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Discussion Starter #1

....I *really* didn't see that one coming. I didn't expect it to do too much of a difference just denting them a bit but seeing how it barely has any effect at all really surprises me.

I wonder if the difference would have been bigger with a full exhaust on it or if they had tried with a "milder" engine.

I'm going to need a bit of time to digest this one because it makes little sense to me
 

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Opeler
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Larger exhaust tubes are not necessarily helping quick evacuation of exhaust gasses. Exhaust system must be designed to increase the speed of exhaust gasses. Bernoulli's formula comes to mind: Fluid speed increased, pressure decreased.

By pinching exhaust pipes, they have increased the speed of gasses and therefore reduced the exhaust pressure and got more power.

We all often say that exhaust system need some back-pressure. This is wrong, we need to increase the speed of gasses in order to lower the back-pressure.

Certainly, there is a limit how far you can go with restriction of pipes as at some point, back-pressure will indeed raise to the point that it will choke the power. It is fine balance as always.
 

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Larger exhaust tubes are not necessarily helping quick evacuation of exhaust gasses. Exhaust system must be designed to increase the speed of exhaust gasses. Bernoulli's formula comes to mind: Fluid speed increased, pressure decreased.

By pinching exhaust pipes, they have increased the speed of gasses and therefore reduced the exhaust pressure and got more power.

We all often say that exhaust system need some back-pressure. This is wrong, we need to increase the speed of gasses in order to lower the back-pressure.

Certainly, there is a limit how far you can go with restriction of pipes as at some point, back-pressure will indeed raise to the point that it will choke the power. It is fine balance as always.
Can't say that I agree with this one. Increased velocity at the collector can increase scavenging but the only way you can get increased velocity in the middle of a tube is to increase the up stream pressure which is what everyone calls back pressure.
 

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Opeler
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Well, as we agree to disagre, I did some Google-ing for another opinion.

Back pressure: The myth and why it's wrong.
One of the most misunderstood concepts in exhaust theory is back pressure. People talk about it with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need back pressure" when discussing exhaust upgrades
.

Interesting reading.
exhaustbackpressure
 

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If you ever get a chance to read up on Ed Henneman's stuff either on his website, his written works or in person (I've done all three), he really was ahead of the curve in terms of header designs.

He is the owner of 'Headers by Ed' and some of his experiments blow common knowledge right out of the water. Example: most headers are WAYYY too big for the design intent. One of his tests included putting headers on a mild 350 Chevy with bolts ons, and he kept shrinking the primary tube diameters smaller and smaller and going faster in the 1/4 mile. 1.5" primary tubes worked best on that combo even though the aftermarket only sold 1-5/8" and 1-3/4" headers for that engine/chassis.

Go figure.
 

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Discussion Starter #6
The headers essentially being too big for this application is something I can understand and accept as an explanation for why it hardly mattered at all what they they did to the header.
 

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The headers essentially being too big for this application is something I can understand and accept as an explanation for why it hardly mattered at all what they they did to the header.
Yup, probably the case.

Opel is no different really. The classic 'Sprint' exhaust manifold first showed up on the twin-carb'd Opel Rekord Sprint (106 PS) in the late 1960's. It was also used in the later EFI equipped 1.9E engines (81 hp US version, 105 PS Euro version). That same manifold, with a larger headpipe, was also used in many competition Opels throughout the '70's. Typically producing 150-180 HP.

So in reality, it was probably overkill for the street engines it was originally intended for.
 
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