All,
Looking for some thoughts and experiences on improving take-off traction. On easy acceleration a rear tire breaks loose. It will also chirp going into second on hard acceleration. It is, as best I can tell, a standard 351C-4V with a Holley 4160 carb. An FMX automatic. A 3.00 open rear. BFGoodrich Radial T/A P235/60 14 tires on the original 14-6 styled steel wheels. The tires have good tread so replacement is not needed any time soon. I’ve tried altering pressure, no noticeable difference. The tires are about 6-1/2 years old.
The obvious choices are:
Stickier tires - if available - these have an A traction rating for what it’s worth
Wider tires requiring wider wheels - the existing tires already are one size wider than recommended for the wheel
Positraction rear
None of these are cheap. I could learn to take it easier on the gas but that’s no fun. I’m not looking to race but the sensitivity is annoying and gets attention that I generally don’t need. I prefer a traditional look so don’t what to get too drastic on bigger wheels. I don’t recall any similar issue on my first Cougar with 195-14 tires but it was a 351-2V.
I don’t know what you have now. But 3.25 should be ok. I ran that behind a mild 351c 4v before and it was livable imho. My 351w 70 XR-7 has a T5-Z and I run a ate of 3.50 gears. But it is not comparable to a FMX Automatic.
Then you should not be cranking the R’s up to much at 70. They were built to cruse at 70 with stock E 70 or F 70 X 14" tires. I don’t recall exactly what it was in my White 70 when it still had the FMX in her. But not much over 2400 at 70 if I recall correctly. But who drives speed limit on the free ways. As long as you are running the factory torque converter.
I’m getting about 3000 rpm at 60 mph. The 3.00 ratio calculates to about 2400 rpm at 60 mph. I’ll need to jack up one rear wheel, put in neutral, mark the wheel and the drive shaft, and count rev’s. I probably need to check the tachometer calibration as well. I’m not certain on prior owner modifications. I didn’t think the engine or transmission had been modified until I found a bit of silicone rubber at the intake manifold and very strangely a wire going through the engine and bell housing joint (it ran into the dash and was cut off there).
70XR7Tom, I’m following your post closely due to having the same setup on my car. I just went through a ton of work to try to get my car running better off the line. I couldn’t even get mine to spin the TIRE (standard axle, no locking). After a lot of work I can now smoke the tire when stabbing about 3/4 pedal on launch.
Mine is a factory 351C with 4V now running an Edelbrock 1407. I have no clue if it’s factory cam. Special Order 3.00 axle ratio with FMX tranny. I turn right at 2400 rpm at 70 mph. I’m running 275/60s on 15s out back and 235/60s on 15s up front.
With my CRS that is most likely the correct numbers. You can cut the RPMs with taller tires and a speedometer calibration too. But you will cut your of the line performance too. Best of both worlds is to get deeper gears and a overdrive transmission of some sort.
But a manual shiftier is just so much fun… Hiding under that stock looking Hurst shiftier is a 5 speed.
I agree that a limited slip differential would be the most satisfactory solution overall. But I’ll also note that tire size plays a role. Your car probably came stock with either E78 or F70x14 tires, both of which are approximately 26" in diameter. 235/60/14 are approximately 25", a difference of 4%. Which means your 3.00:1 axle ratio is effectively 3.12:1. That increases the torque applied to the rear wheels when you hit the gas, and also raises the cruise RPM on the highway. Going to a 215/70/14 (26" dia.) or 225/70/14 (26.5" dia.) would reduce the torque multiplication to the rear wheels, and decrease cruising RPM. Granted, 4 or 5 percent isn’t a huge difference, but would be a step in the direction you’re wanting to go. Unfortunately, 14" tire selection is such that there are no options to keep the width you have while moving to a stock or slightly larger diameter.
Back before I installed an overdrive transmission in my '69, I ran 235/70/14s on the back. Those were 27" diameter, or about an inch larger than stock, but fit with no problems at all. Traction was decent, and they reduced highway RPM by a few percent. Sadly, that size was discontinued several years ago, prompting me to go to 15" wheels and tires, and eventually a 5-speed. Those are relatively major changes, but short of that you can improve the situation just by changing the tires on your current wheels.
Ok, first answer my question, then I’ll answer yours:
When I punch it at around 45-50 mph my car downshifts to 2nd, smokes the tires, the car upshifts on the limiter at 7400 rpm and then swerves slightly left. At 90-100mph with the tires spinning it’s a bit hair-raising so I have to lift out of the throttle. Should I
a) get better tires?
b) take a couple of spark plugs out
c) change the gears
d) buy a Camaro
LOL, my car really does do that but back to your problem… which I hardly consider a problem based on the above. A traction-loc that you install yourself would be the best investment. Gear change? I used to think that 4.11’s on the street were better but now I kind of wish I had my 3.00’s back because even 3.50’s on the highway gets real tiring fast. Personally I would stick with the 3.00s or maybe 3.25s (used on Ebay).
The other thing would be to move to a fatter, stickier tire. The problem being now that the sub-16"-rim tire selections are going away so fast you have few tire choices. You can easily fit a 275/60/15 under the back of the 70 without rubbing but you’ll need a wider rim. I suspect that you can get a 235 to fit on the current rim and, believe it or not, the 6 year old tire you have is pretty much scrap without needing to look at it further. Check what the tire manufacturer recommends on aging
Older picture. It has been changed as well as a couple other improvements like a tilt steering column and a shiftier arm that more matches the factory 4 speed one.
I’ve resolve the rear end ratio question. It is the original 3.00. The tach calibration is off indicating 3000 at an actual 2600 rpm (based on the digital readout on my timing light). That plus an an adjustment for the tires effective road contact diameter vs the unloaded inflated diameter (closer to 23 than the listed 25 inches) and a bit of speedometer error, it is 2600 rpm at an indicated 60 mph. For now I’ll just have to mentally adjust the tach reading down by 20%.
I still spin a tire at less than a quarter throttle from a standing start. I guess I should be pleased with that kind of engine performance. Perhaps there is a engine mod that I’m not aware of.
I’m planning on proceeding with a posi-traction conversion at the current 3.00 ratio (my Christmas present). When I need to change tires I’ll probably go a little taller and get back into the recommended size for the 14-6 wheels.
I just ordered TechnoVersion’s TachMatch TM-02-I-Drive to allow me to calibrate my tach. I installed their MeterMatch-VR last week to better calibrate my fuel level and it looks good. I’m planning on a follow-up post on each.
