Thread: Domed pistons and quench?

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I've never used domed pistons so I've never had a need to know this but can someone explain if you should be concerned as much about quench when using domes? If you go with .035"-.045" then the head combustion chamber will have to be the proper or 'matching' CC's? Or do domed require more quench? And,,,,if the pistons are designed to provide say,,,12.5:1 with 64cc then using a 76cc would give too much quench? Is this right?

Originally Posted by FAYLUR

I've never used domed pistons so I've never had a need to know this but can someone explain if you should be concerned as much about quench when using domes? If you go with .035"-.045" then the head combustion chamber will have to be the proper or 'matching' CC's? Or do domed require more quench? And,,,,if the pistons are designed to provide say,,,12.5:1 with 64cc then using a 76cc would give too much quench? Is this right?

You probably wouldn't be concerned about quench with domed pistons because you'd probably be using a fuel that would be of sufficient octane to prevent detonation in the first place. The quench takes place between the flat crown part of the piston and the flat part of the cylinder head, opposite the chamber. With a dome in the way, the mixture couldn't "jet" across the chamber the way it can with a flat or dished piston.

" if the pistons are designed to provide say,,,12.5:1 with 64cc then using a 76cc would give too much quench? Is this right?"

No, you're confusing static compression ratio with quench. Quench is the clearance between the piston crown and the cylinder head at top dead center and is set at between 0.035" and 0.040" if using steel rods. The idea is to "jet" the mixture across the chamber to mix it up for complete combustion and also to eliminate any "dead" spaces within the cylinder/chamber.

If you had a piston that was designed to give 12.5:1 static compression ratio with a 64 cc chamber, then changing heads to use a 76 cc chamber would lower the static compression ratio to 10.6:1. The quench would remain the same, as long as you used the same piston and the same deck height.

Thanks Techinspector1. So quench would remain the same as long as compression height and chamber cc remain the same. So I guess what I am trying to figure out then is how do professional engine builders determine what dome size would be best for the application,,,,by cam selection? And cam selection is determined by RPM operating range (I think)?
I guess I don't understand why engine builders use a paticular size dome,where in the equation does this come into play? Just to provide a paticular static CR? If so,then why that paticular static CR?

Originally Posted by FAYLUR

Thanks Techinspector1. So quench would remain the same as long as compression height and chamber cc remain the same. So I guess what I am trying to figure out then is how do professional engine builders determine what dome size would be best for the application,,,,by cam selection? And cam selection is determined by RPM operating range (I think)?
I guess I don't understand why engine builders use a paticular size dome,where in the equation does this come into play? Just to provide a paticular static CR? If so,then why that paticular static CR?

Quench is set with the piston deck height and gasket thickness. Piston deck height is the measurement from the piston crown to the flat deck surface of the block with the piston at top dead center and has nothing to do with the chamber cc's. When building a motor, you would bore and hone the block for the oversize pistons, then mock up the crank in the block with main bearings and bolt up the rod/piston assemblies to the crank with rod bearings, but without piston rings. You would then turn the crank so that each piston would be at top dead center, then measure the distance from the piston crown to the block deck. This can normally be done with just four piston/rod assemblies, one at each corner of the block. On a small block Chevy, this would be cylinders 1,2,7 and 8. On a Ford, cylinders 1,4,5 and 8. Having these measurements, you would then cut the block decks to achieve the block deck height that you wanted to achieve. It would be very unusual to find a block that is square corner to corner and I have found some over the years that were off by 0.008". That is to say, for instance, that cylinder 2 might have the piston down in the bore at 0.025" and cylinder 8 (on a Chevy,passenger side of the block) might have the piston down in the bore 0.033". In order to square the block in this instance, that deck would be cut 0.008" more on the end of the block where cylinder 8 is. Let's say that in order to clean the entire deck on one side of the motor, you might take off 0.005" at cylinder 2 and 0.013" at cylinder 8. This would result in the measurement of each piston on the passenger side of the block having a piston deck height of 0.020" This process is repeated on the driver's side of the block. Actually, you wouldn't do any cutting until all 4 corners of the block were measured for deck height and all cut accordingly to the shortest piston deck height. Now, let's assume that all pistons measured out at 0.020" piston deck height after you have cut both block decks. In order to achieve the optimal quench (or squish, whatever you want to call it), we will have to use a gasket that will give us 0.035" to 0.040" clearance between the piston crown and the underside of the head with the gasket installed. With a piston deck height of 0.020", we could use a 0.016" shim head gasket to achieve a quench of 0.036". Again, this has absolutely nothing to do with the chamber size of the head. We would choose a chamber and piston crown configuration to achieve the desired static compression ratio. You NEVER use different gasket thicknesses to achieve your target static compression ratio. Different gasket thicknesses are used to achieve your target quench.

