The MGA With An Attitude

The title line calls up a very large subject. This discussion is only a little about making power and more about not killing your engine in the process. I probably have too much time on my hands, so grab a drink and get ready for a long explanation.

At 05:57 PM 2/3/03 -0800, Larry Colen wrote:
>"This made me hypothesize on why cars are more likely to predetonate going uphill rather than on the flat."

They aren't. Surprise! They will predetonate exactly the same on the level for similar internal engine conditions. Going up hill only tends to decrease acceleration such that it may spend more time under heavy throttle within the speed range of interest, so it might have time to get a little hotter inside the combustion chamber. You can get the same effect on the level while pushing into a strong head wind or towing a trailer with lots of wind resistance at constant speed or LOTS of extra weight when accelerating, both of which will require the heavy throttle setting for longer period to maintain speed or to accelerate.

>".... With more force on the piston (and no predetonation) divided by the same area, there is more pressure in the cylinder. Likewise, a lighter flywheel could slightly reduce pinging, less rotating inertia, less force on the pistons, less cylinder pressure. Am I at all close here?"

It has nothing to do with weight or inertia of the car or flywheel. It does however have a lot to do with cylinder pressure, spark timing, fuel quality, and any hot spots that may be inside the combustion chamber.

Preignition is spontaneous ignition of the fuel from pressure and temperature before the spark is intended to fire it off. This can be caused by too high compression, too low octane of the fuel, hot spots in the combustion chamber, or some combination of these items. Pinging or detonation does not necessarily indicate preignition, but may just be a case of too much spark advance for the rest of the conditions.

The pinging will be worst with conditions of maximum dynamic pressure (MDP) during combustion. This occurs with the highest density of intake charge, most spark advance, lowest octane fuel, and hottest hot spots, or some serious combination of these things.

The most air will be introduced into the engine with full throttle opening at about 2500 rpm. At lower speed the intake ram effect and exhaust scavenging effect of the moving gasses doesn't do so much, and valve overlap allows some loss of charge before the intake valve is fully closed. At higher speeds flow restrictions take over to reduce the density of the intake charge. Starting with the highest density of intake charge and compressing it near TDC gives the maximum static pressure before ignition (MSP). This usually occurs at about 2500 rpm. If you have a belt driven supercharger MSP may occur at lower speed. With an exhaust driven turbocharger MSP may occur at higher speed, depending on a lot of variables.

For carburetors with the vacuum tap at the venturi, the vacuum advance in the distributor will be at maximum shortly off of idle. The vacuum signal will remain strong with light to moderate throttle setting. With full throttle the manifold vacuum drops, and (or) the venturi vacuum signal drops below the threshold point for the vacuum advance unit, and the vacuum advance wil disappear. This gives good throttle response when you poke it from dead idle, and good fuel economy at moderate steady cruise speed. The mechanical advance of the distributor usually reaches maximum by about 3500 rpm. Initial spark timing at idle and vacuum and mechanical advance are added together for total spark advance. Example: 15d static timing +9d vacuum advance +12d mechanical advance = 36d maximum total advance. This will likely not be quite at maximum in the 2500-3000 rpm range, but at higher speed the flow restriction drops the dynamic pressure in the cylinder, so the worst combination of pressure and spark advance usually happens in the 2500-3000 rpm range.

Now you light the fire at about 32 to 36d BTDC with maximum static pressure at around 2500 rpm. The flame front takes a few milliseconds to propagate and burn all of the fuel/air mixture. Since the gasses are contained in the cylinder and cannot expand when burning, the pressure rises rapidly. At 2500 rpm this is likely to be completed very near TDC or just a few degrees thereafter, at which time you have MDP.

The noise (commonly called spark knock or pinging) will occur when the pressure is sufficiently high for long enough time to make an acoustical pulse loud enough and long enough in duration for you to hear it. When MDP occurs shortly after TDC it may drop rapidly with the downward motion of the piston, in which case you hear no ping. When MDP occurs a little before TDC, it will remain high while the piston finishes the upward travel and begins to descend. When this high pressure interval is long enough in duration, you do hear the ping. If MDP occurs well before TDC the acoustical ping may be quite long in duration and may sound quite loud. This more severe noise is usually referred to as detonation. All of these noises may in fact have about the same sound volume, but the longer duration pulse is more audible (sounds louder to the ear).

A very mild case of barely audible pinging for a short period of time may not be detrimental to your engine at all, and may in fact be the most desired set of conditions for best mid-range power, best fuel economy, and lowest emissions. This is because the engine will generally have best thermal efficiency at higher dynamic pressure. This is when it converts more chemical energy to heat energy with less fuel left over, and follows up by best converting the heat energy into mechanical energy. This is also why a small engines may get better fuel economy than a larger engine (running at the same speed), because the small engine will spend more time running at higher pressure and therefore higher thermal efficiency. This is also why high compression engines can put out more power (as long as you can avoid the detonation problem). Fuel economy also has something to do with "pumping loss", but that's a different discussion.

