Exhaust question, possibly stupid

[swalve]

OK, so we know that designing an exhaust system is a compromise between having narrow enough tubing to promote fast flow at low RPMs for optimal torque via exhaust scavenging, and having wide enough tubing to not restrict flow at high RPMs.

I know there are electric and vacuum operated exhaust cut-outs that will open up the exhaust for race-type circumstances.

Is there a valve, or a system, that uses this principal, that is completely automatic?

Say your default system is 3 inches. What you'd do is install a "main" pipe after the cat that's say 2.5 inches. Nice and narrow for high torque, low RPM operations. There would be a spring loaded pressure relief valve, located just after the cat right at the transition point, leading to a second exhaust pipe. During normal operation at low RPM, it is closed, directing exhaust flow to the main one. When you pound on the pedal, and exhaust pressure builds, the spring loaded valve opens up, allowing exhaust gases to flow through both pipes, giving the system more "room" to flow freely.

Maybe I'm missing a key point of the physics of exhaust flow. But it seems like this kind of system would be a great way to solve the compromise...

 

[Gary Allan]

An active system would be something like an exhaust brake that smaller diesels use. Then you would just need a trigger to open the auxiliary exhaust at a given rpm. Otherwise, some adaptation of a flapper from an older V8 heat riser/carb warmer setup would work if you could replace the bi-metal spring with (just) a spring. Who knows what tension you would want.

I don't know how much you would gain ..but I guess if you started with a much smaller primary exhaust you can get more out of the lower rpm range without losing anything at higher end.

 

[02Pilot]

I don't think you're going to get precise enough opening of the valve, nor fast enough to avoid excess backpressure, with a spring-loaded valve that relies solely on exhaust pressure. Instead, I would build a system that opens the valve by some external means, either engine vacuum or an RPM and/or throttle trigger for an electrical solenoid. You will also need to do a LOT of tuning on the dyno and the street to get the opening and closing points correct (you may end up with some unpleasant resonance characteristics if you just have it snapping shut at the same RPM it opens).

 

[oilyriser]

A tunable system like a trombone would be neat.

 

[Papa Bear]

I always thought that an exhaust system that operated like a trombone slide would be cool....
so would intake runners.

 

[paulo57509]

Originally Posted By: Papa Bear
I always thought that an exhaust system that operated like a trombone slide would be cool....
so would intake runners.


I know the early (86-90) Acura Legends had variable length intake runner system. It used a system of butterfly valves inside the intake manifold. Under 3200RPM, the valves would route the air flow thru the longer runner length to the heads. Above 3200 RPM, the valves would begin to close and route the air flow thru a shorter route to the heads.

 

[The_Jeff]

I see a lot of "flutter" of the valve at mid throttle applications that would sound very annoying.

Since you want a bit of vacuum scavenging at low RPMs for torque, I could see that scavenging opening the valve real quick (at the wrong point in the exhaust cycle) and dirtying the airflow behind it.

I rather like the electric cutouts idea, PCM controlled.

While I'm at it, it would be neat to have a 2nd fuel tank, 1 gal capacity, that you could put high-test in, and plumb it up to the fuel rail. When you start getting on it, or when the knock sensor detects knock, a valve inches from the injectors flips over to the high test.

 

[Jim 5]

As you probably know, a number of cars have these, but they're closer to the back of the system. You might be able to use a factory part.

My BMW has this and my neighbor's Corvette Z06 has it as well. It's more a backpressure thing if I'm not mistaken, but the factory solenoids may be able to be adapted to work in your idea.

Papa bear, current generation BMW V8's have variable intake runners. The intake runners change length with varying RPM's. It looks kind of like if you put your hands together with fingers interlaced, then pull your hands apart while keeping the heels of your hands touching and fingers curved. Pretty weak description..I know.

