Dave was co-partner with ace mechanic Demetri Kokkoris,
the team known as SUPERJET SPECIALTIES of Queens, NY.
Superjet rebuilt and blueprinted musclecar engines, specializing in modification
and custom tuning of
Rochester Quadrajet carburetors.
Below is an article written by Dave regarding camshaft and cylinder head selection for the Pontiac V-8, one of several articles that appeared in Pontiac musclecar related publications.
Following is some additional info by highly experienced Pontiac ace, Jim Hand.
Due to time constraints, Dave is no longer able to answer automotive questions.
Blueprint Tuning the Pontiac V-8:
A Guide for Cam & Head Selection
Valve
timing and compression pressure are the two most important factors that effect
an engine's performance. All other
modifications will be futile unless the valve events and compression pressure
have been carefully dialed in per desired state of tune. Through experimentation, we have developed the following
"formulas" which can help when attempting to create an optimum parts
combo.
-
The optimum (maximum) cranking cylinder compression pressure for a performance/street application using 93 octane fuel = 160 -170 psi
-
Changing the position of the camshaft (and thus intake valve closing event) by 4 degrees changes cranking cylinder pressure by about 5 psi. (Advancing increases pressure, retarding decreases pressure.)
-
Changing the compression ratio by 1 point changes cranking cylinder pressure by about 20 - 25 psi.
CAMSHAFT
(all cam and valve event figures measured at .050" lift)
The
first thing is to choose a cam grind that will yield the desired manner of
operation. We know from experience
that a cam such as Pontiac's 068 - "H.O." (211/225 - 116 lobe sep.) or
similar will produce a broad torque band in a 400, and thus excellent all around
power, while yielding a pleasant idle and relatively good fuel economy.
We also know that a cam such as Pontiac's 041 - "RA IV"
(230/240 - 114) or similar will produce max torque at a higher rpm, thus greater
horsepower, but at the expense of lesser low end torque, poor idle, poor
economy, and excessive emissions.
Any
Pontiac enthusiast who seeks improved performance but intends to use his/her car
as a "daily driver" should aim for a maximum camshaft intake duration
of 210 - 220 degrees, depending on the engine/situation.
Exhaust duration can be a bit greater without an adverse effect.
For those who are ONLY concerned with obtaining the lowest possible
quarter mile e.t. ("race" use), cam durations upwards of 225 degrees
will be more effective. Such larger
duration cams are NOT appropriate for use in "daily drivers" because
of their adverse effect on economy and street behavior.
A car that is used nearly everyday, city and highway, heat wave or snow
storm, must be dependable, efficient, and convenient to use.
DO NOT make the age old mistake of creating a "race" type
vehicle which is actually intended for "daily driver" usage.
Remember
that engine displacement is an important factor when choosing a cam grind.
Bigger engines can tolerate larger durations.
A Pontiac 068 grind will act a bit "wild" in a 350, but act
"mild" in a 455. Also,
heavier vehicles, and vehicles with low numerical rear-end gearing will be
better off with a lesser cam duration, where lighter vehicles, and vehicles with
higher numerical rear-end gearing can get away with a greater duration.
COMPRESSION
The "compression ratio" (also referred to as "static" compression) is determined by the cylinder head combustion chamber, head gasket, deck height, and piston face volumes (dead volume), with respect to the total cylinder volume swept by the piston (swept volume).
The intake valve closing event is determined by the intake duration and intake lobe centerline of the camshaft, as well as the actual position of the cam as installed with respect to the crankshaft.
Actual
compression
pressure, also referred to as "dynamic compression", is determined by both the
"static" measured compression ratio, and the intake valve
closing event. The dynamic compression is
the compression pressure the engine actually sees, regardless of the measured
ratio, and is all that really matters. Consideration must be given to the matching of the cam
and heads to yield the desired compression pressure. It is too common an error that a new cam grind is swapped in
without paying attention to compression, resulting in too low a compression
pressure.
Optimum
cranking compression
pressure when using 93 octane fuel is 160 - 170 psi, at least with old Pontiacs.
Too high a compression pressure will yield too high a peak combustion
pressure and thus cause detonation. Too
low a compression pressure will yield a low combustion pressure and thus poor
performance and poor economy.
