How Well Would Equal Length Headers Work Without The Muffler?

[nicholastanguma]

How well would equal length headers work without any muffler? Thinking about them like old school Triumph or Norton or Harley motorcycle drag pipes equipped with internal baffles, it seems like equal length headers on a Type 1 or Type 4 engine would retain necessary back pressure while affording exceptionally light weight. Am I right, or am I missing something?

[Buggsy]

Great. That's why race cars use headers.

[Lo Cash John]

What is "necessary back pressure"?

[Ol'fogasaurus]

I think it is a bit more complicated than that. Dual headers for an ACVW (I forget the slang term for them) (
for example), gives less performance than a 4 tube merged equal length header (for example system. I have used both on the same engine and you can surely feel it.

https://www.roadandtrack.com/new-cars/c ... orsepower/ This is dealing with drag racing more than a combination street and strip.

Lee

[John S.]

4 pipes sound like 2 Harley's.

[FJCamper]

Necessary Back Pressure?

This is one of those un-fun subjects rife with misunderstanding. An engine does not require exhaust back pressure to run, but some back pressure is necessary in "tuning" some exhausts. Because exhaust pulses bounce back and forth inside an exhaust pipe (place your hand over an exhaust pipe to feel this exhale-inhale pulses). The amount of exhaust is something of a constant, the diameter (and length to calculate volume) of the pipe is a variable.

And all this is more applicable to an extractor, or "tuned" exhaust, than a simple straight pipe.

Engineers calculate how much exhaust is coming out of a combustion chamber based on engine displacement, valve size, and valve timing. The exhaust "plug" if you think of the gases as a physical object, comes out of the exhaust valve and rush for the end of the pipe. You might expect there to be a smooth steady stream of exhaust, but there's not. You've felt the exhale-inhale pulse, right? The instant the exhaust valve closes, at supersonic speed external atmosphere rushes back into the exhaust pipe to replace the temporary vacuum. From all this, the diameter of the exhaust pipe is calculated.

Too small of an exhaust pipe diameter creates back pressure restriction on exit, which if too much, harms efficiency and robs power. Too large a pipe diameter and an attempt at scavenging exhaust (meeting the exhaust with a low atmospheric restriction) is ineffective.

So, in every exhaust design, back pressure is present, but only as a result of exhaust pipe diameter and volume. When people say "necessary back pressure" they're referring to it as a single element of a complex system.

You can experiment with your own exhaust by making restrictive exhaust gaskets. If you add restriction and suddenly the engine runs better, you were using exhaust pipes of a diameter mismatched to your exhaust valves.

I apologize for even addressing this back pressure subject and having to do it in oversimplifications. Ricardo's series of books on exhaust systems is the place to go, and maybe even less fun for all the math.

FJC

[VW&MGman]

Hi FJC,

Nice write up. However in practice what is the consensus of the size of exhaust pipe diameter for an given engine size CC?

I came across this chart: https://www.thesamba.com/vw/forum/viewt ... ?p=6415575

What is everyone's thoughts on exhaust size for a 2165?

[FJCamper]

I agree with the chart. I in fact use it myself.

You should be running a 1 5/8th for a 2165cc. Luckily, this is the most common header size. We use it on high-rev 1699's, and on our 2.2 enduro engines.

[woodsbuggy1]

Thanks for posting this, I was about to purchase a 1 5/8th header for my 2276cc offroad buggy, but since my rev limiter is set at 5500 RPM I will just stick with my 1.5".
Kenric

[Ol'fogasaurus]

And a spark arrester of some sort for off-road .

Lee

[DeathBySnuSnu]

I used to run 4 straights.
Sounded good in my vw powered motorcycle.
But zero scavanging effect.
I proper 4 into one seems the best for full performance.

I now am running a 4 into 2 anti reversion can open header.
1-5/8 at the head going to 1-3/4 after the first reversion can. Then steep (tri-y style) collectors. Then a 2-1/2 dia can on the collector with a 2" outlet.
1 and 3 are paired, and 2 and 4 are paired. Which would be a bit difficult to do in a car.

It seems to work and made good dyno numbers.

No mufflers at all......open all the time.....not as loud as expected.....possibly because the 2 outlets are pointed opposite directions