Exhaust Physics

[Drax240z]

David Vizard did a really nice article recently on exhaust system theory and testing in the May 2005 Popular Hotrodding Magazine. "Exhaust Science Demystified" (byline: "The fact is most cars are leaving horsepower on the table. We show you how to get it all back.")

 

Great, now I gotta see if I can get a copy of that magazine! How come you couldn't have mentioned this back in May?

 

So this is in the May 2004 PHR or May 2005?

[SHO-Z]

Here is an article on exhaust design that I read when I was looking at installing a SHO engine in my Z. There are a lot of good points that are brought up about sizing and possible problems by going to large. The SHO is 3.0L so is a good comparison to the Z engine.

 

 

http://www.shotimes.com/SHO4exhaustjohnh.html

[pparaska]

David Vizard did a really nice article recently on exhaust system theory and testing in the May 2005 Popular Hotrodding Magazine. "Exhaust Science Demystified" (byline: "The fact is most cars are leaving horsepower on the table. We show you how to get it all back.")

[Guest tony78_280z]

Not to be a pest... ... well Ok, so I'm a pest. But could someone chime in on my two questions above? please?!

[280zwitha383]

2> Someone said something about somehow (don't you hate it when a question starts off like that) the exhaust valves could be open at the wrong time and actualy suck air/exhaust back into the combustion chamber. How could this be? I think it'd only be possible on miss matched components particularly in cam and heads. Is it possible for a stock motor (like mine) to have this effect? Also, this would be simply a horrible perfomance loss, would it not?

 

Don't know if someone already answered or if this is the answer to your question because I didn't read the whole thread but:

 

I think the condition that you are describing is called overscavenging. It is where the exhaust valve is still open on the intake stroke and in essence the cold air from the intake is sucked out the exhaust which obviously wastes some cold air. I would say there's really no chance of this happening with a stock engine and cam. However, with a large exhaust lift, high ratio rocker arms, and heads that flow well on the exhaust side (and probably a good set of headers) this condition is quite possible and maybe even likely.

[Guest tony78_280z]

I think the condition that you are describing is called overscavenging. It is where the exhaust valve is still open on the intake stroke and in essence the cold air from the intake is sucked out the exhaust

No, this is not what I am trying to describe. I found the quote I was looking for where this is mentioned.

 

"reversion", depending on valve overlap. If both the intake and exhaust valve are open at the same time, exhaust gases will actually be drawn back into the cylinder. This is only a possibility, it probably wouldn't happen

 

The way I understand it is the exhaust port doesn't close soon enough, or opens too soon but exhaust gas is sucked back into the combustion chamber. Ever hear of this one? What are the odds of this happening with a stock motor?

[jt1]

Tony, some of the ceramic coating places can coat the headers inside and out to minimize the amount of heat radiated out of the exhaust gas. Calico coatings in Denver NC is one; They did a high temp reflective coating on the inside of mine, then the shiny ceramic coating on the outside. It makes a big difference, a couple of minutes after you shut the motor off you can touch the headers.

 

John

[SHO-Z]

On back flow into the engine from that article that I pasted in above

 

The reason for this difference in size has to do with reversion, which is back flow of the burned exhaust gases back into the engine. The bigger that the pipes get, the better the chance that flow can reverse and contaminate the incoming fresh air/fuel charge. On a Mustang, the large displacement keeps the total volume of gas going though the exhaust high (at low rpm), and a moderate amount of reversion is even desirable to help reduce peak cylinder temperatures and keep NOx down (the 5.0 having a basic layout and chamber design that is not ideal from an emissions standpoint). With the smaller displacement, bone stock SHO engine, dual 2.25 inch pipes would likely allow too much reversion at low RPMs, hurting low rpm torque. This latter effect can only be mitigated somewhat through additional fuel/spark a la an LPM, but it can not be eliminated completely, irregardless of what you may hear elsewhere.

[johnc]

Think about it, you've got an intake tract that has some level of vacum and an exhaust tract that is slightly pressurized. You open a connection between the two and what do you think is going to happen?

[clint78z]

Good point JohnC, air always likes to move in the direction of lowest pressure. On most cars there is a small amount of valve overlap, very small. On the exhaust stoke the piston is pushing up the exhaust valve is open, it begins to close as the piston reaches TDC. Before the valve is almost closed the intake begins to open. Because the exhaust gas is rushing out the port it creates a venturi effect and pulls in intake air (scaveging. If you have too much valve overlap or high duration cam, you basically start too early for the effect to happen and the exhaust goes into the intake and dilutes it. (Exhaust Reversion)

 

I usually don't like the term backpressure for the exhaust, pressure drop between two points is a little more accurate. That's splitting hairs.

 

Cool thread, let's keep it rolling !!

[Guest tony78_280z]

With the smaller displacement, bone stock SHO engine, dual 2.25 inch pipes would likely allow too much reversion at low RPMs, hurting low rpm torque. This latter effect can only be mitigated somewhat through additional fuel/spark a la an LPM, but it can not be eliminated completely, irregardless of what you may hear elsewhere.

 

Too large of an eshaust system can actualy hurting Horsepower/Torque through reversion. So with a stock 5.7L motor would dual 2 1/4 pipes be too big and cuase too much reversion? Or should I not lay awake at night fretting over possible lost power?

 

What is SHO?

 

What is LPM?

[6speediroc]

I know this is an old post, but here is the Vizard article mentioned:

 

http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/index.html