Another exhaust tweak

Very much looking forward to your test results. Any plans to diagram the placement of your pick-ups?
I too have a magnehelic gage and plan some underbody and radiator airbox measurements - but those tests will have to wait until the car moves under its own power -- and the body is back on ;)

Cheers - Jim
 
Put a few miles on the car yesterday at a test and tune session. Got some "data" and figuring out the significance of the numbers. They were:

Air filter: I had one side of the differential pressure gauge hooked to a tap in the air cleaner base, and the other side connected to the airbox. At WOT with the stock AC 3"x14" filter I only saw 1" H20 pressure drop across it. Due to this low efficiency loss and the limited amount of time I had to do all the other changes and measurements I didn't do the test again with the K&N filter. At the next opportunity I'll hook up the filters again to check for consistency of the measurements.

Front belly pan: Where to obtain the reference pressure was an issue. I initially wanted to use the ambient pressure as a reference but my sealed container reference kept heating up and offsetting the gauge reading even while sitting in the pits, so I said THWI and just used the interior cabin pressure as the reference. I'm not totally happy with that, but I had limited options and supplies at the track. At about 50 mph I would see the gauge start to move and it would reach 3" H2O (below cabin pressure) at about 130 mph when I lost courage and put my eyes back on the track. Not a lot of pressure differential, but directionally correct if accurate.

Carb pressure drop: Ran out of time. I wanted to find out if I was leaving anything on the table with the 850 carb, but I'll have to do this measurement at another time.

Exhaust backpressure: I saw very little dial movement in the pressure gauge so I hooked the line up to the differential gauge. At WOT and high RPM I saw 24" H20. IIRC, 28" H20 equals about 1psi, so I've got almost a psi of backpressure. I've got 2 1/2 " turbo mufflers at the back, so it looks like I might look into 3" units to see if I can help the system breathe a bit better.
A interesting item showed itself during all this. The exhaust was very reasonable in volume at low RPMs, but there was a very noticeable step change in exhaust note and volume at 3000 RPM. It seemed like something was suddenly hitting a resonance, but it continued to stay louder and sharp all up the RPM range. Right now I'm just trying to figure out the mechanism that is causing this abrupt change in tone.
 
69427 said:
The exhaust was very reasonable in volume at low RPMs, but there was a very noticeable step change in exhaust note and volume at 3000 RPM. It seemed like something was suddenly hitting a resonance, but it continued to stay louder and sharp all up the RPM range. Right now I'm just trying to figure out the mechanism that is causing this abrupt change in tone.
Click to expand...

wouldnt you expect that the volume of exhaust gases being pumped into the exhaust system would start to alter the sound as the volume increased? just like a musical horn?
 
bobs77vet said:
69427 said:
The exhaust was very reasonable in volume at low RPMs, but there was a very noticeable step change in exhaust note and volume at 3000 RPM. It seemed like something was suddenly hitting a resonance, but it continued to stay louder and sharp all up the RPM range. Right now I'm just trying to figure out the mechanism that is causing this abrupt change in tone.
Click to expand...

wouldnt you expect that the volume of exhaust gases being pumped into the exhaust system would start to alter the sound as the volume increased? just like a musical horn?
Click to expand...

Yes, but this change in sound is almost instantaneous at 3000 RPM. Just very different than the normal gradual change in exhaust note I'm used to.
 
Chambered mufflers work because the different chambers have different standing wavelenghts. It looks like you really have only one chamber, so you won't have a lot of different frequencies covered.

There is lots of information in acoustics books on the subject.
 
I'll bet 5252 RPM.


POWER (the rate of doing WORK) is dependent on TORQUE and RPM.
TORQUE and RPM are the MEASURED quantities of engine output.
POWER is CALCULATED from torque and RPM, by the following equation:

HP = Torque x RPM ÷ 5252

Maybe where is the Torque Maximum peak? Perhaps the power band starts at about 3000 and that begins to set the resonance?

Jim
 
It's very early but I'll give this a try:

3000 rpm/60 sec=50 rps

50 rps x 8 exhaust pulses = 400 pps

1250 ft/sec (guesstimate of speed of sound at 200 degrees)

1250/400= ~3ft

Do you have sections in the flowpath that are about this length?

Edit:4 exhaust pulses per revolution (I said it was early)! So ~6ft. But a 1/2 wave would be 3 ft.
 
Last edited:
BBShark said:
It's very early but I'll give this a try:

3000 rpm/60 sec=50 rps

50 rps x 8 exhaust pulses = 400 pps

1250 ft/sec (guesstimate of speed of sound at 200 degrees)

1250/400= ~3ft

Do you have sections in the flowpath that are about this length?

Edit:4 exhaust pulses per revolution (I said it was early)! So ~6ft. But a 1/2 wave would be 3 ft.
Click to expand...

Larry, I believe we're on a similar wavelength here (no pun intended). I did some calulations a few weeks ago using the wave speed and time for the pressure pulses to travel from the (just opened) exhaust valve to the termination box and back. IIRC, I got a calculated resonance at about 8000 RPM, which I immediately knew I was never going to be able to take advantage of. :amused:

I'll take another set of measurements and see what I come up with using what you outlined above.
 
rtj said:
Chambered mufflers work because the different chambers have different standing wavelenghts. It looks like you really have only one chamber, so you won't have a lot of different frequencies covered.

There is lots of information in acoustics books on the subject.
Click to expand...

Yeah, I'm still on the steep part of the acoustics learning curve. I've got an old engineering book with a chapter on acoustics, but a lot of it is pretty dry reading.
 
