SPOautos

I've been reading some things about "tuned" exhaust and would like to hear some comments from some of you technical guys.

I was reading about how a tuned Helmholtz setup works, and how a exhaust can be tuned to create a sound wave that moves back and forth creating a vacume that sucks out the a/f miture after its combusted so that the motor doesnt have to waist its power pushing it out. It goes much more in depth than that but I though this might get a conversation going.

who knows much about this? Its sounds pretty interesting to me.

STEPHEN

maxpesce

It is very interesting and well know phenomon Racers have been using tuned exhausts for years
it also is why Yamaha makes Pianos as well as Motorcycles - the science of ACOUSTICS! is the same weather it is being applied to an internal combustion engine's intake and exhaust system or a musical instrument.

SPOautos

I have a basic understanding of how it works but doesn anyone know how to go about implementing something like that??? How would you design it???

Anyone know much about actually making one? How would you know if its working right???

STEPHEN

maxpesce

The old way of designing one was trial and error w/ a flow bench and dyno - now you can get real close using COMPUTATIONAL FLUID DYNAMICS and a SUPER COMPUTER!

j9fd3s

iTrader: (3)

http://personal.riverusers.com/~yawpower/
try that for a start

mike

SPOautos

I just got done reading that entire tech section at that link and it is very informative. Most of this I pretty much knew but I did learn some interesting things. It basically confirmed and cleared up some things I was thinking about. Thanks for the link.

BUT - It still doesnt go into detail about how to implement these ideas. Do most of the exhaust designers take the "wave travel" into consideration? I dont see much in the current exhaust designs that suggests they do. Is it just too hard to design and exhaust for a turbo car that takes advantage of the waves???

How does the turbo and turbo manifold affect everything. That link above as well as others on the net seem to be geared toward a more simplified N/A exhaust that doesnt have turbine wheels, ect getting in the way.

It seems to me the 3rd Gen exhaust manifold would totally screw everything up but I might be wrong.

Does anyone have much experience (or knowledge) with wave travel in a turbo car exhaust?

STEPHEN

rx7tt95

If I remember correctly, the turbine itself has to be designed to take advantage of the tuned header. Most are designed to work more efficiently with the "erratic" pulses of a piston engine. I believe the rotary produces a more regular pulse however, but a very powerful one compared to the pulses in a piston engine. That's all off the top of my head however, gleaned from an article in the distant past, so I could have things veeeery wrong....I do remember them saying that early turbos didn't take into account the messiness of the pulses hitting the turbine blade and later they were changed to take advantage of irregular pulses, making them more efficient. I also"think" that it had a lot to do with the fact most turbo cars at the time ran "log" style manifolds that were NOT tuned. Most turbocharged racing vehicles, especially from the 80s'/early 90's used tuned headers. Go to any vintage race where they have various makes/models such as the Porsche 935's, Stillen 300Z's and Nissan GTP's and you'll see what I'm talking about. Long. tuned headers connected to big *** turbos. :-)
Michel

SPOautos

Ok, so here is the deal. You create positive and negative waves traveling back and forth in your exhaust. These waves actually carry exhaust. When your port closes it send off a positive wave that carries exhaust with it. When that wave reaches a sudden area change like the collector or a larger diameter pipe it reverses its direction and becomes negative. A negative wave sends the exhaust in the opposite direction from the wave travel. This means that even though the neg wave is traveling back toward the port its pushing the exhaust out the back. This is good because what happends is when it gets to the port it sucks the remaining exhaust out of the chamber which in turn creates more efficient ve by allowing that chamber to take in more clean air during the intake phase. If your negative wave isnt in sync with your port opening it wont suck it out and then you have exhaust in the chamber when it sucks in intake air which means you cant get as much of your good clean air into the motor as you would have with the right wave timing.

Now here is the deal, what I'm trying to figure out is this. Where you place the area change (larger dia pipe in my case) is where the wave will turn around at. It has to be timed so that it comes back at the right time for a given rpm. Lets say I want it to be most effective around 6000rpms. How would somone go about figuring all this???

I understand how it all works my down fall is the math. I'm a finance/banker/business man, not a engineer. What I'm wondering is who would you go about figuring the distance that your larger dia pipe need to be from the port to take full advantage of the neg wave at say 6500rpms????

Anyone actually know how to do this???? Rice? Reted? Anyone?

Thanks,
STEPHEN


STEPHEN

SPOautos

Would it maybe be better to set up your area change before the turbo so that its not affecting or affected by the waves? The turbine wheel really just need the exhaust pulses not the sound waves so wouldnt it be better to place the area changer before the turbo???

Can anyone tell that my brain is melting???

