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· Banned
206 Posts
Discussion Starter · #1 ·
Mr. Hack,

Was wondering if you could explain why reducing the length of the intake runner and reducing the backpressure of the exhaust side of the head (installing a header) will reduce the low end torque and move a specific torque and hp point to a higher place on the rpm curve.

From a stock setup, decreasing back pressure and reducing inlet length makes horsepower, but a point is reached when this process becomes detrimental. Engine firmware is not the problem. The solvencies for such a problem is to perform headwork. Port, polish, deshroud valves, camshafts, larger valves, etc. Yes engine management is crucial here, but not the bottle neck.

The question is how does this breakdown explain itself due to intake and exhaust modification

:mad: Razzmatazz

· Rest in peace, Coach
9,614 Posts
Razzmatazz said:
Dan, nevermind.

Obviously you avoid me thinking this was a lure to test your knowledge. I understand your hesitance. The funny thing is, I need the answers. But I know to whom I can call on to provide me the education I require.

Okay here is a blatent barb. "Go back to your assemblies with nuts and bolts"

I'll post the results of my finding when they arrive.

:mad: razzmatazz
I don't think we're on first name basis.

And yes, you are on my ignore list, so frankly, I will admit you succeeded in baiting me to respond again. I'll try harder next time.

As far as your question is concerned, I am NOT an engine designer so I can not offer detailed answer to your inquiry. I believe there are people far more qualified than me to answer this question on this board but I will make a rudimentary stab at it.

The basic concept behind an internal combustion engine is simple. Think of a pipe with a big chamber in the middle. The front of the chamber is the air intake and the rear is the exhaust. Basically changing the intake runner length and the flow of exhaust is a way to control the rate of combustion. More oxygen = higher rate of combustion. Higher rate of combustion = more torque. More torque = More HP.

Now, that's the basics. The more complicated stuff is a little fuzzy to me. From what I understand, longer intake runners and more back pressure means more volume of air available per combustion cycle, thus more torque produced at lower RPM. However, as the engine speeds up and the rate of combustion increase, a shorter intake and a free flowing exhaust will allow for smaller volume of air per combustion but MORE cycles of combustion per timed unit, thus allowing the engine to rev faster, so even if the overall torque per combustion drops, you get more engine cycles and since engine HP is a formula of RPM X torque, you get more horse power out of your engine.

At least that's the way I understand it. I'm sure there are more inner workings that I have no clue about. :dunno:

Let me know if I am wrong. And please don't call me Dan...You haven't earned that right in my book.

· Registered
7,544 Posts
Good stuff HACK. As you say there is a LOT more to look at as well. As Razz says at some point the reduction of backpressure is detremental at some point, try running your car without any exhaust manifold. You also have to consider valve overlap, cam timing, the laminar flow of the air/fuel mixture into the combustion chamber. If the flow is too smooth then your fuel does not mix correctly and you get less than optimal power.

The total explanation will fill a large volume.

· Banned
20,253 Posts
in_d_haus said:
Good stuff HACK. As you say there is a LOT more to look at as well.
Good job guys. Not to mention the further complication resonance effects have on airflow through the intake manifold, due to the periodic, non-uniform nature of basic engine operation (valves opening and closing, pistons drawing air in, etc.). Because of this, like a guitar string that wants to vibrate a modal frequency based on it's length, an engine intake acts much like a set of organ pipes that will work with or against airflow depending on whether or not the path length is resonating with the valve timing.

This is why the E46 has a resonance valve in the intake manifold that changes the effective path length at different RPMs.

This entire subject is exceedingly complex. I've only scratched the surface as far as my own knowlege is concerned. Gee, perhaps that's why one can get an engineering DEGREE in automotive mechanics, eh? :D

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