377Z

iTrader: (1)

I’m struggling with when the theoretically ideal quench clearance of 0.040-0.045” becomes important for avoiding detonation. At what static compression ratio is it necessary to run on pump gas? The block & rotating assembly is currently at the machine shop so I could have it zero decked; but I don’t like cutting metal when it’s not necessary. This was touched upon in several threads I have been a part of, but not to a degree that would make me confident with this issue specifically. From what I’ve learned shim gaskets are a bad idea, and dished piston motors (except d-dish) basically have no quench effect. Since the block is at the shop right now, if we establish that tighter quench is necessary I’ll have the block cut, instead of monkeying with shims. Without decking, the pistons are ~0.035” in the hole. Pistons are flat top, heads are iron. With a 0.039” head gasket, this puts quench clearance at ~0.074”. Now, if you go get some calipers and actually look at a 0.074” gap, there still must be some quench effect with this clearance, but will it be adequate in my situation? With heads at 67cc compression comes in at about 9.7:1. 0 deck would put me at about 10.4:1 with 67cc chambers. I’m sure I can gain a few cc’s in the chambers by being a bit more generous with the unshrouding if necessary, & chambers will be polished too.

To those who have built a few motors, what’s the highest compression you’ve successfully run with a larger than textbook ideal quench clearance on pump gas? Should I have the block decked, or do you think I’ll be okay? If I’m missing any critical info to answer just ask.

RB83L69

On the list of important things in order of importance, "quench" is not at the top. However, the higher you make the CR, the more helpful it is. It's probably a total non-issue at anything below 9.5 or so, and gets more important as the CR goes up from there.

Dished pistons inherently provide "quench" when used with a small piston-to-head clearance, by creating a shock wave forcing all the material into the center of the dish.

If you don't have pistons yet, the best of all possible worlds would be a small chamber head, a D-dish piston that approximately matches the head chamber shape, and zero-decking the block, and running the .039" head gaskets. That way you get the right CR (one of the things near the top of the above-mentioned list), the "quench" that you want, and (another thing at the top of the list) head gaskets that seal well and are long-term reliable.

For that matter, you could get some flat-tops, and have the dish you need milled into them. It doesn't take a whole lot of material removal to affect the CR strongly in a high-CR situation.For instance, a .050" dish over half of the surface of a 4.125" piston, is about 5-6 ccs. That's less than 1/16" of milling.

377Z

iTrader: (1)

Thank you.

Part of the reason I was still struggling is because I didn't really know how to phrase the question; your answer includes pretty much all the information I was looking for, and then some. The order of importance of things I particulary find useful. Also, I was not aware that it was acceptable to mill piston tops; thats cool because I had run across the info that its more efficient to have smaller chambers and a d-dish. I already have all the parts: pistons are L2256's (& they're awful beefy). I'll need to see if the machinist I use is capable of doing this, but first I'll cc the chamber and see where I'm really at with the CR and whether I need to take any action.



Aaargg. I did not destroke a 400! Sorry if I'm a bit sensitive. I really wish I knew that a bast ardized 400 was commonly refered to as a 377; I should have took the screenname 383Z (but I was a nerdlinger about the fact that I didn't overbore the block). I can't believe people who are not under any restrictions would actually shoot themselves in the foot and then argue that they then have more HP potential than the normals.

Thanks again for the info, an please pardon the rant, I had to get that off my chest.

377Z

iTrader: (1)

One more question:

If I were to have the piston tops milled, would the cut go across the entire "chamber area" of the piston top or is there a reason to maintain an unmilled "lip" around the edge? All of the factory and aftermarket dished pistons I've seen have this "lip." How come?

i.e. do I do it like this:

377Z

iTrader: (1)

or like this:

(blue area milled in both pic's, piston viewed from top)

RB83L69

More like the 2nd one.

Look through the JE or Wiseco or Manley piston catalog, look at what they call a "reverse dome". It's basically a "reflection" of the head chamber.

