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  #16  
Old 10-12-2006, 09:57 AM
jsteele22 jsteele22 is offline
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Quote:
Originally Posted by svxfiles
The photograph and #s came from exhaust system #9.
On a STOCK SVX the #s are
From the head to the entrance of the pre-cat roughly 28"
(not counting the roughly 4" inside the head to the backside of the valve)
to the exit of the pre-cat 38"
to the inlet of the two into one cat 59"
and to the inlet of the resonator 79"

All measurements are from the head.
Sorry for any confusion or delay.

Cool, thanks Tom. I had the wrong photo, but the number I grabbed (59") just happened to be right.

Coincidence ? I don't think so....
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  #17  
Old 10-12-2006, 10:15 AM
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Quote:
Originally Posted by jsteele22
Cool, thanks Tom. I had the wrong photo, but the number I grabbed (59") just happened to be right.

Coincidence ? I don't think so....
That would be quite a coincidence!
I believe the Subaru engineers built a well thought out exhaust system, and for most owners its fine.

But there are some gearheads out there who are all about more!
More power, more speed, more braking, more road handeling, more!
More later.
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  #18  
Old 10-12-2006, 11:14 AM
jsteele22 jsteele22 is offline
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Quote:
Originally Posted by oab_au
There seemed to be a few different exhaust mods that had that resonance. The ones where the two header pipes are joined with a Y pipe, instead of joining in the low pressure area in the front of the secondary cat, will have gas pressure problems besides any sonic problems, as the unexpanded gas pressure can travel up the other header pipe to enter the open exhaust valve on that side. While the two header pipes join in the front of the secondary cat, the gas pressure and the sonic wave that enters the other pipe will be at a low intensity.
As I mentioned to Trevor (who, by the way, replied faster than you to my original post ), using a Y-pipe instead of the main cat (or suitable resonator) seems like the "best" way to excite the unwanted "tinny" resonance; I have a (less and less confident) recollection that people who kept the main cat intact and removed the pre-cats also had problems.

(NOTE: things get long and detailed here; bail out if you're not in the mood...)

Actually, as I type this, I'm picturing a second effect that could contribute to resonance at the same frequency. A positive pressure wave from an exhaust port hits the expanded volume of the main cat. From this interface, a negative pressure wave reflects back up the same pipe. This is the desired effect. But at 3k RPM, I'm guessing the exhausting cylinder is coming close to TDC, but not there yet, and the intake valve hasn't opened yet. (i.e., the system is tuned for higher RPM) So that cylinder is gonna look like a rather small. closed volume. And the other two exhaust valves on that side are closed. So, for the most part, this bank will act like a closed pipe, and the negative pulse will reflect (as a negative pulse) and travel back to the main cat. This time, the negative pulse will reflect off of the large volume in the main cat and travel back as a positive pressure wave. What's interesting is that this reflection will coincide exactly with the arrival of the strong exhaust pulse from the other side of the engine.

So, from the point of view of the main cat, every time a strong positive pulse arrives from the LHS of the engine, some (small) portion of it will pass though to the RHS. At the same moment, a negative pulse will arrive from the RHS and reflect back (positive pressure) to the RHS. Both of these will contribute to a positive pressure wave travelling back to the RHS just in time for a new exhaust valve opening. This is assuming we're at the magic RPM around 3k.

Anyway, both effects are weak, but it's interesting that they combine exactly in phase. This is true if the collector pipes are equal in length; making them unequal would break up the critical RPM into 3 different RPMs for the left-only, right-only, and side-to-side mode. Also interesting is that altering the volume of the main cat shifts the importance from one mode to another. If the volume is negligible (i.e., Y-pipe) the side-to-side mode dominates and the one-side-only mode will be tiny.
(There's always some reflection off a discontinuity, though.) If the volume is huge, the side-to-side mode is non-existent, but the one-side-only will be stronger.


Anyway, if anybody has read this far , I'd be interested in hearing some comments.
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  #19  
Old 10-12-2006, 11:22 AM
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Quote:
Originally Posted by jsteele22
Anyway, if anybody has read this far , I'd be interested in hearing some comments.
Run true duals, no bigger than 2'', and use four resonators.

Just a plug for
www.verociousmotorsports.com
very high quality resonators reasonable price.
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Last edited by svxfiles; 10-12-2006 at 11:30 AM.
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  #20  
Old 10-12-2006, 11:23 AM
jsteele22 jsteele22 is offline
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Like I really need to be typing more ....


