Head parameters and how they affect power output

[sicivicdude]

First, this is in addition to the information included in "squish, mystery or not" thread. You'll need to cover some of that ground first to understand how squish in particular affects the other parameters and why the other parameters change the needed squish.

 

Glossary:

 

Squish or quench area or band: The area between the outside edge of the combustion chamber and the hemispherical (or torrodial) area in the center of the head.

 

Squish gap/clearance: The distance between the squish area and the piston crown at TDC.

 

Combustion chamber: The area inside the head that the flame and fire contact. Usually consists of two general areas, the squish or quench area and the chamber proper.

 

Squish percentage: The percentage of the total bore area that is taken up by the squish band and "break" area.

 

Break area: The small amount of material that is used as a transition area from the squish area to the combustion chamber.

 

UCCR (UnCorrected Compression Ratio): The mathematical calculation representing the ratio of swept volume to trapped volume in an engine.

 

Swept volume: The volume that the moving piston displaces as it travels. Basically, bore x stroke = displacement.

 

Trapped volume: The area that all of the swept volume is compressed into when the piston reaches TDC

 

CCR (Corrected Compression Ratio): The mathematical representation of volume above the exhaust port to trapped volume. Usually called the "Japanese method" and only useful for cranking compression comparison on 2 stroke motors!

 

TDC: Top Dead Center the upmost point that the piston travels, when the crank bearings, crank pin, big end bearing, piston pin bearing, and piston thrust are all aligned "straight up and down". Crucial figure for head tuning as it's needed to figure out squish gap thickness

 

BDC: Bottom Dead Center. The lowest point that the piston travels to.

 

Predetonation: The term that refers to when the charge is fired too early. It takes a certain amount of time for the explosion (or more precisely, the pressure wave generated during the explosion) to reach the piston crown. This is the reason the stock ignition timing is "advanced" during higher RPM operation. The time it takes that flame front and pressure wave to hit the piston is constant (relatively) but the piston speed and dwell time is more and more limited as rpm's rise. Predetonation can occur due to either too much ignition timing (controlled predetonation that can be adjusted out) or by other factors present in the engine (too huch heat, lean condition, "hot spots")

 

Dwell: In modern engines, the amount of time and position of the piston while the connecting rod is rolling over and under TDC and BDC. The piston also changes face thrust at this position. In a well worn cylinder, there is a "groove" at the top and bottom of the cylinders where the rings rub the cylinder at the dwell points. This is the area with the most amount of wear in a "normal" engine.

 

Squish velocity: The speed of the homogenized charge (air:fuel mixture) as the piston rises and "pushes" the charge towards the center of the combustion chamber. Pushing the charge too fast (high velocity) will preheat the charge to the point of autoignition resulting in predetonation. Piston speed affects this number GREATLY.

 

Piston speed: The rate at which a piston rises and falls for a given crankshaft RPM. Usually expressed as feet/second. More rpm = greatly increased piston speed.

 

Overstuffing: The rate at which volumetric efficiency rises well beyond "normal" capacity as the engine, pipe, carb, portwork, and head begin to work together.

 

Volumetric Efficiency: The ratio of the engine's ability to move old charge out and new charge in under real world conditions versus the "theoretical" ability at no engine rpm. Generally given as a percentage that gradually drops as engine rpm increases.

Edited November 14, 2012 by sicivicdude

[sicivicdude]

Alright so the first thing we need to understand is that head design is very subjective. It's extremely difficult to compare squish, combustion chamber volume, or really any of the other parameters without knowing all of the parameters and how the affect each other. In this case knowledge is power (QUITE literally).

 

First off, lets dispell a myth. An "smaller" cc head is NOT ALWAYS BETTER than a "larger" cc head. That method of "comparing" head sizes is fundamentally flawed. An 18cc head on a stock exhaust port engine will knock like hell while a 20cc head on a drag ported cylinder running methanol will barely be adequate to fire off the fuel. Comparing ONLY volume is a dangerous and foolhardy method of comparing head sizes and shapes simply because so many other things compound head shape other than volume. Also, "head volume" is NOT the same thing as trapped volume. Trapped volume is actually more important than head volume (which is only semi useful in comparing head to head to head, not engine build to engine build!) because that is what is used to calculate the UCCR or CCR. The piston is domed and properly built, the trapped volume will actually be less than the head volume!

