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John tells me that the pipe molds and other patterns are about done.
In terms of the other stuff that needs to be done before building the exhaust and having the car finished, there's a long list of medium and small things to do. Ordering the right kind of muffler packing materials such that the home grown muffler can match the 1,000,000 mile warranty on Borla exhausts. Patterns for the air filter box cover. Heat shields. Shrouds. Etc.
Next week, John is going to work on putting some 3" dual exhaust parts on another turbo car, not mine. If he sends me photos of that car, I'll post those as well.
Another one of John's twin turbo cars is about to see the light of day. White car with the thoroughly tested system that is sized for that 700 rwhp level.
The white car is on the lift and my black car doesn't move forward in terms of assembly before the exhaust has been installed on the white car.
The supplier recommended these materials after going through the requirements with John. HD stainless steel wool in a mat that doesn't blow out from the perforations and then very durable and corrosion resistant acoustic packing for the outer layer. The supplier and John went thru a whole conversation of how to get the packing to last as long as the rest of the high-quality stainless steel and Borla axle mufflers in the exhaust system. The answer was their Spec 18 for the outer layer.
The outer layer volume is about five liters, and at 150-200g/l packing densities we need 750-1000g for a single rear muffler. That's in addition to two layers of stainless steel wool mat packing on top of the perforated sleeve.
The fixtures will be used to bend an acrylic semitransparent cover for the air filter box. To avoid unwanted resonances and the box acting as a drum, John's going with quite thick material (0.375").
One of the things that non-turbo motors really have to pay attention to is the intake noise. It's surprising to many that a large fraction of the engine noise comes from the intake, especially for high-performance motors with large camshaft overlap and low-restriction straight intake path. For turbo motors, the intake noise is a lot less of a concern. This is because the sound has to travel first thru the intercooler, which functions as a laminar flow muffler similar to a catalytic converter on the exhaust side, or a micro-perforated baffle. Then it has to travel thru the spinning compressor wheel, which especially at high loads has tip speed close to sonic velocities. Sound wave has really hard time swimming against near sonic velocity current. So by my guess/understanding for the turbo engine, most of the intake noise comes from the compressor inlet and the flow induced noise in the intake tract to the compressor inlet. Since my turbos don't have the "compressor map width enhancement" passages, it's likely to be especially quiet. Therefore, with the big filter, for once we don't have a big noise-control challenge with this intake air filter box.
Some general comments on compressor noise suppression effects from Audi here:
While John is working on the white car, I've been in watching JLDC dance recitals and thinking about the exhaust sound problem.
Without any mufflers and just the downpipes collected into a single effectively 5" exhaust pipe that is about 18" long, here's what the simulated sound spectrum would look like at 2000, 2250, and 2500 rpm. Those rpms are especially important because I use that rpm range to climb up the steep hill on our little cul-de-sac.
The first thing to note is that the "loud" sounds appear to be at the 4th, 2.5th and 2nd engine orders. That is, the frequency in Hertz is EO*rpm/60. At 2000 rpm, the 2 engine order is 66Hz, 2.5 engine order is 83Hz, and 4 engine order is 133Hz. The dB scale is highly non-linear, and EO 4 is overall most important but EO 2 and EO 2.5 are also important there.
At 750 rpm idle, EO 4 would be 50Hz. At 3000 rpm, EO 4 would be 200Hz. Therefore, anyone working on a custom exhaust that they want to be "well behaved" under 3000 rpm should really focus on dampening the 50-200Hz band. We probably don't get as much of that dampening as we'd like during this iteration, but we'll see how far we get.
Another thing to note is that after 3000rpm, I want the car to sound sporty. What sporty means to many people is explained in my opinion well here: http://www.sandv.com/downloads/0904cerr.pdf . What I think sporty exhaust sounds like is that the higher EOs are louder than the lower EOs and that the overall loudness increases about linearly with rpms under acceleration. Humans seem to accept loud exhaust just fine as long as it sounds like "it's supposed to sound" which is product of life-time cultural conditioning and general linear control intuition.
The 4th engine order goes over 200Hz at 3000 rpm and ends up at 450Hz at 6750 rpm. What I think one should do to get a sporty sound is to _not muffle_ and instead _let thru_ frequencies 200-450Hz. This way, the engine sound is consistent with the acceleration in the 3000-6750rpm range. Recall that the turbos take out a lot of sound above 500Hz and really muffle above 1000Hz, so it's really the battle of under 200Hz and the 200-500Hz bands for a turbo car.
The problem is that if you just throw in some 14" long Magnaflow straight-thru mufflers in there, they'll take out the very high frequencies fine. But they also take out a lot of the 200-450Hz someone desirable range, while (due to the size of the case) they don't do much to the 200Hz and below undesirable sounds. In a normally aspirated car, that sort of system might sound too loud at low rpms but at least there would be so much high-frequency noise generated that it would sound sporty at high rpms. With a turbo car, you can get the worst of all worlds by muffling the 200-450Hz range and passing thru the sub-200Hz range.
The entire pipework is back from the powder coating. Can you imagine how much head scratching it must have taken from John to fit all that inside the engine compartment (and one more big pipe that's not in the photo)?
05-25-2017, 12:50 PM
Molds are about done
