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Date: 16 Feb 2005 20:27:05 GMT
From: Dave Hinz <DaveHinznospamcop.net>
Subject: Re: new saab motor for 9-3 series


On Wed, 16 Feb 2005 20:12:58 +0000, Colin Stamp <col.dustbinnospamp.plus.com> wrote: > On 16 Feb 2005 15:09:46 GMT, Dave Hinz <DaveHinznospamcop.net> wrote: > >>On Tue, 15 Feb 2005 23:25:20 +0000, Colin Stamp <col.dustbinnospamp.plus.com> wrote: >>> By harsher, I mean more vibration transferred to the engine mounts. >> >>Can you cite evidence for this, specifically in regards to Saab's >>dual balance shaft I4 design that's been in use for a decade or >>more? > > Nope. Can you provide evidence to support this :- > > "the only people who want a V6, are those who > don't know enough to know it's a step backwards." Mine is a matter of opinion, yours was a statement of a specific, measurable physical phenomenon. >> >>> Every time a cylinder fires, the engine gets pushed upwards with the >>> same force that the piston gets pushed downwards. Also, the engine >>> gets twisted backwards with the same torque that twists the crank >>> forwards. >> >>Yes. >> >>> It's not possible to balance either of those forces out. >> >>Counter-rotating balance shafts. > > Completely useless for the combustion forces. They only work for > forces which are constant at a given RPM. And what is the proportion of those forces? >>> Balance shafts etc. can only compensate for the moving masses of the >>> pistons, rods and whatever. The forces from combustion change >>> dramatically from overrun to maximum torque. >> >>How much of the vibration is induced by the combustion vs. the >>reciprocating mass? > > My money is on the combustion forces being heavily dominant at full > power. Right, which would be where it'd be logical to calibrate the balance shafts. I'd be shocked if they didn't set them up to do the most good, and I'm not sure I know why you think they haven't. > You can demonstrate this yourself since they fall to zero with > the throttle closed. Just accelerate at full throttle to 6K RPM or so, > noting how much vibration you get at the top end, then go straight to > overrun and check out how much smoother things are as you coast down. > Also, look at the design of I-2 engines. Both pistons move in unison, > which is the worst setup for balancing their masses, but the best > setup for reducing the combustion-induced vibration. Why is it done > like that, I wonder? Dunno, never owned an I2. >>> One very effective way of reducing this vibration is to >>> increase the number of cylinders. The pulses are moved closer together >>> so they overlap more and their peak value is reduced for a given >>> average power. >> >>Have you driven one of these cars and found vibration to actually >>be a problem? > > Nope, although the most powerful four I've driven my 200ish BHP 9-3. So, you _have_ driven the I4 with dual balance shafts, and you haven't experienced the problem you seem to be saying exists. I wasn't confused at what your point was before, but now I am. >>Theoretical "6 is a bigger number than 4" stuff aside, >>what are you feeling that apparently others are not? > > I haven't felt anything particularly convincing in either direction, > nor did I ever claim to. It's just based on the engineering theory, > but it's a really easy bit of engineering theory to understand. As a > quick plausibility check, I note we're not all driving around in > vibration-free, balance-shafted, single-cylinder cars. Big difference between 1 and 4. Displacement and combustion chamber issues to start. >>Have you >>measured the vibration transmitted to the driver in one design vs. >>another? > > No. Have you? You were the fist to imply that a V6 was "unbalanced" > compared to a straight 4 after all. Hardly the first. Think about those dynamic forces that concern you in the I4, now take 2/3rds of them and put them at a 60 or 90 degree angle to the other 1/3rd of them. Alternate that direction 3 times per engine rotation. There's a whole lotta shakin' going on. >>Have you considered engine mount geometry and dampening >>characteristics? > > Irrelevant to this argument. We're talking about engines, not mounts. You're talking about vibration. If the vibration never gets anywhere, what difference does it make? You won't feel it being 'harsh' or whatever your term is, it's dampened before it gets near you. >>There's a lot more to this than "6 is bigger than 4". > > I never said there wasn't, but a 6 really does have more cylinders > than a 4 (honest) and that makes a big difference. If everything is equal, yes. It's not. >>> I don't do marketing, but I bet it's lack of demand more than anything >>> else. V6s are seen as a "luxury" engine as opposed to a "sporty" one. >> >>If you say so. For whatever reason, nobody offers performance upgrades >>for the V6, so if you want power, the V6 is the wrong engine to buy. > > If it's a Saab, and if you want to make upgrades, then yes. We agree on this, but disagree on the reason. Whatever. >>So, kindly don't destroy the design made for bad weather just because >>you don't get it. I mean, I haven't gone and said "take off the >>windscreen wipers because it doesn't rain much here", have I? > > Don't worry. No-one will take any notice of me, so the FWD is safe. > How about 4WD? Would I be allowed that? 4WD is again a case of marketing taking precidence over actual need. But, go ahead and buy the Saaburu if it would bring you joy. Just don't tell me it's a real Saab. >>> Yep. And the Saab V6 is a niche within that niche. It's niched out of >>> existence. >> >>I thought you said you preferred it? > > I don't recall saying I preferred V6s anywhere. I've never even seen a > Saab V6, nor do I know anything about it and it's problems. I'm > talking about V6s in general, and why they might have their place in > the lineup of a brand like Saab. I'm starting to wonder if I'm wasting my time. >>>>Can you quantify "smoothness" as used in this context, please? >>> >>> I'm still defining it as the quantity of vibration that gets >>> transferred into the engine mounts. >> >>But, who cares how much gets _into_ the engine mounts? It's what >>gets _out_ of them that matters. > > As I said above. The engine mounts are irrelevant. Whatever they do > for a four, they can also do for a six. More vibration in = move > vibration out. If you want to compare engines, you have to restrict > yourself to engines, otherwise you'll go potty worrying about all the > other variables that might change. Engine vibration in a car involves the engine -and- the mounts. They're a system. Unless you're running the engine bolted to a test bench, it's absolutely relevant. >>>>Have >>>>you actually _driven_ a dual balance-shaft Saab I4? Or, even a single >>>>balance-shaft Saab V4? >>> >>> I've got a 9-3 Aero. It's plenty smooth enough for me, but I'm easily >>> pleased as far as smoothness is concerned. >> >>Well then. > > My priorities for engines (which probably coincide with yours, > incidentally) don't have any bearing on what seems to be the one real > bone of contention - that a six, even if it has to be a V6 will cause > less vibration that a four for a given power output (and that effect > will increase as the power output increases, by the way). IF and only IF all things are equal. The existance of the balance shafts indicates that they are not. Unless your hypothetical or real V6 has balance shafts? >> >>> My gut feeling is that the >>> V6 156 did have less vibration on full throttle than the Saab, but on >>> either car at full throttle, you have plenty to take your mind off any >>> vibration :o) >> >>So, it's a guess based on a preconceived notion, with no actual >>measurement to back it up. I understand. I think you're worrying >>about a non-problem. > > Nope. It's a conclusion, based on sound engineering principles and > backed up by some vaguely-relevant personal experience. Haven't seen any of that yet.

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