Exhaust Manifold

[Christ]

Me?

[Shawn D.]

Originally Posted by Shawn D.

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

Originally Posted by theunchosen

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.

Guess what? Exhaust tuning for torque exists! Mandrel bends are also used on exhausts that are tuned for maximum torque, not just HP!

[Christ]

Mandrel bends are used where exhaust routing cannot be made straight. There are space constraints to working with mass production and existing chassis designs. Those are still the only reason for using bends at all.

I've said it once already, a bend slows down flow, no ifs, ands, or buts about it. There is no optimal bend in an exhaust system that cannot be countered by tuning the cross sectional area of the exhaust tube.

Bends in the header are solely there to facilitate equal length tubes to the collector, and, again, space constraints. If the headers could be made straighter, they would.

In all of motorsports history, the ultimate aspiration in dealing with exhausts has been to make them as straight as possible, regardless of the tuning characteristics desired.

It is generally accepted that bends restrict flow, and are not desirable.

You fail to mention exactly (using science) how a bend can be beneficial to anything except space constraints and scavenging (scavenging can still be had in other ways, so even this is not a 100% downfall to straight pipes.)

[theunchosen]

I rest my arguement on the fire hose. Its pretty convincing to be a full-grown adult and get knocked flat on your back because the hose is curved just a few degrees with a serious friction burn that looks like a really bad motorcycle accident.

[Christ]

Simple rule of Physics - All fluids will follow the path of least resistance.

When watching the flow of a fluid through different paths, one thing is clear - it will always try to flow straight.

If it begins to flow in a straight line, unobstructed, and you put something directly in front of the flow, the flow will first attempt to push that object out of the way. When that can't be achieved, the next option is to flow around it.

When you put a bend in a pipe, you're essentially blocking optimal flow - causing it to change direction. When you do that, you're slowing it down. When you slow it down, pressure increases. When pressure increases, pipe diameter must increase to lower pressure. This means more weight to the system, more expansion area for the gasses, and more surface area to leech heat from the exhaust gasses, cooling the exhaust.

Ideally, the exhaust's path will be straight as possible, with as little restriction to flow as possible, and only the correct diameter to flow exactly the necessary amount of fluid (gasses are fluid) from A to B without creating as much pressure. This means that the pipe can and should be a smaller diameter when dealing with low volumes of fluid, so that it can't expand outward as much, and thusly, can't cool as fast. This creates (again) the largest pressure differential across the valve, which aides in scavenging.

Harmonics also apply to scavenging, in exactly the same manner that they apply to intakes.

Tuning the length of the pipe so that the pressure waves are pulling away from the valve as it opens will create a lower pressure gradient at the valve seat, further facilitating exhaust flow with less engine power required. The exhaust tuning doesn't mean you have to make the pipe shorter or longer, it means you have to make a shorter or longer primary tube, which opens to a different mass.

[Shawn D.]

Hmm... I wonder how it is Y'all haven't bothered to address how all of this will actually come together in a real-world street-applicable car. I guess I'll just have to wait to see this real-world street-applicable straight exhaust that Y'all seem to think is possible.

[Christ]

We actually did... that was the beginning of the thread, Shawn. It was the preface to this whole discussion. How hard would it really be to have a straight exhaust pipe?

I venture to say that given the head re-shaping that would be necessary to have a truly straight exhaust flow (or anything close to it), the engine would be laying nearly flat, which was part of the design consideration for the original intent. From there, it's just a matter of piping straight-ness from engine to rear of car. Jeez, that's hard.

You are vehemently opposed to the idea of a straight exhaust, aren't you?

[theunchosen]

Originally Posted by Shawn D.

Hmm... I wonder how it is Y'all haven't bothered to address how all of this will actually come together in a real-world street-applicable car. I guess I'll just have to wait to see this real-world street-applicable straight exhaust that Y'all seem to think is possible.

Actually Christ and Frank Lee already discussed it.

The idea is twist the piping just enough to miss the cams and thats it. Make the pipes just wide enough to accomodate what you want(flow wise, 2K rpm 2.5 or 3K)

Given you lose efficiency at anything but those rpms, but this is true of all systems. If your pipes are designed for racing then you lose power at any rpm below WOT because the air is condensing and slowing creating extra back pressure that the engine fights to clear the cylinder. If the pipes are designed for economical driving you lose anytime you clear 2.5K rpm because the piping contains a higher pressure that depletes too slowly and there is still remaining pressure when the cylinder goes to clear the next round. . .that it has to push out of the way as well.

You always have to chose. Until someone puts dynamic exhaust piping on the market you always will, and I can only imagine how expensive it would be to rig something to allow for this on standard cars.

You could use a system of telescoping pipes with valves and springs and when it runs higher than 2K they open to allow the air to the slightly larger exterior pipe and solve the problem that way, but then you are adding a whole second layer of pipes(obviously the inner would not have to be tremendously thick to resist exterior environs but its still going to have weight). So until some "smarter" material comes along manufacturers are always going to have to chose where there system operates best(like everything else).

[Shawn D.]

Originally Posted by Shawn D.

Hmm... I wonder how it is Y'all haven't bothered to address how all of this will actually come together in a real-world street-applicable car. I guess I'll just have to wait to see this real-world street-applicable straight exhaust that Y'all seem to think is possible.

Originally Posted by Christ

We actually did... that was the beginning of the thread, Shawn. It was the preface to this whole discussion.

Originally Posted by theunchosen

Actually Christ and Frank Lee already discussed it.

Really? I can't find that wheat amongst the chaff. Would Y'all be so kind as to point out the exact posts?

Originally Posted by Christ

You are vehemently opposed to the idea of a straight exhaust, aren't you?

No, I'm not opposed to the "idea" of a straight exhaust.

Originally Posted by Christ

How hard would it really be to have a straight exhaust pipe?

So hard it won't get done on a street car that requires a catalytic converter and/or muffler.

The absolute "closest" one could come to implementing this concept would require one pipe per cylinder, with one catalytic converter and one muffler per pipe. Once the flow enters the catalytic converter (even a monolithic one, which all of 'em are now), there goes the "straight" flow concept which seems to be the point of this thread. The "straightest" muffler is a glass-pack, which most any hot rodder knows flows worse than a turbo- or chambered-style muffler -- what would be the point of having a "straight" muffler which flows worse? Adherence to the idea for adherence's sake? Straight ain't gonna happen except in Ecomodder Fantasy Land.

[theunchosen]

Yeah shawn its on pages 3 and 4 between Frank and Christ. It's also on page 2 where Christ posts a picture and then we talk about it there for a while. Its also mentioned when Christ brings up his caravan.

And yes you can put a muffler and a cat on it. No reason you can't. Both are straight. I don't know how much you know about either but they are both just expansion chambers one with a thin membrane that reacts with unburnt fuel and the other that has baffles that reduce noise. Also the OP intent was not about these items as they may not be in the system at all. I actually had in mind using a new GDI engine lifted off a wrecked 09-10 with the smallest capacity. In which case as long as I can maintain an AFR of 24-30 I don't need any of those things because it produces less emissions than your engine with a cat.