Exhaust Manifold

[theunchosen]

Frank there is no pleasing you is there lol.

The images show more or less how the piping is right now(intake) and how I'd like it(exhaust).

Its probably not anywher near as sharp but curved surfaces are not convenient to draw in sketchup, so I frequently resort to boxes with the same angles of turn overall.

I think what your last post is saying is if you made the intake as straight as possible it will flow more air, but may cause compression may be compromised? I'm not anywhere near so competent on the subject as either of you, so forgive the questions.

I have another idea I would like to couple this one with. I'll have to get sketches to you guys later. explaining it in words is not an effective method.

[Shawn D.]

Originally Posted by Christ

You just contradicted yourself... if you're taking the bends out, flow will have increased velocity and lower pressure than if there are (even smooth) bends.

You say that there are no measurable gains from "taking a few bends out of the exhaust", yet you claim that there are measurable gains from "mandrel-bent curves" in the exhaust.

So, given the OP's question, you're essentially telling him that more power can be had using a mandrel-bent, curvy exhaust, than a straight, bend-free, less restrictive exhaust, correct?

No, I did not contradict myself. As you quoted me, I said "taking a few bends out of the exhaust," not taking all bends out of the exhaust. He would not be able to make a perfectly straight exhaust; changing the angle of the engine would only take out one 90° bend per cylinder, leaving the rest of the curvy exhaust. If bends were such a killer, racers wouldn't go to extreme effort to fabricate "bag of snakes" headers to make equal-length primaries and instead would go for the straightest design -- that is not how it is done. Racers do, however, rely on mandrel-bent tubing. Get it, now?

Originally Posted by Christ

I wouldn't be so quick to shoot it down, is all I'm saying.

I wouldn't be so quick to shoot down my shooting it down, is all I'm saying about your saying.

Bringing up valid arguments against something isn't being "so quick to shoot it down" anyhow.

[TheWildHippie]

Conventional IC engines are designed to operate on the axis they are originally designed for. There are many resons for this. I might also add that simply gaining a straight path out for the exhaust will gain little to nothing, and might have negative effects because of a few factors. The most obvious being back pressure, which a 2, or 4 cycle IC engine needs to an extent to opperate at it's peak performance.

I can also say that unless you had a degree in flow dynamics, there is no way you will be able to design an exhaust system more effecient than the stock one on your car. There will be trade offs involved, such as you might get a quarter HP from making an exhaust manifold straight....completely straight, but then fuel mileage might suffer....you see what I am getting at?

Peace out, and live Green!

[TheWildHippie]

Originally Posted by Shawn D.

No, I did not contradict myself. As you quoted me, I said "taking a few bends out of the exhaust," not taking all bends out of the exhaust. He would not be able to make a perfectly straight exhaust; changing the angle of the engine would only take out one 90° bend per cylinder, leaving the rest of the curvy exhaust. If bends were such a killer, racers wouldn't go to extreme effort to fabricate "bag of snakes" headers to make equal-length primaries and instead would go for the straightest design -- that is not how it is done. Racers do, however, rely on mandrel-bent tubing. Get it, now?


I wouldn't be so quick to shoot down my shooting it down, is all I'm saying about your saying.

Bringing up valid arguments against something isn't being "so quick to shoot it down" anyhow.

To tell you the truth in exhaust design, a straight exhaust is not what would be the most effecinet design anyway. A system with the least "restrictive" number of bends would be the most effecient. Take a look at the design of an exhaust on a Fourmula one car. They have many bends, but all are very gentle and swooping, and end in a system in which every one of the exhaust header pipes are exactly the same length to the collector. That will generate not only the greatest ammount of horsepower, but the greatest effeciency as well.

Peace out and live green!

TWH

[TheWildHippie]

Originally Posted by TheWildHippie

To tell you the truth in exhaust design, a straight exhaust is not what would be the most effecient design anyway. A system with the least "restrictive" number of bends would be the most effecient. Take a look at the design of an exhaust on a Fourmula one car. They have many bends, but all are very gentle and swooping, and end in a system in which every one of the exhaust header pipes are exactly the same length to the collector. That will generate not only the greatest amount of horsepower, but the greatest effective design as well.

Peace out and live green!

TWH

...

[theunchosen]

I kind of disagree. If we consider the highest possible flow engines out there steam turbines and jet engines they are all as straight as they can get them. I've toured a couple of power plants(Kingston/Erwin) and their steam piping coming towards the turbine is smooth and straight as is the "exhaust." Yes of course they have to curve some but if you look at most turboshaft engines that exhaust just curves enough to get the driveshaft out of the piping and thats it.

Yes obviously there are also huge gains to be had from making the pipes the exact same length but you can do both.

just consider a single cylinder engine. Best you can do is get it straight and smooth. Go look at engine exhausts that are single cylinder. It curves to get it facing the right direction and that is it, no other curves. two cylinders is the same for bikes, curves 180 degrees to go under and out no other curves(except maybe for styling to get it to go under the seat). Speed Triples are the same and the L4's that I know of do it as well.

