Once you remove the plastic breastplate covering our long-term 2009 BMW 750i's engine, it's easier to see what's going on in there. Click the image above for a larger version.
Basically, there are two of everything. The flow paths for the two cylinder banks are completely independent--the two flow streams do not converge until after the exhaust exits the turbos.
Green arrows denote the location of the two airboxes where panel air filters reside. There are two probe-type hot-wire mass flow sensors (red arrows).
Purple arrows show a peek at the compressor housing of each turbo. The hard metal line and hose that curls around each turbo are coolant lines--when you shut off the engine, the coolant in the turbo's center housing boils. This is intentional, and is called a thermal siphon--the water vapor then moves up the line and more coolant takes its place, thereby preventing the heat that is "soaking back" (from the exhaust manifolds and downpipes) from coking the oil in the bearings.
The blowoff valves are integral to the compressor housings and are actuated electronically rather than pneumatically.
Yellow arrows point to the oxygen sensors located just upstream of the close-coupled catalytic converters. The closer the cat is to the turbo, the more quickly the cat's substrate can heat up and become functional, which improves emissions. BMW mounted these cats as close as they feasibly could have.
As you might imagine, there's a lot of heat shielding in the engine's vee around the turbos. Exhaust manifolds are dual-walled "tube within a tube" arrangements--there's an air gap that surrounds each exhaust primary tube. This helps the cat "light off" quicker and reduces the transmitted heat load. Still, I'm curious how well the surrounding bits hold up over time.
Blue arrows point to the liquid-to-air intercoolers. These are mounted directly to the front of the engine via isolation mounts. This makes for a very tidy and modular package. Had BMW used air-to-air intercoolers instead, they would have been forced to package much larger-diameter plumbing to the nose of the car, and then back to the intake manifolds. Air-water coolers can be smaller, too, since water has a high specific heat (i.e. it requires a lot of energy to raise its temperature one degree).
What you can't see are the plastic intake manifolds mounted to the outside of the cylinder banks. Again, this approach (the "inside-out" V8) is easier to package since the intake manifolds are relatively small.
Click the jump to get a better idea of the airflow paths of this engine.
Fresh air enters the intake tracts from ducts in the nose of the car. Air travels through a couple of noise-reducing intake honkuses before making a U-turn through the air filter(s). This filtered air then curls around and enters each turbos' compressor.
The now-boosted air (pink) is also heated during the compression process. If you're an engine, pressure is good but heat is bad.
Enter the intercooler. Intercoolers are heat exchangers, and they chill the air by transferring its heat to the (in this case) water. The "water" is probably windshield washer fluid or similar. I'll check. In any case, it has its own cooling loop and another heat exhanger (a "low-temperature radiator") located in front of the engine's radiator.
You usually won't see liquid-air intercoolers on dedicated racing vehicles outside of drag racing. It's difficult to adequately cool off the liquid once you've dumped a lot of heat into it like during an endurance race, plus these systems add a lot of complexity and potentially catastrophic failure modes--you don't want water entering your engine's intake system, unless you like tacoed connecting rods and ventilated blocks.
For a street cars, which never see prolonged full-load operation, liquid-air works great. There's very little pressure drop in their coolers, and the liquid is (initially) more effective at cooling than is air.
Jason Kavanagh, Engineering Editor
bankerdanny says:
02:54 PM, 04/29/09
Great, two of everything to fail. LOL
billt9 says:
03:00 PM, 04/29/09
omg that looks magnificent.
belongs in the art museum.
hondacura4 says:
03:21 PM, 04/29/09
Id like to see this drivetrain with, less restrictive air filters, high flow cats, full exhaust and an ECU flash. Oh the potential!
compliance says:
03:28 PM, 04/29/09
Has anyone felt reduced power due to heat soak with the 750i yet?
uncanny_man says:
03:49 PM, 04/29/09
With all this extra stuff, is this turbo v8 actually any lighter than the old v10?
benson2175 says:
04:17 PM, 04/29/09
This doesn't look like much of a DIY engine.
hondacura4 says:
04:57 PM, 04/29/09
"With all this extra stuff, is this turbo v8 actually any lighter than the old v10?"
Think you mean V12. The 750i's 400hp 4.4L TT V8 isnt the replacement for the old V12, its a replacement for the former 4.8L 360hp unit. The new turbo engine makes more power, has substantially more torque over a broader rev range and uses less fuel while doing so. The 2010 BMW 760i will have a 6.0L TT V12.
