Why round exhaust pipe?

[JeffMcKC]

OOPs and I left out the fireing order issues, and guys not lining up the right header tubes with it................

[TheSilverBuick]

Am I missing it, or what's missing from the math is port and tube size will play greatly in exhaust velocity. 71.63mph into what kind of open volume? Like a venturi, you choke down the volume and the speed could increase, unless the restriction is great enough that it back's up and chokes at less than the calculated 71.63mph and pressure builds. You're right though, there is a crap ton more math than you are showing as you are just using average piston speed rather than absolute along with only the time the exhaust valve is open (like Bob is talking about but with cam timing now a factor). Way different than a fixed speed/pressure HVAC system, and of course as mentioned, that is just one cylinder.

[CDMBill]

While we are at the math, there is one exhaust stroke for every two rotations of the crank, 720*, suck, squeeze, bang blow, but you guys knew that.

Someone with more spare time than me can surely find average exhaust gas speed for a high performance or even DD engine.

[TC]

IMO it all comes down to running pipe sizes that maximize exhaust velocity while creating zero back pressure.......

[MR P-BODY]

You need to know the gas temp to apply to the formula... expanded/heated air has greater
volume.... next you need to know the area of the tubing(greater area has greater cooling
effect over a given distance)...also most exhaust systems are based on about 1260* to 1350*
at 1" from the head surface... so to say that the exhaust flow or even temp at the tip of the
tail pipe will be X is pretty hard to say without all the data needed

[MR P-BODY]

Also when using the fancy rolled in exhaust tips at the tail pipe we seen about 4 HP reduction
just from the rolled tips.... they look nicer(my opinion) but lower HP.... I'm retired now but
this testing was a couple of years ago in the exhaust lab at Chrysler(30+ years in that lab as
well as fuel)

[Beagle]

sooo.... when you toss a turbo into the equation...

[JeffMcKC]

IMO it all comes down to running pipe sizes that maximize exhaust velocity while creating zero back pressure.......

You need to give up the zero back pressure. There is back pressure trying to get past a valve, then there is back pressure trying to get out of the port, then as soon as a pipe is added there is back pressure, then when connected to another pipe it can help pull that lowers pressure. If you wanted zero back pressure on a engine the the exhaust valve would be relocated and bigger and the port would be bigger and there would be no bends in the pipe and it would be huge on a Top Fuel Car even.

[TC]

You need to give up the zero back pressure. There is back pressure trying to get past a valve, then there is back pressure trying to get out of the port, then as soon as a pipe is added there is back pressure, then when connected to another pipe it can help pull that lowers pressure. If you wanted zero back pressure on a engine the the exhaust valve would be relocated and bigger and the port would be bigger and there would be no bends in the pipe and it would be huge on a Top Fuel Car even.

Then just say you want the closest amount of back pressure to zero as possible....... Also I was under the impression that when using headers that a vacuum was being pulled in the header tube assisting in drawing the new intake charge into the cylinder during overlap, but how can this happen if there is back pressure in the header tube like you are saying??......... Vacuum, Pressure, you can't have both, or can you??...........

[anotheridiot]

Am I missing it, or what's missing from the math is port and tube size will play greatly in exhaust velocity. 71.63mph into what kind of open volume? Like a venturi, you choke down the volume and the speed could increase, unless the restriction is great enough that it back's up and chokes at less than the calculated 71.63mph and pressure builds. You're right though, there is a crap ton more math than you are showing as you are just using average piston speed rather than absolute along with only the time the exhaust valve is open (like Bob is talking about but with cam timing now a factor). Way different than a fixed speed/pressure HVAC system, and of course as mentioned, that is just one cylinder.

Thats what I cant understand. HVAC are closed systems. Expansion of the "exhaust" happens in the condenser not to atmosphere. I've been in 71 mph winds, and never felt that coming out of an exhaust pipe.

I think we are missing an important part too, what would the exhaust sound like. We all know the low throaty sound of a 3" exhaust pipe, even on rice burners. How awesome would a 1.125 x 5" exhaust sound?

[SuperBuickGuy]

Thats what I cant understand. HVAC are closed systems. Expansion of the "exhaust" happens in the condenser not to atmosphere. I've been in 71 mph winds, and never felt that coming out of an exhaust pipe.

even the ones you're thinking about aren't closed - they have balance vents to allow for thermal expansion. There's another kind that I suspect you didn't consider - commercial ventilation systems, and they suck all sorts of air or air/liquid products out of spaces... funny aside, or a cooks baseball cap up 40 feet of duct.

I am wondering now what the relative static pressures are - easy enough to measure.... have to take 5 minutes tonight to measure it on the Vette (or whatever is closest and easiest.... actually, that would probably be the diesel) - that will answer your question Randal

[SuperBuickGuy]

Am I missing it, or what's missing from the math is port and tube size will play greatly in exhaust velocity. 71.63mph into what kind of open volume? Like a venturi, you choke down the volume and the speed could increase, unless the restriction is great enough that it back's up and chokes at less than the calculated 71.63mph and pressure builds. You're right though, there is a crap ton more math than you are showing as you are just using average piston speed rather than absolute along with only the time the exhaust valve is open (like Bob is talking about but with cam timing now a factor). Way different than a fixed speed/pressure HVAC system, and of course as mentioned, that is just one cylinder.

whether or not you squeeze 4x4 (bore x stroke) through a 4" hole or a 1.73" hole doesn't matter - it's the same volume no matter what, it's simply the pressure that increases but goes back to whatever the pipe will handle once it is past the obstruction (I realize I'm not being precise, but don't think it's necessary for this concept).

[TheSilverBuick]

even the ones you're thinking about aren't closed - they have balance vents to allow for thermal expansion. There's another kind that I suspect you didn't consider - commercial ventilation systems, and they suck all sorts of air or air/liquid products out of spaces... funny aside, or a cooks baseball cap up 40 feet of duct.

I am wondering now what the relative static pressures are - easy enough to measure.... have to take 5 minutes tonight to measure it on the Vette (or whatever is closest and easiest.... actually, that would probably be the diesel) - that will answer your question Randal

If it's static, change the size of your measuring tube, aka resolution.

[Bob Holmes]

Also when using the fancy rolled in exhaust tips at the tail pipe we seen about 4 HP reduction
just from the rolled tips.... they look nicer(my opinion) but lower HP.... I'm retired now but
this testing was a couple of years ago in the exhaust lab at Chrysler(30+ years in that lab as
well as fuel)

WELCOME!!!!

I hope we see a great deal of participation from you.

[MR P-BODY]

WELCOME!!!!

I hope we see a great deal of participation from you.

Thanks.... I'll be around but I dont like getting into the pissin
matches that occurs on many boards