Variable Exhaust...

[Guest Anonymous]

I have an idea, and it may be a crazy one, but here goes...

 

A fairly restrictive exhaust system gives an engine more torque low in the RPM range, and a free flowing exhaust gives great top-end performance, but hurts bottom end. Right so far? Well, I'm wondering how a butterfly valve in the exhaust pipes would work. Somewhere just below the headers, a butterfly valve could be installed in each pipe. When closed, this valve would block maybe half of the pipe, and could be tuned with some sort of throttle stop-like mechanism. At high RPM (~4500?), the butterfly would open up, giving the exhaust free-flowing characteristics. This setup would be controlled with an RPM switch used for a shift light, a relay, and a power door lock solenoid. How do you guys think this would work?

 

The main question I have about it is regarding the closed position... A restrictive exhaust system is not just restrictive, it has higher velocity and correspondingly higher inertial effects, increasing cylinder scavenging. However, people always refer to "backpressure", which I take as being pressure across a resistive element in the system, i.e. a restrictive muffler or a partially closed butterfly. Either "backpressure" is a complete misnomer, or these are two separate effects that both increase low-end torque. My mechanical engineering and internal combustion engine design education tells me that the notion of "backpressure" is indeed a misnomer, but I could be wrong... Sometimes these things are counterintuitive. Thoughts?

 

Thanks!

 

-Rodney

[Tim240z]

This very toic was discussed in detail a few months ago. Do a search and have a read. There are a couple of products on the market that do this very thing.

Tim

[Guest Anonymous]

Thanks for the reply... I did a search, and I couldn't find anything. If anyone knows of the thread where this was discussed, feel free to post a link...

 

-Rodney

[grumpyvette]

Sparks

do a search , but your main concept is flawed, back pressure (a restrictive exhaust is almost never a good idea!) what your trying to do is keep the exhaust gases speed and inertia in the range that effectively scavages the cylinders, at low rpms the smaller dia. header pipes and longer length header pipes do this due to the lower volume of gas PER SECOND and the LONGER distance that EACH gas PULSE FROM THE EXHAUST PORT can travel BETWEEN EXHAUST PULSES,remember your useing the INERTIA of the fast moveing exhaust gas to form a negitive pressure wave behind the exhaust valve to help scavage the cylinder and help SUCK IN A NEW INTAKE CHARGE as the rpms increase the VOLUME of gas increases and the distance each pulse can travel before the next pulse due to the time between pulses gets shorter, now one way to cheat is to place anti reversion damns in the header primary tubes, this tends to aid the cylinder scavageing at low rpms and not hurt the high rpm flow, look here,

REVERSION

 

Reversion is simply the exhaust gases momentarily flowing backwards during the overlap phase of the camshaft at low cycling rates. During the overlap phase the engine is on the exhaust stroke and the piston is pushing out the last of the exhaust gases. Prior to reaching top dead center the intake valve begins to open. At low cycling rates the intake charge and the exiting exhaust pulse have yet created any momentum. Thus the piston pushes some spent exhaust gas into the intake manifold. This is why engines with big camshafts idle and sound radical. The exhaust pulses shoot up into intake manifold causing a major disturbance. The cylinders receive an uneven mixture of air, fuel and spent exhaust gas. The piston then reaches top dead center and begins the intake stroke. At this point both valves are open, in fact the exhaust valve in some cases may not shut for another 50 degrees of crank rotation. During this 50 degrees of crank rotation the piston literally draws from both the intake and exhaust valves causing the exhaust gases will momentarily reverse. At high cycling rates the inertia of the incoming intake charge and the out going exhaust pulse keep the gases flowing in the proper direction.

BTW they tried building headers with a 1 5/8" primairy tube about 3" long, that was stuck centrally inside a 1 7/8" outer primairy tube that continued for about 12" and slowly narrowed to a 1 3/4" tube, while a total P.I.T.A. to build the made about 70 more ft lbs of tq in the 1000-2500rpm range on a 11"1 compression race engine with a high overlap cam , they worked because the reversion pulses tend to flow along the inner surface of the header tube thus on the reversion pulse the wave tend to follow the outer 1 7/8" tube and not reach the cylinder thus aiding scavageing

btw, headman makes stepped headers a cheaper to build but way less effective version of the anti-reversion pulse , increased scavageing idea a good set of anti-reverion headers will need to be custom made as no one I know commercially builds them as they cost about 3-5 times as much to build and altho they work great the benefits are over a lower rpm and narrow range mkeing them great for building mid-range torque but of no huge advantage in max hp

http://www.hedman.com/pages/hedmanmain.html

[Guest Anonymous]

Thanks, Grumpy!

 

I've always heard that "backpressure" is good for torque. I'm very aware of the intertial effects of a smaller diameter exhaust setup, but this notion of "backpressure" threw me off. To me, backpressure means resistive pressure due to a pressure drop across a restriction in the system. This has always sounded wrong to me. Apparently, although not accurately named, backpressure is the inertial effect due to higher velocities in the pipes as a result of smaller cross sectional area, and this negative pressure scavenges the cylinders.

 

Okay, so the butterfly idea is bad. How about a system where a set of side pipes opens up at high RPM? Any way to do this without having a helacious exhaust leak due to a poorly sealing valve? I've seen the plates with wingnuts that you can buy, but I'd like something automatic. I can't think of a valve that would both seal well and move freely when activated... any ideas?

 

-Rodney

[Oltmann]

Adding backpressure.

 

The last thread on this issue, although the focus was on the idea that you could reduce noise at cruising/idle rpms without reducing performance.

[Guest Anonymous]

Thanks for that link!

 

Hmm... sounds like there are many ways to do this. My main goal with this idea is to make an exhaust that will cruise quietly, but open up when the need arises. I'm liking the idea of some sort of blowoff valve, but running a butterfly on the side of a crossover pipe and into a short length of pipe under the middle of the car may also work. The only thing I'm worried about with that is that it may get clogged and/or have a noisy leak. I could rig up some ~50mm bronze-bushed throttle body on a crossover pipe pretty easily, and it'd be very serviceable... I don't know how this would package under the car though. My original idea of using a power door lock solenoid or actuator would work well here, if activated by a WOT (or 3/4 throttle)switch.

 

Thoughts on this?

 

-Rodney

[Guest m35514h]

go to www.quicktimeperformance.com they have a y pipe that you can with a button or switch go from exhaust system to open headers. Maybe you should try those out and tell us how they work.

[Guest Anonymous]

Actually DSMs in Japan had this sort of thing in their exhaust manifold. I think it was called a Cyclone manifold. A friend was gonna buy one off ebay, but he got beat. He was happy though, because he read that it loses 10-15hp at top end just after he bid on it.

[Guest 73TPIZ]

Sparks, i used a throttle body of an import and made a plate to bolt to it out of 1/8" steel. i tied my 2 pipes to it before they went to the muffler. i was going to run a solonoid but ended up running a lawn mower throttle cable up to the tunnel. glad i did 'cause the change in heat when you start the engine will cause it to stick and i have to really work the lever. works fine when warmed up though. may be tough to work w/ a solonoid. Do some research on the Honda CBR929. i seem to recall they have a valve where the headers tie together under the belly pan to work as you describe. more torque on low end and opens to larger chamber on top end for HP. something like that. Good Luck.