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Old 01-23-2010, 07:11 PM   #31
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. . .Yeah, I've seen that before as well. It would work, but would take way longer to spool up than it should, probably result in greater exhaust back pressure (compared to a manifold-mount turbo) and take way longer to pressurize all the charge piping running from the rear end of the car back up to the engine.
l . . a . . a . a . aa .aaaggggGGGGGGOOOO!

Such a system might actually be more efficient if it is used in it's primary design platform of trucks and SUVs. It's crap for acceleration performance relative to a header setup, but it would substitute very nicely for otherwise unneeded extra displacement when towing, allowing it to freewheel more or less when not needed.
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Old 01-23-2010, 08:29 PM   #32
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Some of the xB turbo systems are mounted below the car because of the lack of room around the header since the exhaust comes out of the head between the fire wall and engine instead of in front of the engine. The problem is the turbo gets wet when you run through deep water puddles. The end result of adding a turbo is more strain on the engine and drive train . . . the Scion tC gets 160hp and they push them to 300-400+ HP and they really go fast but bottom line is you pay a lot for a few seconds less to accelerate beyond the speed limit - eventually blow up the engine or clutch and burn your brakes out.
There is one advantage of a supercharger and that's the low end boost that is there when you want to lug the engine or shift at low rpm. There was a system of compressed air in a tank charged up with a slow compressor that forces air into the intake on demand for the times when you need a burst of speed for 10 seconds that was pretty simple and cheap. Problem with pushing more air is you need to increase the injector volume and remap the A/F mixtures sometimes boost fuel pressure and volume in the fuel pump run intercooler cool things with engine oil, change it more often due to additional heat breaking it down faster. and the costs go on and on. Some turbo setups run $2000-$3000 plus installation labor and then you usually have to get stronger drive shafts and clutch right away. A lot of guys push first gear when turboed and break the tranny cases and blow the lay shafts right away. Money is better spend like I did improving the drive train efficiency so I get more power to the wheels and better mileage all the time.
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Old 01-23-2010, 09:47 PM   #33
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I see what you're saying, but I don't think we're talking about turbo systems that triple the engine's power output. A small turbo will spool up quickly but only boost power and torque by 40-50%. Most drive trains can handle that just fine, so longevity becomes a matter of appropriate tuning.
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Old 01-23-2010, 10:00 PM   #34
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I wish I could remember where to site it from, but it was proven that small turbos increased FE. I think it was on BMWs, or Mercedes.
It's probably the Ford Ecoboost series of engines that you're thinking of.
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Old 01-25-2010, 04:50 AM   #35
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i once watched the guys on Horsepower TV install a twin turbo on a corvette...at the rear of the vehicle.
There is a system for that, but the part that bothers me is running oil lines (to lubricate the turbo) all the way back there, not to mention the increased risk of damage due to speed bumps and road debris and whatnot. Performance-wise, you also have to remember that the exhaust gasses have cooled considerably by the time they get back there, which means they've also shrunk in volume (and velocity), so the turbocharger is quite inefficient.

Having said that, they can still work.

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Old 01-25-2010, 07:02 AM   #36
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Wouldn't the shrunken, slower exhaust gases have more force per volume because of the increased density? I suspect you'd just need the turbocharger to be designed differently to make use of that.
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Old 01-25-2010, 10:16 AM   #37
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Yeah. You would have a more even, consistent exhaust stream, but you're still losing some portion of the pressure, velocity and/or thermal energy that the turbine makes use of. If you could keep the turbine close to the engine but feed it a constant stream of gasses, it would likely perform similarly well. But to do that, you would need a non-cyclical engine of some sort... Something that doesn't use cylinders or closed combustion chambers... Something like this:
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Old 01-25-2010, 11:10 AM   #38
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Old 01-26-2010, 12:13 PM   #39
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Yep. The issue with superchargers is that they're on all the time and boost pressure is dependent on engine RPM rather than load..
This is not true, most superchargers utilize bypass valve assembly which bleeds boost depending on throttle position.
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Old 01-26-2010, 12:17 PM   #40
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yea but you are still spinning the screw (or turbine or whatever)

the pulley isn't set up on a clutch (that I know of) so you still have parasitics.

it may not be compressing but it is still spinning. there may be a reduced load if there is no pressure being accumulated but the load is still there.

(load being drag on the belt thus HP required)
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