deck height / quench

[gwcrim]

I'm building a 400 from a short block I bought from my neighbor. He claims it has very low mileage (less than 1000 miles) and I have no reason to doubt him. It's got 20 over Keith Black flat tops. Piston depth varies from 0.043-0.057. It's a pretty big variance but not my main issue.

I've been looking at Promaxx heads. Promaxx is the reorganized Patriot. They offer a head with 72cc chambers. If you run the numbers with a 0.04 head gasket you get about 9.5:1 compression. That's perfect for my needs.

My concern is that there will be a total quench height of 0.083-0.097. That's excessive. If I run a cam that keeps dynamic compression at a reasonable level, will that much quench cause pinging?

Any other concerns?

This is going to be a street engine, not to be run on the ragged edge.

 

[350runner]

Subscribed.

Sent from the dust in front of you!

 

[SkinnyG]

In my research, I found that you want about 0.040" quench.

My 350 has the pistons 0.025" in the hole, and I chose a 0.015" head gasket to give me 0.040" quench.

I run 10.9:1 with iron heads, and it survives ~because~ of the quench (maybe the cam as well). Better engine builders than me have said that if I went for a much thicker composite gasket, even with a lower compression ratio (9.8, I think it would have been) my engine would have been detonation prone.

I'm no pro, but the engine works on 91.

 

[gwcrim]

I remeasured tonight with a helper and got more consistent readings right around .042. Not great but the variance is .0022. I can tune around that I suppose.

The Summit calculator puts it at 9.74:1. I'm starting to think a bit more conservative with the cam.

 

[SkinnyG]

If you still can, why not take the block out and get it decked so that you end up with .040" with the head gaskets you want to use? (Composites tend to seal better than the steel shim I am using).

Yeah, it'll cost some machining, but the quench will be good.

Hopefully you won't have to have the intake milled as well.....

 

[scottybaccus]

The reason for the variance is careless rod resizing in the last rebuild. Engine shops often just touch the rod to a sander to slightly shrink the big end bore, then the hone it back to size for the rod bearings, without any consideration of the variance in rod length. For a stock rebuild, daily driver it is fine.
If you want better, you are faced with resizing the rods properly, which shortens them more, or replacing them. Several years ago, it was less than $40 difference between resizing and a new set of SCAT rods that were weight matched and sized, with ARP bolts. I just threw my old rods away. I had a variance of about .012 across all 8.
After you have installed the new rods (or resized), you can measure to see what your variance is. If any, it will probably slope from one end of the deck, to the other. Your machinist will need to know this when decking the block in order to true it up. This part can work wonders for valvetrain alignment.

I went with a deck clearance of .008 and a .028 gasket, for .036 quench. Be sure to measure valve clearance to the pistons, with the cam properly timed, using silly putty or clay. I lightly torque the head, without a gasket, and add the gasket thickness to anything I measure. I get pretty uneasy if anything touches without the gasket, but a skilled builder may go tighter.

 

[gwcrim]

This is a build on a budget. I'm not looking for all out HP. Just a nice hot rod motor. So.... Friday I ordered a set of Promaxx heads as I had none at all. Got them direct with steam holes drilled for $825 shipped.

I'll make sure the cam is good for the combo. Don't want to build too much cylinder pressure. Pump gas required!