[SILVY968]

[quote name='RS Barn' date='Nov 30 2005, 09:33 AM']I agree that the resonator is restrictive. I went the route of gutting the stock one at first seeeing the opportunity to gain some power and noise at minimal cost.. But it is a pain and I'm not sure of longevity. The restrictor plates also hold the guts together.

I was developing a cat back at the same time that gave hp and sound with lighter weight and improved looks. The downside is cost. It's hard to get the best of everything without removing, replacing or redesigning.

Pete

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If the mesh sleeve is left in and only the restrictor plate is removed do you still loose the "guts hold together" function?

If so, would a compromise be to cut out the plate, but leave the circular base of it tied to the sleeve, or would turbulence be a problem?

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[S_Cal968]

Hmmm, my drilling idea isn't looking so bad after all... One could increase flow just by some stategically placed .50" holes and retain the retaining plate.

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[RS Barn]

Their are multiple plates holding the tubes together. You have to cut out the first to gain access to the second. To ensure longevity you would then need to weld a brace to support the tubes. I don't think drilling the first plate would help.

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[flash]

that's very likely going to create turbulence, change the velocity of the flow, and probably do some very weird things in the midrange

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[Cloud9...68]

[quote name='RS Barn' date='Nov 30 2005, 10:33 AM']I agree that the resonator is restrictive. I went the route of gutting the stock one at first seeeing the opportunity to gain some power and noise at minimal cost.. But it is a pain and I'm not sure of longevity. The restrictor plates also hold the guts together.

I was developing a cat back at the same time that gave hp and sound with lighter weight and improved looks. The downside is cost. It's hard to get the best of everything without removing, replacing or redesigning.

Pete

[right][post="12978"]<{POST_SNAPBACK}>[/post][/right][/quote]



Pete,



Any comments on the "pause" or "flat spot" described by others? Do you experience it with your set-up? Does your complete system (header/high flow cat/muffler) have a different degree of the flat spot relative to your cat back-only set-up? Thanks.

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[RS Barn]

The mystery flat spot lives.

I just looked through my dyno tests: chips ,air intake mods,exhaust ,header any combination all have a dip in the otherwise smooth hp and torque curve. The less of a mod the smaller the dip only due to the larger jump just after the dip with more mods.

My thoughts are it has something to do with either the dual stage intake or variocam. Exhaust doesn't seem to affect it.

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[apex]

Fascinating discussion!



I for one would love to get rid of that "alien" strainer. <img src="/forum/images/smilies/968/rolleyes.gif" class="smilie" alt="" /> Actually I wanted to remove it last year just 'cause it was annoying me at the time - but it really is socked in there as others have attested.



Just a long way of saying if anyone experiments with alternate methods (drilling) or discovers that a muffler shop will do it for us easily and cheaply do keep us informed! <img src="/forum/images/smilies/968/biggrin.gif" class="smilie" alt="" />

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[ds968]

no dead spot / flat spot / torque loss on mine - seat of the pants does not detect anything like that in that rpm range, or anywhere else on the band for that matter, and the torque graph from my last dyno test shows a pretty consistent curve..



I have the BB ( 2.5" pipe ) catback system, without a resonator.

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[RS Barn]

The problem is what tool to use.The plate is stainless and a bear to drill. I had my exhaust fabrication place start a pilot hole and use a tapered carbide drill (they cost $75.00) and you could only get the first plate and it tended to rip the metal at the end and distort the tubes. For the second one you would have to weld a long rod on the tool to drill and it would be extremely difficult to do.

Sorry!