2008 BMW 750i: 4.8L 32V V8
360hp @ 6300
360tq @ 3400
2009 BMW 750i: TT 4.4L 32V V8
400hp @ 5500-6400
450tq @ 1800-4500
2008 BMW 760i: 6.0L 48V V12
438hp @ 6000
444tq @ 3950
2010 BMW 760i: TT 6.0L 48V V12
544hp @ 5000
553tq @ 1500-4500
Im not sure about the overall engine weights but the turbo engines offer MUCH more power and get better fuel efficiency.
ctpax says:
05:16 PM, 04/29/09
"You usually won't see liquid-air intercoolers on dedicated racing vehicles outside of drag racing."
What kind of intercoolers are used in endurance races?
charlesncharge says:
06:05 PM, 04/29/09
As strange as it sounds, I believe the 750i also has air/air intercoolers - one on each side - in front of each front tire. Something's lurking there, looks an awful lot like an intercooler to me - front fender liners are ventilated - just behind them. Could you please verify and/or refute this claim? Thank you!
hondacura4 says:
06:23 PM, 04/29/09
"front fender liners are ventilated"
Probably brake cooling ducts.
cwc1 says:
06:28 PM, 04/29/09
Very cool.
This inside-out design sounds quite similar to the one that GM designed for its 4.5 liter Duramax diesel engine for its 1/2 pickups, of which, sadly, they just recently put final development on hold due to their dire finances. I hope we do see that engine come to market, because it's very innovative thinking and will extend the diesel's advantages to more vehicles.
kyolml says:
08:06 PM, 04/29/09
dang, then it's basically the same as the X5/6 M engine. I thought they put some new tricks on the M, I guess just a little bit more boost and tuning instead on the Ms.
bimmerjay says:
09:11 PM, 04/29/09
"As strange as it sounds, I believe the 750i also has air/air intercoolers - one on each side - in front of each front tire. Something's lurking there, looks an awful lot like an intercooler to me - front fender liners are ventilated - just behind them. Could you please verify and/or refute this claim? Thank you!"
In the 335i the secondary oil cooler sits behind the vent in the fender liner.
fuhteng says:
06:31 AM, 04/30/09
What a brilliant post! Thank you. I hate the 'breastplate' on my G8's small-block because it hides the pretty internals. This engine is amazing. I love it! I think you should keep the breastplate off so you can show off the design.
joefrompa says:
07:27 AM, 04/30/09
Charlesnchange - One of the big problems with the first 335s back in late 2006 was the lack of an oil cooler on many of the models (supposedly only sport-packaged, manual transmissions models got one). The oil went up to 300 degrees on hot track days, and the car entered limp-home mode.
BMW started putting oil coolers on all their 335s, and their located was near a vented fender liner in front of the tire.
I imagine this puppy has 2 such oil coolers :)
Comments on this blog post:
1. I love the engine bracing. I've long thought that the bracing should flow at an angle from the strut tower to the front center first, and then maybe across the strut towers second. From a safety perspective, and performance, I'd think you'd want the bracing to go diagonally from the sides of the vehicle to the front.
2. I'm actually impressed by how EASY this engine design looks to work on. Sure, there's 2 of everything. But the turbos look far easier to access than most; hence, undo a hose and you can examine the compressor! Ditto the dual water-to-air intercoolers.
MAFs are at hand, 2 oxygen sensors are right there, cat converters....all pieces that are sometimes harder to access than others.
And there is enough room around everything that I feel I could disassemble it, do my work, and re-assemble it without too much hassle initially.
I don't know, I like the layout.
70ss454_man says:
12:45 PM, 04/30/09
I agree completely with joeframpa. It's an excellent layout, and just because it at first looks difficult to work on, doesn't mean it is. I spotted the ease of maintenance early on as well.
I can imagine some other things are difficult, but the vitals seem to be right there.
charlesncharge says:
01:21 PM, 04/30/09
bimmerjay: Yeah, I know that - I've got a 135i w/sport pkg. - but what I'm referring to looks a lot more like intercoolers vs. oil coolers + I highly doubt that only those two dinky air/water coolers would be sufficient for this engine. It's possible that the initial charge air goes air/air, then air/water, to combat the intense underhood temps. that come from having two turbos within the engine's "V". I had a new 750i on a lift recently - can't quite remember, but could've sworn engine's oil cooler was in the front grille opening.
hondacura4: No, that's not what I was referring to - this is so cooling air can pass through these particular two coolers - one on each side.
CycloneRcr says:
10:24 AM, 05/ 8/09
joefrompa - "And there is enough room around everything that I feel I could disassemble it, do my work, and re-assemble it without too much hassle initially."
There is of course enough room because they should make a 6.0L twin-turbo V12 fit in exactly the same place =)