Why round exhaust pipe?

[CDMBill]

Nice car, I should have the side pipes on it tonight (presuming the honey-do list isn't too long).

I like the look of that silver Corvette, must have been hard to part with that beauty.

[JeffClark]

I like the look of that silver Corvette, must have been hard to part with that beauty.

It was easy at the time...... but time sucks I look at it now and wish it was in the garage..... but the old garage is quite full....

[Huskinhano]

Hey, I got a question on header tube size and length. While everyone is interested if all the tubes being the same length, I understand. But as you add more twists and turns in the quest to make that cylinder equal to another that has a straighter over all path, don't all those twists and turns add more resistance to flow? I would think the cylinder would see that as being an over all longer length?

Next, like Mr P says, a header will have a specific RPM it works in just like a cam, what happens when your not in that optimum range? Does it really matter any more at that or those points that all thet ube length are the same? I would think not and that it really wouldn't matter too much or hurt performance so long as the tubes were large enough not to cause a restriction.At least for a street motor. I would think at least for a street motor, that's all over the RPM spectrum, tuned headers aren't really needed.

That's one thing I've always believed in for the street, torque. The low budget motor I'm going to put in my 'stang is going to about torque. I'm not planning on spinning it past 5K and I'm trying to pick the parts that way.

[SuperBuickGuy]

yes every twist and turn adds resistance. Here's a good rule of thumb - every 90* of turn has the same amount of back pressure as 10' of linear pipe (that formula is scaleable)
with that said, resistance to flow is dependant on flow rate. And the resistance at each flow point is not linear, it's a curve - with increased flow comes increased resistance

[JeffMcKC]

Overall I think headers are over thought, if the cam, port and intake are not right, and a guy does not develop the engine as a whole package. Then add in the right gear and converter and run it in the range it was intended.

[CDMBill]

I worry less about equal length when I build the headers than equalizing the bends. Headers in stock chassis cars especially those with cramped engine compartments are dictated more by packaging than ideal configuration. I'm also willing to err slightly high on size, not because its less harmful than too small, it isn't, but because it allows for downline increases in power, displacement, power adders etc. I hate spending money twice if I don't have too.

[Huskinhano]

Being an electrian and having a refrigeration background as well, I know what is going on. I look at everything in the terms of OHM's law, it's all the same formulas and relationships wether it's electrons, gas or liquid just the terms are different. The one thing that bugs me is the term "backpressure" What is it really saying? Shouldn't we be saying "pressure drop"? After all that's exactly what is going on. On a length of pipe or wire, it's a drop or loss which gives an accurate understanding.

Instead of getting all jazzed up by the # reading in pressure drop w'e need to be focusing on percentage of pressure drop over all. The more flow, the more pressure drop. Don't forget Boyle's and Charl's gas laws either. For a given volume of gas, the hotter it is, the thinner it is, the more pressure it has and as such you could reduce the CSA of a pipe and still have exceptable flow and percentage of pressure drop while having a higher reading of pressure drop.

[MR P-BODY]

Hey, I got a question on header tube size and length. While everyone is interested if all the tubes being the same length, I understand. But as you add more twists and turns in the quest to make that cylinder equal to another that has a straighter over all path, don't all those twists and turns add more resistance to flow? I would think the cylinder would see that as being an over all longer length?

Next, like Mr P says, a header will have a specific RPM it works in just like a cam, what happens when your not in that optimum range? Does it really matter any more at that or those points that all thet ube length are the same? I would think not and that it really wouldn't matter too much or hurt performance so long as the tubes were large enough not to cause a restriction.At least for a street motor. I would think at least for a street motor, that's all over the RPM spectrum, tuned headers aren't really needed.

That's one thing I've always believed in for the street, torque. The low budget motor I'm going to put in my 'stang is going to about torque. I'm not planning on spinning it past 5K and I'm trying to pick the parts that way.

If its a all out race engine you want the primary tube to be the same length so the pulse
is at the collector at the right time to help draw in the fresh charge... if the lengths are
wrong it wont assist in that process.... understand the NUMBER 1 thing in headers...
if you cant get them to fit on the engine they are USELESS in a car.... they may make GREAT
power on a dyno but make ZERO power if you cant get them in the car... we have to sacrifice
some to get them in and in MOST cases the tubes wont be equal... the one set I made for
my race car all of the tubes are within .1 of each other(and that was just a error on my part)...
exhaust gasses are just like water.... you want nice big radius's if you have to have them and the
fewest number of bends

[SuperBuickGuy]

If its a all out race engine you want the primary tube to be the same length so the pulse
is at the collector at the right time to help draw in the fresh charge... if the lengths are
wrong it wont assist in that process.... understand the NUMBER 1 thing in headers...
if you cant get them to fit on the engine they are USELESS in a car.... they may make GREAT
power on a dyno but make ZERO power if you cant get them in the car... we have to sacrifice
some to get them in and in MOST cases the tubes wont be equal... the one set I made for
my race car all of the tubes are within .1 of each other(and that was just a error on my part)...
exhaust gasses are just like water.... you want nice big radius's if you have to have them and the
fewest number of bends

all true, plus, the bends need to be equal as well. To analogize by extreme - a straight pipe 2"x18" will have less backpressure then a U shaped 2"x18" pipe.