 

We, being relatively intelligent creatures will move past comparing "dome size" as a comparison for engine tune level (or even head tune level).

 

The single biggest problem with the stock blaster head is that the squish band is cast at a different angle than a stock piston. A stock stroke stock bore engine (we'll discuss that before we get into strokers and big bores!) has a negative deck height of approximately -.008". Head gasket thickness is a nominal .032". The stock head has a positive step (further away from the piston) in it of something like .055" and an angle of approximately 20°.

 

The piston has an angle of approximately 10° (averaged over the outside 50% of the bore size).

 

To begin with, the stock engine closest squish point of approximately .095" or 2.4mm. From there it actually growsin thickness up to approximately 4mm before hitting the break area.

 

JUST shaving off the step until the mating surface is flush with the outside edge of the squish band will yield a relatively close .040" squish at the very outside edge growing larger as it goes. This will, at the very outside edge, push the charge very energetically but with diminishing force as it goes. The very energetic push at the start can actually cause an otherwise completely stock engine to predetonate.

 

The first step to all head mods is to recut the squish angle to ~11°. This process alone, however would yield a squish gap of something like .200" (roughly 4 times the amount desired) and a combustion chamber volume of something like 24 cc's but only because the squish would be sooooo large. That head would run worse than a stock head because it's hardly pushing the charge towards the spark plug at all.

 

Removing the sealing surface to correct the squish gap would yield a head volume of something really tiny like 16cc's which would run great, for about 4 minutes until the engine heat soaked. At which point the glow plug (spark plug but with enormous amounts of charge heat) would begin to predetonate the charge and eventually tear up the engine. This might be optimal for a quad that will never get hot enough to knock? Maybe? Not very useful for most folks....

[sicivicdude]

So this leaves us with a conundrum. In order to correct the deficiencies in the stock head design, we must correct the squish band for angle, remove some of the mating surface, and remove some of the combustion chamber material (to correct the volume).

This is how a stock head, ends up looking like this:

Now, in order to make the changes properly, one must know several things.

First off, the intended use of the engine (and quad really) comes into play. The reason is this; the maximum squish velocity of a given head with a preset squish gap is effected the greatest by RPM. If you completely optimize a head shape and install a higher rev pipe that will allow another 1k rpm of over-rev you WILL get predetonation in that head. It may not be enough to be audible but it will be there.

NOT ALL PREDETONATION IS AUDIBLE. BY THE TIME YOU CAN HEAR IT, IT HAS ALREADY BEEN HAMMERING THE ROTATING MASS FOR A LONG TIME! Tuning a head by "listening" for predetonation is also foolhardy and will end in disaster over the long haul.

[sicivicdude]

Second thing that affects the head design is exhaust port duration. This number GREATLY affects the Corrected Compression Ratio.

Only stating head volume without knowing the exhaust port duration or CCR (either will work) is like knowing the bore and stroke of one cylinder without knowing how many cylinders there are!

The exhaust port duration is used to calculate the CCR which will tell us how much charge is actually getting pushing into the trapped volume. This is very important to know because it will affect the final cranking compression numbers.

A higher amount of exhaust port duration means that less charge is trapped inside the combustion chamber by the rising piston. Less charge trapped means that the final CCR is lower. While this *GENERALLY* (we'll get back to this later!) means a lower CCR and less engine performance, there are obvious benefits. Longer exhaust port duration means better scavenging of the last charge (the fresh charge from the carburetor "pushes" the last charge out the exhaust port).

The total trapped volume needs to be considered with regards to exhaust port duration. The head volume can compensate for the lower CCR by lowering the trapped volume to keep the CCR constant (or increase it).

[sicivicdude]

Third thing we have to consider when designing a optimized head is the squish band area (differing from the "squish area", the name for that area). Here, meaning the squish percentage.

This is very important when calculating the squish gap or "thickness" because as you push a given amount of charge (as estimated by the CCR) at a certain RPM (as astimated by the pipe, porting, and carb setup) the distance that the charge must travel to reach the center of the combustion chamber imparts more energy on that charge. You can have a 40% by area head that can safely run a .040" but a 50% by area head needs more gap to keep the same squish velocity. Remember, velocity = energy. The more energetic the squish area is, the better, more vigorous, and stronger the combustion fire is going be. However, if it charge explodes before the piston comes all of the way up to the top, you're wasting that compression and potentially doing a lot of damage to the reciprocating mass of the engine!