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[flash]

dan - i've driven the B&B - it has it too - it's not something that is easily detected by the seat of the pants - you really have to be paying attention and creep up on it - it's really something you see on the dyno - "flat spot" is not really an accurate term, but i can't think of anything better - it's not like it falls down or anything - it's just a place in the curve where the curve doesn't rise like the rest, and it isn't huge - it's just a thing that says "there is something not quite right here"



i agree with pete that it could easily be related to the intake - to expand on my above theory, i am thinking that it has to do with pulse timing - this is the only car i've tinkered with where i have ever seen this kind of reaction - it's also the only one i've tinkered with that has a resonant induction manifold



to get it right, it seems that a lot of math will have to be done to figure out how many inches down stream the place the resonator, what size, how many, etc - it still may not get it - the "flat spot" is just before the peak torque point on the stock curve - this may be a coincidental place where the header, the exhaust, and the intake are just not happy being messed with



the header itself is designed for low and midrange optimization - the short tubes and 4-2-1 design is what works best for that - but, it could also be the difference between the 1-4 and 2-3 tubes - the disimilar merge shape there could be causing a flow anomoly in the volume and timing as well - one of them is defintely not the optimum shape



it is acting almost like a harmonic



interesting stuff for sure

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[ds968]

In that case I probably did not pay close attention to the graph -

if find it I'll take another look, but if this is a consistent

result with all the other cars you tested, then I'm sure mine won't be any different. Poor recollection of what the torque line looked like I suppose..., but definitely no no dip or flat spots on the

butt-o-meter. In fact, just the opposite effect.



All I know is that I'm marginally pulling ahead of cars with sub-14 (high 13s) 1/4 mile stats, not to mention that at the top end this car now has a highly respectable showing, so that's all I need to keep me content.



But I realize you guys just want to get every little detail worked out perfectly so you're digging far beyond the seat of the pants factor.. Kudos !

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[flash]

well, yeah - you're sure not going to notice it at full throttle 1/4 mile stuff, or even at high speed runs - the spot is well below the peak torque point, and is usually passed without notice - next time i see you in your car, i'll take you out and show you how to make it show up - of course, then you'll be pissed

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[S_Cal968]

I've machined S/S before and true it's not easy, but it can be done. I wouldn't use a tapered drill - I'd use a split-point cobalt. A .25" [x 12" in length] drill can be had for < $13. I don't have the luxury of a torch or air chisel.

I have have to disagree that drilling it would create more turbulence then it already has. If anything removing material will increase flow. Like I said I'm not set on going this route, but it's still on the table.



If I had to guess on the power dip, I'd suggest the intake design. The dip is probably a dynamic transition point in the airflow through the intake. At low rpm the flow is slow enough to utilize plenum pockets and as the flow increases the pockets create a negative turbulence. At high speed these pockets are most likely by-passed as the flow takes a more direct route. Just a theory...



Obviously with all streetable exhaust systems there's a compromise. Unless there's a way to dynamically change the diameter of the pipe, power is lost at the top or the bottom. Also component placement is critcal so as to not distrub the exhaust pulse. The header is a nice design, but I don't think the runners are the same length which would perform better.

The stock system is a nice all-around system. Although, If the RS cheater pipe is the right diameter, that might be better than hacking up the resonator.

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[orphanowner]

I'm gonna chime in here at the risk of more controversy...but I need to clear some things up.



More info on the resonator:



Think of the resonator as a three chambered muffler. The first chamber is the “engineering afterthought”, the most restrictive part of the exhaust system, and houses the diverter and the 70 mm perforated baffle tube located immediately after the 4 bolt flange. The first chamber can be accurately visualized by looking at the image of the resonator and observe the discoloration of the resonator body. That dark area is exactly the size and location of the first chamber. Removal of the first chamber internal structure in its entirety, will have no affect on the integrity of the resonator. In fact, it will reduce the operating temperature substantially, which should increase the MTBF.







The second chamber is really a 8”-10” open chamber in the middle of the resonator and serves as a “storage chamber” for the exhaust that exits the diverter/perforated tube on it’s way to the third chamber.



The third chamber is the remaining part of the resonator and comprises of the two tubes that exit the resonator and feed into the rear muffler. The “after” picture in my original post shows the front of this chamber as viewed through the resonator inlet. You will see a triangle shaped partition that closes off the rear third of the resonator and locates the two smaller perforated tubes that exit the resonator and feed the rear muffler. Between this last partition and the end of the resonator and around the twin perforated tubes, is a fiberglass packing not unlike a glass pack type muffler. You can see in my “after” photo where I punched the partition with my air chisel point and the fiberglass is visible in the hole. I was going to try to remove this as well, but thought better of it.







