How (and why) to Ramble On your domestic shorthair - Page 1187 - Miata Turbo Forum -Boost cars, acquire cats.

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Old 11-02-2015, 10:15 PM   #23721
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Originally Posted by aidandj View Post
Bigger turbos make more power at less boost, because?
More efficient compressor and turbine results in less exhaust manifold pressure for a given intake manifold pressure. Less power from crank going to drive out the exhaust, so more net power.
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Old 11-02-2015, 10:23 PM   #23722
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Originally Posted by DNMakinson View Post
More efficient compressor and turbine results in less exhaust manifold pressure for a given intake manifold pressure. Less power from crank going to drive out the exhaust, so more net power.
Is it easier on the turbo is 2 compressors make the same boost as one?
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Old 11-02-2015, 10:59 PM   #23723
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Quote:
Originally Posted by aidandj View Post
Bigger turbos make more power at less boost, because?
Because a smaller turbo is not optimally selected for that operating range.
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Old 11-02-2015, 11:02 PM   #23724
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There's really no inherent boost/efficiency/etc advantage to using 2 turbos over one. Yes, a half-size turbo is smaller and easier to spool, but you're only spooling it with half of the available exhaust energy. So if you put 2 turbos on a 3.0L V6, you've effectively built a pair of 1.5L I-3s with one turbo each. Assuming the turbos are otherwise-equivalent, it's basically a wash -- at least until one of the turbos fails. Then the boost control will try to make up all of the deficiency using the single remaining turbo and it will also fail in short order.

So twin (parallel) turbo is more expensive and (generally) heavier, but easier to package on a cold-center V engine. Twin sequential offers spool advantages (FD, late-90s Supra) but seems to not really be worth the complexity.

Then there's my hot-center V8 Audi, which nevertheless has a pair of turbos in the valley... That's the German engineers for you -- I suppose I should be glad there aren't four in there!

--Ian
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Old 11-02-2015, 11:04 PM   #23725
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Quote:
Originally Posted by aidandj View Post
Is it easier on the turbo is 2 compressors make the same boost as one?
Not sure what you're asking here. Two compressors on a single turbine shaft? Or just two separate turbos?

So long as we're talking about a parallel configuration, It doesn't really matter how you break it out. You've got a certain amount of turbine powering a certain amount of compressor. You can divide that up into as many packages as you like, and aside from a tiny incremental loss in bearing friction each time you add a turbo, the math pretty much works out the same. Two small turbos, each taking in half the exhaust gas and compressing half the intake air, will do the same job with the same efficiency as one turbo twice as large* on the same engine. They'll have a certain island of efficiency, a certain boost threshold, a certain choke point, etc. Physics doesn't care if you have one turbo on every exhaust valve.
* = not literally twice as large in every dimension, volume increases as the ^3 of scale, and the formulas for flow are non-linear. But I'm handwaving here. You get the general idea.
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Old 11-03-2015, 01:30 AM   #23726
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Makes sense. I wonder if parallel smaller turbos are more reliable. Theres got to be a reason BMW runs parallel instead of single.
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Old 11-03-2015, 01:44 AM   #23727
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I'd imagine because they are cheaper to repair and easier to package. Turbo on each side of the engine vs running the headers together to a single turbo.
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Old 11-03-2015, 01:44 AM   #23728
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Quote:
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I'd imagine because they are cheaper to repair and easier to package. Turbo on each side of the engine vs running the headers together to a single turbo.
Turbo BMWs are 2 on the same side.
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Old 11-03-2015, 09:47 AM   #23729
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I've got a Turbo BMW, it's very nice. Yes, they're on the same side. The engine is an inline 6, not a V6.

My guess would be that the decision is both packaging and spool based. For packaging concerns, you can package two smaller turbos tighter than you can one large turbo. For spool, even though you would theoretically have twice as much exhaust pushing twice as much turbo, there is a greater moment of inertia that must be overcome with the larger turbo. The turbo BMW doesn't feel like a turbo car. It's exceptionally powerful, but my 2560 1.8 feels like a dyno queen in comparison.
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Old 11-03-2015, 10:45 AM   #23730
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Quote:
Originally Posted by fooger03 View Post
For spool, even though you would theoretically have twice as much exhaust pushing twice as much turbo, there is a greater moment of inertia that must be overcome with the larger turbo.
I was assuming someone would bring that up eventually.


