I did not write some of this but took it from my notes,

Gas Burn Rate

Several factors affect the burn rate (flame speed) of the gas. The air-fuel ratio (a/f/r) affects burn rate. Mixtures with a/f/r of less than 11:1 have little chance of burning (to rich), and a/f/r greater than 20:1 have little chance of burning (too lean). The fastest burn rate is at 17:1 but that is far to lean for reduced emissions, and way to lean for maximum power. Best power is achieved at an a/f/r of 12.6:1.

Homogeny of the gas affects the gas burn rate. Homogeny refers to the uniform distribution of air and fuel molecules within the gas mixture. As we mentioned earlier, the a/f/r affects burn rate, so homogeny also affects burn rate. Homogeny also introduces another issue concerning failure of ignition. If the localized a/f/r where the spark plug is located is to lean or to rich due to poor homogeny, then the spark plug will fail to ignite the gas, and that power stroke will be missed. This concept is referred to as the probability of ignition. The better the homogeny, the greater the probability of consistent ignition for each power stroke.

Because poor homogeny can cause ignition failure, a longer duration spark discharge into the spark plug is better than a shorter discharge duration. The turbulence and swirling actions due to the intake port shape and piston motion, may very well replace that lean mixture with a normal mixture while the spark is still arcing. When this happens, then the probability of ignition is improved.

Multiple sparks can help to overcome the failed sparks (due to homogeny problems) but multiple sparks will not make the combustion gas burn any faster. Dual spark plugs could make the resultant gas burn time shorter because of two burn sources. Sort of like burning a candle from both ends. The candle will burn faster this way, and so will the combustion gases. But each end of the candle still burns at the same rate. A rotary engine is the exception, and uses multi-spark dual spark plugs to compensate for poor homogeny due to the abnormally long combustion shape of the rotor in conjunction with the ported intake gas flow.

Turbulence, Squish & Quench

As mentioned earlier, the shape of the combustion chamber can help to prevent detonation in two ways. The shape of the piston crown as it approaches the shape of the cylinder head, can create tremendous turbulence in the gas. This squishing of the gas mixture causes swirling and tumbling actions which causes shear tearing of the air & fuel molecules, which results in better homogenization. This improved mixing of the gas makes the gas burn faster. The same gas when burned faster has less time for spontaneous combustion. The faster the burn, the less time that is available for detonation to take place.

Another advantage of a faster burn is that ignition spark doesn't need as much advance. With less ignition advance, there is less time to build burn pressures before reaching TDC. This reduces braking action to the piston compression pressure, which increases pumping efficiency of the engine. This results in less power wasted to pump the engine cylinders.

Quench is quite another story. It is reasonable to expect that the gas in direct contact with the metal cylinder walls, piston crown, and the cylinder head surface; would be cooler because the metal absorbs heat from the gas (the metal is cool as compared to the burn flame temperature which can reach 5000F degrees). Because this thin layer is cooler, it does not burn and results in what is called a boundary layer of gas attached to the metal surfaces. This boundary layer is only a few molecules thick, but acts as an insulator which keeps the burning gas temperature from direct contact with the metal engine parts. This contains the gas burn temperature and prevents imparting excessive heat directly into the metal engine parts, which could melt aluminum parts. Like all insulators, it leaks some combustion heat into the metal parts and the engine cooling system must absorb that heat.

At TDC, portions of the piston crown get within about .040 inch from the cylinder head (squish region), and the close proximity of boundary layers quenches any attempt for gas in that region to burn. The .040 inch gap is hundreds of times thicker than the boundary layers, but the cooling effect quenches any gas trapped there. When that gas cannot burn, it reduces the chamber temperature which results in less heat available to cause detonation during the time from TDC to 16 degrees ATDC (after the squish time). This cooling effect is referred to as virtual octane because the cooler gas escaping the squish area as we leave TDC, steals heat from the burning gas, which reduces the chances of spontaneous combustion. This quenching effect results in a virtual octane increase. It has been found that the squish region has little effect if the piston to head squish clearance is 0.060 inch or greater. The optimum quench clearance is 0.040 inch.

perhaps I failed to explain the answer well enough.
you MUST KEEP THE CORRECT QUENCH DISTANCE (.037-.042) REGAURDLESS OF STATIC COMPRESSION RATIO ONCE YOU GO ABOVE ABOUT 9:1 STATIC COMPRESSION,OR YOU ENGINE CAN RUN INTO DETONATION PROBLEMS
THE ENGINE ONLY SEES DYNAMIC COMPRESSION NOT STATIC COMPRESSION
that will depend on the cam your useing, look at these diagrams and keep in mind that the piston compresses NOTHING untill both valves are closed
you don,t need to lower your STATIC COMPRESSION , what you need to do is lower your DYNAMIC COMPRESSION RATIO, a cam with a wider LSA or a cam with slightly greater duration or both will do that for you[/b] here read this http://www.mercurycapri.com/technica...e/cam/lca.html http://www.mercurycapri.com/technica...e/cam/lca.html http://ourworld.compuserve.com/homepages/axelg/cams.htm http://ourworld.compuserve.com/homepages/axelg/cams http://tru-442.tripod.com/camselect.htm
http://tru-442.tripod.com/camselect.htm http://victorylibrary.com/mopar/cam-tech-c.htm http://victorylibrary.com/mopar/cam-tech-c.htm read this info http://www.newcovenant.com/speedcraf...camshaft/1.htm http://www.newcovenant.com/speedcraf...camshaft/1.htm >lessons 1-8
these are the valve timeing overlap ranges that are most likely to work correctly
trucks/good mileage towing 10-35 degs overlap
daily driven low rpm performance 30-55degs overlap
hot street performance 50-75 degs overlap
oval track racing 70-95degs overlap
dragster/comp eliminator engines 90-115 degs overlap
but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range.


