[242177P]

Originally Posted by slowbird

For everyone saying .050 to .100 is the danger zone, why would GM set up their LS engines in that danger zone? Wouldn't it be wiser of them to stay away from it if it really is an issue?

Simple answer, when you think about it. LS engines have
knock sensors. Without them, GM might've had to lower
compression ratios across the board. Conservative tune?
Perhaps both?? MPG and emissions would take a hit too.

[steve25]

It depends on the heads.

With the LS the engineers knew they had enough flow to meet there target hp goals and they could have got more flow if they gave up some swirl, but the swirl contributed a lot to low speed throttle response and to real good fuel usage .

The swirl made up in part for the deck clearence.

This all being said and in terms of the basic Pontiac wedge chamber, the lower the motors compression sinks below 9 to 1 the more the motor reacts positively to a tight deck clearance, and yes even the heart shaped wedged chambers in aftermarket Pontiac heads.

[Half-Inch Stud]

Applying some fact and logic:

1. You got an Engine pinging.
2. You back the timing off and the pining diminishes greatly, even gone.
3. You tink about piston position for 34* BTDC and realize the quench is closer to 1".
4. You tink about piston position for 26* BTDC and realize the quench is closer to 0.5"

5. You get mad because the quench cannot be directly blamed for pining...yet.
6. So you figure the ping is involved in the kernel's flamefront PROCESS being "interrupted".
7. So you figure there must be a critical time in the kernel development that either compression too high (pressure, temperature, volume, charge count) or kernel is interrupted.
8. So you blame the quench pad system. Driving along you ponder the deal for ping-free power.

9. Oh, but other potential issues arise:
9a.mixture leanness
9b, 87 octane, or what you've tried.
9c. Orange Glowing Plug ground strap. Plug Heat range. Running temp. Hot irn manifold sinking heat into head.
9d. Hot edges like valve reliefs, chamber cuts, CYL top edge, piston top edges.

10. You take time to optimize all of the above, and it still pings.
11. Still pings when timing is advanced for responsiveness and power.
12 you second-guess your cam's small duration and very high LSA.

13. You go to PY, find this Thread, and realize a few tings:
The cam does matter. You ponder why cams don't advertise their propensity to ping.
Items 1-10 matter, but the cam matters too.
Also, the chamber shape & piston shape must be a system that matters.

So you conclude that proper research, a belt sander porting tool, and a teardown is needed to optimize the chamber-piston system shape, and that'sa why you read this far.

[Stan Weiss]

Originally Posted by Half-Inch Stud

Applying some fact and logic:

1. You got an Engine pinging.
2. You back the timing off and the pining diminishes greatly, even gone.
3. You tink about piston position for 34* BTDC and realize the quench is closer to 1".
4. You tink about piston position for 26* BTDC and realize the quench is closer to 0.5"

Mark,
In a Pontiac 428 those distance are less than half of that.

Stan

[performerrpm]

Originally Posted by slowbird

Just conversation sake, GM LS7 stock is around .060 with a compression ratio of around 11:1. Why wouldn't GM tighten it up more?

ive been working on a ls7 z06 the last week or 2 replacing a head from a dropped valve.

i didnt think to measure deck clearence because my part was just put this basket case back together....had a new head ready to go. when i was looking up head gaskets....stock is .051 mls gasket. i thought the ls7 pistons sit slighltly above the deck tho like .005 to .008 or therabouts putting the quench at .04x?


i wont swap my pontiac to an ls...and the ls7 has some trouble areas....but those motor flat get with it even stock

[Jay S]

Where is the information coming from that LS engines do not have very good quench? All the LS engines I can think of are about .040” to .045”, not .060”. Generally the pistons on most of the LS engines are slightly above the deck .005” to .007” and they have a .051” head gasket..Quench is tighter than .060”.

I have a 461 which the pistons are .015” down with a .039” Felpro for .054” quench. It is a mid 11s SCR iron head engine on 91 octane., 6X-4 heads milled .015”, 92cc, with 15cc dome pistons. I shoot for .040” when I can, but am comfortable with up to .060”. On the engines I have done .07” to .09” are bad areas to be in, we made them work but tuning seemed more difficult. For above .07” quench and try to use a cam that opens the exhaust valve earlier. I have done 9.8:1 on a 510 CID on 87 octane with an open chamber iron head with no quench at all.

[Stan Weiss]

Remember that at any give CR. The squish velocity is a function of not only the squish clearance, but also the squish to bore ratio.

Stan

[Jay S]

Originally Posted by Stan Weiss

Remember that at any give CR. The squish velocity is a function of not only the squish clearance, but also the squish to bore ratio.

Stan

That is complicated to predict when it is a function of multiple inputs.

[Jay S]

At 8.0 compression a .020” deck height adjustment to tighten quench on a 461 changes the static compression about .2, which is helpful. At 9.5 compression on a 461 at .020” deck height adjustment is 1/3 of a point of compression, makes it about 9.8. For a pump gas combo, no big deal, probably should go for it. On an 11:1 engine the same adjustment is 1/2” point up to 11.5. Not as warm and fuzzy of a mod as it was at 9.5. Somethings to consider when the engine is dancing with the devil on pump gas.

[grivera]

Originally Posted by lust4speed

The stock engines I've torn down have all been .005" to .008" in the hole, and I haven't come across deeper ones. Maybe just luck of the draw.


The couple tech papers that I've read indicate that there is a no-man's zone between .050" and .080" that is very prone to detonation, and staying below or above the zone is necessary. This would explain why hemispherical heads are fine. The math also says that with a standard .039" thick head gasket then the piston should be no more than .011" down in the hole before additional risk of detonation occurs. Something gets nasty with flame travel in that zone that doesn't happen with less or more clearance.

With factory head gaskets at .045, seems they'd be detonation prone as built.

[steve25]

We really also need to be talking about the rate of cylinder pressure rise as it changes with a given motors piston speed when talking about what makes a motor detonation prone or not.

Just like your work shop air compressor when it’s running it creates hot air all on its own

[Half-Inch Stud]

Originally Posted by Stan Weiss

Mark, In a Pontiac 428 those distance are less than half of that. Stan

34* is 0.44" & 26* is 0.26" per your chart. I was winging it wrongly with 1" & 0.5" by 2x.
"oh yea the piston velocity is slowing down on TDC approach and departure.