Tranny & Rear End Ratio Question

[Whodeeny]

I have my 427 Cobra order placed with ERA and I expect delivery in September this year. I am beginning some of the planning and wondering which transmission and rear end ratio I should select. The car will be used strictly on the street and I would like to do some cross country cruising with it when I get it completed. I am planning on installing an alloy block 427FE (about 500hp) with 15 inch pin drive wheels and 295/50 X 15 rear street tires. I am thinking of using a Tremec 600 with a .82 fifth gear ratio and a 3.31 rear end. That would provide approximately 2575 engine rpm. at 75mph. I could also select a Tremec 600 with .64 fifth gear ratio and a 3.54 rear end which would provide 2150rpm at 75mph. The fifth gear would just be used for highway cruising with the .64 ratio.
Any thoughts, recommendations or advice on this subject would be appreciated.

[1795]

A lot depends on cam selection if you decide to go with either of those setups. If it is going to be primarily a street car with some highway driving, you are probably going to want a mid range cam with a power band that starts lower. The reason I say this is because you are focusing more on the highway driving end and do not seem to be considering the city/town driving conditions that may predominate your driving. When you are in town where the speed limit is 30 mph with the 3.31 you will be chugging along at about 1600 rpms in 3rd gear (unless you want to be cruising around in 2nd). With the 3.54 you will be running at 1750 rpms. Most aggressive cams do not like to run below 2,000 rpms and their longevity may be compromised by inadequate oil flow at low rpms.

I would suggest a more moderate cam and the 3.54. I had a 3.54 in my ERA with a 428 pushing about the same horsepower and a 4 speed wide ratio top loader. Thought that it was a good setup for being able to have enough power to go around town at 30-35mph and not stress the engine.

[strictlypersonl]

I tend to look at it from a 1st gear perspective. The TKO 600 has a 2.87:1 ratio, which is fairly short. Combining that with a 3.54:1 differential ratio makes 1st gear not very useful unless your aim is to make tire smoke.

[DanEC]

There seems to be 2 schools of thought on the 5th gear ratio. I've heard some owners say they got the .64 ratio just for the highway and they only use it on the highway. Otherwise they drive it like a 4 speed.

The other group - I call the canyon carvers - want their gear ratios close without a big rpm drop from 4th to 5th and they drive their car in town and highway like a 5 speed. So they choose the .80 fifth gear.

Just need to figure out which group you fit into.

[DonC]

428 in front of a TKO 600 with 0.80 fifth in front of a 3.07 rear end but with 17" wheels (better selection of tires). 22 - 2300 rpm at 70mph. Wind noise and exhaust noise are about even on highway.
A very tractable combination for a cruiser and can run in 4th comfortably down to 30mph with the big block "grunt" available.
DonC

[Grubby]

ERA 755 has a TKO600 with .64 and 3.54 rear. The engine is a stroked 428 with 480hp.

We went on 3 Hot Rod Power Tours in the car and several longer trips. The jump from 4 to 5 was pretty big, but it was well worth it for the lower RPMs on the highway.

Cruising down the freeway at 80mph was just fine.

I put more than 13,000 on the car and wouldn't change the setup.

John

[jhv48]

If you actually want a useable fifth gear, get the .80 with a 3.31 rear end.
I had the .64 fifth and a 3.46 rear with a mild 427 and the jump from fourth to fifth was about 1000 rpm. I found myself rowing back and fourth in and out of fifth on the freeway. Want to pass? Downshift. I swapped out the .64 for the .80 and now have a very useable five speed gearbox. 75mph is in my engines sweet spot. If I want to pass, I just floor it. No more downshifts.
That 1000 rpm drop is really an engine killer. If you're hot footing it and you rev out in fourth and then hit that overdrive fifth, your engine feels like it hit a wall and acceleration takes a big hit.
A lot depends on what your normal hiway speed will be. If you rarely go over 75 mph, get the .80 fifth gear and the 3.31 rear. However, if you normally cruise at 85 mph, then the .64 overdrive or a 3.09 rear end with the .80 might suit you better.

[ERA713]

Whodeeny, those gears work good in era713 with stroked 427. The .64 is too tall, Ask me how I know.Good luck with your build.You will love your ERA.

[eschaider]

Whodeeny,

If you have side pipes like most of us do you might want to take a moment and have someone measure the distance from the outlet on the sidepipe to your ear while you are sitting in the car. You will be surprised, I suspect.

Depending on your height the number will come in somewhere between 24 and a low 30's figure. At 2500 rpm on the highway for any length of time that will produce an impressive sound in your ears — well one ear anyhow.

I have a TKO600 with the 0.82 OD because that particular trans combo makes 5th gear just feel like the next gear, instead of the big rpm drop with the .64 version of the trans. If I were building it again I would definitely use the T-56 Magnum. Much stronger trans, much better shifting trans, and it seems like it has a perfect gear for literally any driving situation you find yourself in.

The double overdrive available in fifth and sixth make extended highway driving, like you are contemplating, actually less painful than enhanced interrogation events. Shucks it even borders on tolerable.

Think about that dimension from the outlet of the side pipe to your ear at 70 mph, one more time ...


Ed

[Xmas]

The high 5th:

Gear 1 Max Speed - 50 MPH at 6000 RPM
Shifting at 6000 RPM puts you at 4000 RPM in Gear 2
Gear 2 Max Speed - 76 MPH at 6000 RPM
Shifting at 6000 RPM puts you at 4100 RPM in Gear 3
Gear 3 Max Speed - 112 MPH at 6000 RPM
Shifting at 6000 RPM puts you at 4700 RPM in Gear 4
Gear 4 Max Speed - 144 MPH at 6000 RPM
Shifting at 6000 RPM puts you at 3900 RPM in Gear 5
Gear 5 Max Speed - 224 MPH at 6000 RPM
Cruising RPM at 70MPH in Gear 5 is 1900

Vs the smaller .82 over drive that is this:

Gear 1 Max Speed - 50 MPH at 6000 RPM
Shifting at 6000 RPM puts you at 4000 RPM in Gear 2
Gear 2 Max Speed - 76 MPH at 6000 RPM
Shifting at 6000 RPM puts you at 4100 RPM in Gear 3
Gear 3 Max Speed - 112 MPH at 6000 RPM
Shifting at 6000 RPM puts you at 4700 RPM in Gear 4
Gear 4 Max Speed - 144 MPH at 6000 RPM
Shifting at 6000 RPM puts you at 4900 RPM in Gear 5
Gear 5 Max Speed - 175 MPH at 6000 RPM
Cruising RPM at 70MPH in Gear 5 is 2400

I'd prefer the mallee 5th. I like the 3.31 though