Some fellows will go ahead and cut both block decks so that the piston is exactly even with the deck at top dead center. This is called "zero decking" and you would then use a gasket with a compressed thickness of 0.035" to 0.040" to achieve your quench.

Different domes together with different chamber sizes would be used to achieve a target static compression ratio. Conventional wisdom says that with iron heads on a street motor having to use readily available pump gas, somewhere around 9.0:1 is the best choice. With aluminum heads, you can run another full point, up to 10.0:1. Now, as we've stated on this forum quite a few times, a fellow can get away with a lot higher than 9.0:1 on pump gas by choosing the proper quench and the proper camshaft. Running 11.0:1 on pump gas is doable with the right combination.

The reason for using different static compression ratios is to optimize the motor for power. Each full point increase from about 8:1 up to about 11:1 will result in a power increase of about 4% for each point increase. So if a guy is going racing and/or has access to some good high octane fuel, he would want to start with a higher static compression ratio.



I love explaining this stuff. If it isn't clear yet, keep asking questions.

I understand matching the SCR & DCR to the camshaft for 'optimum' performance,but from my understanding the 'bottlefed' and supercharged motors don't use domed pistons? but yet produce more HP. Is this because the domed would cause too high of pressures and not be dependable and go kaboom ? If that is the reason then would it be proper to say that domed pistons and superchargers are just two ways of skinning the same cat? But then chooseing which one would be based on 'class rules' or upon whatever would be practical for a street vehicle?
One last question For a drag race motor application (brackett not a 'class' car) what would determine whether I would use 10:1 or 13:1 ? Is this all based on cam spec's. and whatever power I want the motor to make? If so,,,then why doesn't everyone that builds a race motor run 13:1 or much higher?

everyone does try to go for as much compression as their fuel will allow. this includes the nitrous guys. nitrous does really care about compression like forced induction does. the more compression the more power for n/a and nitrous motors at least. when you get into forced induction, then its a bit different, instead of trying to squeeze the piss out of the air you alreay have in the cylinder, you try actually get more air in there and out. you could run a blower on a higher compression, but then you could only run a little psi. if you drop the compresion ratio down, then you can safely run higher psi, which equalls more air, which is more power.

to decide on which compression ratio to run depends one 2 things more or less. what type of power adder are you using, if any... and what octane of fuel you will be running.

Hi Faylur,

Your choice of whether to run SCR 10:1 or 13:1 will depend upon two things I can think of and they are both related to each other:

a) octane of the fuel you'll be using
b) cam profile or more generally dynamic compression ratio

The idea is to create maximum cylinder pressure without causing detonation...

on ~92-93 octane pump premium you want to run DCR in the 8-8.5:1 range while you can run higher (9.0-9.5:1+) with high octane race gas (100-110-116 octane etc)...

Richard (techinspector1) is definitely the expert on this stuff and I'm just parroting what I've learned from him in the past...

-Chris

"Is this because the domed would cause too high of pressures and not be dependable and go kaboom ?"

Yep. In the case of the supercharged motor, here's a chart from Blower Drive Service showing the limits of scr versus boost for use with pump gas:
http://www.blowerdriveservice.com/techcharts.php
It clearly shows the "8 for 8" rule (8:1 for 8 lbs of boost with pump gas). On alternate fuels, you'd be limited by the components you chose for your buildup. You could use a very high scr with nitrous or a blower, but it would be a matter of which component would fail first. Will it break the crank or split the block in half first?