STEPHEN

SPOautos

Ok, I think this is the formula to find out how long of a pipe you need for a given rpm

Can someone use this formula and base it off of 6500 rpm's???

Length (in inches) = (CID x 1900) ÷ (rpm x pri.OD2)

I'm not really sure what the "pri.OD2" stands for but maybe some of you math gurus know.

STEPHEN

SPOautos

Well, after talking to someone else on another forum about this he think that the formula above doesnt have enough detail to be accurate. He says he thinks the formula is this based on something in one of his books....

I DID A CUT AN PASTE AND HERE IS WHAT HE HAD TO SAY ON THE SUBJECT.......


I haven't seen it in this context before, but if you analyze it a little, the wave is traveling at a fairly constant velocity. It is definitely disrupted by a turbine wheel. I would end the length of the runners there. Remember it includes the port in the head too (all the way to the valve or outlet).

You are making me drag the book out....

The simplified Helmholtz equation is

RPM = (955/K)*a*(A/l*V)^.5

a is the speed of sound (unaffected by pressure in this instance)
A is the cross sectional area of the inlet (cm^2)
l is the effective length of the inlet system (cm)
V is the effective resonator volume (1/2 the displaced cylinder volume plus the clearance volume)
K is a constant. This varies between engines and is what you would try to find experimentally on the dyno. Once K is found for your engine the equation is extremely accurate It is usually around 2, and 2 can be assumed.

END OF CUT AND PASTE

So what do you guys think? I'm by far no engineer and am not really sure how to do some of this stuff. I also dont know some of the values to use.

ANYONE KNOW THIS STUFF?????

Thanks,
STEPHEN

j9fd3s

iTrader: (3)

i would think that you want to tune the pressure waves so they interact with the turbo, you want it to spool faster right? another thing ive been thinking about is the piping to the wastegate (i'm thinking single turbo, external wastegate) when the wastegate is open it acts just like an open header on an na car, seems like its worth trying to get it to flow well.

mike

SPOautos

I dont think that actuall sound waves help with turbo spool, from my understanding that more a function of the exhuast pulses itself.

The negative sound waves if timed right will hit the exhaust port while its open sucking out all the exhaust so that none is left. This is effectively raising the ve.


STEPHEN

j9fd3s

iTrader: (3)

yes, but i think the turbo blocks those so you would have to tune it for the turbo. i'm not sure i agree with what im saying but, maybe its valid?

mike

SPOautos

I THINK what would happen is this....

When the sound waves leave the exhaust port they are positive waves, they travel down the pipes untill the reach a "area change". When those waves reach a area change they turn around and go back as negative waves. I know all that for sure, the part I'm a little unsure about is how much area change is needed to reflect the waves back. I think (and have been told) that the turbine housing SHOULD be enough area change to reflect the waves back. This means as soon as the waves reach the turbo's turbine housing its just going to turn around and go back toward the motor. If that is the case the turbine wheel should be any problem at all. The positive wave would travel down the pipe come to the turbine housing then reverse and go back as a negative wave.

Then you just build the pipe between the motor and turbo to be a cetrain length so that the wave will be timed to reach back at the exhaust port when its open so that it can suck all the remaining exhaust out.

My major problem is figuring the length, that second equation I posted is asking for info I dont know and the math is too advanced for my skills. I'm finance/business and have been out of school for 5 years.

STEPHEN

waynespeed

If we are talking about volume we have the length but the missing info sounds like the diameter.

(primary Outside Diameter x 2) ?? just a guess.

SPOautos

Yea, that is the first equation, I've been told it doesnt seem very accurate.

I've heard this is the one to use.....

RPM = (955/K)*a*(A/l*V)^.5

a is the speed of sound (unaffected by pressure in this instance)
A is the cross sectional area of the inlet (cm^2)
l is the effective length of the inlet system (cm)
V is the effective resonator volume (1/2 the displaced cylinder volume plus the clearance volume)
K is a constant. This varies between engines and is what you would try to find experimentally on the dyno. Once K is found for your engine the equation is extremely accurate It is usually around 2, and 2 can be assumed.


What do you think about that one??? Anyone know how to figure that? Say I wanted it to be tuned for 6000rpm's, can anyone here figure what the exhaut pipe length should be???

STEPHEN

setzep

I really think we need Rice Racing in here.. I remember him saying the primary tubes should be 13" long but I also know he has his engine set up for higher RPM's than 6000. I'm sure he could give us some information.

HWO

Peter said 13inches long for a extended port or stock port, thats the length for good power delivery, 11 inch for PP turbo he reckon'd, that'd give torque and HP higher up the range.