When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Here's a blackberry video of the engine running. One can't figure out the true sound from it, think of this as just a "proof of life"! Better microphone and with the car running on road will give a more accurate picture. The objective for this exhaust was to dramatically reduce back pressure, remove harsh sounds, and reduce drone. Over the next couple of weeks, we’ll see how well those goals were met.
I’m sure it will fare far better than any iteration of Shelly’s monster. But, still, have to do this:
The sound has gone from the bride of Frankenstein to Frankenstein's monster. It's a lot lower now than with the old exhaust. It's a little quieter at idle if you're close to the car, but you'll still be heard from pretty far away. There's no snap-crackle-pop anymore. Rythm is different, more even.. DEEPER, LOWER, SMOOTHER. It sounds like a really big motor, if you try to describe it in a nutshell.
What remains to be heard is whether there's any cruise drone, and overall loudness at full throttle, full boost, and high rpm.
Some 928s seem to leak like the Fed before policy announcements, but to my surprise there’s absolutely nothing dripping from anywhere from the black car. The towels on the old Benz are completely spotless.
Here’s a more informative exhaust sound clip, start at 1:20. It’ll be a little quieter with the intake side fully hooked up. Sounds like a really powerful car, but quite refined.
I like the way it sounds now. The high frequency irregular noise is gone, and the pulse sounds very even. The old exhaust sounded like four Harleys.
It’ll be yet quieter once the intake piping is fully assembled.
A lot of nerdy design calculations went into the exhaust for both sound and flow. It has shock wave traps, fully combined pulses before divorcing the paths, one set of mufflers, unequal path lengths in the rear, fully recombined pulses again, and then a combined absorption/expansion chamber. The only things that are still left on the table are active valves and fully custom hydro formed axle mufflers instead of off-the-shelf Borlas.
The same exhaust ideas should work well in a cat-back exhaust of a normally aspirated car with H-pipe and cats in stockish location. For a race car, one would have to think carefully about the weight vs. specific track noise level requirements.
The butcher’s bill came from the exhaust materials. 3.5” US made high quality stainless pipe is very expensive. It’s a surprisingly high step up from 3”. Also, the inconel bellows are expensive, as were the Borla mufflers and all the custom custom muffler components. All in all, just from materials, it was very expensive. Now, it was also worth it! The sound is great, the exhaust will outlast the car, and there’s no restriction to speak off. Since nothing bigger fits, I will never need to replace it. I’m super happy with the exhaust, and the car really needs an exhaust of this size.
For anyone else who doesn’t actually need a dual 3.5” exhaust, dual 3” system with sufficient cross-overs is going to be both quieter and a lot cheaper. From practical point of view, what you want is an exhaust that is sized right for power, and nothing smaller or nothing bigger.
Another update is that at some point I’ll probably need a new mini starter. A mini starter is needed with the oil pan spacer, as the stock starter doesn't fit. Oil pan spacer is needed to keep the oil flowing correctly inside the crankcase. The issue is that my starter is too slow and doesn’t sound “turbo powerful”. So when everything else is finished, I'll probably replace the current starter with the latest and greatest version.
Here are photos of the stock S4 piston and an aftermarket piston for Subaru EJ257 engine. S4 is cast from high silicon alloy. Subaru piston is forged from alloy similar to 2618. S4 piston had to handle about 40 hp per cylinder, this Subaru piston is designed for short stints of 230+ hp per cylinder. The redlines are 6700 rpm vs 8000+ rpm. Same bore and stroke. 575g vs 417g in weight.
So which one is better in a boosted 928 engine? It depends! The stock S4 is worse in every other respect other than thermal inertia and ability to dissipate that heat thru wider rings, and wear. So the Subie piston on nikasil bore is better, but thermal inertia and ability dissipate heat makes the S4 piston surprisingly relevant for a turbo engine. Running very long and very hot but without detonation at below 6000 rpm, I’d guess stock S4 is going to be close to the quarter century later and much more expensive Subie piston.
Image upload from a phone still doesn’t work on this site.
so you are saying the s4 piston is good for your appllicaiton or that you installed subi pistons in your s4 that is now twin turbo charged????
The blue engine has stock pistons with small, additional soap dishes cut into the crown to lower compression. I am just rereading the Mahle piston book and have some single pistons around which I am trying to understand. One question in particular is why the S4 stock rotating assembly can seemingly take any sort of boost (short of persistent detonation) without breaking. I’m thinking it’s the weight. New piston design would only be needed if going to high rpms, which this engine doesn’t need to do.
04-03-2018, 07:12 AM
Trying this