People mill domed pistons alot, for CR adjustment or whatever; and of course, for valve clearance. THere's no reason you can't do it. Mostly, you'd want to make it as consistent as possible from one to another, for consistent CR and also balancing; and you'd have to remember that the cyls are turned opposite ways, so you'd have 2 left-bank pistons that would be "left-handed" (1 & 5 for example), and 2 others that would be "right-handed" (3 & 7); and the same on the right bank.

377Z

iTrader: (1)

Thank you.

Auggie

If you mill the block deck surface .035" to get "O" deck remember that you will probely have to mill the intake surface on the heads to get your intake manifold to fit. Just my .02!

Auggie

RB83L69

True dat, any time you mill anything, you need to at least check your intake.

Don't deck your block until you know exactly how tall the pistons you're going to use are.... they're not all the same, by any means. Plus, rod length varies; stock rods get shorter every time they're re-sized for example.

Kind of like "What length push rods do I need?" The answer is, whatever your engine asks for, that's what it needs. You can't really predict it unless you have accurate measurements of all the parts.

As far as milling the block, I'd recommend buying all your parts first; then install the crank and the pistons and rods without the rings, and measure how far they are down in the hole; and mill the block exactly that much, or at least, down to a deck height of exactly 9.000", I wouldn't go any farther. Leave the heads as they are if possible. Then check the intake.

377Z

iTrader: (1)

Yeah, I took the whole shortblock to the machinist because I was getting different readings for depth in the hole not only from cylinder to cylinder, but the readings differed slightly at front and back of the same piston along the pin c'l in several holes. I'm kinda new at this so I don't know if the parts are screwy or if its due to operator error , so I figured it was time to hand it over to a professional. The rods are resized stockers, crank was a new import (originally thought it was a stock crank; machinist says its chinese).

In light of the info on the role of "quench" clearance wrt CR & the duty this motor will see:

...the more I think about it the more attractive dropping compression to about 9.5:1 becomes. Adding a d-dish to the piston tops seems to be the best way to do it unless this block's decks are reeeally on the high side of the tolerance. I think achieving 0 deck height will still require cutting the pistons to get under 10:1 (must run on pump gas) & there won't be enough payoff (power) to justify it based on the rest of the combo; its for a weekend street car so I doubt I'll notice a couple tenths of a point of compression. I'm just hoping my machinist is capable of this because it may be a big hassle to take them somewhere else. I'll see what he tells me; I haven't had a chance to talk to him since I dropped it off and if something is wrong with the hardware, I'll need to replace parts & start over.

Thanks again.

377Z

iTrader: (1)

...or I could stop overlooking the obvious and look into a head gasket that's a bit thicker installed.



Using 0.035" for piston depth, 0.050" installed gasket height, L2256 pistons, and 67cc chamber, CR works out to just under 9.5:1.

Any potential problems with this? I would assume slightly thicker gaskets would seal well, true?

Auggie

Here's what I did.

400 block bored .030" and decked to 9.010".
Bore = 4.155"
Stroke = 3.750"
Rod length = 5.7" (stock 350)
Cyl. head CC's = 75
Head gasket Thickness = .039"
Head Gasket Dia. = 4.190"

Forged Pistons = Manley ( L2352-30)(stock 400)
Piston Head Thickness = .280"
Piston Pin Dia. = .928
Piston Comp. Dist. = 1.560"
Top Ring Dist. to the Top of the Piston = .263"
Piston Dia. at the Top = 4.125"

So: 5.7" (rod C to C dist.)
1.560" (piston comp. dist.)
1.875" (1/2 crank stroke)
----------
= 9.135" minus 9.010" = .125" to be milled off the piston
top for "0" deck.

With this set up you would have a C/R of around 11 to 1. Not good for the street.