With respect to the diverter plate and removing the same…this is a straightforward modification with little chance of screwing anything up. You can use a standard drill and make holes around perimeter of the diverter, or use a hole saw, air chisel or flame wrench if you have one. While tedious and frustrating at times, it’s easy work and very rewarding.



It’s all a matter of time and patience and what tools you have available. I once took a cast off my arm with a cordless drill and a ½” drill bit (and lots of wine!)…. Yes, removing the entire first chamber will be more work, but the payoff is well worth it IMHO. The only factor is your time and if you have to take the part to a muffler shop to have some cutting done for you, it’s still an easy project and very cost effective. I still highly recommend it as a virtually free HP/TQ upgrade with only a very subtle change in exhaust noise output. In either case, I don’t believe that the integrity of the resonator is compromised to any degree by this modification. I’d do it again without any hesitation.

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[Cloud9...68]

[quote name='S_Cal968' date='Dec 1 2005, 10:29 PM']If I had to guess on the power dip, I'd suggest the intake design. The dip is probably a dynamic transition point in the airflow through the intake. At low rpm the flow is slow enough to utilize plenum pockets and as the flow increases the pockets create a negative turbulence. At high speed these pockets are most likely by-passed as the flow takes a more direct route. Just a theory...

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With all this discussion of the "pause" being intake related, I have another question for Pete - I noticed on your web site a link called "intake manifold", which leads to a tantalizing picture of what looks like a modified manifold, but no description. What do you have cooking in the intake department, and could it address this flat spot in the torque curve? Thanks.

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[BruceWard]

Guys, Orphanowner has a great thread going about the progress of his low cost exhaust mod. Could we please take the flash exhaust, RS Barn Exhaust and "what causes the flat spot that is found in stock and modified 968s" stuff to new threads?



Personally I want to hear more from Orphanowner.

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[flash]

having personally driven his car, and talked to him about it and the mod, and found the anomaly to be present in this mod too, it is entirely appropriate to have that part of the discussion in this thread

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[Cloud9...68]

[quote name='BruceWard' date='Dec 2 2005, 01:29 PM']Guys, Orphanowner has a great thread going about the progress of his low cost exhaust mod.  Could we please take the flash exhaust, RS Barn Exhaust and "what causes the flat spot that is found in stock and modified 968s" stuff to new threads?[/quote]



As a perpetrator of diverting this thread a couple of times, I do see Bruce's point (but thanks, Flash, for acknowledging that perhaps the diversions were justified). This falls into a gray area - when an interesting point is made within a thread about something that is related, but different enough to technically justify a new thread, do you start a new one, with the risk that it is of limited enough interest that it will quickly die off, or do you stay within the existing thread, where it may survive longer on the coattails of the main thread, but annoy people who want to keep the focus on the main topic?



It would be nice if there were a feature called something like "reply==>start subtopic", where the reply would fall into a new thread, but the new thread would remain under the existing thread's hierarchy, like a subroutine. C is the farthest I ever got in programming classes, so I know nothing about this web based stuff, but does anybody know if this would be within the realm of possibility? Actually, given all the flexible features available to forums these days, I'm a little surprised this isn't currently available.



Since not everybody is following this thread, I'll go ahead and post this suggestion on the "General forum specific Q&A" forum. Thanks.

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[austin944]

Instead of cutting/drilling the resonator, why can't you replace it with a straight pipe? I'm sure there's probably a good reason for not replacing it, but I can't think of it. Would it be too loud?

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[flash]

it is very loud when you do that



the bigger issue is a dramatic reduction in necessary back pressure - the results are a loss in low end torque - the same thing happens when you go to a 3" pipe, or delete the cat - the high end hp goes up a smidge, but the low end loss is no fun - this is fine for a race car, but not the best for a street car



steve - in looking at the above post (nice job by the way) am i correct in understanding that the 2 tubes are not present in the center section of the resonator, and are only present in the last section?

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