Some
well-blueprinted Pontiacs have been known to handle pressures upwards of 170 psi
when using 93 octane fuel without detonation. But considering the inconsistent and often poor quality of
today's fuel, it is much wiser to aim on the low side of what might technically
be the maximum useable pressure. Detonation
must be avoided at all costs. We have found that pressures of 190+ psi will cause
detonation, overheating, "run-on", etc., unless 98+ octane fuel is
used, or ignition timing advance is reduced way below optimum. Remember that you should expect to often receive fuel with an
actual octane rating of less than what you're paying for.
Also, at any given time, a few cylinders may be running a tad hot/lean
due to one of several common problems (uneven coolant flow through the block,
uneven fuel mixture distribution, etc). For
street use, we vote to dial in a "safety zone" and stay
detonation-free.
EXAMPLE
Begin
your project by measuring the existing cam and compression specs as a starting
point, and then use the previously mentioned "formulas" to get an idea
of what parts are needed to create an optimum combo.
It is very important to measure all the variables of your own project
engine. Do not rely on existing
texts/manuals for measurements. Texts
are often inaccurate, and original parts of your engine may have been modified
or swapped prior to your ownership.
We
recently disassembled an excellent bone stock 1975 Pontiac 400 from a Trans-Am.
Actual compression ratio was measured at 7.8:1.
Cam was a Pontiac 066 and measured in at 197/206 - 112, as specs call
for. Even with the original
stretched timing chain, cam was found positioned at a 107 degree intake lobe
centerline, also as specs call for. This
places the intake closing of this cam at 26 degrees abdc, as measured. Before we had disassembled the engine, cranking cylinder
pressure was measured at 135 psi across the board.
In
order to increase performance to the desired level, we decided to replace the original 066 cam with a
Pontiac 068 cam. If we install the
068 cam "straight up", the intake closing event will be at 39 degrees
abdc. This is 13 degrees LATER than
with the original cam (13 degrees retarded from original). Refer back to our
formulas, and note that the engine would now produce roughly 15 psi LESS
compression pressure with the 068 cam due to the later intake closing - from 135
psi down to 120 psi. A compression pressure of 120 psi is too low to support optimum
efficiency. Desired pressure is
160-170 psi, so we need to increase pressure by 40-50 psi.
Refer back to our formulas, and note that it is possible to gain 40-50
psi by increasing the compression ratio by 1.5 to 2 points.
Our target compression ratio is thus 9.5:1.
In order to reach this compression ratio with a .030" overbore and
typical flat top replacement pistons, we'd need heads with 82cc combustion
chambers. Since we cannot readily
get our hands on heads with such a chamber size, we've chosen a pair of cheap
and plentiful 93cc "6X" heads (off a `77
350). These heads will be
milled .040" to achieve a chamber size of 86cc.
The compression ratio will be about 9.2:1. Estimated compression pressure is 150-155 psi - close enough,
considering the ease and low cost of this plan.
To
gain a bit more psi, we can advance the cam.
Or, we can try a different cam grind with similar duration to the Pontiac
068 cam, but with a tighter lobe separation which will automatically advance the
intake closing event with relation to the other events.
The amount of overlap gained by reducing the lobe separation from say 116
to 112 degrees on cam with 068 size duration will not cause any measurable
adverse effect on economy.
A
pair of older "16", "62", or similar "high
compression" heads could have been chosen, but with their small 72cc
chambers, estimated pressure would be up in the 185+ psi range with our combo -
too high for our purposes.
Keep
in mind that if the wild Pontiac 041 cam was installed "straight up"
in the above engine, the intake closing event would be 47 degrees abdc, 8
degrees later than with the 068 cam. This
alone would drop pressure by about another 10 psi, and thus the 72cc heads (10.4:1 c.r.)
might just slip by as detonation-free. Estimated
pressure is 175 psi - right on the line.
OVERLAP
A
common misconception is that camshaft overlap reduces cranking cylinder
compression pressure. Not so.
Overlap indeed reduces peak combustion pressure by diluting the incoming
intake charge with exhaust, and by sending some of the combustible mixture out
past the exhaust valve before ignition, but this does not affect compression
pressure. The intake valve closing
event is the only cam related factor that noticeably affects compression
pressure.