 

[02Pilot]

Rather than use a complete blocking butterfly, I would think you might get better results (smoother transitions, for one, plus you wouldn't get the flutter mentioned) with a perforated or otherwise not completely sealed flap. Progressive opening (perhaps over a 500-1000RPM range) might be more desirable than simply snapping open and shut.

 

[Gary Allan]

How about something like a Spence valve? You spec an ideal exhaust pressure ..and the valve opens or closes the valve to match that preload? Same as gas line regulators ..etc..etc.

 

[postjeeprcr]

Maybe something like this would work for you.

http://ultimategto.com/1970cars54.htm

 

[BearZDefect]

Originally Posted By: paulo57509
I know the early (86-90) Acura Legends had variable length intake runner system. It used a system of butterfly valves inside the intake manifold. Under 3200RPM, the valves would route the air flow thru the longer runner length to the heads. Above 3200 RPM, the valves would begin to close and route the air flow thru a shorter route to the heads.

The F22A6 engine of the 4th generation Honda Accord had this kind of variable intake too, vacuum activated at a certain rpm.

 

[wiswind]

FORD uses the Intake Manifold Runner on some of their engines.
My 3.8L windstar has it......the runner butterfies open at about 2900rpm.
You can feel the boost when they open up.
I read someplace that some years used a lower opening RPM, but I don't know for sure.
I don't know what RPM they open at on the pickup trucks.

Another thing that they do is to have 2 catalytic converters, one for each bank of cylinders, then combine into 1 tail pipe.

A RPM tuned exhaust is an interesting concept.
Thing about spring loaded valve is that it will need to be quite heat tolerant.

 

[tom slick]

I believe the newest Lamborghini has an exhaust valve like you describe.

 

[beanoil]

This may work for you. As RPM is increased, the tip actually opens up for more flow. During part throttle use, backpressure is controlled by fan RPM. Just a thought.

 

[artificialist]

I always figured that the way to do it would be to have an engine with 4 valves per cylinder, and one intake and one exhaust pipe per valve. In certain conditions, one intake and one exhaust would be shut off in order to make the engine behave like it has a mild intake and exhaust. Then, all 4 valves would work at the same time, making it ideal for maximum RPM.

However, building an engine that has 2 intake manifolds, 2 entire exhaust systems, and variable valve gear seems very expensive.

RPM sensitive intake manifolds are very common on cars, and have existed for a long time. The Toyota 4AGE that was used in the last RWD Corrolla had a variable intake manifold.

 

[[RT] ProjUltraZ]

header primaries is where it's at (primarily)

exhaust pipe dia secondarily/cat/muffler(s) or not

 

[Gary Allan]

Originally Posted By: artificialist
I always figured that the way to do it would be to have an engine with 4 valves per cylinder, and one intake and one exhaust pipe per valve. In certain conditions, one intake and one exhaust would be shut off in order to make the engine behave like it has a mild intake and exhaust. Then, all 4 valves would work at the same time, making it ideal for maximum RPM.

However, building an engine that has 2 intake manifolds, 2 entire exhaust systems, and variable valve gear seems very expensive.

RPM sensitive intake manifolds are very common on cars, and have existed for a long time. The Toyota 4AGE that was used in the last RWD Corrolla had a variable intake manifold.


Isn't that how the ZR1 Corvette worked, at least on the intake side?

 

[PandaBear]

Check out these articles in the Exhaust section by Michael Delaney

According to the articles, it is not the back pressure but the resonance of the exhaust frequency and the pipe length and size that scavenge the exhaust gas from the engine. So, adjusting back pressure is not going to help. What you can do is having a switchable runner on the exhaust side to adjust the length and/or the size of the flow to have adjustable peak position for the max resonance.

Dual runner intake manifold uses the same idea, and is very common. The problem with exhaust side is going to be heat and cost. It is much more expensive to have a dual runner exhaust than a dual runner intake, you have to consider the temperature and the location of the emission components like CAT, O2 sensor, and parts clearance.