You can get nearly straight and have equal lengths. We all agree that straighter smoother is better, and we all agree that making sure the engine scavenging and harmonics work out. So we all agree that a system that is as smooth and straight as possible while maintaining equal lengths is best.

[Christ]

Frank - The first quote is speaking of flow from the cylinder back into the head's flow tract. The "shrouding" it reads of occurs in the combustion chamber design, not the area behind the valve.

The second quote actually says exactly what I said. Straighter is better.

Shawn D - The "Bag of snakes" you refer to is an equal length header which is designed to improve pulse scavenging. Reading back through my replies to Frank and theunchosen, you can see that this has been addressed - No, a straight exhaust will not be able to scavenge flow in the same way that today's exhaust systems do, however, pulse scavenging can still be acheived by tuning the length/volume of the exhaust. (Pressure waves do the same thing on each side of the engine. Timing them creates either positive or negative pressure at the valve face.)

Before saying that a particular design "must be the most efficient", you should look into why it is used. Those headers can be tuned for length and volume to shape the torque curve.

It's a given that "less restrictive bends" will be more beneficial than sharp radii in the exhaust flow - think about that though. The "less restrictive bends" are commonly of less radius, and progressively, the less radius the bends have, the more velocity the flow has. It's a widely known, commonly accepted fact of fluid dynamics.

Common practice is to widen the radius of existing bends, so that the final product still fits within packaging constraints, but has better flow characteristics. A wider flow path creates a straighter available line in the exhaust flow. Straighter is better.

By tuning the cross sectional area of a flow tract, you can increase or decrease velocity, based on the volume of fluid you need to flow. Obviously there is an optimum flow volume per shape/size of exhaust pipe. This is the reason that the best tuned exhausts will still only create gains in certain parts of the RPM range. They can create a specific resonant gain based on volume manipulation, and they can create an optimum shape/size for a given volume of flow, decreasing pressure at the valve face by increasing velocity for that range of flow.

To address engines needing back pressure - Um... prove it. It is commonly accepted that engines of any common type do NOT need back pressure to operate. Pressure at the valve's backside cone only allows for the valve to be partially opened before flow begins exiting the cylinder. Providing an optimum exhaust means utilizing negative pressure at the valve's rear face to increase VE, so that the engine doesn't need to provide any power to expel the exhaust under pressure.

Here's an experiment - apply vacuum to a non-running engine. When the vacuum gets strong enough, the piston will rise. This means that the engine would have had to expel no energy in releasing the exhaust gasses. This would increase volumetric efficiency. Obviously, you're not going to create vacuum at the exhaust without added energy, but the higher the pressure differential across the valve, the faster the exhaust can exit, with less energy needed to do so.

[theunchosen]

Originally Posted by Shawn D.

Racers do, however, rely on mandrel-bent tubing. Get it, now?

No, they are out for HP. I'm not today.

Your beloved Mandrel Bends and other components are great for running at or near WOT. I'm not planning on being there.

Alright, try this on. Go to your kitchen and get a straw. Suck some water out of a glass.

Now put a little bit of bend in it. Try again. its probably not that much more difficult but you are only talking a few grams of water over a very short distance. Lets multiply that by say 2 feet same diamter piping and its roughly equivalent to the exhaust in the pipes.

If you still disagree go down to your local fire department lay some hose out and ask them to cut it on. I promise you all but the last few feet of that hose will be in a straight line from the pump in an instant and anyone in the way will have serious friction burns(its happened to me).

Now the gains to be had from making a fire house straight are obviously greater than my exhaust because water is much more dense. That said there are still obviously gains.

[evolutionmovement]

Rotary valves. The sealing issue, I read a few years back, has been solved by 2 independent companies, but I digress. You have two rotating horizontal tube "valves" across the tops of the head(s) for intake and exhaust (that could be controlled electronically for what would essentially be variable lift and duration, perhaps not even needing the tube to fully rotate, only variable degrees back and forth, but the mechanics of that sound unreliable at best). Within each tube, for each cylinder, there's a vertical tube valve that opens to the cylinder when the tube is rotated to position. The air would enter or exit at the angle determined optimal for the shape of the combustion chamber within the head. Potentially, there would be much more air flow as the openings would be much less restrictive and larger, but I don't know what that actual dynamics of the air flow would be, especially while the valve was rotating and creating an irregular chamber, but it's all just an idea anyway.

[Shawn D.]

You people just don't get it! We're NOT talking fire hoses, jet exhausts, or anything else -- we are talking street-use automotive applications!

So, are Y'all actually telling me you are suggesting a single pipe per exhaust port, straight only, like a Top Fuel dragster? With no catalytic converter or muffler?