Quote:
Originally Posted by oab_au
The interference pipe will extend the period of low pressure returned to the cylinder, but it will take a longer time. This is OK for low reving engine that can spend the time .... (snip)
Or if the pipe was "too short" in the first place ? Wouldn't this accomplish the goal of allowing a shorter pipe distance from header to main cat and still having a favorably timed (albeit smaller amplitude) low pressure wave ?

I guess the one thing that's not clear to me is how "strong and sharp" the exhaust pulse is from a modern engine (like, say a Subaru ?) compared to the ancient beasts that Robertson, er, I mean Morrison, worked with. Maybe that's a factor that isn't considered in his work. But a sharp pulse reflecting off the closed end of an interference pipe will still produce a fairly sharp spike in the middle of the extended waveform travelling down to the resonator. It should be a copy of the original, more or less, reduced by half.


Cheers,
Jeff

Last edited by jsteele22; 10-12-2006 at 11:33 AM.
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  #21  
Old 10-12-2006, 12:32 PM
Hose92SVX Hose92SVX is offline
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Thanks Trevor, your a wealth of information. I get it now. While both valves are slightly open the backpressure helps keep the new fuel&air that's coming in the cylinder from escaping out the exhaust vavle. Get that part, now I'll have to think about what your saying about the affects of overlap at higher RPM's. Is it that the exhaust flow at higher RPM's helps suck the fuel&air into the cylinder from the intake, or did I get that wrong also. Wish I could re-pay you for your time explaining, but New Zealand is kinda far away. I'll buy more Kiwi to boost New Zealands exports
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  #22  
Old 10-12-2006, 02:30 PM
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i think the other problem is that the factory main cat is not meant to have the pressure change that you get with the pre-cats removed, i believe the added back pressure from the pre-cats are also part of keeping the sound down, so in effect, with the less restrictive exhaust flow going into a cat that is made for a different application there is bound to be some sort of sound problem, not to mention the missing baffling and sound deadening provided by the two pre-cats. i was talking with my professor about this today infact, and his thoughts were to run the pipes off the headers a little larger size and run them like duals and then join them into a single cat or into a single pipe.if this makes sense at all

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  #23  
Old 10-12-2006, 06:57 PM
oab_au oab_au is offline
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Quote:
Originally Posted by Hose92SVX
I'm a little confused as to how a smaller pipe helps torque. I understand that less backpressure is good and I know on newer sportbikes they come with butterfly valves in the exhaust that open and close as the rpm changes. I've always thought the reason smaller pipes help torque is because as the gases reach the rear of the exhaust pipe they start to cool off and slow down and keeping a smaller pipe keeps velocity higher which helps keep the gases from slowing down and creating turbalance in the pipe. Turbalance in the pipe would cause backpressure and that's what you try to avoid. Is that correct or am I still not getting the picture. I know at laundry mats where the dryers exhaust pipe are real long they purposely keep the diameter small so that the velocity of the air keeps the lint airborne. Is this the same prinicple with car exhaust? Thanks for any info that clears this up for me. Ya'll might have answered this above but to be honest you guys are talking above my head.
The short answer is yes the smaller dia header pipe maintains the gas velocity that is needed to allow the exhaust gas inertia to extract the cylinder at low rpms, I'll explane back to Jeff.

Harvey.
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  #24  
Old 10-12-2006, 07:15 PM
dynomatt dynomatt is offline
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Here's the next problem then.

Based on Tom's post, true dual system is the best...that's all well and good, but for packaging reasons, I can't do that.

What's the best option for a single sytem, with no cats and the standard headers (I have to use them).

The other thing in the back of my mind is the 4 cylinder Subaru engines have different pipe lengths from the extractors...is this again a packaging exercise or are there reasons that they've done this?
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  #25  
Old 10-12-2006, 07:40 PM
oab_au oab_au is offline
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Quote:
Originally Posted by jsteele22
Like I really need to be typing more ....




Or if the pipe was "too short" in the first place ? Wouldn't this accomplish the goal of allowing a shorter pipe distance from header to main cat and still having a favorably timed (albeit smaller amplitude) low pressure wave ?

I guess the one thing that's not clear to me is how "strong and sharp" the exhaust pulse is from a modern engine (like, say a Subaru ?) compared to the ancient beasts that Robertson, er, I mean Morrison, worked with. Maybe that's a factor that isn't considered in his work. But a sharp pulse reflecting off the closed end of an interference pipe will still produce a fairly sharp spike in the middle of the extended waveform travelling down to the resonator. It should be a copy of the original, more or less, reduced by half.


Cheers,
Jeff

Development of resonate tuning in cars has been slow. I have been using it for two strokes engines for some time. It was started back in the late 40s by Walter Kaden at CZ. After the end of the war, CZ became MZ, and the resonate tuning was developed to a very high state.

