Need some help.....

[RustyRatRod]

But the KB400 isnt designed for the 516 head either is it ? And when figuring compression your not calculating at zero deck, your adding the positive displacement of the dome.

But yes I understand how the flat portion of the piston needs to be at zero deck to properly measure how many CCs the dome would take up.

The flat portion is also where the quench action takes place. Since the dome is opposite the head deck, the flat portion is the quench area. With a 9.980 (stock) low deck deck height, the piston sits down in the bore .024" (measured from the flat quench area), with the dome protruding out of the bore. This is why I am going to zero deck height the block. Then the head gasket will be the quench distance at .043". That's perfect. The dome on that piston is only .200". That's not very tall. It only measures -6ccs. While it's true that a domed piston will not be as efficient as a flat top in a quench situation, it will still work fairly well.

All of the calculators I have used online say around 10.5:1 static compression and 7.7 dynamic compression. The threshold for a pump gas motor is 7.5-8.5 dynamic compression. Also, that does not take into account any quench that will be helping against detonation. I could probably go even higher on the dynamic compression, but I am not going to chance it. It will work great just like it is. Might even run on 89.

 

[RustyRatRod]

A word of caution, most online calculators require you to enter a negative number for a pop up (-6cc) and a positive number for a dished piston.

That's right. I did it backwards once or twice and wondered where all my compression went. lol

 

[Garys1969RR]

Hi Rusty, I like calculating the C/R on these engines, so I checked your engine using the .024 deck height of the KB 400 piston, and a .020 steel shim gasket. You still get .044 quench distance, and C/R comes out to 10.47 to 1. And you dont have to spend the money to deck the block. Also I think what ever heads you use will need to have the intake face cut also, if you cut the block deck .024. So that the intake manifold ports and bolts line up properly. Just a thought. Good luck.

 

[RustyRatRod]

Thanks dude. I agree...but here's the thing. Chrysler was horrible about deck heights. Knowing that going in, I am gonna have it decked to at least blueprint spec.....so I may as well go ahead and go zero. It's always been my thought that not having any bore above the quench area of the piston helps.....I might be slam off base, but what the hell? It sounds god, right? lol Plus, I will know beyond the shadow of a doubt that the gasket thickness is the quench distance. I'll know more when I get it all together and start mockin it up. I'll start a build thread when I do. It should be fun. Thanks for the interest yall. I appreciate it and the advice too.

 

[RustyRatRod]

Adam, maybe this will help. This quench stuff is new to me, too. This is the basic concept using a domed piston. It's somewhat oversimplified, but I think you can get the idea.