We will resume our discussion on intake theory now with part two, starting at the intake manifold.
The next stop on our trip towards the engine is the intake manifold. There are three factors that determine if a manifold is helping or hurting your quest for more power: volume, distribution to cylinders, and the runner openings. Ideally the plenum will equalize flow to all cylinders. This is done by slowing the air down as it enters the plenum and being shaped so that it doesn’t direct air towards one cylinder more than others. Often times with a stock manifold, this is the problem. Because, due to the desire to have an appealing looking engine bay, symmetry, and even cost factors, the plenum is usually a uniform size across it’s entire length. This means pressure drops more the further away from the inlet the air goes. The result of this is a suction action towards the furthest cylinder from the inlet. A well designed manifold will have a slight taper to compensate for this pressure drop which helps to keep pressure equalized across the entire plenum.
We are not out of the woods yet. Just having a well shaped manifold at the runner entrance can undo all the hard work we have done to this point. The idea here is to take the air in the plenum and accelerate it into the intake runners. To do this, the runner entrance or, plenum exit, if you will, needs to have a taper to make sure the air moves into the runners without turbulence. The taper does this by taking off the edge of the entrance and it also serves to help accelerate the air into the runner.
Intake runners dictate where the engine is going to make power and torque. It’s hard to free up horsepower here per-se, but if the runners aren’t doing the job you would like them to, a new set is probably in order. For a moment, I’d like to step away from talking about freeing up airflow and talk about how the runners effect engine performance.
By changing the diameter and length of an intake runner, you can move the power curve of an engine around. Of course, you can’t physically change the diameter or length of the runner, so you have to design or buy them according to what you are looking for. The diameter of the runner fixes peak horsepower. A larger diameter will raise it, a smaller diameter will lower it. This means you have to rev the engine less to get to peak power. The length of the runner will rock the power curve around the peak that the diameter dictates, meaning a longer runner boost bottom end, while a shorter runner boost top end. A street engine will run better with a longer runner with a small diameter. This combination results in low end torque and a lower peak horsepower. It’s important to realize all of this if you are going to be changing the cam profile. You can use some tuned runners to compliment the cam profile you have chosen for maximum drivability or all out horsepower. The combinations here are mind boggling, so it’s important to do your homework if a new head or cam is in your future, since the wrong combination can make a car nearly undrivable on the street.
The head itself can be misleading in this whole equation. It is entirely possible that maximum air flow here does not equate to the most efficient combustion, which is what we are after. I realize that flies in the face of everything I’ve said previously, but, in the head, efficient delivery to the combustion chamber is key. Several factors play a key role in this. Namely, inlet port size, valve size, cam profile, and inlet port shape.
[I]*disclaimer: at this point I had to make an executive decision, do I drown you in equations that likely won’t mean anything to you, unless you are going to PnP and rebuild your own head, or, do I try to keep this simple and plain spoken like I have so far? I decided not to drown you in equations, hoping that some theory can sink in here and that your eyes don’t just glaze over as you read this section. [/I]
The cool hot rod, car guy, tuner, thing to do here is to go over sized. Meaning to bore out the inlets as much as possible, go with oversized valves, a huge lopey cam, etc. Do not get caught up in bigger is better here. As usual, simpler is better. Often times, a simple port and polish will result in significant gains, if done correctly. Round ports flow best and the ideal diameter is about 80-83% of the valve diameter. This keeps air flow uniform and stops it from slowing down as it goes around the intake valve and into the combustion chamber. It is also ideal that the inlet diameter is consistent. This may not always be possible, so if it isn’t, it’s best to taper down to the 80-83% number as the inlet reaches the intake valve. This will help accelerate the air towards the combustion chamber.
If a rectangular port must be used (most GM heads use a rectangular port), then the area of the port should be about 68% of the valve size. The smaller size will help over come the drag that the rectangular port causes.
Well, we have completed our journey from entrance to engine. The idea here was to give a basic theory of where power can be gained and lost, hopefully without completely boring you to death or getting too technical. Of course, left out of this write up was any mention of forced induction, which introduces a completely new animal to the subject. Maybe in the future we can discuss this, but considering the different types of compressors and specific ways to tweak and improve them as well as the secondary systems involved, a write up on that would be quite massive. For the same reason I glossed over the section on heads, I would like to, at least for now, avoid talking about forced induction. Keep in mind the same principles apply, efficiency is the key, as is minimal restriction.
As always questions and comments are always welcome. This is a discussion, not a lecture, so if you have anything to add, please feel free to do so. The idea here was not to definitively tell you how to modify your car, but to get you thinking outside of the box that the modifying community has created. Too often, people only think in terms of bolting on power, big gains, and following the crowd. In fact, there are very nice gains to be had by simply examining the system as a whole and working on the weak spots. It’s important to look at the engine as a total package, and for every place you make the system more efficient, it will make the modifications you do more effective. If you have a forced induction car and feel that this does not apply to you, think again. I assure it does because engine theory is the same no matter how your engine makes it’s power. Every engine is as simple as air in, combustion, air out, that’s all there is to it really. This applies to every internal combustion engine made. Thank you for reading and happy modding.