69427 said:
rtj said:
Chambered mufflers work because the different chambers have different standing wavelenghts. It looks like you really have only one chamber, so you won't have a lot of different frequencies covered.

There is lots of information in acoustics books on the subject.
Click to expand...

Yeah, I'm still on the steep part of the acoustics learning curve. I've got an old engineering book with a chapter on acoustics, but a lot of it is pretty dry reading.
Click to expand...

OLD Helmholtz resonators, Corner horns, Folded horns, Acoustic suspension, all them terms back when I played with stereo speaker designs....

Used to have a HUGE McIntosh stereo system....stacked ML2c's in each channel...make that FOUR 12" woofers per channel, and 400 watts RMS per channel pushing them....the amp had it's own 20 amp breaker feeding it....the system was tuned to the room resonances.....

sold it maybe 4 years ago, never listened much anymore....

:nuts::clobbered::tomato:
 
Summit racing on YouTube has some good video on corsa and flowmaster mufflers.
 
I e-mailed the tech guys at Dynomax to request some flow information on a couple of their 3" turbo style mufflers. I picked two models that would probably package decently, and asked about the flow rates (and at what pressure drop), and why one of the models was twice the price of the other. I got a short reply saying that both models flow 430 CFM. It frustrates the hell out of me trying to figure out wth is the purpose of a tech line if they won't answer simple technical questions. A flow rate number without the pressure drop or head pressure number is meaningless. I could take a guess that Dynomax flows their mufflers at the same pressure drop as Holley flows their carbs, but I'm not inclined to spend hundreds of dollars (in mufflers and my time) installing these things, and finding out I guessed wrong. I see that they have a phone number to their tech area, so I'll give a call this week to see if I can get some complete answers this time.

If I can definitively reduce the backpressure significantly with some new mufflers I'll do it. If the answer is fuzzy my backup plan is to retain the current mufflers and make a simple tap in the underspring Y pipe to be able to hook up an additional pipe and muffler/glasspack and dump some of the exhaust out under the license plate area during track days. I could install/remove this extra exhaust shunt in five minutes. I would prefer not to carry the additional ten pounds of this system (although I could weld up an aluminum glasspack), but as I mentioned earlier, I'm also not interested in spending money and time installing a new set of mufflers that don't "pay for themselves" in performance.
 
BBShark said:
Mike, Chances are the "tech" guy does not know much. Thats been my experience with this. Dynomax does have a PDF that describes the test perameters

MODEL (Flow Rate- @ 20.3" H2O) Muffler Inlet Size

http://www.dynomax.com/assets/2008catalog_pdf/superturbo.pdf
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Damn, Larry, that's the exact info I was looking for! Don't know where you found it, but I greatly appreciate it. For whatever reason I was never able to find this type info on their website, and the tech guy's reply also neglected to say that one of the 3" mufflers I was asking about is actually 2.5" tubing inside. The chart also contradicts his claim that the two part number mufflers flowed the same.

Looking at the chart gives me the indication that it's doubtful I can package a pair of sufficient-flow mufflers back there. The pressure drop numbers are also darn close to what I currently have at my "rule of thumb" flow number, so the performance improvement per dollar ratio is pretty slim.

Again, thanks a bunch. It's probably saved me from a lot of wasted time talking to the "tech" guys.
 
A while back, I was searching for info on induction tuning. What I came up with was very little info on induction and a lot of questions about exahust (flow rates etc.). The Dynomax info was a result of that. It would be really good if that was a third party test that included other brands but I haven't seen that yet.

This is another example of when marketing people sell a product. The bigger, better, faster hype starts and the science is forgotten. For automotive, there is probably no more abused term than CFM. From radiator fans, to carbs, to pumps, to exhaust, it's all the same.
 
Titanium muffler (temporarily)

Been going a couple different directions in the exhaust mods. With the info Larry provided I was able to make a comparison with what (muffler flow) I currently have and what better flow units are available from Dynomax. I ordered a pair for the car.



The muffler on the top is a spare like I've had on the car for several years (2 1/2 inch), while the bottom is one of the replacements (3 inch). The flow rate numbers are 410 CFM for the 2 1/2" part, while the 3" muffler is rated at 554 CFM, for a 35% improvement at the same pressure drop. The down side is the new mufflers are 8 pounds (per pair) heavier than the current mufflers.

I was trying to get ready for a track weekend in Indiana but I knew I couldn't get the new mufflers installed in time so I did a quick modification of the current setup to reduce the backpressure a little. I was going to buy a glasspack from the local parts store but I happened to remember the titanium muffler on my lawnmower. (Okay, it used to be on my Yamaha R1 but recently it's been a quiet replacement for the cheap & loud muffler that B&S puts on their engines.) A little bit of cutting and welding:

IM002285_zps89606b6b.jpg

Didn't get a chance to test out the system at speed as the track event sold out pretty quickly. I had held out for a few extra days while I kept checking the weather forecasts. I wasn't in the mood to pay multiple hundreds of dollars for the entry fee, and pay the fuel to haul the car 1100 miles (roundtrip), only to be standing around in the rain for two days. Oh well, I'll pull the extra muffler off and put it back on the mower, and weld up the new ones.

I'm hoping the new mufflers don't end up being louder than the current ones.
 
73c34me said:
How Does The 3Rd Muffler Effect The Sound? Looks Interesting!
Click to expand...

Unfortunately I didn't get the opportunity to put any track miles on this, just a couple throttle blips in the garage. No significant difference with that modest test.

I'm just going to unbolt everything after the exhaust termination box and put the stuff on the shelf. In the event I'm not happy with the new mufflers (I believe Dynomax has a 30 day return policy) I'll throw the old setup back on.
 
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