Posi-Tractrion

[davidathans]

For guys that race there cars...

So i've got a 347 stroker, a C4 auto with 3500 stall converter, and a ford 9 inch rear end with auburn limited slip differential & 31 spline axles.

I raced the stang a month ago and ran 13.42 at 101.5mph at LACR, a track with not so good conditions at like 2500 ft elevation.

My biggest problem is not the traction coming off the line (im using hoosier 26x9-15 slicks on 14lb aluminum rims and Cal-tracs traction bars, Rancho RS9000 adjustable rear shocks) and im power breaking to about 2600 rpm then flash stalling by putting my foot to the floor and its hooking.. i had a 1.94 second 60 foot time.

MY PROBLEM IS...

After the launch as first gear is getting to about 5500rpm's the tires break loose and im shifting early (the cam/head combo is pulling strong to 6700rpm in second gear) im having to shift out of first around 6000 and in the shift the car fishtails. The back end tends to jerk and i find myself weaving down the track through my entire second gear, by third gear im going straight again and crossing the lights at 6000rpm.

I believe im losing at least a tenth of a second because im losing traction, not going straight, and shifting earlier than necessary out of first gear.

My question is: Is there something wrong with my limited slip diff? With posi shouldnt the car hold straight on that shift?

Thanks
David

 

[walkupg]

My 68 runs 11.60 in the quarter at 117 mph. I use 26x8.5x15 mickey thompson slicks. C4 transmission with either 3.25 gears or 4.11, doesn't matter same et. The only changes are the length of time I'm in 1st gear before I shift out of first gear. I use tubes in my slicks with 14-16 psi. Using tubes makes them more consistent in my opinion, I know some guys run them tubeless to save weight, then you really have to watch the tire pressures. My sixty foot times are lower than yours, I utilize New York Machine Lift bars, which are probably not as good as Cal Tracs. Cal tracs requires very, very stiff leaf springs to work properly, if you're running standard leaf springs, and extremely low tire pressures, this contributes to that squirm feeling, I don't feel you should be breaking loose unless your using a standard transmission and your shift is not smooth. I would adjust my air pressures, never below 10 psi, and if that didn't help, borrow somebodies else's slicks, they do wear out, or you are over heating them in your burnout, limit your burnout to five seconds at 5000 rpm, to much chemicals can raise, from the tire rubber, to the surface of slicks if they get to hot. The purpose of burnouts are to clean the slicks of debris and get some chemicals to the surface, but it can be overdone. Good luck and I hope my advice helps.

 

[davidathans]

Thanks for your advice. 11.60 is very fast, what elevation was that? What engine? The transmission is an A-1 Racing C4 transmission with 26 spline output shaft and a 3500 stall continental converter....what converter did you use? what rpm do you leave the line at?

im going to leave the line at 3500 next time i race, for the 13.42 at 101.5 i was flash stalling from 2600rpm.

 

[walkupg]

I'm using TCI 10 inch torque converter, they claim about 3000 rpm stall speed. I don't know if this correct. Over the years I have used Aamco rebuilt transmissions, Jackson racing transmission out of the Carolina's. Right now I'm putting in a TCI transmission from Jegs. I foot brake the car, left foot on the brake, hold engine rpm at 2000, when the third yellow illuminates, I release the brake and mash the gas to the floor. I bracket race this car and this has proven very reliable and consistent. I race mostly at Temple Academy Race Track, outside of Temple Texas. It is roughly 1000 feet in elevation. The track is very narrow, but it has really good hook. I've raced at other places, while they had nicer facilities, the starting line is no different. At least not the speeds I run. Good luck on your car.

 

[et9secstang]

hello,what front shocks do you have on the car? Sounds like your car is unloading the suspension.Cal tracks are good if you can get your car set up.I know a guy that that has a old maverick with no trick suspension, mostly used wore out parts ,if he leaves at 2600 he puts the car on the rear bumper.small block with c-4.He had to put wheelie bars on the car to try to keep it under control.I personally know the guy and have seen the car,it doesn't look like anything special.If he can keep it under controlit will run 10 flat.

 

[davidathans]

Im using carrera 90-10 front shocks...and softer springs V6 springs.

Im getting hook off the line, but in the shift at high rpms in first to second i cant hold the car straight.

Do you think there could be a problem with my limited slip diff. ?
It also might have to do with my steering. It is kinda loose (if you know old mustangs) The guys at mustangs etc seem to think that it is fine, but i do not believe i have replaced the steering box or gears or whatever. Barely moving the wheel turns the car, shouldnt there be some play so that its easy to hold the car straight?

 

[et9secstang]

Hello,when you say you can't hold the car straight,does the car spin the tires?Are you breaking traction.Their could be a problem with the posi? It sounds like the the suspension is unloading .It sound like you have the car set up good.If you can get someone to take a video of you going down the track and watch what the car does and try to see whats going on.How much air pressure do you run .Have you measured the circumference of each of the rear tires?Make sure they are close to the same size.You may have to play with pressures to get them right. In a tire that size we play with pressures,we run as low as we can, if the car waddles down the track bump the pressure up by 1/4 or 1/2 pound every time. I know on that maverick he just has slapped bars and he has them adjusted so when the car launches the bar then hits the spring. I can't beleive how that car hooks with no special or trick parts. I wish I could help you some more.Good luck, Keith