and love them, but I think I would be just happy with 355's.

Dick
 

·
Senior Member
Joined
·
5,559 Posts
Discussion Starter · #7 ·
Richard,

I had thought about that, but the 3.55's I have are all set up in a freshly rebuilt solid axle, so I decided to just get new gears rather than dismantle the existing rear. I will just sell it as a complete unit.

Brian
 

·
Registered
Joined
·
858 Posts
We'll assume you are not going to the drag strip, considering the change to the IRS......

With a 3:35 first gear ratio, and 25.8" diameter tire:

3:55 rear gear ratio = 38.8 MPH @ 6,000 RPM
3:73 rear gear ratio = 36.9 MPH @ 6,000 RPM

With a 1:1 fourth gear ratio, and 25.8" diameter tire:

3:55 rear gear ratio = 64.9 MPH @ 3,000 RPM
3:73 rear gear ratio = 61.8 MPH @ 3,000 RPM

If you are planning on the stroked 351 in the near future, you ain't gonna be able to put a big enough tire in the rear wells to keep from rendering them totally useless (in first gear) with the 3:73s, considering that 3:35 first gear ratio. If it were me, I would stay with the 3:55s. The torque of the stroked 351 will like that much better, and also let you loaf along the interstate at 70 MPH, while only turning about 2,200 RPM.

td
 

·
Registered
Joined
·
3,154 Posts
Dick Smith's CSX 3035 has 3.55s. 198 mph at Datona. His newest 499 sideoiler has been broken in with some street miles and he recently led a few of us on a tour through the countryside. His driving skills never induce wheelspin on street tires.

Reports are, 3035 will be at Snakes, COCOA and the BIG BORE BASH at Willow Springs running 3.55s.
 

·
Senior Member
Joined
·
5,559 Posts
Discussion Starter · #11 ·
Thanks for the help and advice guys! I guess, I'll try to find a good deal on some 3.55's like I have in the FOX setup. I'd probably have a blast using 3.73's with my current engine, but I have to "plan ahead" so I don't end up re-doing too many things! :rolleyes:


Brian
 

·
Registered
Joined
·
2,158 Posts
Brian,
If you want to get a ton of info, you can also try doing a search. Seems a lot of people ask 3.55 vs. 3.73. For the 2 cents it's worth, my opinion is that you really probably would barely notice the difference between the two. The biggest difference you'd notice is as your speeds approach 100+ mph and you're really rev'ing more with the 3.73s. Or on the open track you'll run out of gear quicker. But if you want just a little more off the line fun, you'll be happy with the 3.73s.

Steve
 

·
Charter Member
Joined
·
972 Posts
3.73s really woke up my stock 302. If I had more power I would have chosen 3.55 or lower. You might find that the 73's are too high if you are going to change to a 351.

Dennis B.
 
1 - 13 of 13 Posts
Top