Many years ago, I happened across a paper in which some rather bored graduate student decided to calculate the fuel savings which could be realized if the operators of commercial 747 passenger jets were to optimize the loadout of the galleys, carrying only the minimum of various supplies required for each flight segment. Presumably on a lark, he went to far as to determine the amount of fuel required to transport a single sugar packet from New York to Sydney (or whatever route he was measuring.)

That's the realm into which we are delving when we speculate about the mass of the turbine wheel in this context.

(And, to humor the notion, you're also ignoring the increased friction from having two sets of center-section bearings, and the increased weight of the vehicle overall from adding the plumbing necessary to support a second turbo.)


From the point of view of the automakers and their decision to use one turbo vs. two, particularly in the case of the Germans, don't discount the marketing appeal of being able to print "Twin Turbo" on the glossy advertisement. Same reason vehicles with two exhaust pipes wind up with four exhaust tips.
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Old 11-03-2015, 11:07 AM   #23731
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Quote:
Originally Posted by aidandj View Post
Makes sense. I wonder if parallel smaller turbos are more reliable. Theres got to be a reason BMW runs parallel instead of single.
They aren't. More turbos means more parts, more things to break, more cost, more pluming, etc.

The N54 was twin turbo, but the N55 which replaced it is a single turbo (twin-scroll). The N55 supposedly spools better, but that's likely related to the specifics of the single turbo they used, rather than anything inherent to single vs twin. The N55 is also supposed to be more reliable, although most of the N54 failures aren't in the turbos themselves, AIUI.

Speculating, the only reason I could think of why BMW might have chosen twin turbos for the N54 is that they didn't have an off-the-shelf single turbo available that met the power goals they had in mind at the time that they shipped it.

As for weight, I wouldn't be surprised if running twin turbo was 15-20 pounds heavier than single, once you added up all the plumbing. A pair of 2554s is about the same power as a single 3076, but the 3076 doesn't weigh anything like twice what a 2554 does.

--Ian
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Old 11-03-2015, 11:29 AM   #23732
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But ze Germans are always right. What are we misunderstanding?
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Old 11-03-2015, 02:39 PM   #23733
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Supra twin turbo manifold.
It's more compact than I imagined it being.
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Old 11-03-2015, 02:42 PM   #23734
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And some of the fancy ones have the turbine housings cast into the manifold.
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Old 11-03-2015, 02:59 PM   #23735
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Gold

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Old 11-03-2015, 11:20 PM   #23736
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Old 11-04-2015, 01:10 PM   #23737
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So recently I kind of realized how shitty my posture was. And have been trying to fix it.

Mother of god I am in a world of pain right now. I started 2 days ago. Conciously pulling my shoulders back, and straightening my back. It causes really bad soreness and tension, but mostly because my shoulders are incredibly inflexible.

Well yesterday I relaxed to my normal shrugged shoulders position, and holy ****. It just kinks my neck and hurts even more than pulling my shoulders back.

So now I'm alternating between the back of my shoulders screaming at me, to my neck wanting to kill me. I hope this doesnt take long.

If anyone has any other good tips for posture improvement I'm all ears.

Last edited by aidandj; 11-04-2015 at 01:46 PM.
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Old 11-04-2015, 02:20 PM   #23738
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lift weights with good form. your posture will correct quite a bit when your muscles are pulling your bones back into the right place.
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Old 11-04-2015, 02:25 PM   #23739
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Ugh. I hate going to the gym. College rowing beat that out of me. I play soccer every day instead now and bench transmissions every month or so.

I found stretches and excersizes I can do at my desk so I'm going to work on that.

Basically I have really rounded shoulders. My palms naturally rest flat on my thighs standing up.
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Old 11-04-2015, 10:27 PM   #23740
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They say that breast-feeding is healthy and natural, and that a woman should be able to do it anywhere.

I agree, but would like to point out that masturbation is also healthy and natural, not that you'd know it from the reaction I got on the uptown 6 train this evening.
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