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Jim Hand is to the Pontiac hobby what Warren Johnson is to NHRA Pro Stock Racing. Jim's meticulous approach to engine building, respect for empirical data and trial & error is the reason Jim's 4,000 pound wagon runs 11.30's on street tires, through the exhaust.

Jim has been involved with and raced Pontiacs longer than I have been on this earth. He has amassed a considerable amount of  knowledge on how to make a Pontiac run. His articles have appeared in Pontiac Enthusiast, High Performance Pontiac and he has conducted performance seminars at the GTOAA National conventions.

The following article is about compression ratios and related factors pertaining to compression in a Pontiac engine.

 Eric Douthitt, June 1998

 

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Update 10/2000: Jim runs 11.3299 at
117.24 MPH.

* See Jim's new motor at Pontiac Street Performance, click here.

 

Factors Affecting Selection OF Optimum Compression Ratio, And Detailed Suggestions On Engine Prep For Optimum CR.

Jim Hand, June, 1998


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We regularly hear admonitions in print about the dangers and hazards of running ideal compression ratios. Unfortunately, most of the comments about adequate or optimum CR are negative, and donít inform us of how to actually build an engine to live with the desirable and performance enhancing higher ratios. The following information was prepared to provide an insight on all the factors that must be considered when designing and assembling an engine for best performance on the selected and available gas that each plans touse. No one can tell you what is the "safe" CR, and this series is no exception. However, it will provide a detailed look into the factors that must be considered.

Part 1, Introduction:

When preparing and engine for high performance output, selection of the optimum compression ratio (CR) is one of the important factors in the process. However, CR by itself is somewhat meaningless. What we are really interested in is compression pressure, because that is what the engine sees. High compression pressure increases the tendency towards detonation, while low compression pressure reduces performance and economy. Maximizing cylinder pressure benefits power, and one sure way of increasing cylinder pressure is to increase the compression ratio. The cam selection and intake system can also have a major affect. In my opinion, the most
important factors in selecting the optimum CR is the deck height, the cam timing, the shape and finish of the combustion chamber and pistons, the operating rpm range, and of course, the fuel available. All of these factors can be controlled to some extent, especially during initial build and assembly of an engine.

Deck Height or Deck Clearance:

This most important step consists of measuring the distance from the assembled piston tops to the surface of the block deck (deck clearance), and milling as necessary. The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. As most of our Pontiac head gaskets compress to about .042", that means we want about 0 deck clearance. The quench area is the flat part of the piston that would contact a similar part of the head if you had .000 assembled quench height. In a running engine, the .040 quench height decreases to a close collision between the piston and the cylinder head. The shock wave from the near collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. The shock wave also provides better fuel/air mixing, and this allows the fuel to ignite better and burn faster. A faster burning fuel charge means less timing is required for optimum power output. An example of this - after running my 462 for years with a factory deck height of about .020, we set the deck to 0. There were no other significant changes to the engine (new rings and bearings, but same cam, heads, intake and exhaust systems). The optimum timing setting prior to the change was 34 degrees - that provided the fastest MPH and quickest ET. After the change to 0 deck, the optimum timing using the same Amoco gas changed to only 30 degrees total mechanical. Not only did the lowering the deck raise the CR by several tenths of a point, but by retarding the timing 4 degrees, we were later able to increase the CR even higher due to the optimum lower timing setting.

Note: Since it is the close spacing between the piston and cylinder that reduces the prospect of detonation, never add a shim/head gasket, or flat cut the pistons tops to reduce CR. If you have proper quench with 10 to 1 CR, and then reduce the CR to 9.5 by one of these two methods, you will create more ping with the 9.5 CR then you had with the 10 CR. By all means, deck the block first and under all circumstances when building an engine for optimum power output, and then determine what chamber volume will be needed in the heads to arrive at the final CR.

Part 2, Cams And CR

Most cam companies recommend increased CR to be used with their "higher performance" cams. Why is this necessary or recommended?

Letís review CR and what it really means. "Static" or "rated" CR is a ratio of the fixed volume of space above the piston top in the cylinder at Top Dead Center (TDC), to the volume of space displaced by the piston when moving from bottom dead center (BDC) to top dead center. When we look at actual cam timing specs, we find that the intake valve does not close until sometime after BDC. There can be no compression until the intake valve closes, so actual CR will be less then the static. How much less will be determined by the closing point of the intake valve. This actual CR will be referred to as the "dynamic" CR. These two different CR values are predictable and calculable. There is a third type of CR which represents the total cylinder pressure, and is the result of many more variables, but this value is unpredictable and for all practical purposes, unmeasurable. We will discuss it in later sessions.

