Urgent Reply, IRS QUESTION!

1NCE BITN · #1 (**permalink**) 07-03-2007, 08:08 PM

I found a 8.8 t-bird supercoupe center section, half shafts, spindles,brakes that I'll need for donor parts. My question is, the manual tranny cars came w/ 3.27 gear ratio, the auto. trannys ran 2.73's. The one I found is the latter ratio set-up. All super coupes within 89-97 are posi-trac and I know how to verifty that part by tag on the diff bolt and 8.8 cast on the top of the diff.

I have a 427 s.o. and will be running a top loader. Will this ratio hinder performance dramatically? Should I pass on this one and find a 3.27 ratio. center section? Is it cost effective just to buy the 2.73 ratio and swap a ring and pinion set to the 3.27 aspect? When I split the diff. cover I'm hopeing to see good wear patterns and lash movement and not have to do a tear down/replmt. if it's safe to run the 2.73 ratio set-up!! Thanx guys in advance.

Three Peaks · #2 (**permalink**) 07-03-2007, 08:46 PM

The 2.73 gears would give you a lot better top end since you aren't overdriven with the toploader 4th gear. Should also make 1st gear more useful so you don't have to shift after 10 feet like some of the 3.35 first gear/3.55 rear end drivetrains.

Your call, but I think I would stay with the 2.73's for now and see how you like them. Easy enough to change them later if you decide to.

Just make sure you keep all the bolts from the rear when you get it, including the bolts for the pumpkin, hubs/spindles, etc.... you'll need almost all of them.

1NCE BITN · #3 (**permalink**) 07-03-2007, 09:01 PM

Thanx Three Peaks, I know about retaining all fasteners, Now I can call this guy and commit 50% dwn and rest assure, ck. lash and wear, look for the the tag on the diff: 2L73, the L is for lock, (posi-trac) 2.73 ratio and the 8.8 cast on the diff. THANX A MILLION!!!

By the way, I'm getting the center section, shafts,boots, joints, spindles, brakes and brake lines for,,,,,,,,,$275. Gas,road trip w/ the wife, $30, Dropping the wife off @ the diamond store 3 miles down the road while I pull the rear and parts,,,PRICELESS! THANK U,,,,THANK U VERY MUCH!

fostereast · #4 (**permalink**) 07-04-2007, 04:58 AM

The 2.73 is a good candidate for a Richmond 5 speed. If you go through the calcs you'll see you can get a braod range of compromise for road racing, streetability and 1/4 mile track.

nevermind65 · #5 (**permalink**) 07-04-2007, 05:46 AM

I'll say this. It kind of depends what you are going to do with the engine. I built a truck a few years ago and thought the numerically lower axle ratio would save me on gas for longer trips. The thing sucked fuel. Performance was pretty good. I changed the rear axle ratio to a 3.55 and it was night and day. The mileage didn't change, but the truck ran a LOT better. The low gear ratio was out of the range of the cam/intake/carb combination when driving down the road at cruising speed.

Granted the truck was a lot heavier than a Cobra, but it is something to think about especially since it isn't a 9" where you can just replace the third member.

Just a thought.

Cobrabill · #6 (**permalink**) 07-04-2007, 08:22 AM

Quote:

Originally Posted by fostereast

The 2.73 is a good candidate for a Richmond 5 speed. If you go through the calcs you'll see you can get a braod range of compromise for road racing, streetability and 1/4 mile track.

Sorry but 2.73s & a 5 speed would put criusing rpm over 70 mph.Timing wouldn't even be all the way in cruising city streets.Not a good idea.

I have 3.27s & a WR toploader which was changed by David Kee from a CR.This is the perfect compromise for a 4-speed set-up.

Five speeds:To put 5th gear cruising in the right spot,3.55s or higher.But this gives you a mostly useless 1st gear(3.27 1st & 3.55 rear)I BELIEVE Tremec is making a box with a 2.80 something 1st gear-this would cure that.

Roger Stine · #7 (**permalink**) 07-04-2007, 08:30 AM

Four years ago I retro fit the IRS option into my carbon fiber FFR MK 1 Cobra. The aluminum 99 Cobra pumpkin had 3.27 gears. 3.55s better suited my small block and after one session at Willow Springs I took out the 3.27s.

Last year I built a small block FFR with IRS for my friend, the late Dick Smith. This street only FFR was for coastal cruises and other social events. He was pleased with the standard 3.27 gears.

Dick's 427 race car #198 has 3.50 rear gears. The ultra built 427 side oiler was not well suited for street (cooling) but he did drive it on some NorCal coastal cruises that I participated in. The car had 3.31s at Daytona in 1967 when radar proved the 198 miles per hour record laps.

I was given the honor of doing some laps in the #198 car last year at Willow Springs. It's the most powerful car I have driven.

In my opinion 3.27s would be ideal in a street 427 Cobra with a 400 hp.

Roger

fostereast · #8 (**permalink**) 07-04-2007, 10:27 AM

Richmond does not have overdrive, cruising speed is over 2K, maybe you could elaborate?

1NCE BITN · #9 (**permalink**) 07-04-2007, 08:10 PM

Thanx for the follow up guys, My intention is to put a mild cam in, hyd. lifters, full roller, vintage duel carb,/intake, most likely blue thunder heads, close ratio top loader, tweak 500 rear whl. hp out of it, 90% street/highway, 5% road/rally race, 5% strip/1/4 mile!

I commited to the deal already w/ a paypal deposit! I can't pass up the center section, posi-trac ,spindles, half shafts, brakes, fasteners for the price.

Just as long as I can keep the idle under 1k with a nice lobey came, whatever David Kee recommends, cr/wr top lodr, when he spots my torque/hp curve dyno sheet etc,, etc. I'm hoping it'll work, Yet if it doesn't then the cost effectiveness for getting it right, (ring&pinion) on the first time around is out the window. I'll probably buy a new set, (ring&pinion), Thanx again guys for the input! Jim

fostereast · #10 (**permalink**) 07-05-2007, 06:07 AM

Ask David Kees about close ratio vs wide for street use, you may find yourself thinking that the wide ratio is better for the street.

David talked me into a wide ratio, but I am also considering a Richmond 5 speed.