"If that is the reason then would it be proper to say that domed pistons and superchargers are just two ways of skinning the same cat?"

Yep. You can use high scr naturally aspirated with no power adder or na with nitrous or low scr with a blower although you'll make more horsepower with either nitrous or a blower than you will just naturally aspirated with a high scr. By the way, some guys consider a nitrous motor to no longer be naturally aspirated. I disagree because the air going into the motor is still being pushed in there by atmospheric pressure although there is fuel and an oxidizer under pressure at the port.

"But then chooseing which one would be based on 'class rules' or upon whatever would be practical for a street vehicle?"

Yep.

"what would determine whether I would use 10:1 or 13:1 ? Is this all based on cam spec's. and whatever power I want the motor to make? If so,,,then why doesn't everyone that builds a race motor run 13:1 or much higher?"

Making horsepower is a matter of moving air through the motor. The more air you move through it, the more horsepower you make. Spinning the motor faster dictates a long cam and the long cam dictates a high scr. But spinning the motor faster means using the best components money can buy so they can withstand the tremendous forces generated with very high rpm's. Any cam grinder can grind a cam that will feed any motor at any rpm's, but the cost of the components will require a second mortgage on your home.

The stumbling block with building a high scr, low rpm motor is the cam. Let's say you want to build a 13:1 motor, but don't have the coin for the high buck components. You figure you'll keep the r's down. Problem is, the cam that will work with 13:1 is going to make power from about 4,000 r's to about 8,000 r's.

Can you see how all of this fits together? You must sit down and figure out your combination before you start to build a motor. If you're planning to make big power at low rpm's, then start with the largest motor you can, 500 inches minimum. If you plan to make big power at low r's with a small motor, then include nitrous or a blower in your plans. When I say low r's, I'm referring to a motor that will see a limit of about 6,000 r's.

Going back to domed pistons for a moment.....

I remember experiments conducted by Ak Miller on the Ford 2.0 and 2.3 motors. He made more horsepower with a flat-top piston over a domed piston although the scr was higher with the domed piston. It was a matter of the dome getting in the way of the flame kernel as it travelled across the chamber and effectively "putting out the fire". Same thing with the Chevy Vortec head #12558060 L31. Chevy recommends a flat-top piston with that chamber for the same reason. Domed pistons show less horsepower.

The answer for a naturally-aspirated high scr motor is a flat-top with small chambers. Include a tight quench if you plan to run pump gas and keep the scr at 11.0:1 or less. Proper matching of the intake closing point on the cam is mandatory.

Originally Posted by techinspector1

The answer for a naturally-aspirated high scr motor is a flat-top with small chambers. Include a tight quench if you plan to run pump gas and keep the scr at 11.0:1 or less. Proper matching of the intake closing point on the cam is mandatory.

I wonder what Eric will say to this ? OH, good article Tech

Originally Posted by southerner

I wonder what Eric will say to this ? OH, good article Tech

We'll find out soon enough

Good job Tech! That explained alot to me. Have 1 question though, you referred to the cam being long, you mean in duration? I got lost there, sorry. Could you go into a little more detail on how the lift and duration parts of the cam work in correlation with high and low compressions? That would be awesome. Thanks.

Originally Posted by FAYLUR

I've never used domed pistons so I've never had a need to know this but can someone explain if you should be concerned as much about quench when using domes? If you go with .035"-.045" then the head combustion chamber will have to be the proper or 'matching' CC's? Or do domed require more quench? And,,,,if the pistons are designed to provide say,,,12.5:1 with 64cc then using a 76cc would give too much quench? Is this right?

I will give you my 10 cents on the subject.
What Richard says is true.
I really don't care that much about quench when we use a high comp. ratio.
I will say this.
If it where a "full on" race engine I would put the piston down in the hole .005 to .010 that way you have some room to clean up the block if you have any issues.
If you want compression try and make as much as possible with out shoving a dome in the combustion chamber.
When you do this you create a kinds of problems trying to "light off" the fuel and air charge you also create all kinds of vortec's when you do this.
In a 383 a 6.0 rod and a 58 cc combustion chamber head and a flat top piston you can make 11.8-11.9 for a comp. ratio,no this won't run on pump gas.
I would never fart around with a 12.5 to 1 comp. ratio.
If you are going to shove a dome up in the combustion chamber it might as well be a big one.
One of our USMTS engines this year will go over 15.5 to 1 on meth.
This really pushes the limits of everything.
If this engine was on "gas" you would have to run 116 octane.

