Which is better for my Cobra: Tremec .62 or .82 OD?

[timsullivan]

I'm building a Superformance with most likely a 351W stroker of around 418 or 420 CI that I'm shooting for 450 to 500HP. I will be running 15 inch BFG's or Goodyear GT2's. My rear end ratio is 3.46:1 I am unable to afford changes to the wheel size or tires and don't want to change the rear end ratio. I will be doing occasional freeway driving here in SoCal and occasional longer trips on the freeway but the vast majority of my driving being simply around town.

Which TKO 600 should I get .62 or .82 OD and why in your judgment? Thank you in advance for any input you can give me on this.

-Tim

[Excaliber]

Generally speaking you want the .8 OD because the .6 drops your rpm by almost 50%, 60% in fact. That can lead to the motor not being comfortable at a reasonable cruise rpm. To determine which is better in your case you need to know two things:
1. At what rpm range will the camshaft profile, induction system, timing, etc be within accepetable range?

2. What typical cruise mph are you going to go?

An engine that "comes on the cam" at 2500 rpm and your cruise rpm is 1800 is NOT going to be fun. Gotta a BIG cam? Go with more cruise rpm to match it's characteristics. .8, or 80% of cruise rpm at mph vs 4th gear (straight through, 1 to 1) will increase your rpm more than a 60% drop in rpm will. Which is better? Re-read above! I would design the gearing and camshaft for best performance at the highest interstate speed limit in your area, not to much over that or you risk a ticket.

[madmaxx]

.62 and you will have to go 80 mph to get the rpm's right, the .82 and you are about 2500 rpm at 72 mph. too bad they dont make a .72.

[kitcarbp]

You will be happier with the 0.82 for driving on the highways below 80 mph for the reasons stated above, its about matching the ratio(s) to the cam/intake rpm band your engine is optimized for. With that said, have you considered the T56 6 speed which is a game changer a bit and offers you more versatility. search threads here on CC for T56. Its been done in a Superformance and since you are still building, cost wise should be similar. In my build I decided on the 0.82 using a 3.25 rear gearing but I'm not on the road yet to give you feedback.

[xlr8or]

You'll be happier with the .82

I had the .68 version with a 3.31 rear and was pulling 2200 RPM at 80 MPH

[Excaliber]

Myself I would also carefully consider the FIRST gear ratio as well. Often with a five speed it is way to low. Resulting in serious wheel spin, while fun, it gets hold shifting really early or just using 2nd gear to take off.

My favorite "recipe" for gearing is:
The first four gears would be equivelant or close to the ratios you find in a top loader 4 speed. The icing on the cake would be a 5th gear at .8 Rear gear ratio can vary depending on what you like. 3.5 is a good call for my "recipe", in general. I might tend to go a bit higher IF the first gear ratio in my chosen trans is fairly low. I like to see about 40 mph in FIRST gear before shifting. That is just about optimum in my opinion.

[RICK LAKE]

TimSullivan Tim more important is what is the first gear ratio? The other thing is where your torque band starts. FORGET HP. You are not going to spend your time at 6,000 rpms all the time. Torque is what moves the car down the road. Ask for 550 ft of torque at 2,500 rpms. The idea is to be about 400-500 rpms into the torque band for cruising. You are building a stroker. Would recommend calling Keith Craft for a custom hydro roller in your motor. Unless you plan to race the car all the time a 6,500 rpm limit would be great and give long life to the drive train.
A 2.70's first gear and .82 overdrive would be a good combo. You don't want the motor to luggin 5th gear on the highway. 1,900 would be a min to have good power in 5th to accell to a higher speed and not have to downshift.
As for first gear I have run 3.25 and 3.00 with 3.31 gears, It's a waste of time starting out in 1st. 2nd gear is 2.45 and the car still pulls away with little effort. Have to remember you and the car weight is under 3,000 lbs. Good luck with the build. Rick L.

