help with tire pressure

[quick 52]

went to the track Sun and car was very lose it was all over the track even out 330'... Ran the car on motor and it ran straight but with extra 275hp hit couldn't keep on the track... 78 Malibu 427 sbc #3250 with driver tires are MT 29.5x9x15 slicks... Try to make 1/8 miles pass on hose ran 6.10 @ 103 had to abort run.. On motor pass 1/8 6.62 @ 105 straight down the track no problem the 1/4 was 10.55 @ 126... Tire pressure was @ #12 and tried 11 didn't help have run the car before with 12lb without trouble best pass 9.41 @144

[1bad68]

I'm up around 17-18 in my 28x11.50 15 et streets down around 15 it all over the place

[squirrel]

I run 18 in my 7" slicks, seems to help it run straighter.

[yellomalibu]

What size rim are you running your 9" slicks on?
I don't recall how my M/T slicks acted - but I do know that different brands acted different. I think the M/T's were nice and tight feeling. Some tended to feel loose going down the track, while others acted just like radial street tires. The goodyears I used toward the end of my racing career were very steady... down to about 10 lbs... but I had the best contact patch in my 3300 lb car with 10x29" goodyears on 8.5" rims at 12 psi - and they hooked just as well at 12 psi as they did any lower that I tried.

Another thought: is the car/ suspension setup new or tried and true? Posi or spool? Front end alignment good?
I ask because when I switched to a spool from a posi, my car was a handful - I had to abort both runs the first night. Turns out my rearend wasn't straight; by trying to add preload with adjustable upper trailing arms (like a true 4 link would) it moved one tire 5/16" further forward than the other, causing my car to be extremely loose feeling.

Yet another thought: Have you measured the circumference of both rear tires? Sometimes tires with the same size markings on the sides can be very unequal in size.