Posted by jimmyjeep:
"you referred to the cam being long, you mean in duration?"

Yes.

"Could you go into a little more detail on how the lift and duration parts of the cam work in correlation with high and low compressions?"

Concerning lift: If you are looking to optimize your build and make the most horsepower per cubic inch that you possibly can, then use the highest lift cam you can bolt in the motor. There is a price attached though. You must be willing to very closely check every component of the valvetrain to insure that everything is going to work together. You'll want to measure the exact valve to piston clearance through both intake and exhaust cycles with lightweight checking springs, clay on the pistons and solid lifters and rolling the crank through a complete 720 degree cycle. You'll also want to check for spring coil bind at max lift, rocker to retainer interference and binding of the rocker on the stud. The other downside to doing this with a street motor is the maintenance required on the springs. When using very high lift, the springs are going to take a beating and will fatigue over time. Now, there have been some advances in spring technology and they are getting better, but you still have to check them every so often. Your cam grinder will have exact information on this. If you are looking to put the motor together and not remove the valve covers again except for the occasional valve adjustment, then back off on the lift. Again, consult with the tech at your favorite cam grinder.

Concerning duration: It takes time to fill a cylinder with mixture. As engine speeds increase, the available time the valves are open becomes less and less. How do we extend this time at high rpm's? By keeping the valves open longer with more duration.

The most important event on the cam lobe is the intake closing point. Compression in the cylinder cannot begin until the intake valve has closed at some point after bottom dead center with the piston ascending up the bore on the compression stroke. With a low scr, you'll need to close the intake fairly early to trap enough mixture to make a good bang when the plug fires. If you use a long cam with a low scr (exactly what most newbies do), most of the mixture is blown out the open intake valve and back into the manifold and out the carburetor bores before the valve closes and you have a motor that won't pull your hat off your head due to low cylinder pressure. This is called reversion or standoff and you can see it as a white, foggy cloud that stands off just above the carburetor with the air cleaner removed. Reversion upsets the signal to the carb so that it doesn't know how to meter the fuel/air properly and things go to **** in a handbasket. It won't make any power, but man she sure sounds good at idle. Conversely, with a high scr, you must delay the intake closing point so that some of the mixture is lost back into the intake tract or you'll make too much cylinder pressure for the available fuel. It is a very delicate balancing act and is the reason you should have ALL, and I mean ALL the information about your motor before you talk to a cam tech at the grinder for a recommendation. I, as well as others on this and other boards will give advice based on available information, but it is only a recommendation that is meant to get close so that you understand what a cam in that neighborhood will do for you and why. Under no circumstances should you take anyone's advice but the cam tech's for your final decision.

There is a new wave of thinking in choosing a cam being talked about as the result of extensive testing by David Vizard. He suggests that the cam should be chosen for the correct overlap first, based on the cylinder displacement divided by the diameter of the intake valve, then duration will be determined as a result of that figure. It's taking me some time to get wrapped around this thinking and I'm not there yet. As soon as I do, I'll try to explain it to you guys. If you get it first, please help me to understand it.

That suggestion by Mr. Vizard sounds interesting and I can understand part of his reasoning in that there may be as much as 80% of the 'hotrodders' running very expensive motors with an imbalanced equilibriam between the insky and the outsky And with this imbalance the volumetric efficency is way down. As we all know,there are mismatched parts combinations,insky and outsky sides both,and maybe this is the reasoning behind his suggestion? (I don't understand half of what I know about this so correct me if I'm wrong). Or maybe his reasoning is to match the overlap to the flow capability of the motor .
But I can't even start to understand why he would suggest designing around overlap unless it would pertain to a specific class race motor?