Tire pressures and traction

Currently I have a 104 cammed 76 block with 70 Stage1 heads. I am hitting the ground through 3.90 geared posi and 255/60R15's. I have that wonderful problem of traction. Right now I am probably around 30-32 lbs of air in the rear. I am lookin to air down some. Where would you recommend that I put them at for street use and cruising? Then this upcoming week, I am looking to use a G-tek to see how good I am doing, where would you suggest I drop down to for that?

Thanks for the help, Mike

:beer

 

I think you could go down to 25 psi and that would help. But with street tires, unless you have a drag radial of some kind, lowering the pressure isn't going to help that much.

 

I have another set of rims that I am considering getting a set of tires for them.

What do you reccommend? What kind of prices should I look to pay as well? And, is there something special to do when mounting them to assist in operation? As in, I used to be on a Toyota 4WD board, and the mega rock crawlers would screw through the rims into the tire. Of course by drilling te rims and then putting the screws through the hole into the rubber.

Thanks, Mike

 

I'm not so sure that dropping pressure on a standard radial tire is going to help much.

Depending on car weight, lower pressures can work ok.
My 2400# roadster runs BFG 285/70R-15's on the back.
Since it's such a lightweight car, 20# pressure works well.

Here's a couple of different ways you can see if the pressures are right for your car - lightweight or not.

For general street use, park the car on a dry cement driveway.
Wet a small part of the driveway.
Roll the rear tires into the water and roll them out.
Note the pattern left on the cement by the wet tires.
(You may have to go several feet out so as to get a not-so-wet pattern.)

If the pattern is dark on the outer edges and light in the center that indicates underflation.
Conversely, if the pattern is dark in the center and light on the outer edges that indicates overinflation.

Keep in mind that one reason radials (standard, but it works on the radial slicks as well) grab so well is the longer footprint they put down.
Longer footprint as compared to bias-plies - which most do not run nowadays.

The other way to check for correct inflation - at least you'll get a good start at it - is to do a short burnout on asphalt and note the pattern left behind.
Same deal as above as far as over or underinflation goes.
Note as well if both tires are leaving about the same pattern and putting down the same shade of rubber marks on both sides.

We used to do this with my 335" Olds Rocket powered 50 Ford coupe with home-made Traction Masters. (Traction Masters being an early traction aid. They look identical to the lower bar of a 4-bar setup.)
Since we were running an open diff - high 13.90's ETs and 100 mph - you could see the same pattern coming off the slicks, but the left side would be darker than the right.
That indicated torque was raising the right rear wheel lessening bite on that side.
Best way out was to install Air-Lifts on the rear axle and run more air pressure in the right side than the left.
An additional help was to wedge the left front coil spring so as to shift a little weight.

You shouldn't have the unequal (shade-wise) pattern if you're running a locking diff and a reasonable for drag racing rear suspension setup.
We never did in this particular car that also ran the street, but the Air-Lifts and watching the burnout pattern allowed us to set up what we had so it was the optimum.

 

I think your best bet would be getting the new MT Drag Radial 275/60R15. Those hook great I'm told after "warming them up". With that 3.90 gear, your prone to spinning the tires regardless of what street tire you'd put on.

 

You wont get eney traction with street tires with a BBB and 390 gears, i had tration problem with a car that had a wore out 455 and a 3.08 posi. DRAG raidials work great but they wear out fast so if you put alot of miles on i would mount them on seperate rims and put them on when you need to. also i have a modified suspention on my GS to keep the rear level and help the weight tranfer when am at the track useing slicks, but this doesnt help at all when i have street tires on, the car has just has to much low end torque for sreet tires. also you usually wont spin on the rim untill you are running in the 10's . just my 2 cents