So: .280" (stock piston head thickness)
-.125" (amount milled off of the piston)
----------
= .155" (now piston head thickness)
-.030" (Depth of milled "D" shaped dish)
--------
= .125" (new piston head thickness)

So: Cyl. Head CC's = 75
Piston Dish CC's = 3.30 cc's
Head Gasket Thickness = .039"
Head Gasket Bore Dia. = 4.190"
Deck Height = "0"
Piston Ring Depth = .138"
Piston Top Dia. = 4.125"
Piston Comp. Height = 1.435"

This will equal a Dynamic C/R (the most importent one) of 7.81 to 1 with an intake opening of 74*. The Mech. C/R will be 10.52 to 1.
This engine has ben running on the street with iron heads and a tight quench (.039) on pump gas (92 oct.) sence 1991 and really stroung I might add. The Dyno numbers are as follows: 421 hp at 5200 rpm with 470 lb. ft. of torque at 3700 rpm. This engine idles at 650 rpm and passes emissions.

Auggie

AllGoNoShow

I'm running .072-.078 quench on a 8.9-9.0 Compression small block with a 246/262 advertised cam which puts Dynamic Compression way high. I keep it cool around 180-full timing(36 in before 3K) and it runs like a champ on 93. I wouldn't be too worried.

OMINOUS_87

Depending on your budget you might get some good work done while the motor is ripped apart.

.030 over with TQ plate hone.

Zero square decking.

Lastly the right pistons.

Just my .02 though.

377Z

iTrader: (1)

Ominous, I'd totally agree with that if my goals were for a wilder combo, but I already have all the major parts and I can live with compression in the low/mid nines. So far the only reason I’ve seen to zero deck is to achieve “proper” quench clearance which should play a very minor role at this level (from RB above). I’d rather go a bit conservative on the compression this time around anyway because I don’t want to end up with a motor that needs to run rich/get shortchanged on timing or some other crutch to run on pump gas.

The cost of an overbore, plus new pistons is too expensive a way to gain 6ci for me, I should be okay with just a hone.

From what RB and Auggie's responses, it seems like sometimes all that separates the wrong piston from the right one for the application in question is some machine work, plus I like the idea of being able to taylor yet another thing to suit your own application.

AllGoNoShow; depending on the other specs of that cam, you may no be too far from a factory motor (which I suspect do not typically have 0.040-0.045" clearance) in terms of DCR, have you ever worked it out? Did you have issues with lower octane gas?

DTL504

iTrader: (1)

Read This:
Quench

Use this calculator by Pat Kelly to determine your quench. Scroll to bottom of page and download to your computer.
DCR Calculator

AllGoNoShow

I have Pat Kellys DCR calculator and I know the part# on the pistons, have calculated deck height, have roughly measured combustion chamber volume(which turned out to be 2-3cc bigger then I thought) and I get 9.0 Compression and 7.99 Dynamic Compression ratio....ask anyone and everyone usually says that 7.99 DCR with .07+ quench on iron heads will be ping city but I have no problems even with agressive timing curves.

I have tried 87 octane when I was young and stupid and didn't notice any adverse effects but I'm not sure I would have at that age, plus I was not running an agressive timing curve at all.

I really think a key to a strong mild motor like mine is the high DCR...In my 81 camaro that wieghs 3800LBs plus I've gone as fast as 14.1@98.2mph with a 2.1 60ft time(get it to hook and easy 13s) and I get 18-19mpg on highway with a double pumper carb.

377Z

iTrader: (1)

Cool, thanks for the info guys. I think I'll shoot for around 9.5:1 static; I figure I can play around with different cams a bit if needed once the motor is running, though hopefully I won't need to. I'll also be running FI so I should be able to keep close tabs on knock once I get some data logging software and such. I figure the motor's hardware is only a third of the big picture, with the other two thirds made up of the rest of the drivetrain/suspension and proper tuning. I don't have a specific ET goal, I just want to make the best use of what I've got.