Wisconsin S2k

I know I have read several articles in various mags or auto documents stating that the less backpressure the better. Some people, however, claim that you have to have some backpressure because it somehow creates torque. I know this is a myth, but I need to know how to prove it. Help me out here??

Bobway

Not a myth.

Great book on it: The Scientific Design of Exhaust and Intake Systems.

marcucci

"Backpressure" is something of a misnomer. The science behind the "need" for it is simple- pipe diameter for intake and exhaust systems relates directly to flow, but what people fail to consider many times is that it is also related to gas velocity. Opening up an exhaust in diameter (or in length, like open headers) drastically affects the gas velocity. Lowering it reduces the effect of scavenging which helps "pull" exhaust out of the cylinder. Essentially, for a given flow rate (i.e. xx displacement motor spinning at yy RPMs) there will be an ideal size for the exhaust primaries and pipe to yield the ideal flow/velocity characteristic in terms of peak power gain or power band.

What this doesn't help you with is what makes power where. That's very hard to determine scientifically, which leads you back to some exhausts/headers making more power (or power elsewhere in the band) than others. Suffice to say that the flow/velocity/"backpressure" theory does explain why running an open header or a 4" exhaust yields worse performance than stock in most cases.

j2k

Backpressure is not directly related to velocity... and adding more backpressure for backpressure's sake will NEVER produce more power. It will only cause a power loss. That is a fact. Tying backpressure to torque/power is indeed the myth.

Now it is impossible to have ZERO backpressure without sacrificing other important elements of an exhaust system because without a pressure differential, there is no flow... but the backpressure doesnt do anything beneficial by itself.

Bobway,
I have "Scientific Design of Exhaust & Intake Systems"... and you're going to have to tell me how, after reading it, one can come to the conclusion that backpressure somehow equals torque.


cheers,
jason keeney

ultimate lurker

Thank god people don't believe the backpressure myth.

To add to what Jason and marcucci have said, I suggest picking up the April 2002 issue of Circle Track magazine (it may be hard to find now as the new issue is out).

David Vizard (one of the truly great flow experts who publishes results) essentially rehashes a lot of his old research in an article entitled "No Loss Exhaust".

In it Vizard talks about the entire exhaust system, from header primaries all the way to a muffler setup for zero loss (his sample setup is a 700 hp small block Chevy on open headers which still made 700 hp with a proper muffler setup producing streetable noise levels).

Particularly relevant to this discussion is his research into the amount of CFM flow required per hp of output. He notes (through research and empirical testing) that if your muffler flows at least 2.2 CFM per hp of output, you will see no losses from muffling. At 2.2 CFM/hp backpressure is about 0.2 psi. Dropping flow to 1.625 CFM/hp increases backpressure to 1.2 psi and will reduce peak hp by around 8%. Interestingly enough, when you apply this rationale to pipe sizing, a 2.5" pipe flows about 560 CFM (115 CFM/sq in is the equation) at 1.5" Hg. Our S2000 needs about 525 CFM according to the 2.2 CFM/hp number and I believe our internal piping diameter is about 2.4", which should be good for about 520 CFM - wow, pretty close!

Vizard also give equations for chosing header primary and secondary pipe diameters (at least ball park numbers) based upon exhaust port flow. According to his info (and flow numbers obtained on stock heads for the F20C), our F20C should be using primaries with a 1.83" internal diameter (or about a 1 7/8" outside diameter) and a collector of a little over 3" in diameter. Of course, our collector length gets all messed up by the cat, so streetable systems probably have to be different. But sizing pipes in this manner ensures maximum flow velocity and minimal restriction in the critical operating ranges of the engine. Could such a setup compromise low rpm torque? Potentially, but it doesn't have to.

UL

Bobway

Oh, did I say that backpressure = torque? Sorry, meant that there was a definate correlation between finding the optimum pipe length for the best cylinder fill and the amount of backpressure that happens to be there at the time. From what I gathered in the book, the least amount of backpressure did not equate to optimum cylinder fill.

I thought if wisconsin wanted to know, he should read that book, or at least the first couple chapters.

I think the one of the best examples in the book was the single cylinder Crossley engine experiment where they examined the pressure waves for scavenging effect, and found the optimum pipe length to be something like 13'.

j2k

YES!! that David Vizard article should be required reading! That article actually appeared in Circle Track many years ago and I'm glad to see it reprinted... and even updated a little.

On a slight tangent... for those who might not know, Circle Track is often an excellent source of information and data on high performance automobile concepts... concepts that often go way beyond the limited appeal that the title might suggest. Do underestimate this magazine because of the title... its a source of some real gems.

cheers,
jason keeney

JaminBen

Great post, great thread!! I have nothing more to say, despite having attended a 2 day course given by David Vizard.

Anybody recall the "NOLOGY" backpressure device? It might still be on the company's website.

There IS something in varying backpressure in correlation to RPM's. The Modena's variable flow exhaust is a current illustration.

Is the aforementioned book still in print?

Wisconsin S2k

Thanks for all the info. I will have to look into this circle track mag. I'm glad I have a site that I can come to for so much technical information. I am impressed!!