An
experiment was conducted: a given cam, as installed in a custom 9.9:1 c.r. 455,
yielded about 185 psi. Detonation
was evident. Cam had only 1 degree
of overlap, and the intake closing was at 39 degrees abdc.
A new "larger" aftermarket cam with a full 14 degrees of
overlap, but SAME intake closing as the previous cam (39 degrees), was
installed. (New cam had greater
duration and a tighter lobe separation.) Cranking cylinder pressure was exactly the same - 185 psi.
Other engine characteristics changed, but detonation was still present.
(Fuel metering and ignition timing were correctly tuned for each
application.)
The
first cam was then refitted, but was retarded 10 degrees from the original
position so that the intake closing was now at 49 degrees abdc.
Pressure dropped from 185 to about 170 - the tendency of detonation was
noticeably reduced. (The optimum
set-up for this particular engine was ultimately achieved by reducing the
compression ratio and running the smaller cam slightly advanced.)
Cylinder
compression pressure effects the amount of peak combustion pressure to a greater
extent than does the amount of overlap, per degree of change.
So, for those trying to reduce detonation problems in original "high
compression" engines by merely swapping the cam, be sure to get a cam with
a wide lobe separation and thus a relatively late intake closing.
Note that TOO late an intake closing event will adversely effect street
performance. Such allows an
excessive amount of fuel mixture to be pushed back into the intake, creating
significant reverse pulses which will hurt low end performance and can even
upset carburetor operation.
The
above information is general, and is intended as a guide.
Figures are approximate, and will vary with different situations.
Also, remember that "budget" measuring devices often have large
error margins. (Inexpensive
compression gauges will vary from each other by as much as 15+ psi, etc.)
It's
always a good idea to consider tech articles and recommendations, but you can
never avoid experimenting with your own project when optimum results are
desired. If you want it done right,
you must make your own educated decisions based on your own specific data, and
do some trial & error testing, because
every situation is unique.
Concept
and article
by Dave Miranda
Mechanical
tests conducted by Demetri
Kokkoris and Dave Miranda
ABOVE: The Official "Elf-Mobile" - 1968 G.T.O., owned and raced by Demetri Kokkoris.
Due to time constraints, Dave is no longer able to answer automotive questions.
Additional
info directly related to the above subject, emailed to me in 1998 by highly
experienced Pontiac ace, Jim Hand.
Thanks for the input, Jim! -Dave
Factory
Grind 066 Intake is 197 with LC at
107, dynamic CR = 8.21
Factory
Grind 068 Intake is 212 with LC at
113, dynamic CR = 7.25
Factory
Grind 744 Intake is 224 with LC at
113, dynamic CR = 6.67
Factory
Grind 041 Intake is 230 with LC at
112, dynamic CR = 6.65
Comp
Cams 268 Intake is 218 with LC at
106, dynamic CR = 7.62
My
Wolverine 234/244 Intake is at 107
with LC at 107, dynamic CR = 6.84.
What
is the significance of these numbers? The
066 and the CC 268 would be real pingers in an engine with higher rated static
CR. The 041, 744, and my Wolverine
would be less likely to detonate. The
earlier lobes (intake closing points) of the 066 and CC 268 make the engine
think that it has higher static CR. Longer
durations tends to minimize this tendency, but does not preclude it.
For
example, the same Ultradyne cam grind - 239-247 at a different lobe separations
(LS) show the same characteristics. The
normal 239/247 has the intake at 104 (exhaust at 116) and an LS of 110, and has
a dynamic CR of 6.90. However,
using the same lobe profiles but placing the intake at 100 and the exhaust at
112 for a 106 LS (which is essentially advancing the intake closing event by 4
degrees compared to the previous), the dynamic CR jumps to 7.19.
If the engine was marginally OK with the 110 LS cam, it could be ruinous
to install the 106 LS cam.
Be
very careful of any aftermarket versions of Pontiac grinds. Invariably, they
have advanced the intake lobes in order to improve low rpm throttle feel, but
give the engine more dynamic CR!
