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.
Douthitt, June 1998
10/2000: Jim runs 11.3299 at
See Jim's new motor at Pontiac Street Performance, click here.
Selection OF Optimum Compression Ratio, And Detailed Suggestions On
Engine Prep For Optimum CR.
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
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
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
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
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,
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
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.
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
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
2. Valve size or more correctly, amount of possible air flow through the
valve - Same general affect as throttle opening except that this will
engine at all rpm/throttle opening.
3. Altitude, or air density - the denser the incoming air, the more
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,
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
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
7. Cylinder head - Can affect cylinder pressure in several ways -
airflow at appropriate rpm, chamber design not optimum, sharp or
surfaces that could cause surface ignition and resulting knock/poor
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
will control engine efficiency at various rpm points and engine rpm
10. Engine timing - Best torque development is dependent on optimum
spark timing, not only at peak rpm, but at peak torque point.
11. Finally, the most significant of all, the engine temperature, and to
lesser extent, the incoming air temperature. Engine temperature, and
specifically the temperature immediately adjacent to the cylinder head,
critical to peak power production. High HP per cubic inch engines not
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
maximum push on the piston. However, the chamber parts must cool down to
reasonable level between cycles, so that the incoming air is not heated
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
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
Part 4, Positive
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
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
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
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
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
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
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
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
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
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
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
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.