High rev engines (drag/ dune motors) need less percentage. Simply put, a smaller squish percentage means better scavenging of the last charge (less burnt up fuel and air left in the head) but it also means a slightly less energetic "push" from the squish area.

Lower rev engines can get a great torque advantage by having a larger squish percentage because the slower piston speed means not TOO much velocity in the charge.

Most engines (gasoline trail bikes for example) do well with 50% squish percentage. In other words, the squish band is 50% (by area, NOT diameter!) of the bore area.

 

There are a few more things to consider:

 

The size of the "break area" makes a lot of difference to predetonation resistance, actually. The aluminum in the break area is at the junction of all of the energy being put into the charge by the rising piston and all of the heat being generated by the explosion. Considering 2 strokes never get a "dead stroke" to cool off, too sharp a break area means some aluminum is "sticking out" into the combustion chamber with less material around it to "wick" that heat away. A super sharp break area can be quite a bit hotter than the average temperature of either the squish area or combustion chamber! A less rounded break area is good for keeping the squish velocity the highest for the longest period of time HOWEVER they're prone to predetonation. Really wide break areas, are less prone to predetonation but also allow the squish velocity (and the energy imparted into the charge) to diminish as the charge passes out into the combustion chamber.

[sicivicdude]

I said that not all predetonation is audible. What I mean by that is that the rider would not necessarily be able to hear "knock" at low levels and by the time the rider can hear and diagnose the "knock" the engine has been predetonating for quite some time. Possibly to the detriment of the engine. An improperly designed head can predetonate and hammer away at the big end and small end bearing forever and barely be audible outside of the engine. Doesn't mean it can't damage the rotating components.

 

Install a knock sensor on the side of the motor and THEN test for predetonation if one should choose to "experiment" their way to hard squish numbers. However, I caution against that too, as soon as you have hard squish data numbers and change one parameter you are no longer perfectly optimized again.

 

The lesson to take away from this post is that specific numbers are only specific for a specific build. More general numbers are slightly less powerful (still significantly more so than factory heads) but also "safer" for more conditions and mishaps. Also, it's easier to discuss and understand if we all get on the same page about the process of design and building of a rechambered head.

 

 

 

It was suggested I include some more information (all of which is true and relavent!)

 

Number one is that the faster, more energetic charge burns faster. This has the same basic (flame hits the piston earlier) effect as advanced ignition timing. Igniting a lazy charge early and igniting a really excited charge later end up at the same point. Generally, there is less (or even no need at all) need to run advanced ignition timing with a rechambered head.

 

Second thing is that the value (increased charge speed) diminishes somewhat with increased compression. At some theoretical point, the compression will be so high, that the squish area isn't "exciting" the charge any more than it would be naturally using the higher compression. This limit, I don't believe, is applicable to air cooled singles as heat and fuel choice is the limiting factor on these engines anyway, but an interesting note none the less.

 

Some er...... light reading on the subjects (and many many more). SOME of the data, theories, and methods used are slightly outdated (engine's designed in the 60's and built in the 70's) but generally it still applies in theory and concept!

 

A reference of books:

Design and Simulation of 2 Stroke Engines, Gorden P Blair 1996

Basic design of the Two Stroke Engine, Gorden P Blair 1990

Two-Stroke Tuner's Handbook, Gordon Jennings, 1973

Two Stroke Performance Tuning, Alexander Graham Bell, 1999

The High Performance Two Stroke Engine, John Dixon, 2005

[sicivicdude]

Another thing I feel I need to elaborate on is the "overstuffing" of the engine.

 

Volumetric efficiency suffers as engine rpm rises. The same air and fuel has less and less time to make the journey into the engine as the engine turns faster. To this end, anything that can be done to augment the amount or speed of the scavenging can be used to greatly augment the power output at rising engine rpm.

 

We know them as pipes but what they really are is an exhaust driven supercharger. Some are more efficient than others and some have different engine rpm's that they operate at better than others.

