Posted: 12/25/13 09:41 AM
What is "Detonation", "Knock", and "Surface Ignition"? "Knock" is the name given to the noise which is transmitted though the engine structure when essentially spontaneous ignition of a portion of the compressed air/fuel charge occurs prior to the arrival of the propagating flame front (flame front that was correctly ignited by the ignition spark). "Surface Ignition" is ignition of the fuel/air charge by a hot spot in or on the chamber walls, that is, by any means other then the normal spark discharge. Surface ignition can occur before the normal spark or after. "Detonation" is a term that generally includes all abnormal events within the combustion chamber.
The goal in building a high performance engine is to develop maximum compression pressure (within the physical limits of the short block). As the term cylinder "compression" could be confused with mechanical or dynamic compression, I will use the term "cylinder pressure", and that represents the final result of good air and fuel flow into the cylinder, good fuel/air mixing, and optimum compression ratio for that engine. High cylinder pressure results in high power.
What additional factors affect cylinder pressure?
1. Throttle opening - More air will be allowed to enter the chamber with wider throttle opening, and therefore more air will be available for compression.
2. Valve size or more correctly, amount of possible air flow through the valve - Same general affect as throttle opening except that this will affect engine at all rpm/throttle opening.
3. Altitude, or air density - the denser the incoming air, the more pressure that will be developed when it is compressed.
4. Compression ratio - Determines the relative amount that the available mixture in the cylinders will be compressed. As we discussed earlier, rated CR and dynamic CR are different with dynamic CR being a function of both rated CR and cam timing.
5. Carburetor size - A carb that is not large enough for the engine can limit airflow at higher rpm and will degrade cylinder pressure. Caution: Oversize carbs may cause as much or more degradation of power by the adverse effect on fuel mixture and lower rpm power development.
6. Intake manifold design. Without getting into the pros/cons of intake types, the incorrect intake design for a specific engine application can inhibit cylinder filling at optimum rpm and proper intake tuning at various
7. Cylinder head - Can affect cylinder pressure in several ways - inadequate airflow at appropriate rpm, chamber design not optimum, sharp or irregular surfaces that could cause surface ignition and resulting knock/poor power
output, and incorrect chamber volume and resulting incorrect CR.
8. Fuel - Knock resistant of the fuel will determine the maximum safe cylinder pressure at the rpm desired.
9. Cam selection - Not only will affect the dynamic CR, but will control the airflow through the head, will aid or hurt ram tuning of intake flow, and will control engine efficiency at various rpm points and engine rpm range.
10. Engine timing - Best torque development is dependent on optimum engine spark timing, not only at peak rpm, but at peak torque point.
11. Finally, the most significant of all, the engine temperature, and to a lesser extent, the incoming air temperature. Engine temperature, and more specifically the temperature immediately adjacent to the cylinder head, is critical to peak power production. High HP per cubic inch engines not only develop more total HP, but they make more heat that gets dumped into the engine cooling system and block.. The heat balance in the chamber is
critical - we want maximum heat during the firing cycle in order to develop maximum push on the piston. However, the chamber parts must cool down to a reasonable level between cycles, so that the incoming air is not heated and caused to expand to the point that it would degrade cylinder filling. Excessive heat in and around the chamber can also cause any sharp edges within the chamber, spark plugs, exhaust valve edges, or even pieces of
carbon within the chamber to overheat and cause surface ignition prior to the planned ignition event.
12. Incoming air temperature will directly affect the cylinder pressure - Hotter air is less dense, exactly as is air at higher altitudes. Less density means less air to be compressed, and less power developed.
Some of the above factors are controlled by engine design, and there is not much we can do about them. However, some can be modified to some extent, and some we have direct control over. Note; The above 12 factors are described as related to the effects on cylinder pressure. Unfortunately, optimizing some for best cylinder pressure may be counterproductive in controlling knock or vice versa, or best heat control may mean less power. In the next segment, we will discuss some things that can be done to help develop good cylinder pressure while staying within reasonable limits on "knock" risk or heat development.
In the previous three posts, we have discussed factors that affect effect cylinder pressure, engine power output, and possible knock or detonation. Detonation can be broken into two major groups - Spark Knock and Surface Ignition.
Spark Knock is a knock which is recurrent and repeatable in terms of audibility. It is controllable by the spark advance: Advancing the spark increases the knock intensity and retarding the spark reduces the intensity.
Surface Ignition is ignition of the fuel/air charge by any hot surface other than the spark discharge prior to the arrival of the normal flame front. It may occur before the spark ignites the charge (pre-ignition), or after normal ignition (post-ignition).
