Gene Fiore
Famous Member
I keep hearing that it's a good idea to zero deck your block to improve quench...and presumably make more power. What is quench and how does it affect power? 
Quench
- Thread starter Gene Fiore
- Start date
Gene, the area ahead of the valve reliefs on the flat of the pistons is the main area of concern when speaking of quench. However, any flat area around the entire surface of the piston with the exception of the valve relief area can be considered in some way a quench area for those that will try to correct this desription. It is usually the part of the piston that actually sees the tightest compression space with the flat of the deck of the head. It gives the firing gases its "pop", so to speak. You could probably compare it to an air gap, and the closer or tighter the gap, the tighter the gases are compressed in that space. It is a very worth while modification to do, and the results will be extremely beneficial to any engine, whether street or race. I would highly recommend you do it. You can easily see that if you were to sit down and try to develop an engine package for yourself or someone else, that the more flat area in the quench, the better the compression of the gases. So pistons with a dish or champher around the circumference are going to be less likely to produce any significant quench. However, dish pistons that mirror the combustion chamber are very effective just like a flat top piston is. So selecting the correct style of piston is critical for an engine package to produce its peak power , and is just as important as selecting the right camshaft.....hope this helps.
8) what the quench area does is provide turbulence in the combustion chamber forcing a more complete burn of the fuel mixture. this is one of the reasons that the early hemi's were not a great street engine, but when you added a blower you really woke them up, as the early hemi's had little to no movement of the fuel mixture.
Howdy Gene and All:
"Quench" is the relationship between the flat, uppermost part of the piston- not the dish, and the flat, head surface- not the oval combustion chamber. When the flat of the piston comes up to the flat surface of the head any A/F mixture lingering there is squeezed out at a high velocity, creating turbulence, which improves combustion. The distance, at the closest point needs to be .040" or less to generate an effective quench effect. Some clearance must be maintained to allow for wear, piston rock and heat expansion.
Our production engines have several strike against them when it comes to maximizing quench effect. 1st is the amount of deck cleanance, which includes the compressed thickness of the head gasket and the deck height of the piston, or how far the top of the pistons are below the deck of the block. With modern, aftermarket composite head gaskets measuring .045" to .055" thick. The other factor- deck height, is typically .025" on a stock 200, much more on a 250
2nd is the miss-match of the oval shape of the stock combustion chamber to the round shape of the dish in the OEM pistons.
3rd would be the fact that most aftermarket pistons, in overbore sizes are typically down on deck height some amount. If you don't know that and deal with it on a rebuild, clearances get bigger.
Ideally, for the highest ratio of quench, you will need a very small chamber in the head, most notably a "Heart-shaped" chamber, and a piston dish to match that shape. You will also need a very clear understanding of the dimensions of all parts to keep a tight quench of between .022" and .034". Is it worth it? Almost all modern engine makers think so. An idealize quench is not only more powerful, but more efficent and economical too.
How to achieve max quench in our engines? Start with a small chamber, pre-'72 head, unshroud and reshape it. Next acquire a set of HSC 2.3 flat-topped pistons and machine a dish into the tops mirroring the shape of the new combustion chambers. Now, how to maximize the rest of the engine????
Thanks for asking the question Gene.
Adios, David
"Quench" is the relationship between the flat, uppermost part of the piston- not the dish, and the flat, head surface- not the oval combustion chamber. When the flat of the piston comes up to the flat surface of the head any A/F mixture lingering there is squeezed out at a high velocity, creating turbulence, which improves combustion. The distance, at the closest point needs to be .040" or less to generate an effective quench effect. Some clearance must be maintained to allow for wear, piston rock and heat expansion.
Our production engines have several strike against them when it comes to maximizing quench effect. 1st is the amount of deck cleanance, which includes the compressed thickness of the head gasket and the deck height of the piston, or how far the top of the pistons are below the deck of the block. With modern, aftermarket composite head gaskets measuring .045" to .055" thick. The other factor- deck height, is typically .025" on a stock 200, much more on a 250
2nd is the miss-match of the oval shape of the stock combustion chamber to the round shape of the dish in the OEM pistons.
