Combustion Contraption

I think we are all familiar with variable intake geometry, most on this board in relation to the OE GSR intake manifold, so i wont have to explain that.

I guess the question im posing to those perhaps more experienced with this subject, is that why havent we seen much (any?) variable exhaust geometry? I read an interesting peice from one of Steve Matchett's books and while he was working as a mechanic for then Benneton-Renault, it was suspected that both Ferrari and McLaren were testing the idea, though they most likely never found any obvious benefit, as it didnt become 'the next big thing'. Though it is tough to quantify what F1 holds as 'the next big thing' because their 1% increase in HP might have been offset by the extra weight the system brought with it, and so on.

Anyway, just thinking out loud here...

JCushing

the infinity fx45 i belive (the infinity version of the murano) incorporates a variable geometry exhaust. some other pretty nifty stuff to to bump the power a bit compared to the murano.

D-Rob

Are we talking more about variable length, variable diameter, or a different path by itself for the exhaust?

Maybe the F1 engineers didn't see the benefit of targeting power at a certain RPM lower than what they were racing at, so the variable diameter exhaust idea got the boot?

- Derek

P.S., Combustion Contraption, did you recieve my PM?

Natural Aspirations

Doesnt the VW R32 do this already?

mgags7

Randy Monroe was talking about this some time ago, talking about variable size collectors on headers....

I believe he was saying that the headers for jet ski race engines that he builds already use these "tunable" collectors and it shouldn't be long before they become available to us.....

I found a patent on one, but it sounds pretty simple, probably just one adjustable size "valve" that goes inline

http://www.freepatentsonline.com/6799423.html

b18bEKcoupe

I recently read about an australian lad who did extensive testing on variable back pressure on an NA engine. He conducted many expiriments and varied the ammount of back pressure. Every time that the back pressure was increased the power suffered.

black_EM1

Is this anything like the new 7.0L Z06s? The mufflers have the equivalent of throttle bodies that open up for 0% restriction after a certain RPM(I think 3k), or under full-throttle conditions.

D-Rob

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by black_EM1 &raquo;</TD></TR><TR><TD CLASS="quote">Is this anything like the new 7.0L Z06s? The mufflers have the equivalent of throttle bodies that open up for 0% restriction after a certain RPM(I think 3k), or under full-throttle conditions.</TD></TR></TABLE>

On the same note, didn't the DC5 ITR have the same kind of idea with the muffler design?

black_EM1

Can't say off hand.

*goes to look it up*

JCushing

i belive the evo has something like that (valve in the muffler)

D-Rob

https://honda-tech.com/zerothread?id=1410979

DC5 ITR has a valve to quiet it down under low loads.

cjames235

i wanted to do this type of thing with my car but not for performance reasons but rather for sound reasons.......... take a vacuum operated butterfly valve like alot of companies put in exhaust systems, cut the butterfly in half or 3/4 to allow exhaust to flow when closed and then at wot the valve opens all of the way up to allow it to flow well

revolt_allmotor

I believe most of the cars mentioned use the exhaust valves to help them pass government-mandated noise tests, rather than to help power or torque. Since the noise tests are conducted below a certain RPM, they can open the valve after that point and achieve the benefits of a more free-flowing exhaust (less backpressure).

In fact, this is something I've wondered about for a while. As a motorcycle journalists, I get these interesting tech briefings on the latest motorcycles, where engineers describe the technical features of the bike complete with illustrated powerpoint presentations. Several of the motorcycle companies use valves in the exhaust system. I think Yamaha started it with the EXUP system, and other companies have followed suit.

Some of the valves are located back at the muffler like the Z06, and others are in what we would probably call the midpipe. Some are even right at the collector, and the really cool ones change the flow configuration of the collector by directing air through it in a different way in different parts of the RPM range.

Pure speculation on my part, but I know that engines with big ports and big cams with lots of overlap (all designed to breathe at high RPM) typically make poor low-end power - I guess because the big ports don't have much velocity at lower RPM, and the overlap allows lots of blow-through, although I'm by no means an expert. Could adding more backpressure at lower RPM help reduce this problem? Exhaust guys, this one is for you!

blinx9900

i too wondered about the motorcycle exhaust flap, my uneducated opinion is that it does indeed have more to do with sound than flow, however i believe it can be used for power aswell:

say you have a car with huge pipes... say a sohc civic with 3.5" exhaust, its going to take longer for the exhaust to reach the end of the pipe and exit the system, there may infact be a period (at lower revs) where the engine is strugling to push all this dead exhaust to the end and at that point it maybe loosing power as there is little to no velocity at this point, but put a 2" exhaust on the same car and exhaust travels a shorter path (dosent have to build up as much in smaller pipes), velocity increses sooner but yes at some point (higher revs) it will be restrictive... so i gueses you have to find a happy medium, perhaps the butterfly aids in this...

Jasper_db1

Yamaha's EXUP

http://en.wikipedia.org/wiki/EXUP

Rocket

There is a method of achieving variable tuning with out physically changing the pipe dimensions.

Combustion Contraption

I am aware that some OE manufacturers do this, but you dont see anything for the aftermarket high perf. aspect. Thats more of what i was getting at.

blinx9900

perhaps the amount of R&D is too extensive... it would need some kind of monitering device plus electronic control... just a thought...

mgags7

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Don Lackey &raquo;</TD></TR><TR><TD CLASS="quote">There is a method of achieving variable tuning with out physically changing the pipe dimensions. </TD></TR></TABLE>

Well out with it then!

Combustion Contraption

Thats not roberts style.

Revolt, i have an interesting SAE paper regarding variable exhaust geometry written by some members of Honda's R&D staff. PM me if youre interested.

D-Rob, yeah i got your PM, good stuff! Glad everything got worked out for you.

Rocket

It has to do with "fooling" the pressure waves in thinking that the pipe is longer. Hot gas molecules are dumb and can be readily fooled by some simple physics.

Natural Aspirations

Audio resonance?

alterdcreations

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Don Lackey &raquo;</TD></TR><TR><TD CLASS="quote">It has to do with "fooling" the pressure waves in thinking that the pipe is longer. Hot gas molecules are dumb and can be readily fooled by some simple physics.</TD></TR></TABLE>
How would you fool gas molecules?
i know that header tape traps them inside of the pipe and increases the exhaust gas velocity it the tail pipe but how do you fool them?

Rocket

When you sit in traffic it makes you think your trip is longer, right?

So you basically have to create a "traffic" jam in the header pipe.

alterdcreations

thats called stepping the header or one of those anti reberation chambers, right?