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OK, but you asked for it.... (long) (archive)

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Posted by Shawn Fogg on January 11, 2000 at 13:44:45:

In Reply to: Educate me on short velocity stacks... posted by The Other Tom on January 11, 2000 at 11:44:57:



You never ask the easy quick to answer questions do you? ;)

It all has to do with your intake manifold tuning and the airflow through that and into your head and cylinders and where in the RPM band you want to maximize your volumetric efficency.

I'll try to explain this based on a single cylinder 4 stroke engine as it is easier to visulize.

At lower engine speeds you want a much longer intake 'pipe' leading to your head. The reason for this is because your engine is taking in much less air at lower speeds and at lower RPMs your intake valve is closed a greater amount of the time. Remember, at half the RPM your engine only needs about half the airflow.

If you think of the air moving in your intake manifold at lower speeds it would tend to have a sort of start and stop motion to it based on your intake valve opening and closing. You engine 'breathes' in pulses, it isn't just constantly drawing in air. Each time your intake valve opens the air needs to start moving into your cylinder again. The intake valve is only open for a short period of time so the time in which the air has to start moving again is time wasted in filling your cylinder with air. That lowers your VE. So what you want to do is try and minimize the time it takes for the air to start moving again when your intake valve opens. If you consider all the air in your intake manifold (no matter what the length) and remember that air can be compressed, how can you accomplish this?

Answer: You put in very long intake runners. Remember any moving mass has inertia. When your intake valve closes the air right at the valve of course stops dead. But all the air in the manifold behind it is still moving. It kind of crashes into the air in front of it and actually slightly compresses the air right at the valve which increases it's pressure. If the pipe is long enough the intake valve will open again before the air has totally stopped moving forward in your manifold and the slightly greater pressure at the intake valve will force a little more air into your cylinder quicker. This increases your VE.

Now if the pipe is very short there is little air in the manifold and very little inertia so it stops moving very quickly. This means that when your intake valve opens again the air needs to start moving from scratch again.

BTW, really long intake pipes are only possible with fuel injected engines. This is because just air flows through them. If you tried this with a carb. (which would be at the start of the manifold) the air and fuel would need to pass through the entire length of the manifold. The fuel would tend to fall out of the air and collect on your manifold itself.

At higher RPMs you need to flow much greater amounts of air. A long intake manifold would restrict the much higher volume of air you want to flow. The intake valve will be open more often so you don't need to have as much moving airflow to keep inertia up. So for higher RPM applications you would want a much shorter pipe which can move a much greater volume of air at higher RPMs. This would make your VE up high better.

So you should see that your intake manifold is going to be a trade off between low end torque and high RPM power. It usually will pretty much be designed to flow best in the midrange so that it still does fairly well (though not optimum) at low and high RPM. But there are exceptions to this. Look at any of the original 'M' motors (or the Euro motors of today) and look at their intake manifolds. They all have short velocity stacks, is there any wonder they are known as high end screamers? Of course they also have cam profiles that favor high RPM breathing and the rest of the engine is built to survive high RPM operation.

And of course with fuel injection and DMEs and such there are all sorts of tricks you can do. The DISA manifold on all 1.9l Z3s is a good example of that. Below about 4200rpm or so (it actually varies based on load) the DISA valve is closed which basically means each cylinder has a roughly 31" long 'pipe' leading to the head. This of course if for lower RPM torque. When the DISA valve opens the length of the 'pipes' leading to the head is basically cut in half to allow for better breathing at higher RPM.

Got it?


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