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Date: Wed, 16 Feb 2005 20:12:58 +0000
From: Colin Stamp <col.dustbinnospamp.plus.com>
Subject: Re: new saab motor for 9-3 series


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." > >> 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. > >> 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. 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? > >> The flywheel won't help >> either. > >Well, it has some effect, but there's no differentiation there >because (as far as I know) all cars have a flywheel of some sort. > > >> 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. >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. >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. >Have you considered engine mount geometry and dampening >characteristics? Irrelevant to this argument. We're talking about engines, not mounts. > >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. > >>>> Robustness doesn't come into it. There's some >>>> very flimsy engines with thriving aftermarket tuning businesses >>>> attached to them. >>> >>>Yes, and Saab engines aren't among them. One would think that those >>>who buy the V6 because it has more pistons, would be _more_ likely >>>to demand performance upgrades - and yet, they're still not out there. >> >> 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. >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? > >>>If you say so. Me, I see it as a sign of what's do-able or not. >>>Saab tuning is enough of a niche market in the first place, after >>>all. >> >> 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. > >>>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. > >>>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). > >> 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. Cheers, Colin.

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