My headers are about 1/16 to 3/32 from my steering box, is this enough? I ask because when I get on the car hard, I get a pop or knock that I feel in the floorboard or firewall area, it doesnt repeat, might happen 2 or 3 times, but again only when getting on it hard. I have stock GM motor mounts, engine is over 400hp. It isnt the exhaust under the car, clearance is very good there.

I just suspect the header to steering box clearance, myself I thought the engine would "torque" the header away from the steering box.

Thanks,
Jerry

More than likely, the header tube is hitting the steering box. But, if the engine is rated at 400 hp, and you have standard engine mounts,ie, rubber isolated type, driver side is probably broken.

You should either replace the driver's side engine mount with a solid one, or tie down the driver side of the engine with a turnbuckle from the cylinder head accessory bolt to the control arm shaft.

If you see a small dimple or dent in the header tube there than you know it is hitting.

Like Everett said, if you are running stock type engine mounts they will break sooner or later with that much HP so IMO you might as well get a set of poly mounts and be done with it.

The dimple is already there, I put it there, before there was no clearance. As far as the motor mounts I dont know if it is broken yet, the car has 10 miles on it since finishing the engine. But I am sure those mounts can flex alot.

I switched from the stock rubber ones to the polys and noticed that the rubber ones DID flex a lot -- but my new poly ones are pretty stiff.

The engine should torque away from the steering box when you jump on it. Unless the header sits under the box for some reason I don't think it's the source of your knock. I have about .030" - '045" clearence and don't have a problem with rubber mounts... Is your trans mount tight?

They all appear to be tight. I cant think what else could be hitting, maybe the motor is torquing enough that the left crosstube coming out of the header to the x-pipe is hitting the tranny crossmember. Would Energy suspension urethane mounts hold the motor better from torquing? There is almost 1/2" of clearance from the exhaust tube to the crossmember? Enough?
Thanks,
Jerry

You make a good point Dennis, but when my drivers motor mount broke last fall the drivers header hit the steering box -- don't ask me why though . . . because like you said it should torque up on the drivers side not toward the box. Maybe my broke mount let the engine shift around a little.

Have you tried shimming the block at the motor mount? Hedman Hedders recommended that for me because my header is wacking my steering box too. You mentioned dimpling your header...is it a ceramic coated header cause mine are and I was wondering if I should dimple it.

Eric - I could see a broken mount letting the block shift around some.