Here
are the steps I take to minimize high-CR detonation related problems:
When
preparing the heads, as part of the porting we lay back the shrouding on both
sides of the intake valve. We start
cutting at the base of the overhang (chamber floor), and straighten and tilt
back the overhang such that it does not get closer to the valve as the valve
lifts. All chamber edges are
rounded/smoothed. The entire
chamber, including both valves, are polished to a bright luster.
The piston tops are prepared by rounding the edges of the piston and the
valve relief's, and then they also are polished.
This practice is based on my desire to eliminate any potential hot spots,
as well as suggested by KB Piston Co. KB
states that the polished surfaces will tend to reflect heat back into the
chamber thus generating more combustion pressure and power.
We also carefully prepare each spark plug by filing all edges to a
rounded surface. So do any of these
things help? They certainly can't
hurt, and by minimizing every possible hot spot, I feel even safer.
Now
for the most important step of all regarding CR.
Set the engine deck to 0. This will vastly improve quench/squish, which
in turn improves fuel air mixing. Better fuel mix burns faster and more
predictable, thus requiring less ignition advance.
Presto! We have won twice!
More CR due to the 0 deck, and less timing required so we can run even
more CR. When I made this simple
change (about .020 off to get to 0 deck), my engine ran quickest and fastest
with 30 degrees total timing as opposed to 34 total with the .020 deck.
This should be the first step in setting the final CR in every serious
engine build-up! On the reverse
side of this effect, cutting the pistons tops (and thus increasing effective
“deck volume”) may aggravate a detonation problem because of the poorer fuel
mixing! Even though the static CR
is lower, the timing has to be advanced to get any power, and we are back into
the detonation problems.
And
don’t forget temperature control! I
was at the drag strip last night in 86 degree weather.
While in line, the engine got up to 190, so I sped the rpm up to about
1500 for 20 seconds. It dropped to 180, and by the time we hit the finish line
at 111.5 mph, it had dropped down to 175.
-Jim Hand, May 1998
Jim’s above approach using “dynamic CR” figures (via an analyzer or calculator) as a guide for dialing in the optimum cam/head combo (optimum peak pressure) is essentially the same premise as using measured cranking compression pressure as a guide for the same. In both approaches, we are studying the relationship between the actual physical ("static") compression ratio and the timing of the intake valve closing event... which together make up the "dynamic compression", or compression pressure the engine actually sees.
The method of calculating dynamic CR on paper (or on a computer) is nice since it can be done without actually assembling and testing a physical engine. Of course, assembling a proposed combo and then measuring the results will deliver an actual physical indicator of what is really taking place. As well, it is good for comparing the measured stats of your current set-up to a new proposed set-up in order to observe the true degree of change and establish benchmarks for your own engine.
As Jim has pointed out in the past, the “optimum” measured cranking compression pressure figure will vary from engine to engine… while 170 psi may be “optimum” in one set-up, 200 psi may be optimum in another set-up, and so on… and then of course we must remember that there will be a good deal of error with typical compression gauges, and differences in the measurement procedure itself (condition of battery, rpm of starter motor, ambient temperature, ambient pressure, temperature of engine, etc… all may effect a cranking compression pressure reading by easily 10% or 15%.)
So it appears that the only way to arrive at the optimum dynamic compression ratio figure for one’s own situation is to experiment… which may mean more than one swapping of cam/heads. Many weekend hobbyists may not have the time or budget to reconfigure their engine more than once, so this is why, when in doubt, I personally vote to aim on the low side when plotting “optimum” dynamic CR. Just my personal opinion.
For STREET performance use where no serious competition is involved, running a slightly lower than “optimum” dynamic CR will still net you excellent performance and economy, even if you might be down on power by a tiny amount compared to true "optimum" dynamic CR. HOWEVER, running a slightly higher than optimum dynamic CR may yield realistic headaches such as detonation, run-on, hot operation, etc. If you can only rebuild your engine one time, which scenario would you rather have? I personally vote to stay detonation-free at all costs, even at the possible expense of a tiny amount of performance.
However, needless to say, if one is serious about squeezing out every
last drop of performance for competition use, then experimenting is absolutely
necessary to arrive at the true optimum state... be prepared to swap parts more
than once. But after all, isn't
that what hot-rodding is all about?
Due to time constraints, Dave is no longer able to answer automotive questions.
MORE INFO COMING SOON!
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