[Cliff R]

NEVER for any reason use a thick gasket to lower compression on one of these engines for pump gas. Here I will not for any reason build one of these engines with over .040" quench. These crappy combustion chambers don't like that deal at all and you'll just knock all the balls right out of it and make it want to run hotter, overheat, and ping harder all else being equal.

I've tuned scores of 400', 428's and 455's pre-1971 with higher compression to run fine on this new fuel. Not exactly sure why yours is so octane sensitive? I've ran into a few where I had to shorten up the timing curve and reduce the amount added by the VA, but have always been about to get a good end result IF they were using factory camshafts.

I have ran into a good many that you were NOT going to make happy with any sort of tuning. Most of those were 455's and a few big block Chevy engines.

Having had several 428's in the Ventura over the years I will say that the dished piston versions are finicky due to the added quench area. One of my 428's was only 8.8 to 1 compression and would ping if you tried to run very much total timing on it. It also heated up very quickly and the temps would "creep" up in slow traffic and at stoplights making it rattle even worse till you got moving and cooled it back down some. This was WAY back before I knew about tight quench and selecting the right camshaft. It ended up having a bad crankshaft and had to be removed but I was glad because it was not happy anyhow.

I did some research and talked to the folks at HO Racing and installed the HC-01A camshaft in the next engine. Since I cracked the #96 heads with the overheating previous engine I used 6X-4's instead. That engines was an absolute HOME-RUN and powered the car for nearly 20 years until I built my first 455 for it.

In any case as it pertains to what you are doing here I would NOT lower the compression. Change the cam instead and pick one with a wide LSA and about 15-20 degrees more seat timing than the 067 cam. Moving up to a real 068 would really help, and even on up to the Summit 2802 would fit the bill nicely and still have good idle quality and street manners while lowering dynamic compression at the same time.

I'll add here that static compression is only one "player" in this game, and whoever came up with the proverbial "brick wall" of 9.5 to 1 for pump gas should be banned from posting on the Internet for at least 100 years. That is one of the dumbist statements I've ever heard when it comes to this hobby......FWIW......