Mufflers or Cats ?

[Redbob]

That inexpensive tool would be a steel tape.
If I could recall the exact frequency of the boom problem, it's a fairly simple calculation to predict the required position of the X-over pipe, based on quarter-wave theory. I'd also have to guess at the gas temperature, which affects the speed of sound in the pipe.

You have to know, however, that the problem noise is coming from the exhaust. Assuming that it is, I think the best position is just ahead of the rear axle, near the point where the two pipes split apart to run out to the two mufflers. It does not have to be exact in order to be effective. Plus or minus several inches should be OK since the wavelength of the sound involved is quite long.

I've attached a very crude sketch I made to illustrate some of what I'm describing. Note that the temperature 'T' in the formula is the gas temperature, and a reasonable guess at this is 450F.
You can see, though, that the frequencies will change depending on engine load, since that will change the gas temps.
- R

 

[Redbob]

Where did that attachment go?
(I'll try again)

 

[Alex D]

I just bought and installed the Magnaflows on my LT1. they are absolutely georgeous and sound great. No resonance with my performance axle car.......I think the standard axle and M6 coupes have the resonance problem when cruising at low rpm (under 2000). I can only recommend them . They come with all parts necesssary for installation and I got them for $421 plus shipping from Madhatter.com Great shop, excellent service and quick delivery.

 

[vms4evr]

Alex,
This is great news. Cause guess what I just ordered this morning My Magnaflows will be here next week. I saw a picture of a set on a 94 ragtop. They look awesome. Sent him email and he said they sound great and in the ragtop only minor resonance. He gets some at 1500rpm. He's got the auto with 2.59 so unfortunately for him that is highway cruise rpm... but he says it is liveable.
If you've got the G92 option (3.07) and that keeps you away from the rpm that causes the annoying resonance then i should be in good shape. I'm running 3.54 gears with my auto. So 1500rpm is something I see for about a second when I leave a traffic light. I cruise intown in Drive and run 2000-2500rpm. Then on the highway I'm in OD at 2500rpm for cruise.
I'm still going to look into an X-pipe for a couple of reasons, mainly resonance. Redbob has some good ideas about that and the materials for insulation and all. I checked out those websites today. I'm going hunting for pricing on some of the stuff.
I'll let you know how I like them in about a week or so and I'll try and post some pics.
Graham

 

[Redbob]

Graham:
Sounds like you're rolling!
Just for your reference, a resonance at 1500 engine RPM would equate to a 100 Hz (or cps) sound; you would get the same effect at 3000 RPM as well, but by then there's so much else going on you'd probably not notice it!
That 100 Hz corresponds to a cabin linear dimension of around 5-1/2 feet. Seems to me like that number should be just a little higher. Could that be the distance from side window to side window?

It could also be that 1500 RPM is the second mode; I wouldn't be surprised if the car was pretty boomy at around 750 RPM as well.

Regards,
- R

 

[Alex D]

Graham,
You'll love them! They are absolutely high quality fit and finish!! Too bad that MAgnaflow doesn't make a full cat back for the LT1 C4s.
I talked to some of my buddies with the B&B system, also plagued by resonance and it confirms that the resonance is mostly present at 1500 to 1800 or 2000 rpm. It quicly fades as you surpass 2000 rpm. Even with the stock exhaust system you can notice the resonance in that rpm range. Good luck!! Let me know how you like the Magnaflows....I think they are a greatly underestimated system.....
If you leave your stock resonator in the systems, it will act like a x-pipe since the exhaust flow joins within........if you are taking the resonator out, let me know how it looks inside. Is it straight through or are there baffles????
Magnaflow also sells nice x-pipes. check their website !!!

 

[Redbob]

I can answer the question about the resonator internal construction:
It's straight through, no baffles. The internal tubes are smaller than the connecting tubes, about 1-7/8" diameter, and they're perforated for almost their entire length.
There is no packing material inside the case, so there'd be no change in tone over time unless the internal tubes rusted out or broke due to vibration. The internal materials are I think 409, a low grade of stainless that will still corrode, but relatively slowly.

This resonator is not located in the best position for cancelling the cabin resonance on the C4, however, and does not take the place of a good cross-over pipe. Because of the small internal tubes, and the abrupt change from system tube size to internal tube size, there's a significant reduction in system backpressure if you remove this device - if you put in a crossover as I've discussed previously.

 

[Alex D]

Thanks redbob!
Is the resonator more restrictive than the stock mufflers???
I figured that a little backpressure is good for mid range torque. All wide open systems I have seen, includingthe MAgnaflow for the C5, have more top end power, but lose some mid range until 4,500 rpm.........

 

[Redbob]

Backpressure has no effect on midrange torque - except to reduce it! - over a rather broad range of pressures. Unless the airflow improvement is so great as to cause the engine to lean-out excessively.
This thing about backpressure helping torque is a result of poorly-understood experiences with carbureted engines back in the foggy mists of time...
If you made to big a change to the exhaust system in a carbureted car, you'd screw up the carbuuretion. If you didn't know how to rejet, the car would run rather poorly, especially in the mid-range, until it got fully onto the main jets, and the secondaries opened.

But the resonator is somewhat restrictive; you'd probably pick up may 4 - 8HP by removing it. This of course is only important at max RPM, but with the gear you're running, it might be noticeable at for example the drag strip.
- R

 

[Topless]

Thanks Redbob.

As soon as they figure out why my belt is jumping (and FIX it), I will be heading to the exhaust shop for an X-pipe. I think I will skip the muffler eliminators. I enjoy my hearing, and listening to good music.

_mike

 

[Redbob]

Thanks Redbob.

<snip> I think I will skip the muffler eliminators. I enjoy my hearing, and listening to good music.

_mike

Not to mention the respect of your neighbors, when you head out at 6 A.M. !

 

[vms4evr]

Topless,
Some folks like the muffler elims and think they aren't that loud. I guess it's all a matter or perspective. I'm with you. I think they would be too loud.

Redbob,
Someone told me a quick way to find out where to put an X or H pipe. Wanted your input on this idea. I was told to spray paint the exhaust pipe. Then run the car and look for the first place the paint starts to bubble and blister. This is supposedly where the maximum pressure is from the sound waves. And therefore the place to put the pipe. Sounds questionable but I have heard this more than once. What do you think?
Btw, I'm taking measurements on the exhaust and cabin so we can try plugging them into the formula and see what it comes up with. I may need your help on that. My Algebra and Calculus gets very little use these days :Roll

Graham

 

[Redbob]

I've heard the same story about painting the pipes; that goes back at least to the '60's that I know of, and it may well work. Peak sound pressure would produce a wave action that would disturb the boundary layer on the pipe walls, increasing heat transfer and discoloring or burning off the paint.

The question, though, is under what conditions does this happen? As you can see from those formulae, the speed of sound varies significantly with the gas temperature, so the position of peak sound pressure would change significantly as well.
I've measured gas temperatures from 300 F, at the end of the exhaust at part throttle, to 1950 F, just down from the manifold at sustained WFOT.
So, mostly this is done on track cars being run at wide open throttle all the time.
I don't know; it might be worth a try for you, but I'd be inclined to work with the formulae first. When you do, remember that a pipe does not end exactly at the muffler entrance, although this is probably unimportant unless the pipe is very short.
You want to put in the crossover pipe ('H'-pipe) where sound pressure is high, but it doesn't have to be at an exact peak to be effective.

Regards,
- R
BTW: After having my car off the road for 6 months, following a minor "Hyundai event", it's all back together and I just started driving it again yesterday! I'm happy!