 

Generally, pipes operate to "help" the engine not lose power as the time interval for scavenging decreases. However, some pipes actually operate SO efficiently that they can actually put more volume into the combustion chamber at high engine rpm than the engine naturally traps at lower rpm (without the help of the pipe). Drag pipes are a fine example. If one simply calculates the theoretical volumtric efficiency of a drag engine at a given rpm, the output would be dismal. The charge wouldn't have enough time (at the high rpm's) to get the old charge out and new charge completely in before the engine "closed up" and the next combustion cycle began. The tuned pipe is actually so efficient at its job that it can suck the old charge out and even some of the new charge out too and then, due to its design, push that new charge back in before the exhaust port closes.

 

If the pipe is good enough at its job that it does manage to get more charge into the cylinder than can be trapped at lower rpm's, it is overstuffing the engine. It's not uncommon to see highly tuned race engines exceed 110% of their "theoretical" volume. A 100cc combustion chamber (the area above the exhaust port) could potentially contain 110 cc's of fresh charge before the piston compresses it into the combustion chamber and even begins the squish phase of the stroke...

 

This is another reason why head designs for "most" engines are kept "safe" instead of to the raggedy edge of their possible design. Change elevation with the exact same engine (no chagnes at all) and even rejetting may not be able to help predetonation that could come on from the increased volumetric efficiency.

[sicivicdude]

Provided for all those who feel the need to learn WHY and not just be told how.....

[sprinklerman]

Thanks for the reading sicivicdude.

 

But, "pre" detonation??..just say'n

 

but thanks again,it was interesting.

[Coupelx]

Predetonation is the unicorn of the mechanical world. Often talked about but never seen.

[sicivicdude]

Thanks for the reading sicivicdude.

 

But, "pre" detonation??..just say'n

 

but thanks again,it was interesting.

 

I'm assuming that you are referring to my seemingly incorrect grammar. If not, please elaborate on your concern.

 

It can also be pre-detonation as the "pre" is a prefix (note prefix DOESN'T use the hyphen), however it is not incorrect to use the term as "predetonation" either. You MUST use the hyphen before anything starting with an "e" but it NOT required before a consonant. Let's not get bogged down in grammar class, however.

 

The point is, there is A LOT going on inside of a combustion chamber while an engine is running. Really, the two threads I've posted are just a "primer" to the real processes anyway in an effort to avoid confusion.

[sicivicdude]

Clearly, that "article" was written regarding blaster heads (where I cracked this nut to begin with) which are fundamentally flawed from the factory. The banshee head is a MUCH better design as far as combustion chamber size, shape, break area, and squish area are concerned.

 

I will be doing a full write up on a stock banshee head maybe tonight so I can tailor the article more directly towards banshee engines.

[sprinklerman]

im not a grammer expert,,believe me. what im refering to is the fact that is simply detonation..not "pre detonation) detonation,by its very definition is this (cut and paste) >

Detonation Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

 

Pre-ignition Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

 

So you see, there IS no such thing as "pre" detonation. Im not trying to nit-pick you,im just asking that when great info is being dispensed ,that it be accurate. The point of dispensing info is to educate guys like me and others,,so lets make sure we are being educated with ACCURTE info. Thats all im saying. Honestly,Thanks again!

[sicivicdude]

I see what you are referring to but take a look at the Merriam Webster (arguably the final point in all definition arguments) definition:

 

Detonation:

 

1 : the action or process of detonating

 

2 : rapid combustion in an internal combustion engine that results in knocking

 

According to their definition, we are both incorrect and the detonation is actually the event you refer to as "pre-ignition" and I am referring to as "predetonation".....

 

How about we chock that up to a grey area as far as the terms are concerned? Is it safe to say that we can assume that all references in the information listed above to "predetonation" are referring to "detonation" as in the harmful "preignition" of the fuel and air mixture before the normal timed ignition event by the spark plug?

[sprinklerman]

Actually , the industry standard definition of the words in question are what i cut and pasted. No where in a legitimate two stroke tech article or forum will you see the term "predetonation" used...But i do know what you were trying to say,and do appreciate the info shared.

The reason i even brought this up is, if we are going to share ideas on technical subjects,we need to adhere to the PROPER (industry standard)definitions and terminology so we are all on the same page.

 

And Merriam Webster IS NOT the final word in defining two stroke terminology. Unless Webster was a gearhead! haha