DCR and overlap are related but not directly related. However, both must be correct for the best performance.
Overlap, which is determined by the advertised duration and the lobe separation angle (LSA), has a profound effect on idle characteristics and high speed scavaging. I believe it was Ed Iskenderian that said we don't have a 4 stroke-4 cycle engine, we have a 4 stroke-5 cycle engine, with overlap being the 5th cycle. It is that important. Use the list posted to help select the proper overlap.

BTW, I include bracket cars in the "oval track racing 70-95degs overlap" catagory.

DCR is affected by the same advertised duration and LSA that overlap is. However, another cam factor enters into the equation, the installed "intake lobe centerline." This is how far the intake lobe centerline is offset from the LSA. If they are the same, then the cam is said to be installed "straight up" (say a 108º LSA cam installed with the intake lobe centerline at 108º ATDC). If they are different, then the cam is said to be advanced or retarded. Advancing or retarding the cam changes the cam timing in relation to the crankshaft. Nothing on the cam changes, just the relationship of the cam to the crankshaft. Getting back to DCR, advancing the cam causes the intake valve to close earlier than it would if the cam were straight up. When this happens, the piston is lower in the cylinder at intake valve closing increasing the sweep of the piston in the cylinder causing the DCR to be higher. Retarding the cam decreases the DCR for the same reason, namely the piston is now higher in the cylinder at intake closing since the intake valve closes later.

Changing overlap requires either changing the advertised duration or the LSA or both (both are ground into the cam and cannot be changed once the cam is made). Tightening up the LSA (say from 110 to 108º) without changing the adv dur increases the overlap. If the same amount of cam advance is maintained (say 4º), the DCR will increase. If the intake lobe center is maintained in the same location (say 106º, which is 4º advanced for a 110º LSA and 2º for a 108º cam), the DCR will not change (changing duration while maintaing the same LSA has a similar effect). So you could have a number of cams with a varity of different LSAs, durations, and overlap values yet all could have the same DCR. Overlap and DCR are related but, as I said above, not directly related.

first look at this simplifyied example<the V-8 is near the bottom of the page the A pushrod engine diagram http://www.howstuffworks.com/camshaft2.htm http://www.howstuffworks.com/camshaft2.htm
now in that example if you carefully watch the valves they have no overlap as one closes before the other opens but in the real world the exhaust valve is still open altho closeing when the intake valve starts to open, here read this http://www.newcovenant.com/speedcraf...camshaft/2.htm http://www.newcovenant.com/speedcraf...camshaft/2.htm http://www.newcovenant.com/speedcraf...amshaft/3.htm" http://www.newcovenant.com/speedcraf...camshaft/3.htm http://www.newcovenant.com/speedcraf...camshaft/4.htm http://www.newcovenant.com/speedcraf...camshaft/4.htm
now notice in the diagram below how the exhaust valve is still closeing as the intake is opening , this allows the fast moveing exhaust gases that have alread left the cylinder to DRAG the intake chager into the cylinder by negitive pressure (SUCKING THE INTAKE CHARGE INTO THE CYLINDER AS IT TRIES TO FOLLOW THE EXHAUST OUT THE EXHAUST PORT)
now the closer or tighter spaced the valve angles are (LSA)like 104-108 the more the valves timeing that they are both open OVERLAPS and the greater the amount of suction from intake port caused bye that fast exiting exhaust gas can occure but keep in mind that the piston starts back up and as it compresses the fuel/air mix in the cylinder the mixture can,t compress untill both the valves are closed, if the intake remains open to long the intake port has a bunch of the cylinders volume pushed back into the intake port causeing a reversion pressure wave, if the exhaust stays open to long excess fuel air mix flows through the cylinder and follows the exhaust into the headers. not only does the cylinders displacement and cam timeing have an effect here but also the rpm level that the engine is spinning at, that one big reason that "hot racing style cams have that lope sound at idle" the low rpm cylinder mixture is poor because the valve timeing is very efficient at lets say 5000rpm- 7000rpm due to longer durration that allows the cylinders to effectively fill in the 1/42nd-1/58th of a second the cylinders have available to fill at those rpm levels but its allowing much of the fuel/air mix to push back into the intake ports at low rpms where the mixture has time to reveres direction as the piston starts back up at low rpms<p

 

Oh boy! I'll have to take the time to read all this tonight when I get a chance.

Thanks! :thumbs:

 

Careful Grumpy, you're gonna make some peoples heads explode :conehead

[the quench stuff goes much deeper than that, but thats about as good an explanation as I've seen]

 

the statement's:
"This is how far the intake lobe centerline is offset from the LSA"
and
"(say a 108º LSA cam installed with the intake lobe centerline at 108º ATDC)"

should read:
this is how far the intake lobe centerline is offset from "top dead center crankshaft rotation"

(say a 108º intake lobe centerline cam installed with the intake lobe centerline at 108º ATDC of crankshaft rotation)..

the term "straight up" is used to describe the installed relationship of the cam to the crankshaft.

I'm sure you know this grumpyvette :jester (the rest of the paragraph prove's it)

And I'm impressed! you did a bang up job writing all that. :thumbs:
RJ


[Modified by ol,RJ, 1:59 AM 3/14/2003]

 

That's some interesting stuff, especially on quench, etc.

However, I've read that a thinner head gasket, say .030", reduces the quench volume (as would a decked block) and so reduces octane sensitivity. Presumably most of this comes from the added velocity of gasses caught in this volume as they are squeezed tighter than with a larger quench volume.