The Test Data

With the Edelbrock castings, the smaller combustion chamber raised the compression ratio to 9.98:1 from 9.1:1. When going from cast-iron to aluminum, even with the exact same chamber design, the compression ratio needs to be at least one point higher to maintain the same thermal efficiency. Aluminum transfers more heat to the coolant and thus, has a lower thermal energy conversion. So in theory, forgetting about the different combustion chamber design and spark plug orientation, the just shy of one point more compression ratio almost makes things equal in that sense.

With the increased intake flow, the aluminum heads did reward us with a nice gain in torque. The Pontiac picked up 15 lb-ft on the average between 3,300 and 5,300 rpm. Average horsepower in the same engine speed range increased by about 12. Reference the dyno sheet and graphs for the complete test data.

What did confuse us was the Edelbrock cylinder head wanted two degrees additional advance (34 versus 32). Also, the Edelbrock heads with the current engine configuration had the power ramp down quickly after 5,000 rpm, while the 6X pulled more evenly to about 5,300 rpm. This is more apparent when viewing the dyno graph than the raw data, since a projected trajectory can be seen. By 5,400 rpm the aluminum heads only had an advantage of 2.7 hp.

There is no denying that the Edelbrock cylinder heads in as-shipped form made more power than the 6X casting, but something was going on that we did not fully understand. In theory, the more advanced combustion chamber design and exhaust valve biased spark plug should not have required more ignition advance. It appeared as if The Mule was balking at the additional airflow. Was the cam too small for the better-flowing heads?

A Possible Analysis

The following theory is the author’s alone and is only conjecture. It’s based upon a search to assign reason as to why the engine did not fully use the additional airflow and its need for more spark advance.

If one were to compare the intake port flow at approximately 0.550-inch lift you would see that the Edelbrock cylinder head flows about eight percent more when on the test bench. It must be understood that the flowbench, though the industry standard, cannot replicate the true dynamics of cylinder filling and emptying that goes on in an engine. It is only a static test. In addition, the airflow equation is only an estimator and it leaves many important facts out, but can be used to get you into the ballpark. With the 6X heads the equation came very close to the actual horsepower produced, but it now indicates that the engine when equipped with the better flowing cylinder heads is not putting the additional air to work.

Now let’s use the equation backwards. Let us predict airflow from horsepower. It now becomes:

Horsepower / # cylinders/ 0.257 = cfm

502/8/0.257= 244 cfm that is being used with the Edelbrock heads

Now for the 6X calculation:

492/8/0.257= 239 cfm that is being used with the 6X castings

If you divide the 6X airflow by the Edelbrock airflow, the result is a difference of 0.020 percent. Now take the 492 (6X hp) and multiply it by 1.020 = 501.84 hp. It is almost the exact number we saw on the dyno for the E-head (502.8).

A possible explanation may lie with the larger intake port volume that may create lower port velocity as the valve starts to lift along with the lower exhaust flow rate. In theory if the port velocity at low lift is lower then the VE will suffer in the beginning stages of the cylinder fill.

The other wild card is the exhaust flow. If I had to guess, I would say that it had a minimal impact on the pumping losses in the engine, but would help to limit the engine’s ability to fill the cylinder bore with charge if energy is being used to push the inert gases out.

In my final analysis, the Edelbrock cylinder head right out of the box has the ability to produce around 535 hp when matched with a more aggressive cam profile and a slightly higher compression ratio. I may be wrong, but the cam grind may need the intake valve opened a little earlier and the duration increased along with the overlap to help scavenging. But this is only a guess on my part.

Regardless, the D-Port E-heads performed well and contain even more potential to be tapped. It’s impressive that a docile street Pontiac makes this kind of power.

Port Flow

Edelbrock D-Port As Shipped
Flow in cfm at 28-in/H2O

Lift Intake Exhaust
0.100 67 54
0.200 139 106
0.300 203 138
0.400 240 154
0.500 258 162
0.600 268 164

Ported 6X
Flow in cfm at 28-in/H2O

Lift Intake Exhaust
0.100 66 55
0.200 133 106
0.300 191 149
0.400 224 170
0.500 236 182
0.600 250 186

If you divide the 6X airflow by the Edelbrock airflow, the result is a difference of 0.020 percent. Now take the 492 (6X hp) and multiply it by 1.020 = 501.84 hp. It is almost the exact number we saw on the dyno for the E-head (502.8).

A possible explanation may lie with the larger intake port volume that may create lower port velocity as the valve starts to lift along with the lower exhaust flow rate. In theory if the port velocity at low lift is lower then the VE will suffer in the beginning stages of the cylinder fill.

The other wild card is the exhaust flow

The other wild card is the exhaust flow. If I had to guess, I would say that it had a minimal impact on the pumping losses in the engine, but would help to limit the engine’s ability to fill the cylinder bore with charge if energy is being used to push the inert gases out.

In my final analysis, the Edelbrock cylinder head right out of the box has the ability to produce around 535 hp when matched with a more aggressive cam profile and a slightly higher compression ratio. I may be wrong, but the cam grind may need the intake valve opened a little earlier and the duration increased along with the overlap to help scavenging. But this is only a guess on my part.

Regardless, the D-Port E-heads performed well and contain even more potential to be tapped. It’s impressive that a docile street Pontiac makes this kind of power.