Here are some examples of static CR and dynamic CR of a given engine using different cams. The dynamic values are as calculated by a Performance Trends Engine Analyzer program, and they may vary slightly from the absolute values. However, they will suffice for comparison purposes. There are equivalent math formula for calculating these values, but are quite involved. Assuming a 462 engine with different static CR values as noted, the real (dynamic) CR will be listed by cam type.

Comp Cams 268H (Intake 218 duration, 106 LC, Exhaust 218 duration, 114 LC). Intake valve closes at 35 degrees ABDC.

8 Static CR, Dynamic CR = 6.14

9 Static CR, Dynamic CR = 6.87

10 Static CR, Dynamic CR = 7.6

Comp Cams P-306R (Intake 275 duration, 102 LC installed,, Exhaust 278, 106 LC
installed). Intake valve closes at 59.5 degrees ABDC.

10 Static CR, Dynamic CR = 5.09

11 Static CR, Dynamic CR = 5.55

12 Static CR, Dynamic CR = 6.00

13 Static CR, Dynamic CR = 6.46

What do these numbers mean in regards to practical CR? What they show is that a shorter duration/ advanced intake lobe cam, will have much higher real compression then a long duration high performance cam in the same engine. Note that the 268 provides higher real CR with 8-1 rated CR then does the 275/278 at 12 - 1 rated (static) CR. How can we use this information? First, if we have a factory high-CR engine, we sure as heck donít want to install a cam like the 268. We want to look for a cam that will provide the desired power range while keeping the dynamic CR as low as
possible/practical. Note the intake closing points of the two cams above - the 268 closes at 35, and the race cam closes at 59.5. If we can find a cam that has the intake closing later then 35 but before 59, we should have a better chance to live with the resulting CR. How abut the Pontiac cam grinds?

744 (RA III) (Intake 224, LC 113, Exhaust 236, LC 118) same 462 engine. Intake valve closes at 45 degrees ABDC.

8 Static CR, Dynamic CR = 5.6

9 Static CR, Dynamic CR = 6.25

10 Static CR, Dynamic CR = 6.91

041 (R IV) Intake 230, LC 112, Exhaust 240, LC 115). Intake valve closes at 47 degrees ABDC.

8 Static CR, Dynamic CR = 5.42

9 Static CR, Dynamic CR = 6.05

10 Static CR, Dynamic CR = 6.69

How about the cam that I use - Wolverine 234/244? (Intake 234, LC 107,
Exhaust 244, LC 117). Intake valve closes at 44 degrees ABDC.

8 Static CR, Dynamic CR = 5.55

9 Static CR, Dynamic CR = 6.19

10 Static CR, Dynamic CR = 6.84

Again, a review of these numbers shows us that a RA III or RA IV cam will provide the actual CR of the CC 268 while running a full point higher rated CR. None of this is to recommend or discount the use of any cam, but is only to show the relationship of the intake closing points on true engine CR. Another way to use this relationship is to see what cam might work best if we have low static CR and want improved performance in the driving rpm range. The CC268 provides almost a point higher real CR then does the RA III or RA IV, so on an 8 to 1 engine, we would have much better throttle response and engine power within the operating range of the CC 268. Do be aware that this type of cam has a much shorter rpm range then either of the Pontiac cams.

While the Wolverine 234/244 increased the dynamic CR of my engine slightly over the previously used 041, the change was minimal, and the resulting increased mid range power increase of the Wolverine provided about .1 ET gain, and 1 MPH gain over the 041 on my setup with no adverse effects on the CR/detonation relationship.

In summary, the cam has a direct control over the engine operating CR, and the controlling factor is the intake valve closing point. By selecting a cam with a later closing point that will provide power in the rpm range needed/desired, the tendency of the engine to detonate will be minimized. As previously mentioned, other factors will affect the engineís cylinder pressure, and some of those will be discussed in the next segment.

Part 3, Misc.:

In the first part of this series, we talked about the importance of cutting the block decks to about 0. Part 2 discussed effect of cams on actual or dynamic CR. In this part, we will cover some miscellaneous items that contribute to high cylinder pressure, and possible detonation or "Knock". In the final part or parts, some of the steps that can minimize the adverse effects of Knock will be discussed.

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.     
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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
rpm points.

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.

11A. 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.

Part 4, Positive Steps

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 letís 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, donít 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.

Part 5, Additional Considerations

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 wonít 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, donít try to increase CR on an oil burner. Second, donít 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. Hereís 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 reliefís 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 canít 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.

Part 6, Conclusion And Summary

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 wonít 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 canít 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.

Jim Hand

 

 

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