So if you use the oval lower IM then you lose the split port design. That means that there is less tendency to have the benefits of short/long intake runners which was supposed to optimize low rpm torqe and high rpm torque.
I guess I have some problems with the claim that the plastic manifolds are better.
Cheaper, hell yes! Better, not necessarily. In order for Ford to make an aluminum manifold perform as well as a plastic, they have to spend more money to alter the stock casting, i.e. extrude honing to make it smooth and wide enough.
With plastic, the longevity is much shorter. Durability can bite you in the butt when you least expect it. Lets not forget about all the cracking that accompanies even high-temp polymers that are subjected to long-term high-heat environments.
And why forget about extrude hone? I agree that the plastic manifolds are a bit smoother, and with the oval instead of split port you gain a bit more area.
However, a great man once said that you don't want to polish your intake passages too much since that would actually be detrimental to performance. Something about a static monolayer of gas that acts as a thermal barrier.....
And another thing. Plastic DOES transmit heat although that isn't the most relevant issue. In fact, ALL solids have roughly the same heat capacity which approaches 3*R where R is the ideal gas constant and 3*R = 24.9 J/K-mole
What does this tell you? It should tell you that if the volume (not density) of the plastic manifold is the same as the aluminum manifold then they will store roughly the same heat.
Now the aluminum WILL conduct heat faster internally than the plastic, but the whole engine warms up at the same rate. Therefore the plastic manifold is essentially always at the same operating temperature as the rest of the motor. Thus either material will have the same effect and will tend to radiate or transmit the heat they contain into the airstream.
This says that at operating temperature the plastic will be just as hot as the aluminum manifold was and the intake charge will still pick up heat.
So, is the "smoother" plastic actually better even though it has been reported that "too smooth" is not beneficial due to above mentioned reasons.
The plastic is less durable and is guaranteed to need replacement before the aluminum manifold.
The single oval design defeats the purpose of the split port design, and will likely NOT generate as much low rpm torque.
Don't forget that it is a heck of a lot easier to modify or widen out the aluminum manifolds to match the gasket size, or even wider, than it will be to modify the plastic manifolds.
My conclusion is to keep the split port design and widen the passages so that they flow the desired amount of air. This will retain all the benefits the original engineers designed into the engine plus enhancements.
It will be interesting to see the dyno of a fine tuned 3L using split port manifolds and a fine tuned 3L using oval port manifolds.
Although there is so much CONTRADICTORY evidence out there now, trials over time will sort it all out.
So, I have to end by saying, "I don't buy it"
but to each his own. Neither approach is wrong.
It's only wrong if you have an idea and don't try to make it work.
warmonger
