Before we return to a discussion of positive steps to run optimum CR, I would like to clarify several subjects we have mentioned before.

The purpose of this discussion is to help each of you run the optimum CR with the pump gas that is available to you. Everything discussed to this point applies equally whether you have the best pump gas in the country, or whether you have to buy from the only station in town down on North Main Street. These steps will allow you to develop the best power from what you have. We have not and will not talk about what CR you should run, or what octane your engine might develop the most power with. While I am safely running 10 CR on Amoco Premium 92 octane, you may be able to run 10.25 in a lighter car with a looser converter and even better gas. Conversely, you may only be able to run 9.6 with the 2.56 geared five speed tranny and 4500# customized Bonny.

A second point not yet discussed is the cumulative advantage to higher CR. Just changing the CR by several tenths of a point wont make huge differences in power. However, a modest change in CR may very well allow you to step up one cam size, and still maintain excellent cylinder pressure, while gaining the additional cylinder filling benefits of the new cam. My Performance Trends Engine Analyzer thinks the following changes will occur on my engine with CR of 8, 9, and 10: From 8 to 9 CR, a gain of 4.4% in HP and a gain of 3.1% in torque. From 9 to 10, it thinks that HP will increase by another 3.8% and torque improve by 2%. While these numbers, if true, are worthwhile by themselves, when the flexibility of more radical cam grinds are added, even more power could be realized with higher CR.

Now, back to the steps, and two new ones have been added, so we will discuss these two first:

The engine load will clearly effect the resistance to knock. A soft load, such as very low gears ( 4.11 and lower) allows the engine to accelerate quicker in each gear, and it is less likely to knock. A looser converter does the same, and the engine does not have to labor as hard at any one rpm point. Conversely, a manual transmission locks the engine solid, and any knock due to fuel/cylinder pressure will be more pronounced and noticeable. So if you are trying to pull a very heavy car and use a lock up system in the auto, or a manual transmission and overdrive, less total CR, less ignition advance, and/or higher octane may be required.

Any oil in the combustion chamber will cause added heat buildup in the chamber. The residue from burnt oil will accumulate in the chamber, and add to the risk of pre ignition from hot spots in the chamber. What can be done about this problem? First, dont try to increase CR on an oil burner. Second, dont be shocked by this one - Perfect compression seal of the two compression rings can cause additional oil in the chamber. A slight compression leak tends to blow the oil from the oil rings back into the crankcase, thus preventing it from migrating into the chamber. This certainly does not mean you want to purposely build-in poor ring seal, but it does mean that a perfect seal may cause more problems in oil control then it solves in adding a slight amount of cylinder pressure. Reference the Technical Manual from "KB Performance Pistons" for added data on this subject.

Now to the heads/chambers. We know the heads/ports/chambers are the among the most important components of the engine. What can be done to improve the knock resistance? Any step that will eliminate or minimize any possibility of hot spots that could cause pre ignition are worthwhile. Any step that will improve the burn rate or progress through the chamber will be worthwhile. The complete interior of the combustion chamber, including pistons tops and spark plugs, should be so smooth and all edges radiused, that an 12 month old child could safely rub their hands anywhere on/in the chamber with no risk of cuts or scratches. Heres how: Inspect each spark plug before installation, and using a small ignition or pattern file, break every edge on the plug base and ground electrode. Yes, I know that electricity jumps best from sharp edges, but ignition sparks jump from the center electrode to the underside of the ground electrode, and not from those sharp edges generated when the parts are stamped. Next, carefully break the top edges of the pistons, and the valve reliefs using either steel wool or plastic type rubbing pads. Carefully inspect the valve heads, especially the exhaust, and assure that the exposed edges are not sharp. Finally, polish the entire combustion chamber, including valve heads, and the tops of the pistons to as glossy a finish as possible using appropriate polishing discs. 3M has a great selection of the small 2""disks with varying degrees of coarseness and sanding capability. The polishing has an added benefit: It provides some of the attributes of the newest parts coatings, in that polishing to a shiny surface improves reflectability. Heat will be reflected back into the chamber rather then being conducted into the heads/block and then into the water, even after some carbon buildup. Higher temperatures in the chamber (when under control) add to the power of the fuel air burn! K&B Pistons estimates that as much as several percent of power can be gained by the polishing. I tend to doubt that there would be that much gain, but so feel some gain will be realized. What about fuel fallout due to the polishing? I doubt that after the compression stroke, especially with excellent quench/squish action, that much fuel will have separated due to the polishing. In any case, the benefits of the possible higher/safer CR will override the possible fuel/air separation.

