Blueprint Tuning the Pontiac V-8

Dave was a co-partner with famed ace mechanic Demetri Kokkoris, the team originally 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.  

 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

Once you have a cam duration figure in mind, attention must be paid to compression pressure.  Optimum compression pressure 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. 

Compression pressure is determined by both the compression ratio, and the intake valve closing event.  The compression ratio 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.  Thus 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 an engine possibly less powerful or less efficient than originally.  

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 atdc, as measured.  Before we had disassembled the engine, cranking cylinder pressure was measured at 135 psi across the board.

In order to increase performance, 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 atdc.  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 atdc, 8 degrees later than with the 068 cam.  This alone would drop pressure by about 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 atdc.  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 atdc.  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.

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