Help on 408 build
My 9.5 comp 340 with Ported J heads will knock on Winnipeg pump garbage...I don’t think you should jerk around with any of these half *** fixes, put pistons in it or run AV or race gas.
Half *** fixes! Well, I guess I'll close the doors and put the shop up for sale. I'm sure Cometic will want to follow suit for making a .098" thick gasket for absolutely no good reason. Some of you guys are right ready to spend someone else's money, when they are desperately looking for an easy solution to a mistake they made. Sometimes it can't be done, but in this case it can. There is nothing half assed about adding chamber volume to a head to lower compression, which is all a thicker gasket does when there was no quench to begin with. Stop beating this guy up over his mistake, and have a little compassion for his dilemna. This is where a good engine builder actually SAVES you money by avoiding these problems in the first place.
I have one of these problem child's in the shop right now. A 496 stroked big block with Stage 5 iron heads and supposedly 10.5:1 compression. The cam was 302/308 advertised duration 244/244 @.050" on a 108 spread (custom grind btw), and couldn't tolerate more than 20 degress of timing. Turns out it was actually 11.6:1 and even with the long seat duration, it wouldn't run on pump gas, even with an octane booster. Once everything is fixed, it will have more power and driveability than it could ever gain from the ridiculously high static ratio. Unless the motor is injected and the timing is controlled by a computer and knock sensors, pushing the limits is just asking for trouble, imo.
Your right on that. I quickly just pulled that number from my last stroker build where the piston was .010 above the deck and it had 55cc chamber. But it has 230-240 cranking compression. That’s way above the 180 psi pump gas limit. And for the guy who’s motor pings at 9.5 to 1 something is very wrong. My street 360 has 11.14 to 1 and doesn’t detonate at all even with a 100 shot of nitrous. Mopar small blocks have a very good chamber design that is very tolerant to detonation. Putting better heads on a motor is not a “half *** jerk off” solution. It’s killing two birds with one stone.
You assumed he had "well over 13:1 CR" based on anecdotal evidence of your own motor, without knowing if his specs matched yours? Assumptions are what caused his problem in the first place.
I don't care what head is on the motor, putting a bunch of compression in a pump gas motor is not wise. I hope your 11:1 motor lives a long and happy life, but I certainly wouldn't want to be an internal component in that motor. Just because you don't hear it rattle doesn't mean it isn't rattling. Ever driven a late seventies 8.2:1 Mopar and listen to it sing it's death song?
A good chamber doesn't require 35-36 degrees of timing to make the best power, and that's what most SBM motors with factory heads need.
And one more time, volumetric efficiency is what determines cylinder pressure at any given rpm, not DCR.