how do you know if you need these?
thanks

Here is some good reading......... subframe article

Kel

If you're running a posi and are putting as much power as a stock 400 I'd highly recommend it. It happens much faster on convertibles, but if you enjoy launching the car once in a while, overtime you'll actually torque the body out of shape. It's subtle, but you'll notice in the door gaps. Convertibles are especially bad. It also hurts ride and handling having body flex.

i have a coup. it has a 3.42 posi, 350 auto trans. #16 heads on a 400. 268H comp cam (218 duration) 2 1/2 dual exhaust. and i believe my rear tires are 8" wide. thanks for any info.

When you have to stop after a hard left corner, open the trunk and pop the left rear quarter back out - my suspension was set up a little too stiff! I was autocrossing the car ('69 conv.) and it would "tincan" in right over the left rear wheel well when I got on the throttle in a left turn. And that was before we had sticky tires!

Bolt-ons are about half as effective as welded in ones, but you have to make that decision!

If you're getting stress cracks in the quarter, you already needed them a while back.

It all depends on how solid your car is. My vert is as stiff as a board. It has a stock 400. I can hammer it every which way and it doesn't flex. I think Pontiac's original design is fine as long as you are running a stock configuration. I do not 'power brake' my car though. I think that a lot of people are using frame ties to solve other structural problems. I like frame ties for race cars. The bolt-in are just fine if they are done right. Just my opinions.