3rd would be the fact that most aftermarket pistons, in overbore sizes are typically down on deck height some amount. If you don't know that and deal with it on a rebuild, clearances get bigger.
Ideally, for the highest ratio of quench, you will need a very small chamber in the head, most notably a "Heart-shaped" chamber, and a piston dish to match that shape. You will also need a very clear understanding of the dimensions of all parts to keep a tight quench of between .022" and .034". Is it worth it? Almost all modern engine makers think so. An idealize quench is not only more powerful, but more efficent and economical too.
How to achieve max quench in our engines? Start with a small chamber, pre-'72 head, unshroud and reshape it. Next acquire a set of HSC 2.3 flat-topped pistons and machine a dish into the tops mirroring the shape of the new combustion chambers. Now, how to maximize the rest of the engine????
Thanks for asking the question Gene.
Adios, David
I'll jump in on this one.
Quench chambers increase low to medium torque, but increase emmisions. They are identified by flats on the head, whereas an open chamber head generally has a recessed chamber diameter about the size of the piston. The flats are called quench pads. They are primarily designed to quench the temperature of the end gas and compressed mixture by increasing contact patch area with the water jacket, making for less detonation and thus a better flame propogation with decreased ignition advance requirement:- although not entirely true the greater the quench area the less maximimum power ignition advance required. The greater the quench area the less the knock tendancy.
The quench pads secondary advantage is that they act as squish pads.
The distance between the piston at top dead centre and the squish (quench) pads is called squish or squish height. It derives it's name from squish top pistons, which are the dished types as opposed to the flat tops. The squish top pistons are advantageous in being able to develop greater IMEP with lower octane fuel than flat tops with higher octane, although build pressure will lag . The idea is that the mixture is squeezed into the dish which reduces the amount of fuel in the initial flame path, reducing flame velocity, then releasing the stored mixture later in the cycle causing a rapid increase in cylinder pressure. Turbo engines can benefit from squish pistons.
There are various rules of thumb, but the general one is that the gasket provides the gap = squish height. The actual clearance should actually be less than 0.005 times the bore diameter, which means some of the old chestnuts are in fact nonsense. For example a 4" bore would need a squish height of 20 thous or less. Don't be shy installing pistons overdeck.
Quench chambers increase low to medium torque, but increase emmisions. They are identified by flats on the head, whereas an open chamber head generally has a recessed chamber diameter about the size of the piston. The flats are called quench pads. They are primarily designed to quench the temperature of the end gas and compressed mixture by increasing contact patch area with the water jacket, making for less detonation and thus a better flame propogation with decreased ignition advance requirement:- although not entirely true the greater the quench area the less maximimum power ignition advance required. The greater the quench area the less the knock tendancy.
The quench pads secondary advantage is that they act as squish pads.
The distance between the piston at top dead centre and the squish (quench) pads is called squish or squish height. It derives it's name from squish top pistons, which are the dished types as opposed to the flat tops. The squish top pistons are advantageous in being able to develop greater IMEP with lower octane fuel than flat tops with higher octane, although build pressure will lag . The idea is that the mixture is squeezed into the dish which reduces the amount of fuel in the initial flame path, reducing flame velocity, then releasing the stored mixture later in the cycle causing a rapid increase in cylinder pressure. Turbo engines can benefit from squish pistons.
There are various rules of thumb, but the general one is that the gasket provides the gap = squish height. The actual clearance should actually be less than 0.005 times the bore diameter, which means some of the old chestnuts are in fact nonsense. For example a 4" bore would need a squish height of 20 thous or less. Don't be shy installing pistons overdeck.
CZLN6":38pqh3hi said:
I'd just caution unshrouding valves at the detriment of the mixture not entering tangentially to the cylinder. Sure the exhaust valves are not so much a concern, but the banking height on the inlet valve should be maintained. The swirl that that subsequently occurs helps to maintain uniform compression/density, especially with peanut quench heads.
Putting flat tops on a quench head will cause the cylinder pressure to rise rapidly early in the cycle and reduce peak pressure. It does however allow for earlier blowdown and thus longer duration cams.
