TKO 600 Ratio question

[Great Asp]

I know someone out there has already addressed this issue, and I am looking for your experiance.

The new SPF rollers are shipped with a 3.46 Dana rear, I am using the Avon 295/50/15 rears (26.7") and will need a TKO 600 for the FE motor.

To me, neither the 0.82, or the 0.64 5th gear seem very good with this rear ratio/tire diameter for cruising at 70 mph. (2,500 rpm and 1,951 rpm).

What about a 0.68 5th?

Thank you for your wisdom in advance

Eric

[Barnsnake]

It would depend largely on your cam and intake. What is the lowest RPM at which you could pull a hill without "lugging" the motor? If it won't pull safely below 2200, your question is answered. If it will pull stumps at 1800, then go for the .64.

[xlr8or]

I say go with the .82

I have the .64 and it's worthless below 80 MPH

[Archrms]

I have the .82 in mine (same tire size and rear ratio as yours), and it is great for the highway - no regrets.

[Got the Bug]

Agree with Archrms, the .82 is the way to go.

[STL Mark]

I've got .82.
Engine runs about 2700 rpms at 80 mph.
Plenty of room for punching it to 125mph+

If I had the .64 - I guess I'd have a bit more of the torgue band and throttle to work/play with above 80 - which also means I'd probably kill myself PDQ.

STL Mark

[Hammer65]

I have a Superformance and selected the 0.62 5th gear. With my tire size (28.0", 4000 RPM in 4th produces 96 MPH and 5000 RPM produces 120 MPH.

In your case with 26.7" tires, 4th gear will produce 92 MPH at 4000 RPM and 112 MPH at 5000. The 0.062 5th gear will get to 143 MPH at 4000 RPM if the torque is there.

My Roush 427SR has a fairly flat torque curve from 3000 RPM, the lowest measurement, through 5500 and I couldn't see myself wanting to quickly accelerate past 100+ MPH.

[wtm442]

I have a 3:73 rear gear with the 0.68 fifth gear and 15 inch Goodboot tires. Very mild 351W with about 425 HP at crank. Idles nicely at 900 RPM. Fifth gear is only useful above 70 MPH. So unless you intend to do a LOT of "illegal" high speed highway driving, go with the 0.82 ratio 5th gear.

The only benefit of the 0.68 is when doing a very long highway cruise at 70 to 75 MPH ... it gets great gas mileage.

At 100 MPH, my motor is only turning at 3000 RPM. That would be great if I lived by the Bonneville salt flats.

Here are some nice "calculators" to play around with ....

http://www.cobranet.com/calcs.htm

[greg schroeder]

I'd get the .82 5th if I had to have the new 3.46 Superformance rear end gearing. I'd want a 3.73 rear gear more than I'd care about which 5th gear though.

My car was with the older 3.73 rear gear. The 28 inch tall 275/60 15 tires worked great with the .64 5th gear at 1600 rpm or higher if going up hill, about 1800 rpm flat or about 2000 rpm if going down hill. The more decline the more the car would get ahead and try to unload the drive line if at lower RPM. The motor had quite a bit of low rpm torque which probably helps. Under acceleration the 4th gear with 28 inch tall tires was good for about 140 mph, so the option of .82 or .64 5th was sort of a prefference wether you wanted to fart along with a cruiser 5th or have a more sporty feel with the 4 to 5 shift. I can't imagine continuing to accelerate after 4th though.

[4pipes]

I have about the same setup and a .64 5th. I love cruising at 1900rpm @ 75mph. The noise is low and I get 17mpg on the highway. If I want to accelerate or pass, I just downshift to 4th. The 5th gear is overdrive for me.

[Great Asp]

Originally Posted by Archrms

I have the .82 in mine (same tire size and rear ratio as yours), and it is great for the highway - no regrets.

If I can not get a 0.68 in the TKO 600 for 5th, I will go with the 0.82. But I do not like the idea of cruising at 70 mph and 2,500 rpms. 1,900 to 2,100 rpms at 70 mph seems like a happy place for me.

The post above about the cam selection is spot on (Barnsnake) and I will ask Keith. I am planning on using the Classic injection from DynaTek. (any of you DynaTek guys trolling this forum?, no one at DynaTek answers my e-mails).

All useful information!

E

[jwd]

Here's another vote for the .82. That's what I have with 295/50/15 tires and a 3.25 rear gears. 2000 RPM is exactly 60MPH. The gears are perfectly spaced (no big drop going into 5th). I couldn't be happier. In a word "perfect".

JIm

[Great Asp]

Originally Posted by greg schroeder

I'd get the .82 5th if I had to have the new 3.46 Superformance rear end gearing. I'd want a 3.73 rear gear more than I'd care about which 5th gear though.

My car was with the older 3.73 rear gear. The 28 inch tall 275/60 15 tires worked great with the .64 5th gear at 1600 rpm or higher if going up hill, about 1800 rpm flat or about 2000 rpm if going down hill. The more decline the more the car would get ahead and try to unload the drive line if at lower RPM. The motor had quite a bit of low rpm torque which probably helps. Under acceleration the 4th gear with 28 inch tall tires was good for about 140 mph, so the option of .82 or .64 5th was sort of a prefference wether you wanted to fart along with a cruiser 5th or have a more sporty feel with the 4 to 5 shift. I can't imagine continuing to accelerate after 4th though.

Your right Greg.

I wonder why SPF doesn't put the 3.73 in the car anymore?

I know the trade-offs to the lower gearing, but big horsepower with a taller rear end doesn't seem to make sense. I agree also, on the street you wouldn't keep the hammer down in 4th, and even so if you reach just above 6000 rpm's, you will still be in 4th when you trap through the 1/4 mile even using the 3.73.

They must have had old guys wanting the taller 1st gear

E

[Silverback51]

I have the .64 5th gear and hate it.

One of the winter projects will probably be to swap it out to a .82 tranny.

[wtm442]

Originally Posted by Great Asp

I wonder why SPF doesn't put the 3.73 in the car anymore?
E

The 3.73 rear end gear was in the 8.8 Ford IRS. Now that they have switched to the BMW rear end, the std gear is 3.46

[Great Asp]

Originally Posted by wtm442

The 3.73 rear end gear was in the 8.8 Ford IRS. Now that they have switched to the BMW rear end, the std gear is 3.46

It's a Dana right?

[ztarum]

Go with the .82

Cruising much faster than 80 mph isn't comfortable in a cobra anyway. I spend most of my interstate driving at around 75 mph, and the .82 doesn't bother at those speeds.

The real benefit is that there is not a big drop between 4th and 5th. The spacing just feels right to me.

[iwantacobra427]

I'm finishing my build now.. I have the 0.64 with a 3.77 differential. It was that combo or the 0.82 with a 3.54 diffy. The speed limit in west Texas is 80 mph on some highways..

[greg schroeder]

Here's a calculator page.

http://www.pro-system.com/calc.html

It's useful to figure stuff.