[milner351]

Being an electrian and having a refrigeration background as well, I know what is going on. I look at everything in the terms of OHM's law, it's all the same formulas and relationships wether it's electrons, gas or liquid just the terms are different. The one thing that bugs me is the term "backpressure" What is it really saying? Shouldn't we be saying "pressure drop"? After all that's exactly what is going on. On a length of pipe or wire, it's a drop or loss which gives an accurate understanding.

Instead of getting all jazzed up by the # reading in pressure drop w'e need to be focusing on percentage of pressure drop over all. The more flow, the more pressure drop. Don't forget Boyle's and Charl's gas laws either. For a given volume of gas, the hotter it is, the thinner it is, the more pressure it has and as such you could reduce the CSA of a pipe and still have exceptable flow and percentage of pressure drop while having a higher reading of pressure drop.

that's why exhaust valves are smaller - right?

Great discussion - man we've got a lot of smart people on this site. I have not built my own set of headers yet mostly because of the time required. Making the exhaust from the collectors back is time consuming enough for my schedule right now.

[TC]

Being an electrian and having a refrigeration background as well, I know what is going on. I look at everything in the terms of OHM's law, it's all the same formulas and relationships wether it's electrons, gas or liquid just the terms are different. The one thing that bugs me is the term "backpressure" What is it really saying? Shouldn't we be saying "pressure drop"? After all that's exactly what is going on. On a length of pipe or wire, it's a drop or loss which gives an accurate understanding.

I not sure I can agree with that, seems like your talking apples and oranges and trying to make them the same thing...... A pressure drop would indicate a vacuum, back pressure would indicate a "pressure" in the exhaust system that can actually push exhaust into the cylinder and back up the intake track during overlap..... Basically "pressure drop" will increase HP, while "back pressure" will decrease HP......When looking at this on a pressure gauge you will see a negative value for the "pressure drop" and a positive value for "back pressure"...... So how can a negative value and a positive value be the same thing??

[JeffMcKC]

TC your over your head, quit while your behind..............

[Barry Donovan]

that's why this discussion interests me so much - no one asks why. The engine is a system, and you can buy all sorts of hero parts; but if you don't design the system from air intake to tailpipe, you'll never get what you expect. I still think, and evidence supports, that square pipe could work quite well for exhaust pipe. That the reason it's round is convenience.

Consider - how many actually believe the "vortex" air intake swirl device (aka money-from-fool-remover-device) works? yet, that same argument is used here - that a vortex creates better flow in the exhaust. Square is devilishly hard to make go in the same directions as round, but for the same available space (again with the system approach), my hypothesis is (now) that square is better from all points after the turbo flange.

While the original idea - square on the vette - isn't going to happen (I have side pipes now), the Spider is begging for square because I still need to fit 10# of stuff in that 5# italian bag.

any shape can be used..
as long as there is the impedance factor of positive flow. the tail tip can even measure as a coil.

excellent advice is intake to tailtip. Some cars are stubborn enough to need the tube for air nearly sticking out the front, inert, and a very lively hard steel just barely sticking out the back. A four cyl in a tin can is huge lessons in this insanity.

at .25 framing and 16 gauge body american version..to this day not many in the older gen pay attention to that at all...and I miss it just as much.

[TC]

TC your over your head, quit while your behind..............

So your saying a pressure drop and back pressure are the same thing??....... Please explain.......

Did you know that you can get such a pressure drop in the header tube that during overlap it is considered a whole different cycle in the combustion process?? In fact the pressure drop in the header tube can cause the intake charge to be pulled into the cylinder at as much as 90mph during the overlap events when the piston is at TDC and starting it's downward travel, thus prefilling the chamber/cylinder at the beginning of the piston's downward travel where piston speeds are very low and not creating much of a pressure drop.....

Funny thing is everyone concentrates on the intake side of the equation, not realizing the real power comes form the exhaust side........

[jcharliem]

^^^^^^ Blah, blah, blah...