"Pontiac Intake Manifold Tests on a well tuned stock type 455 in a 3900# vehicle.''
Note; Following as a preliminary report on the intake manifold tests featured in "High Performance Pontiac" magazine several years ago.
After testing six different manifolds, I have began to understand the Aftermarket's definition of "High Performance". It generally means a series of engine modifications that allows/causes the engine to run at much higher RPM. It does not necessarily mean increased performance. Example: A 350 engine is modified with heavy duty internal parts to allow it to run at 6400 RPM without exploding. A high RPM cam is added along with a high RPM (High Performance) manifold. The engine will now develop more horsepower than stock but at about 1500 higher RPM. When installed in a lightweight vehicle with deep gears and a loose converter, it will provide the performance level expected. Unfortunately, when installed in a streetable 3700# Pontiac, it will be a real bow-wow because of the lack of low RPM power. However, the engine is now a genuine ''High Performance" unit. In contrast, my 455 engine that accelerates 4050# of driver and car to mid 12's at 109 MPH is not a classic "high performance" engine! It is a very well tuned stock type engine because it only runs to 5400 RPM and uses primarily low RPM components.
After I understood the above, the test results obtained with the High Performance manifolds began to make sense. Of the five Aftermarket units tested, only the Edelbrock Performer, which is sold as a stock replacement, ran as well as the stock cast iron unit. The other four, sold as "High Performance" caused a loss in 1/4 mile performance, as well as losing throttle response. Why? The ''High Performance" manifolds are designed to allow much greater air flow and to accomplish that, they have larger diameter and shorter length runners, and larger plenum areas. Most are also single plane (plenum) units. These design characteristics hurt the air/fuel mixture velocity at low RPM and the carburetor metering accuracy, which in turn, adversely affect throttle response and low speed power. Since my wagon is setup to need the high torque/power from about 1400 to 5350 RPM, and the engine does not operate long enough in the RPM range for which the HP manifolds are designed, the predictable happens; Loss of performance. Following are the actual test results. Each test was initiated with a stock cast iron manifold installed and at least two runs were made to get consistent times. Then, the manifold to be tested was installed at the strip. (The water crossover was removed from all manifolds to allow this change at the strip.) Nothing else was changed. The transmission is setup to shift automatically at 5350. The only difference in the engine was the manifold.
The results are presented as (gained) or (lost) in reference to the stock cast iron manifold. With it, the wagon ran in the range of 12.5 to 12.75 at 107 to 109.
|* Pontiac Aluminum||HO Lost .08 sec. and ,4 MPH|
|* Edelbrock Performer||No change in ET or MPH|
|* Edelbrock Performer||RPM Lost .05 sec. and .5 MPH|
|* Edelbrock Torquer II||Lost .0l sec. and .1 MPH|
|* Edelbrock Torquer||Lost .29 sec. and 2.85 MPH|
|* Holley Street Dominator||Lost .25 sec. and 1.9 MPH|
Note: None of the Offenhouser manifolds would fit due to the HEI. None of the single plane specialty manifolds from various sources were tested because it was obvious they would cause a performance loss on this low RPM torque engine.
So what manifold should you run? Unless you can safely run your engine to at least 6000 RPM and develop some power in the upper RPM ranges, and your car weighs no more than 3400#, and you have a four speed or a converter that stalls to at least 3000 RPM, and you are running 3.90 or slower gears, your vehicle will run better with a stock cast iron or a Performer manifold. (A Torquer II may run as well but will lose low speed throttle feel). My wagon has remarkable traction which allows for very consistent runs, and there is no question about the above test results. If you are presently running one of the "HP" manifolds on a stock Pontiac with a 5000 - 5700 RPM engine, try a stock or Performer unit even if you have to use a carb adapter. You will probably discover an increase in performance and throttle response. Watch for a future issue of High Performance Pontiac for more details including air flow tests on the seven manifolds.