You can have the nasty detonation problem if the fuel octane rating is too low, which may cause the fuel to ignite too soon or to burn too quickly. The expedient fix is to retard the spark timing, which may cause a loss of efficiency and power. The better fix is to use higher octane fuel. Higher octane fuel is actually a bit harder to ignite, burns just a little slower, and is likely to yield slightly less energy when burned. This is usually more than made up for by the increase in efficiency that comes with higher compression. At least running with the proper ignition timing will be much more efficient than running too far retarded. Also, just incidentally, higher octane fuel may have a little lower vapor pressure, so your carburetors may be less susceptible to vapor lock in hot conditions. Using fuel with octane rating higher than necessary to serve the purpose will be a waste.

You can also get the nasty noise if you have preignition caused by spontaneous combustion of the fuel from pressure and heat before the intended time for the spark (dieseling). This is the same as having the spark advanced too far, except that retarding the timing will have no affect towards a cure. Here you must increase octane rating, or lower the pressure, or get rid of some errant hot spot in the combustion chamber (usually sharp edges of carbon buildup). Hot spots in the chamber are also likely to cause run-on (also dieseling) after you turn off the ignition, unless you can also cut off the fuel entering the cylinder.

So the summary answer is that when you get too much pressure too soon you get the pinging noise, and the worst and loudest case is detonation. The case of detonation is definitely bad for the engine on two counts. It will be holding very high temperature gasses in the combustion chamber too long, which can burn steel valves, melt aluminum pistons, crack ceramic insulators and burn the tips off of spark plugs. If the high pressure occurs too far before TDC it will also cause very high loading on the connecting rod bearings, and may also like to blow out a head gasket. Conditions which cause preignition under heavy throttle will almost always lead to detonation.

Another condition to avoid would be too much spark advance at high speed. If you set the idle timing with a lot of advance, and you have lots of mechanical advance in the distributor, you might have spark timing more than 40d BTDC (or worse) at 4000rpm or any speed higher. Because of the higher engine speed and the short time duration near TDC, this may not cause any audible ignition knocking noise at all. However, when the fire happens too early, and you get MDP before TDC, this causes kickback on the piston trying to drive it downward when it's still going up. The end result is a terrible loss of mechanical efficiency, giving a loss of power while burning the same amount of fuel. This means very low thermal efficiency, which means you get too much heat and not enough mechanical output. Much of this wasted heat will go to heating up the internal parts of the combustion chamber, which in turn can burn valves and melt pistons. You might not ever notice this condition until thousands of miles later when the compression drops and the power starts to go away. By that time you need a valve job or a new set of pistons (or both).

Another way to make it run (a little) hot is to get the fuel mixture too lean. This drops the power some for lack of sufficient fuel, and the excess air cools the flame, which lowers thermal efficiency. To get the power you want or need you then give it more throttle. Now you may have the power, but the thermal efficiency is still down, so it ends up generating extra waste heat. But the even worse consequence of running lean is the left over oxygen in the combustion chamber. With sufficient heat and the left over oxygen in the chamber, the aluminum piston can actually burn. This effect has the aluminum piston being consumed as fuel for the fire, similar to using an oxy-acetylene cutting torch on a piece of steel. So if you're building a "spirited" engine you might want to run it somewhere between slightly rich and flooded, but never lean.

The excessively hot parts can also in turn lead to preignition and detonation in a much shorter period of time (like a few minutes), and by that time the engine may have already incurred significant internal damage.

By now it should be obvious that very little of this has much to do with what car might happen to be attached to the engine. You can easily mistune any engine regardless of the application. And the only affect weight or inertia have on the engine would be the very fleeting short term effect of reducing acceleration enough to keep the engine in the danger speed range a little longer while accelerating, or maybe to keep it from accelerating at all going up a hill. That is called lugging when it is done at low engine speed, and the only cure for this is to select a lower gear to allow the engine to run faster or with less torque load. That is simply a driver discipline that everyone should lean during early driver training.

A very tiny pinging noise for short term at full throttle may be no problem at all. On the other hand, if you get it all wrong, a perfectly quiet engine could suffer internal meltdown at higher speed (without even overheating the cooling system). Any time you vary from standard specs on compression, camshaft, valve train, ignition timing, carburetors or fuel type, you can get yourself way out on a limb. You should know what you're doing, or give the job to someone else who knows, or otherwise be ready to endure the unknown consequences. With all original type components you can usually fix a small pinging noise with better fuel or a minor ignition timing adjustment. With any significant change of parts a serious tune-up run on a dynamometer might be in order.


Barney Gaylord

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