[Ron61]

I had 3:27 gears in my Cobra and the .62 Tremec. I wish I had gotten the .82 as I hardly ever found a place that I could drive in 5th gear without lugging the engine. By the gear, tire, RPM ratio chart, in 5th gear my gears were about 2:10s. My coupe has 3:56 gears in it and the difference is quite remarkable.

Ron

[patrickt]

Tim, I have the .64, but, I have a 3.54 rear. With a 3.46 rear, I'd go with the .82.

[Chilibit]

Here is a handy tool.

http://www.secondstrike.com/Technical/GearingHelp.pdf

I elected to use the 0.82 and after 25k miles I have never regretted the choice.

[392cobra]

I have a T-56,so it covers both ratios.Rear is 3.70.

Fifth gets the use most all of the time. Sixth is for at least 80 mph and tickets.

[ztarum]

I had a SPF with the same set-up you're contemplating. I went with the TKO 600 RR w/ the .82 fifth gear. It was perfect. Get the .82, you will be happy you did.

[209]

Hi Tim,
My engine is Roush's 427sr/tw with Tremec 5sp with .82, 3.46 rear, and 15" BFG's. In 5th my revs are 2800 at about 75-77 mph and 12 mpg. Cliff

[DonC]

I'd highly recommend the 0.82 fifth. I'm running a slightly different set up (3.07 rear with 17" wheels) and find myself cruising at 2300-2400 RPM at 70. My cam comes in around 2000 so the drive train seems pretty happy there. The 2.87 first gear in the 600(R) is very usable compared to the much lower first gear ratio on the older TKO models. Even with the higher 1st ratio I can still break the rear end loose at will in 1st and 2nd.
I do mainly street driving with a couple of track days per year and find the 600 to be very comfortable across the board.
DonC

[mikeforte]

Hi Tim,
Your gear ratio could work with the TKO-600 .64. It really depends on your cam profile. The gear ratio with be about 67 mph in 4th gear. When you shift into 5th the rpm will drop to 1920 at the same speed. The .82 would be 2480... It depends on what speed on the highway and cam profile.
If you have any questions, feel free to call me weekends 781 248 8946 or weekdays 508 875 0016.
I stock all the TKO models and could supply you with all related parts to assemble the driveline. You could also use the T-56 Magnum and have the best of both transmissions with 2 overdrives....

[Jim Norris]

I have the TKO with .64 and 3.73 rear gears. I don't like it. I'm constantly hunting for the right gear on the freeway. Fifth gear drops the rpms so much that I'm always on the verge of lugging.
I'm thinking about either changing the overdrive gears in the existing trans or swapping the trans out altogether.

[mikeforte]

Hi Jim,
I stock the .82 gearset for your TKO-600. You could also sell the TKO you have and purchase the TKO-600 .82. One other option would be to install a camshaft that doesn't lug at low rpms.

[WarrenG]

I had my cobra in SoCal for a while and used the 0.64 on the highway all the time with no problems or issues.

[mikeforte]

Hi Jim,
I sold a guy a TKO years ago to install in his 428 Shelby Mustang. It had 3.89 gears, 2 4 bbl Holleys & a nice Crower cam. The problem was the cam wanted to go 80-90 mph to run clean and not buck. We installed a much more docile camshaft and he then loved to drive his car at normal speeds. The cam we used lost a tiny at redline and gained total driveability....

[Jim Vander Wal]

OK, I'll go against the grain.

I have the .64 with 3.27 gears. I like it on the highway as I cruise at 2,200 @ 80mph with 315/35/17 tires.

Where I don't like the .64 is 55-75 cruising backroads, the RPMs are too low and 4th forces the RPM range too high for just cruising.

My use may be different than yours. I run lots of highway (hours/days at a time) and find that 2,200 is at the bottom of my usable RPM range. I also play on the track and find that I can run to 157 in 4th if needed so I don’t shift to 5th on the track.

I've looked at the .82 and run the numbers and just don't like the higher RPMs for long distance highway cruising.

There has been a new post about putting the T56 into a SPF. This really interests me as it offers the best of both worlds.

Jim