The closer the piston gets to the head the more it squeezes the fuel/air into the combustion chamber/dish area. Better mixing and less area for the flame to travel. The standard theory is .050" or less between the piston and head. Compressed gasket .039" + .010" below deck = .049". At zero deck it's just the gasket thickness. The piston rocks so you can't go too tight. I doubt you would notice a difference between .010 and zero.

 

Completely related, remember when the Powerblock guys did the 300 build up? They used a flat top piston, but the compression height put them, I think, ~.25" below deck. So they actually managed to drop the compression ratio with a set of flat tops.

Moral, figure out where you want the piston in the bore then find a piston with the right size compression height and dish to get the CR you want then have the machinist measure the block and machine it.

 

If it came out of a 78 then it's probably a D5TE block.

Zero decking improves the quench height. Which helps give you a more complete burn of the air/fuel mixture and also helps reduce detonation. Zero decking with a Fel-Pro 1024 headgasket gives you a close to perfect quench height.

 

Great information! But also consider (I may have tuned in late and missed it) the volume of the c.chamber in the head, and the volume of the D recess/round recess or valve reliefs in the piston top. Quench kind of goes out the window with a piston with a 22 cc dish, does it not? I mean, if you deck the block to get a nice tight quench, and then have a piston with big dish ... what is the benefit?

I found it very difficult to find a piston that would get me in the .040-.050 quench range, yet maintain a 9 ish c.r., without using a piston with a huge dish.

I had a thread on the other forum and was discussing it with pmuller. We threw out some good articles, tech magazine articles, and he mentioned that the quench range of .040 was 'old thinking', and ideal for a high revving engine with higher c.r. than our engines, and that a quench of around .052 was more homey for a dd 300.

 

No quench doesn't goes out the window because of the dish. There is still the area of the top of the piston around the dish. And the flat of the head deck surface, that is outside the combustion chamber but inside the cylinder area. So you still have quench. You don't need a flat top piston to accomplish it. When the whole piston is down below the deck surface you have a whole lot less quench.

It's your engine so build it how ever you want.

 

My point is this: Quench is the distance from top of piston to bottom of head. It is used or mentioned, to my understanding, as a rough guide for having a nice, compressed charge of a/f for the spark. With a 300, quench is nothing more than a measuring concept used and adjusted to determine c.r.

Quench is fine and good, but when it comes time to decide how much to shave a block or head, and to buy pistons for a dd 300, it is cr that determines quench.

 

Quench is completely different from the compression ratio. You can have two similar engines with the same c.r. but very different quench distance. It's not something to stay up at night and worry about on our engines. But when I'm doing a full rebuild and am already having machine work done to the block I personally prefer to zero deck it.

Plus as a bonus zero decking removes one variable out of the formula for figuring out the compression ratio.

 

Squish and CR are related only in the sense that distance below deck affects both.

No, a big dish isn't optimal for squish - that would mean a domed piston matched to the chamber. Set CR aside and I would choose a large dish at zero deck vs. a flat top .040" below. Again though, I don't think there will be a noticeable difference between zero and .010".

 

AB: Begin by having your machinist measure your block, b/f you think about pistons or q. or c.r. Know first where you're at. Then, plot where you want to go with the q., piston, c.r. equation.

 

As far as the compression height, quench, etc. I honestly don't think I'm too worried about it if it's one of those things where the difference is there, but it's only subtle. I'm making a daily driver / offroad engine and not a racer, so I probably wouldn't even know what I'm missing.
Does zero decking remove any sort of reliability from the engine? Valve clearance? Durability? Or is it simply a matter of not being able to resurface it again should I need to?

Either way, he should be boring it this week, so I'll be able to get some actual numbers and measurements from him here soon.

 

No need to worry about valve-to-piston clearance on a stock head. Plenty

 

A safely-tight squish (say .035-,042" street, .045-.048" street performance), assuming the piston/head actually have some squish area, allows you to run a little more compression than you can with an ineffective squish or no squish. The squish head was Harry Ricardo's anti-detonation invention, and a great gift to engine designers (until the small amount of unburned HCs became an emissions target, resulting in the gutless no-squish, low-compression smog motors of the Seventies and early Eighties). Squish (for him, Quench) got SIR Harry knighted by the king.

 

Thanks for all of the great information everyone. It really helps in my decision making.

I'd forgotten about the cam degrees. If I go with an aftermarket cam, am I going to have to worry about that? Or would cam gears that correct that still be worth looking into?

Also, I DO still have to pass emissions and am able to do so with my current 4bbl setup. How much is changing that going to screw with it? I only have to pass the sniffer test on HC and CO, if I remember right.

Heard back from the machinist yesterday. Crank cleaned up at .010/.010 and the block at .030. So it's ready to get going.

 

Most aftermarket cams have 4° advance ground in and should be installed straight up. The difference between intake centerline and LSA is the amount of advance (or retard) ground in.

 

Well, went over to the machinist this morning and spoke with him. Also, took some pics of the current progress. It's been bored out, but not honed yet. He also said the center line was in spec.









The rods were out of round so they need to be reconditioned. I wish I had a set of forged rods, but I have no idea how long it'd take me to track some down.

Otherwise, I was discussing the head. He said he actually recommended the head with the pressed in studs vs. the newer pedestal rockers. He said since I'm using an aftermarket cam, shaved head and/or block, etc. with different dimensions than stock, it would allow a lot greater control over valve adjustment.

Thoughts on this? I like the design of the pedestal rockers. First, they're bolt-in, so they seem stronger. And second, their motion seems so much smoother. I remember reading multiple times that the older style was prone to stress and cracking.

As for Port and Polishing, he said it generally runs around $600, which seemed a tad high. However, it seems like they do some quality stuff, as they provide flow rate data when they're done.

Next step is to bring a head down as he wants to measure it and see how much to shave/deck to get the preferred compression ratio.

Moving forward!