If you're an engine geek, this one might intrigue you. Pictured here is Brabus' twin-turbo conversion for the Mercedes-Benz/AMG 6.2-liter V8. Nothing out of the ordinary here.
However, I had to take a closer look at this display on the Brabus stand at Frankfurt after I caught a glimpse of the turbo side of things.
Hit the jump; get the brain dump.
Brabus cast new exhaust manifolds -- complete with integrated turbine housings -- for their biturbo conversion. The unexpected part of the execution is that only three of the four cylinders of each bank actually feed the turbine. The fourth exhaust port simply dumps into the exhaust downstream of the turbine.
From the engine's standpoint, this layout introduces some fascinating challenges. One; you're harnessing exhaust energy from six cylinders, but all eight intake ports are boosted. It's a situation that can be ameliorated to a degree through careful turbocharger matching. This engine has custom turbos, and that might be why.
More challenging is the large imbalance in the exhaust backpressure faced among the cylinders. Under load, three cylinders of each bank will experience substantially elevated backpressure compared to the remaining one. This means that the fourth cylinder will have much greater volumetric efficiency than the others and will require cylinder-specific fuel and ignition calibration. Not a big deal, unless that capability is missing. Still, the crankshaft's torque loading will be unbalanced no matter what.
None of which is to say that this is bad kit, just a curious one. According to Brabus, this layout was settled on "for maximum efficiency." It's not clear to me how this layout achieves that.
Bonus related historical footnote -- Cadillac's ill-fated Northstar LMP race program (2000-'02) employed a similar layout in its turbocharging system.
isaacl says:
09:07 AM, 09/14/11
wow, good catch.....and an intriguing system. I wonder how long it took through design, testing, re-design and more testing to get this to work.
greenpony says:
10:11 AM, 09/14/11
Interesting.
indy720 says:
10:29 AM, 09/14/11
Hard to argue against the decisions of an established performance company, but that seems like a lot of additional work. It looks as the the hot side of the turbo had the exhaust manifold of the front 3 cylinders cast to it. That looks like it had to be installed after the 4 cylinders header that also collects the exhaust gasses after passing through the turbo. This looks like added headaches to production, assembly and tuning.
But, again, these are just armchair observations made from 3 photos.
I also don't see a wastegate. Any idea if it is tucked behind the header for the 4th cylinder?
fordson1 says:
10:45 AM, 09/14/11
OK, I'll bite...yeah, I'm sure there are fascinating challenges, and I'm sure many of them can be met...but why do it in the first place?
What are they gaining here? Packaging?
csubowtie says:
11:53 AM, 09/14/11
About the only advantage I can see is exhaust scavenging out of the turbo, but this would serve to spool the turbo better. And I would guess just pushing that 4th cylinders worth of exhaust through the turbo would aid spooling better than some exhaust scavenging. What did the other side look like? Is it the same cylinder (i.e. the back cylinder) that doesn't flow through the turbo? If they reversed it, it would take at least some of the imbalance out of the crankshaft. Man, and could you imagine trying to tune those individual cylinders, because now that one has another variable for backpressure based on how much the wastegate is bleeding into the exhuast. Looks cool though.
Jason Kavanagh replied to comment from fordson1
12:36 PM, 09/14/11
fordson1, yes, I suspect you're spot on that packaging is the primary 'get.' I don't think it's doing perfomance or "efficiency" any favors.
miamifan1 says:
01:41 PM, 09/14/11
Question nobody asked, is my take on it.
zr1man says:
08:42 PM, 09/14/11
No no, that just won't do. Forget the turbo chargers and go with a Super Charger, like on the ZR1. It's quicker. Trust me.
afdriver says:
10:28 PM, 09/14/11
I honestly think packaging would be easier with all 4 into the turbo. It looks like they have to put the turbo farther out to let the tubing from the 4th cylinder to bypass it. The only thing that makes sense to me is that it somehow lets the turbo spool faster for better lower end power while allowing the use of a larger turbo for more power on the top end.
cobraboy1 says:
06:16 AM, 09/15/11
This setup confuses me, I understand what they did, but I don't understand why they did it. I also don't understand the purpose, because what you have here is a contradiction in production.
They're using larger turbos to supply boost for the four cylinders in each bank on the motor, but then they're canceling that effect by only supplying 3 cylinders directly into the turbines. Which means, because its a larger turbo, it will take longer to spool up and the power will peak late, and there will be lots of lag. Then it will lose power because only three of the cylinders are supplying the boosted exhaust gases to four cylinders. But then they try to make up for it by redirecting the 4th cylinder exhaust directly into the boosted exhaust.
This also creates an efficiency issue because normally after the turbine you have just one route that's sealed going back to the intake. With this setup you have the 4th cylinder exhaust entering the pipe after the turbine propels the boosted air out which might cause a pressure issue, that might cause a loss of significant power before the air even makes it back to its destination.
The only truly good item that I can see this set up doing is using less fuel, I just think this kind of modification is going to do more harm than good.
tonkatoytruck says:
06:27 AM, 09/15/11
I think there is some efficiency to be gained on the rear cylinders because of the more direct route of the exhaust from the port to the exhaust collector. If it flowed directly into the main manifold as typically done, the route really is longer and full of bottlenecks. None of these factory turbos are seeing boost levels that they are capable of and this may justify not using as much exhaust pressure.
indy720 says:
07:03 AM, 09/15/11
The exhaust gas does not increase in velocity after spooling the turbo. Energy is removed from the gasses to spin the turbine. That means the 4th cylinder exhaust gasses are meeting up with slower gasses from the other three. Perhaps they designed it in such a way that the faster moving gasses from the 4th cylinder create a lower pressure and actually "sucks" the exhaust from the other 3.
That is a lot of fluid mechanics added to a 100+ year old tried a true system like turbocharging... if that is even the case.
viss1 says:
07:51 AM, 09/15/11
Saab did this a few years ago on one of their V6's IIRC.
hybris says:
12:01 PM, 09/15/11
This whole set up just screams "DEALER ONLY PARTS AND REPAIRS!".
angry_mushroom says:
03:36 PM, 09/15/11
Hmmm... Turbo'd cylinder deactivation?
smallfield says:
02:47 PM, 09/16/11
@indy
That is a pretty cool idea. The faster moving air from cylinder 4 could result in a lower pressure and "suck" from the back end of the turbo with the right layout?
I'll bet this makes this break more though.