SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: Ralph Brunt on December 13, 2008, 07:24:24 AM
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Excessive cylinder pressure will encourage engine destroying detonation, and no piston is immune to its effects. An important first step is to set the assembled quench ("squish") distance to .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance (the distance your piston is down in the bore). If your piston compression height (not dome height) is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of this piston that would contact a similar flat area on the cylinder head if you have zero assembled quench height. In a running engine the .040" quench usually decreases with RPM to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity across the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, and speeds flame travel after TDC to increase power. On the exhaust cycle, some cooling of this piston occurs due to the closeness of the hopefully cooler cylinder head. The power increase occurs because the shock wave occurs at exactly TDC on all cylinders, every time. It tends to make all cylinders alike and receive more identical flame travel speed. Spark scatter tends to be averaged with the TDC kick received from a tight quench.
Some non-quench engines, such as '68 and later Chrysler V-8's, can be converted to quench type with pistons such as the KB278, KB280, KB372, and KB373. Most Mopar cylinder heads recess the quench area into the head, so a raised area on the piston is necessary to get the close collision. If you are building an engine with steel rods, tight bearings and pistons, modest RPM, and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Unfortunately, the operating quench height varies in an engine as RPM and temperatures change. If aluminum rods, loose pistons (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit most of the benefits of the quench head design and can push the engine into severe detonation.
The suggested .040" static quench height is recommended as a good average dimension for stock rod engines up to 6500 RPM. Above 6500 RPM, rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or thick head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will likely create more ping at 9.5:l than you had at 10:1. One way to cheat the system is to make sure the piston of choice is light on quench side and to make sure the piston of choice is light on quench side and heavy on spark plug side. As RPM increases the piston tries to cock away from quench surface, allowing a tighter quench at most all RPM. The suitable way to lower the compression is to use a KB Dish Piston. KB Dish Pistons (reverse combustion chamber) are desinged for maximum quench area. Having part of the combustion chamber in the piston can improve the shape of the chamber and flame travel. The Step Dish is sort of an upscale version of our reqular configuration. It allows some piston weight reduction and allows the quench action to travel further across the chamber. It is especially favored when large dish cc's are required.
from keith black
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Good info on the KB site Thanks Ralph !
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ok i'll play, .005 in the hole and .038 head gasket. i will be zero decking the block before i run it again.
the liberty is 0 decked and a .038 head gasket.
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didn't they go out of business
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didn't they go out of business
who ??? and what does it matter, the info is still solid.
either nobody here knows what their quench is or its a big speed secret :-\
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either nobody here knows what their quench is or its a big speed secret :-\
Sorry Ralph, it been to long to remember the actual numbers (and it wasn't my job). But I do know I had a positive deck height (very scary looking but it works).
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I had .013 in the hole and a .026 thick head gasket on the 383 SBC that's sitting in my boat.
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Ralph heres some info that might help (similar to the KB tech) re KB btw , not sure that they are quitting business , just "reaaranging" least thats what I got last time I talked to my Buddy Jerry
Deck Height or Deck Clearance:
This most important step consists of measuring the distance from the assembled piston tops to the surface of the block deck (deck clearance), and milling as necessary. The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. As most head gaskets compress to around .042", that means we want about 0 deck clearance. The quench area is the flat part of the piston that would contact a similar part of the head if you had .000 assembled quench height. In a running engine, the .040 quench height decreases to a close collision between the piston and the cylinder head. The shock wave from the near collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. The shock wave also provides better fuel/air mixing, and this allows the fuel to ignite better and burn faster. A faster burning fuel charge means less timing is required for optimum power output. An example of this--after running my 462 for years with a factory deck height of about .020, we set the deck to 0. There were no other significant changes to the engine (new rings and bearings, but same cam, heads, intake and exhaust systems). The optimum timing setting prior to the change was 34 degrees - that provided the fastest MPH and quickest ET. After the change to 0 deck, the optimum timing using the same Amoco gas changed to only 30 degrees total mechanical. Not only did the lowering the deck raise the CR by several tenths of a point, but by retarding the timing 4 degrees, we were later able to increase the CR even higher due to the optimum lower timing setting.
Note: Since it is the close spacing between the piston and cylinder that reduces the prospect of detonation, never add a shim/head gasket, or flat cut the pistons tops to reduce CR. If you have proper quench with 10 to 1 CR, and then reduce the CR to 9.5 by one of these two methods, you will create more ping with the 9.5 CR then you had with the 10 CR. By all means, deck the block first and under all circumstances when building an engine for optimum power output, and then determine what chamber volume will be needed in the heads to arrive at the final CR.
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Ralph ,I should have learned by now never to sick my Son on a research "motor head subject" heres some more very interesting info ,he (Josh) is a bull dog when it comes to this stuff ! Dad ;)
http://www.popularhotrodding.com/enginemasters/articles/hardcore/0311em_power_squeeze/index.html
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Maybe I'm a crazy bastard ;D I run a 0.025" of quench on the last "high performance" motor I built.... sounds a little scary but it runs bitchin at 7500 rpm. And in my "boosted" atv motor I run .038", that I specked out as well on purpose. Good read Ralph
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Nice work Ralph. Last engine I built for a guy was a 427 chevy at .015. "OUT of the hole"
Simply used felpro gasket .051 thick 3piece to conpensate. Got it back to 0 deck
no problem. Think this block was decked a few times or what?
anyhow it beats the hell out of scrapping a bitchen block don't it ;D
It now sits in a 64/vette 4speed w/456 gears and halls ass >:D
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Last engine I built for a guy was a 427 chevy at .015. "OUT of the hole"
Simply used felpro gasket .051 thick 3piece to conpensate.
That's one Half Assed way to do it!!
Make sure you let us know when that chevy blows a head gasket!
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That's one Half Assed way to do it!!
Make sure you let us know when that chevy blows a head gasket!
What about when one sticks a piston out of the hole 0.015 on purpose, is that half assed? :-\
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Marcsollin... You are welcome anytime to come down and show me how to build engines and or
build the engines at customer specs... Other wise Do not insult my workmanship.
Educated responces on the other hand are always welcome.Thank you for your kind words
and have a good day. :)
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IN that scenario would it have not been acceptable to machine a "smidgy" off the tops of the pistons/quench pads to compensate for the deck - issue ? versus the HG ? just asking ?
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Absolutely Tom...and I suggested that option to my customer.,but he brought in his own pistons etc.
and these pistons based on the manfacturers did not recommend machining .
Not only that but his rod/piston assy was brought in to me already mounted/balanced along
w/the rot.assy...So what to do ::)
you and I both know that .020/.025 off the flats would have hurt nada... but the possibility of damaging the pistons during dismount existed and may have made matters worse, and of course re-balancing the assy was almost a certain
possibility as well...Take care!
Thanks Tom for your input :) :)
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::)