Along with the above step, we drastically move the curl in the chamber that shrouds the intake valve. By removing that overhang, or lip, in the chamber that curls back over the intake valve seat, air flow at all lift points is increased, and in so doing, the sharp edges caused by the lip can be eliminated completely. The exhaust side can also be smoothed, but our tests show a reduction in exhaust flow if the overhang is totally removed.

So in summary, make the finished chamber as smooth and shiny as possible. I cant prove it adds power, but know it certainly did not hurt power, and for sure, it will minimize any possibility of pre ignition.!

Carefully fit/measure the head gasket to be used to assure that it does not protrude into the chamber. Any protrusion will serve as a built in pre igniter and about guarantee combustion problems. The old Fel Pro black gaskets with orange colored water seals was a fine gasket, but was/is not large enough for a +.060 455, and would protrude into the cylinder/chamber.

We discussed the importance of temperature control. Maximum heat is desired in the chamber, provided it is caused by the current fuel burn. Leftover heat from the previous firing cycle is not desirable, nor is heat generated from incorrect timing, oil in the chamber, or simply an engine that runs too hot. Incoming air to the intake system can tend to modulate the chamber operating temperature to some extent, and if it is cooler, it will have more oxygen per unit. Outside air induction, good shielding of the carb, isolation of the carb from engine/exhaust heat, and even cooler fuel will all tend to hold down the chamber heat until the actual fuel/air charge is ignited.

We mentioned the cam timing as affecting actual CR. That is only one important criteria in selecting a correct cam. The duration will determine the minimum and maximum rpm points that the cam is most effective in. A modest duration of 200 to 220 degrees intake will provide good low end power, and will generally allow the smaller engines to easily run to 5400-5600. A 455 may like up to 230 degrees for that rpm range while retaining the good low end. The lobe positions will determine how the power is concentrated within the operating range. Intake lobes that are positioned fairly early (104-108) tend to concentrate the power more in mid range but shuts down power earlier in rpm. Later lobes (110-116, more typical of the larger factory cams) do not usually have quite as strong mid range, but typically will run strongly to a higher rpm. The lobe separation also tends to control the power spread as well as the idle quality. Tighter lobe cams in general have more overlap, and that will degrade idle smoothness and vacuum. Wider lobe cams will usually idle better, have a smother and wider power range, and provide better fuel economy. There are certainly exceptions to these general rules, and the cam makers constantly strive to combine the benefits of one type with the best features of another type. One fact remains: If the cam provides excellent high rpm power, it will not have strong low rpm power, and of course, the opposite is true. An incorrect cam selection can and usually does make the engine work harder at some rpm point/range. By "working harder," I mean it takes more throttle opening, more fuel, and thus more heat may be generated and wasted, in order to provide the power needed at that rpm point. Excess heat in turn, is transferred into the block, heads, and water, and the engine is more prone to knock or detonate. Selecting a cam should not be done by simply reviewing cam catalogs, or talking to the guy that sells them. Find out which cams are doing what you need in similar weight, geared, and type of vehicle. Spend a lot of time reviewing what Pontiac did with similar size engines. Then work from that point. The RAIV cam is considered a baby by many on this board, but it is the same grind as the McKellar #10 solid lifter cam that powered the Super Duties at Daytona Beach! That cam was never installed in a 4000# vehicle, nor was it ever used with a 3.23 gear! I have seen it called "slow acting." I believe that means it does not have all its power concentrated in the mid range. As a result it is still pulling hard at 5400-5600 when the "quick acting," similar duration units, are dead in the water. None of this is to tell you to use a factory grind, but the factory grinds provide a good foundation from which to evaluate other cams, and to take that first step up into more radical units. In summary, the cam characteristics of duration, lobe position, and lobe separation determine how the cam will act in each engine. Added lift (within reason)will usually add torque, but does not generally change the power range. In my experience, more problems are caused by improper cam selection then about any other mistake we can make in designing our engines.

Next time, we will summarize the information covered, and will add any material inadvertently omitted to this point.

In the last two parts of this series, we have discussed some of the things that affect ignition knock/CR, and also steps to take when designing/assembling an engine to minimize the bad effects of higher CR. In this, the final segment of the series, several more subjects will be covered, and a brief review of the complete series will be presented.

