Ported intake manifolds... Do you need one?
If you have ported or purchased ported cylinder heads, YES. I say that with the utmost confidence. That goes for 400's as well! You don't want to go crazy with that for a short stroke combo, but let's have some common sense. Now, if your combination is already marginal due to a poor camshaft choice or too low compression ratio, then a larger intake port will make matters worse. It really does come down to the total combination. Without sounding self-serving, our "Stage" cams are designed to perform WITHOUT adding excessive amounts of intake reversion like many cams out there. In that case, a ported intake just plain works.
It has been found, on some applications, that a stock intake manifold installed on a set of ported aftermarket aluminum heads actually helps performance. Typically, if the use of an undersized intake makes the engine run better, it is correcting a flaw (or multiple flaws) somewhere else. One might say a woefully small intake is good to deter reversion. I say, use the CORRECT cam, heads, and intake manifold to deter reversion. When the intake port or the carburetor is too big, or the cam is too long, or there isn't enough compression, then yeah, the ported intake will continue to hurt the combination! Yeah, THE INTAKE DID IT!!! THAT'S THE TICKET! How does one know their engine is experiencing intake reversion? That popular rough idle, or soggy performance until a higher rpm are sure signs. Upon disassembly, darkened intake ports are a dead giveaway.
I don't expect the typical enthusiast to run a lash or timing loop after every component change... they pay good money and want their new heads or cam to work well right out of the box. So if the introduction of a stock intake gets the engine to run "better", I can't blame a guy for going that direction. But at what cost?
Volumetric efficiency. On a natually aspirated internal combustion engine, atmospheric pressure is the only "force" helping the air/fuel charge enter the cylinder. Granted, too large of an intake manifold will hurt performance at low speeds the same way too large of a camshaft or cylinder head port will. But that's why one considers the performance and drivability goals of the engine before the build is underway. We want to make it as easy as possible for the engine to draw in air on the intake cycle, yet, keep the port and valve sizing appropriate so the charge will have MOMENTUM and keep filling the cylinder AFTER BOTTOM DEAD CENTER.
There is another thing to consider: Pressure differential. It is what keeps the air and fuel flowing the CORRECT way through the engine! I won't go into too much detail, but, you don't want too much intake manifold vacuum at WOT. This may be where some guys will add a 1,000 cfm carburetor to his 570hp stroker. Okay. If one wants to take cues from a certain racing style of engine, drag racing isn't necessarily the best for a hot street car. Nascar or a circle track style of engine build may be better because they are concerned about coming out of a corner.
So how do you know if a ported intake will help your combination? It can be a little tricky for a 400 with its 1.77 rod ratio, but if you have an off-the-shelf, low acceleration flat tappet, it probably isn't for you. If you are looking for every ounce of low end torque or manifold vacuum because your cam is marginally too big or too slow or your carb may be too big, then it may not be for you. But if your engine is efficient--- towards the middle-to-upper limit of compression--- and you are using a high lift (.550"+) cam and are looking to make big torque numbers, numbers that you can FEEL, then it may be for you. Ultimately, you know your combination better than anyone else, so go for it and let me know how it works for you!