Is my tranny to blame??

gotzoom · #1 (**permalink**) 08-26-2003, 03:38 PM

I love driving my Lone Star Cobra. With a mild 351W it will put me back in my seat and oh I love the rumble. But once I am out of the gate I just watch the RPMs keep rising.

It has a three speed C6 that I know nothing about. I don't know the gear ratios but I am on a mission to find out. The rear end is a 3.77 for 9inch. The other thing that worries me is that it shifts at about 2500 RPMs. That seems low to me.

I want to keep my automatic, but wasn't sure if there was a 4 speed or AOD tranny that would drop right in the place of the C6.

I am seeking some advice: What should my tranny shift at? I am I just babying it if I keep it below 4500 RPMs? Any consensus on the best auto tranny?

Mr.Fixit · #2 (**permalink**) 08-26-2003, 03:55 PM

Do you want a different tranny so you can have overdrive? The C-6 is ford's strongest automatic. You can have the upshift/downshift RPM adjusted to better suit your desires. You can also manually upshift an automatic with a ratchet shifter, that's what the drag racers do. What exactly are you dissatisfied with?

gotzoom · #3 (**permalink**) 08-26-2003, 04:13 PM

OOPs, I left that part out. What I am concerned about is a low top-end speed. At about 100-110mph I am at about 4500RPMs. Is that right?

Junket · #4 (**permalink**) 08-26-2003, 04:21 PM

Gotzoom on your C6 go to http://www.smokemup.com/ auto math & for tire & gear ratio that should give all the spec. on the trans. and any good trans. shop can make it shift at 5000 with a little work as for the 4 speed it(would go with a TKO 5sp) will work with enough $$$$ that means every thing new from the back of the block to the yoke on the 9" + peddle ass. the AOD is not quite as bad a small block AOD & drive line change with a 4sp shifter belive shold do that one I belive.
I belive you are not going to hurt a C6 with a small block some are runing C4 & belive most stock trans shift about 3500-3800 the end of a stock cam torque range.
Sure there will be some one else out the that is better in formed than I on auto trans that will chime in.
Ken

Mr.Fixit · #5 (**permalink**) 08-27-2003, 10:43 AM

Do you never rev your motor past 4500? It should be fine until the power drops off or it hits 6000 rpm, whichever comes first. Not to question your motive, but how often do you cruise at over 110 mph? I think your speed estimation may be wrong also, unless you have really tall tires. My (non-cobra) car with a 3.73 is doing 108 mph at 5500 rpm which just happens to be it's speed after 1320 ft worth of acceleration. Slow, I know, but it weighs 3700 pounds and only has a 396.

ozaiss1 · #6 (**permalink**) 09-05-2003, 10:53 PM

I have the pretty much the same set-up 351w with a c-6 tranny and I would not worry about the rpm. When I'm cruising along at 75 on the interstate I run at about 3000 rpm but my red line is 7000 and I still get 22mpg. The rpm is fine I would not worry.

Excaliber · #7 (**permalink**) 09-05-2003, 11:53 PM

3.77 rear gear with an automatic is a VERY low rear end ratio. Heck thats "low" for even a stick shift! I'm surprised your ONLY turning 4500 rpm in the 100-110 mph range. I bet it's higher rpm than that, unless you got some VERY TALL tires!

Auto trannys are NOT a one to one ratio in high gear. A stick shift is (assuming no OVER DRIVE unit). Expect 10-20% "slip" due to the torgue converter. Therefore an Auto Trans will turning 10-20% more rpm than the same car with a stick.

Typical Ford or GM rear ratio with an auto would be 2.73, 3.0 would be "low". Same Ford or GM cars with a stick might be 3.3, 3.5 would be "low". Anything "shorter" than 3.5 rear ratio is getting into "serious" performance numbers, and THATS with a stick shift! I know, we ALL run low rear end numbers, but hey, we drive Cobras, we are NOT NORMAL.

Auto trans have a LOT of torque multiplication due to the higher engine rpm from the "slip" in the converter (stall speed is an example of this). Because of this "multiplication" you can run a "tall" gear ratio (2.73) that would be "unacceptable" with a stick shift. To high a gear ratio with a stick (2.73) would result in a burned clutch plate, sluggish accelleration, tough to get under way on a hill, etc. A 3.7 gear ratio with a stick, just like an auto trans WILL LIMIT TOP SPEED at "normal" rpm ranges (normal being defined as 5,000 or so). You can go "faster" if you spin the heck out of the motor, 7,000 rpm and like that!!

Modern Auto Trans have a "lock up" torque converter, thus the "slip" factor is eliminated, but it only locks up under light throttle and cruising speed conditions. Some (like the AOD) also have a 4th gear, an "over drive". 3rd gear would be one to one ratio, 4th (OD) might be .6 or .7 to one ratio. Add the "lock up" feature and you get LOW rpm at cruising speed.

Want to save wear and tear and get decent mileage? Go for a higher rear ratio, opt for an AOD auto trans or put in a stick shift.

22 mpg with a 3.7 rear ratio and a standard C6? Well,,,,,,that IS impressive, I've never seen it myself. 351W's are not known for great mileage under any conditions! I expect those numbers could only be achieved under "ideal" conditions and a low cruising speed. But, I could be wrong!

I'm running 3.55 gears with a stick and a 302. Top speed? 115-120 maybe. Engine rpm 5000-5500, stock motor. I could use 5th gear but the top speed won't change, only the engine rpm will drop. It takes horse power to "pull" that .8 over drive gear, and the stock 302 aint got it! But it DO accellerate nicely and it's the perfect gear for the short track I run! My best mileage cruising in 5th gear? About 18 with EFI. Real world mileage? 10 to 12.

Ernie

EDIT: The shift speed can be adjusted a number of ways. If your "deep" in the throttle and it's shifting THAT early, it clearly needs adjusting! It SHOULD shift around 4800-5000 with your foot buried in the carb.

jordanatkins305 · #8 (**permalink**) 11-06-2003, 07:57 AM

i've heard from many people and read articles that claim that the c6 is the strongest transmission for automotive period. Compared to anything! the stock rating for torque is 750 ft l/b of torque. it's got plenty of muscle for what you got. i'd stick with it and try adjusting your shift modulator on the back of the transmission. there is a vacuum line that you pull out and a slotted screw that adjusts shift points. i'm not sure how to do it but any good tranny shop can tell you how to adjust it for more rpms. good luck.

HighPlainsDrifter · #9 (**permalink**) 11-06-2003, 09:19 AM

Hi,
The C-6 should have a kickdown linkage from the carb to a 3" lever on the driver's side of the trans. This lever controls fluid pressure and shifts at wide open throttle [ WOT ] .
If you don't have this then you need to get it before doing anything else.
If you have this ,I would take it to a good trans shop and get it adusted and change the springs in the governer so that it shifts higher.
Perry.

Excaliber · #10 (**permalink**) 11-06-2003, 09:36 AM

I think the C6 does use a "TV Rod", (throttle valve, kick down rod) instead of the more common vacuum modulator.

C6 strongest tranny? I think that honour would go to the old GM two speed, the Power Glide.

Ernie