Spark Plug Heat Range: This is an area with much incorrect information floating around, so will try to clarify it. The function of a spark plug is to fire the compressed fuel/air mixture within the combustion chamber. It must function when the engine is cold, when the mixture is too rich, too lean, or when the engine is very hot. If the cylinder misfires for some reason, or there is oil in the chamber, or the mixture is simply too rich, a fuel/oil residue will be left in the chamber and on the plug. If the residue builds up around the electrodes on the plug, the high voltage will be shunted to ground and there will be no spark. The plug designers try to design the plug to be self cleaning, just like a self cleaning oven. If the plug electrodes can be allowed to get so hot that all residue will burn off on each cycle, the plug will always stay clean and fire as intended. However, if the design allows the plug tips to get too hot, they will begin to melt. Thus, the different heat ranges of spark plugs. The physical design of the center electrode holder, as well as the electrode materials, determines how quickly the electrodes will cool after each firing cycle, how well they are cleaned, and how well they last. The perfect heat range is that range that will keep the plug tips/electrodes clean under every driving condition your car experiences but will last indefinitely. Heat range has absolutely nothing to do with spark conduction, engine power, or how strong the engine runs. The exception is that if the incorrect heat range is selected, the plugs may foul and cause a loss of power, or if too hot, begin to miss after the tips burn away. Almost all of our street and street/strip cars should run plugs equivalent to the original factory heat range! If the engine is mostly race, a cooler range can be used, but will not make the engine run any better. If the range is too hot, the plug electrodes can act like glow plugs and cause self induced ignition. If they are too cold, fouling will regularly occur, causing a loss in performance.

For higher CR, there is no reason to vary in plug selection. Pick the ones that stay clean in all driving conditions. I run Champion RJ 12C plugs, and in fact, have had the same plugs in service for over one year (at least 250 drag strip runs). I substituted a new set of NKG this spring, and there was absolutely no change in operation or performance, so the Champions went back in. The correct heat range for the engine in question will do miracles for plug longevity!

Carburetor Metering: Correct metering is absolutely vital in order to run optimum CR. A fuel mixture that is too lean or rich, will not provide peak power output, and this will cause the engine to run hotter than would an ideal mixture. Remember that hotter engine temperatures and higher CR are not good mates! Correct metering means as close as optimum as possible at idle, part throttle, cruise, and full throttle. A too lean mixture will almost guarantee ignition knock, regardless of CR. If in doubt, run slightly rich rather than slightly lean. When testing at the track, if it is found that the car runs essentially the same with a several thousands range of rods or jets, select those in the middle or richer part of the range, and not the leanest. Surprisingly, most engines will deliver better gas mileage on the road if the mixture is shaded towards the rich side of perfect, and the engine will run slightly cooler!

Valve Stem Sealing: We mentioned oil in the chamber as a detriment to optimum CR. How do you keep oil out of the chamber? Obviously, good overall ring seal, but oil can also enter via the valve guides. Case in point: We run Rhoads variable lifters. When the oil is warm, these lifters drastically reduce overlap at lower rpm. Less overlap means higher vacuum in the cylinder/chamber. As we improved the performance of our engine, we begin to notice a puff of smoke at startup when warm. Various types of oil stem seals were tried. Finally to cure the problem we did several things: Installed solid bronze guides, and set the clearances very tight (I wont discuss the numbers - consult with your machinist for recommendations), and installed positive Fel Pro neoprene type oil seals (PN SS 70014) on both intake and exhaust valves. No smoke at all at any time with these on the wagon engine. The oil was settling on the exhaust valve and being drawn through the guide at startup. The bronze guides need less lubrication and so the exhaust valve can be run tighter and dryer than with iron guides. The Teflon seals are designed to meter oil to the guides, and I would not recommend using them for any street/strip engine.

I am sure that we and others will think of additional items that will affect optimum combustion, heat control, VE, and the other various factors that provide peak power with minimum temperature rise. (Minimum temperature rise means we can safely run CR that is close to optimum for each of our engines.) However, this will be the last chapter on the subject.

In summary, this series was prepared to give each of you some things to consider when designing/building a new engine - factors to be considered when selecting the optimum CR for the engine. The effects of cam timing, carb tuning, ignition timing, heat control, deck height, intake manifold selection, plug heat range, load on the engine, poor compression and oil control, chamber finish, and various other subjects have been discussed. If you were waiting for me to tell you what CR is optimum but "safe" for you, neither I nor anyone on this planet can tell you what is safe. This exercise was intended to provide you with some knowledge to help select the optimum CR based on fundamentals, and not some "Pontiac authority" saying "you cant possibly run over ___ CR". Each case is different in that different quality gas is available, the vehicles are of different weight and have different transmissions and gears, different cams are used, the chambers are prepared differently, and so it goes. This has been a great review for me, and hopefully has provided some helpful information for each of you.

[Webmaster's note: Want to do some quick checking? Do you know the key values to determine your engine's static compression ratio? If you do, try out the compression ratio calculator. If you really know your engine's values the calculator will provide very close data for you to consider.]