Our goal in this series is to discuss how/what we can do to minimize the bad effects of higher compression. We know that optimum CR will provide the best cylinder pressure, and that high cylinder pressure will produce more power. We also know that too high cylinder pressure may cause abnormal combustion, and the related knock. We have listed parts/functions that affect cylinder pressure, so let’s discuss how we can control or modify these to allow optimum CR on the available gas. IMPORTANT NOTE: We assume that race engines will be run on higher octane fuel, so this series is for street and street/strip engines to be run on pump gas. That means the engines will be driven on the street, and normal evaluation of engine performance and knock can be made.
1. Without any question, the most important step to take is to cut the deck to 0. This step increases CR, but more than that, it allows higher CR to be run with the same octane of fuel.
2. The next most important step is to gain positive control over engine cooling. I am not talking about installing an aluminum water pump, four core radiator, fan shroud, 7 blade fan, or all those other things we talk about, although they may be required. I mean to control the engine cooling by
whatever means is required in every case and in every temperature extreme you plan to run your engine in. The optimum operating temperature for higher CR engines is around 180 degrees, and if you plan to increase the CR towards the maximum level, dont let the temperature exceed 195 at any time you load the engine at full throttle!
The next steps are not in any order, but are all important.
3. Know exactly where the timing is set, what the curve is, and what the maximum mechanical advance is. Run vacuum advance for improved idle, better engine cooling, and gas mileage. As vacuum advance is strictly load dependent, and retards to zero as quickly as the throttle is depressed quickly and firmly, it has no effect on full throttle power and knock. Light throttle knock caused by the vacuum advance is not a hazard to the engine, and is not a reason to not use the vacuum advance in all street engines. It does not matter if it is hooked to full time vacuum or ported vacuum, but one or other should be selected for best idle of a specific engine. Vacuum advance is vitally important in controlling engine temperature on street/strip vehicle!
Total mechanical timing is the controlling factor for engine performance and/or knock in our performance engines. It should be selected to compliment the available fuel, the characteristics of the engine in question, and not some wild fantasy about peak HP or MPH on a dyno or at the drag strip. If you want to run race fuel, fine, and if you do so, none of these suggestions are really necessary for you. However, if you want to run well with your car on your available pump gas, read on.
There is no magic timing value that is OK for all engines. As mentioned, my engine runs best at 30 degrees total mechanical timing. This was found at the drag strip by varying the timing up and down. At 28 degrees it begins to slow down, and at 32 degrees it also begins to slow down. There is never any spark knock at any rpm under any conditions at 30 degrees total, and I have deliberately advanced the spark to make sure I was not overlooking it. It will knock with advanced timing, and the sound is very clear and identifiable when driving on the street. As most do not have easy and immediate access to a drag strip (or dyno), the same kind of testing and setting can be made in normal street driving. Pick some total timing and drive! Check full throttle acceleration (with vacuum advance disconnected so as not to mislead you) and listen for ping/knock. Note engine response, and if possible, use some form of speed/acceleration measuring technique to compare performance at different timing points. You have to keep total mechanical timing below the point at which full throttle knock/ping is heard. However, after you have heard and can recognize the knock caused by advanced timing, it is safe to run at the timing just under that point. After the optimum total mechanical timing is found, you may need to adjust both initial and mechanical advance values in the distributor to provide good starting and idle whole retaining that optimum total timing. Those of you familiar with the computer controlled engines realize this procedure is identical to what the computer does, only it regularly (many times a minute) advances the timing to the point of knock and then slightly retards it after knock is detected. That timing point just shy of knock is where the engine will develop maximum torque, so it is important for performance to get close and stay close to that timing point.
After best total mechanical timing is found, reconnect the vacuum advance, and drive normally. A small amount of light ping is normal when beginning to climb a hill at steady throttle, or when accelerating at light throttle, but the ping/knock should quit when the throttle is opened further or quickly. If it does not, adjust only the vacuum advance - never retard the mechanical timing to cure this pinging. Either limit the amount the vacuum unit can pull, or obtain an adjustable vacuum advance unit.
4. We mentioned throttle opening, carb size, and altitude as affecting final cylinder pressure. As there is little we can do with these factors, they can be overlooked. Carb size has a major effect on the power range and throttle response of our street and street/strip engines, and therefore, should be sized for these factors and not just Max cylinder pressure.
5. Intake manifold design was also listed as affecting cylinder pressure, and it does especially at peak rpm. It, as does the carb size, has a major impact on overall power though the driving rpm range, and should be selected on that basis as well as for developing maximum cylinder pressure.
In the next segment, we will continue with the remainder of the suggestions for building and running optimum timing on pump gas. I do want to emphasize one more time that if you do not understand these basic steps, or are unwilling to commit to set your engine up accordingly, you probably should leave your CR at a "safe level", whatever you believe that to be.
When The Flag Drops.,.
The Bull ***t Stops.,.
P. Engineer, Engine Builder