The start of the wave is vital to the action of the wave. The faster the wave can be established, the stronger the effect. In strokers, we use a port with a very straight opening edge, to provide a sudden start.

The auto engines are getting the message, as the the quest for more power from the same capacity, is needed. The EG33 has very fast opening cam profiles, that can establish a strong wave action. This action intensity increases with engine speed. The faster the opening, the stronger the wave.

In the EG33 the resonate action becomes strong enough at about 4000 rpms. Below this rpms the wave action is weak. So the engine depends on the low. speed inertia action of the exhaust gas to pull a low pressure in the cylinder as the piston is rising. We don't have to worry about creating a low pressure at TDC (overlap) below 4000, as the Inlet Inertia system provides a positive pressure as the inlet valve opens.

The point is that you do need the longer header pipes to allow this action to operate. If you were to use the interference pipe to shorten the headers tuned length, you will lose the low rpm torque.

Harvey.,
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  #26  
Old 10-12-2006, 07:50 PM
oab_au oab_au is offline
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Quote:
Originally Posted by dynomatt
Here's the next problem then.

Based on Tom's post, true dual system is the best...that's all well and good, but for packaging reasons, I can't do that.

What's the best option for a single sytem, with no cats and the standard headers (I have to use them).

The other thing in the back of my mind is the 4 cylinder Subaru engines have different pipe lengths from the extractors...is this again a packaging exercise or are there reasons that they've done this?
Depending what your engine tuning is. I would just use the two header pipes, set to the length, to suit your torque rpms, into a two in chamber, muffler, to allow for the sonic wave to reflect from, followed by a resonator in the same position, for expansion, then what ever you want Matt, as long as it is free flowing.

Harvey.
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  #27  
Old 10-12-2006, 08:48 PM
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Then again, you could wait until a certain set of SS headers and custom "mufflers" are fabricated and dyno tested Right, Harvey?
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  #28  
Old 10-13-2006, 11:41 AM
jsteele22 jsteele22 is offline
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Thanks Harvey, great answer. If I finally start to piece this all together, I'll have to start picking your brains about IRIS.....



Quote:
Originally Posted by oab_au
Development of resonate tuning in cars has been slow. I have been using it for two strokes engines for some time. It was started back in the late 40s by Walter Kaden at CZ. After the end of the war, CZ became MZ, and the resonate tuning was developed to a very high state.

The start of the wave is vital to the action of the wave. The faster the wave can be established, the stronger the effect. In strokers, we use a port with a very straight opening edge, to provide a sudden start.

The auto engines are getting the message, as the the quest for more power from the same capacity, is needed. The EG33 has very fast opening cam profiles, that can establish a strong wave action. This action intensity increases with engine speed. The faster the opening, the stronger the wave.

In the EG33 the resonate action becomes strong enough at about 4000 rpms. Below this rpms the wave action is weak. So the engine depends on the low. speed inertia action of the exhaust gas to pull a low pressure in the cylinder as the piston is rising. We don't have to worry about creating a low pressure at TDC (overlap) below 4000, as the Inlet Inertia system provides a positive pressure as the inlet valve opens.

The point is that you do need the longer header pipes to allow this action to operate. If you were to use the interference pipe to shorten the headers tuned length, you will lose the low rpm torque.

Harvey.,
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  #29  
Old 10-13-2006, 11:54 AM
jsteele22 jsteele22 is offline
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Quote:
Originally Posted by oab_au
Depending what your engine tuning is. I would just use the two header pipes, set to the length, to suit your torque rpms, into a two in chamber, muffler, to allow for the sonic wave to reflect from, followed by a resonator in the same position, for expansion, then what ever you want Matt, as long as it is free flowing.

Harvey.

So what are the options for a 2-into-1 chamber ? Clearly a Y-pipe is not the ticket. I've seen mention of the Magnaflow 2-in, 1-out cat, the Stebro Euro-resonator (I think that fits the description), and that's about it. It seems that for such a generic application there would be a ton of options. Is there a brand or website that I haven't seen yet ?

Q) Would a 2-input muffler be appropriate (i.e., more than just expanded volume, but also baffles) ? Usually it seems that a muffler would come after a resonator.

Q) If a 2-input cat is used, then you want a larger resonator very soon after it, right ? The cat by itself isn't gonna have much volume.

Q) When someone says "resonator" I've kind of learned to substitute the word "glasspack". I've only come across one or two products actually called "resonator".
Is this just a marketing issue, or is there something about a "real" resonator that's better than a glasspack ?
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