What's the penalty of silence? I'm not talking about your Miranda rights when Officer O'Malley strides over to your passenger window after you've powerslided up the wrong sidewalk.
More to the point, does a quieter exhaust necessarily result in less power? It's a question of particular relevance to Project Evo X, our longterm 2008 Lancer Evolution GSR, as part of its mission is to retain a modicum of street-friendliness.
I told you that we would be doing a dyno comparison of the AMS twin-tip exhaust to the single-exit one. What I didn't mention is that we'd be measuring interior sound levels too.
Join me on the other side of the jump to see what we learned.
After the GST Motorsports upgraded boost pill was installed and tuned, it was time to swap exhausts.
The AMS single-exit exhaust is 3-inches in diameter and pretty much a straight shot from the axle out to the rear of the car. It uses a "straight through"-style muffler with minimal packing, which explains its vocal nature.
While on Dynamic Autosports' dyno, in the cabin with the windows up, the AMS single-exit exhaust registered 90.7 decibels on one run and 89.7 on the next run.
As you've seen, the AMS twin-tip exhaust (also 3-inch diameter) has a large muffler that sits crosswise in the rear of the car, similar to the stock layout. Plus the AMS twin-tip comes with silencer discs that you swap to dial in the amount of unmuffled flow you want traveling through the passenger side's tip.
We started with no silencer disc, aka the max flow / sound configuration. In this guise, I measured 89 decibels, very similar to the single-exit exhaust. Oddly enough, this was subjectively much louder than the single-exit. It goes to show that perceived sound is more than just sound pressure (i.e. decibels); it's about crafting a mix of frequencies. NVH engineers have it tough.
Here's how the two compared on the dyno. Click the thumbnails for larger images.
Based on the bump in output, the twin-tip (red, in the chart to the right) appears to offer less backpressure when the silencer disc is wide open.
After peak power, you can see the difference shrinks to nil, though a few more horsepower could likely have been squeaked out through manipulations of the MIVEC variable valve timing. We didn't bother since the comparison was mostly academic due to the sound level. In the name of science, you see.
Next up, we completely blocked the passenger tip of the twin-tip exhaust. All the exhaust is now forced through the muffler. Sound levels dropped to 85 decibels. Much quieter, in other words.
As for output, the newly butt-plugged version of the twin-tip exhaust (purple, in the chart to the left) basically split the difference between having no silencer disc and the single-exit. Consider run-to-run variation and the differences among these three configurations are very minor overall.
Summary: the three-inch exhausts we compared here have diametrically-opposed design intent but don't make a difference in power. At least not on an Evo X equipped with an intake, exhaust, cams and a reflash.
In swapping exhausts, we have no less power and a lot less cop bait in Project Evo X. If, you know, you ignore the sidewalk thing.
Jason Kavanagh, Engineering Editor
dips200 says:
01:11 PM, 03/13/09
You learn so much about science reading these posts...
felonious says:
02:10 PM, 03/13/09
I'm curious, which sidewalk is the right one to powerslide up? Because I've always wanted to try. :)
church123 says:
05:51 PM, 03/13/09
Great test data Jason. I think far too many people associate sound with power and many exhaust manufacturers play to that perception - as wrong as it may be.
It is nice to see AMS offering a quiet alternative that doesn't cost any meaningful power. I suspect the differences would be even smaller on a non-cammed EVO. Then again, you still have the cat too, so maybe the exhaust isn't quite as important as it might be. :)
farmerbob says:
08:20 PM, 03/14/09
Great writeup Jason. Couple q's. Is the car running with the stock cat converter still?, or is there a high-flow cat from AMS down there, or a "test pipe"?
What do you plan to do next to increase the power, and try to get EvoX into the 11 second 1/4 mile range (maybe a 11.90)?
Lastly, I thought Bryan at GST normally gets the EvoX to 310 awhp/tq on GST's dyno, so how come this EvoX with upgraded Cosworth Cams/exhaust/intake/flash/boost pill only puts down 265-275 awhp?
Lastly, if you're selling it in the future, please let me know.