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Date: Wed, 16 Feb 2005 23:15:00 +0000
From: Colin Stamp <col.dustbinnospamp.plus.com>
Subject: Re: new saab motor for 9-3 series


On 16 Feb 2005 20:27:05 GMT, Dave Hinz <DaveHinznospamcop.net> wrote: >>>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. Oh, I see. It was just an opinion was it? You really should state your opinions less categorically if you're going to pull others up for it. I stand by my "statement of a specific measurable physical phenomenon" even though I haven't measured it, am never likely to measure it, and have never implied that I did. I presume you haven't either? Yet you continue to argue vehemently that the reverse is true. So we're both as bad as each-other. >>>> 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? The proportion is not constant. That is the point which you seem to be missing here. The moving-mass forces have a fixed waveform for a fixed RPM. That allows simple timing-chain driven balance shafts to go some way towards canceling them out. The combustion forces vary wildly and independently of RPM. Perhaps you can explain how you think the balance shafts, whilst rotating at a constant RPM, might be able to cancel out forces which can vary by an order of magnitude or so at the twitch of the right foot. >> 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. If they were calibrated for full power (assuming it's even possible) they would be hideously out of balance on overrun, or even just off the cam. I dread to think what would happen at idle with a simulated minus god-knows-how-many BHPs worth of vibration going on! > >> 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. Only because you've jumped to the conclusion that I think the extra vibration of a straight 4 is a "problem". It's there alright but, no, I don't find it a problem at 200ish BHP. Maybe at 250BHP I might. Others might get annoyed by it even at 150BHP - who knows? From my point of view, this is a theoretical discussion of how much vibration a straight 4 produces compared to a V6. That appears to be the only difference of opinion we have. I think the four will vibrate more, you seem to be saying that's not the case. I'm sure you'll correct me if wrong on that. > >>>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. Yep. Exactly the same issues are there going from 4 to 6. Just in smaller quantities. > >>>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. I'll take your word, at least as far as straight sixes having some advantage over V6s is concerned. I've never disputed that the moving-mass forces exist in a V6, and are awkward. All I did was point out the (IMHO much larger) combustion forces that also exist in both engines, but which are worse in a four than a six. > > >>>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. > If there was an engine mount that could remove *all* the vibration across *all* frequencies, then it would make the whole argument academic (which it pretty-much is anyway). Such a beast doesn't exist. >> 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. If 4WD is the only way I can get some power to the back wheels, then 4WD it will have to be. I'm not thinking of buying one but If I do, I promise to cover all the badges up with gaffer tape. > >>>> 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. Yep. you're getting nowhere alright ;o) > >>>>>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. I'm totally baffled by this engine mount thing. Are you saying that the engine mounts might somehow absorb more vibration from a four than a six? How does an engine mount know how many cylinders there are? If you're saying that the engine mounts might be tuned to the particular frequencies that a four might produce, then that can equally be done for a six, canceling out any possible effects. >> >> 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? > See above for 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. "My mind is made up. Stop trying to blind me with the facts!" Cheers, Colin.

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