Date: 17 Feb 2005 21:40:07 GMT From: Dave Hinz <DaveHinznospamcop.net> Subject: Re: new saab motor for 9-3 series
On Thu, 17 Feb 2005 20:35:04 +0000, Colin Stamp <col.dustbinnospamp.plus.com> wrote: > On 16 Feb 2005 23:34:50 GMT, Dave Hinz <DaveHinznospamcop.net> wrote: >>Ehhh... (I'm seeing this as a friendly joust, by the way, if I'm >>really pissing you off that's not my intent at all). > > Don't worry, I'll let you know if you piss me off. Just checking. >>Saying >>that vibration (a real, mechanical thing) is greater (a quantitfication >>of an amount) with (a) as opposed to (b) is talking about real, >>measurable things. > > It's also possible to measure the number of people who happen to buy > V6s and who know an awful lot more about the subject than you and me > put together. If that number is greater than zero, then your statement > was incorrect. Care to take a bet? I saw it as a generalization rather than an absolute. >>>>And what is the proportion of those forces? >>> >>> The proportion is not constant. >> >>Right. At maximum engine output, it's biggest. When the engine is >>at maximum output, that's almost certainly where the balance shafts >>are set up to provide maximum benefit. As I've said. So I haven't >>missed your point, I'm just asking how important it is. > As I've said before, if you can somehow make the engine balanced at > full power, then whatever is doing the balancing will have an equal > and negative effect on overrun. Actually, thinking more about it, you'd want it to be most effective somewhere below full power, so the area under the curve of reduced vibration, across the entire operating range, is minimized. > That would be really easy to spot > since the vibration would increase when you lift off the accelerator. > My balance-shafted straight four certainly doesn't behave like that, > does yours? Nope, nor does my non-balance shafted I4. (2.0L in an '86 900). So there's more to it than either of us are considering. I'm thinking the engine mounts are coming into play. >>> 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. >> >>I'm not saying that, at all. The balance shafts almost certainly are >>set up to do the most good when the most balancing is needed. It's almost >>insane to have to write that. IF that's a large part of the forces, >>then it'll be tuned/balanced accordingly. If, however, it's mostly >>about reciprocating mass, then the balance shafts' effect is mostly >>constant regardless of output vs RPM. > > You seem to be forgetting that it isn't possible to turn off a balance > shaft. If it's generating enough force to cancel out the combustion > forces at full power (Not that that is very likely to be possible) But, have we determined how much of the overall vibration that force is? That'll be pretty colinear with the axis of the bore, wouldn't it? The "jumping up and down" that someone else mentioned. Linear forces should be easier to dampen than forces in several directions. > then it will still produce the same large forces on overrun at the > same RPM, with no combustion forces to balance them out. The effect > would be no improvement, Just an engine that vibrates as badly on > overrun as a *real* engine vibrates on full power. > >> >>IF, and only if, the combustion forces are that much more than >>all that spinning mass going up and down. > > And at high powers they are. Stick your right foot down and feel it > for yourself. That's the thing - tried it this morning in my non-balance-shaft 2.0L, and the absence of vibration to the pedal, or the gearshift, is equally low regardless. >>> 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! >> >>And yet, from empirical evidence, there isn't a problem. Therefore >>your assumption must be flawed. > > You need to look a bit further up to your incorrect assumption about > balance shafts in order to see why you're wrong here. I think you assumed what I was saying. They're obviously optimized _at some point_, most likely where it'll do the most good. >>>>>>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? >> >>Probably the folks who designed the engine, they might, all >>things considered. > > Yep. Or even the dreaded marketing morons. I assure you, marketing morons don't care about anything other than what color it is. > >>> 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. >> >>You're oversimplifying. WHICH 4 more than WHICH 6? If the question is >>"CAN a V6 be made more smooth than a I4", maybe the answer is yes. >>If the question is, as I thought it was, "Is the GM V6 more smooth >>than the Saab I4", I think the answer is up in the air at best. > > If you're going to get to the bottom of this the you *have* to keep it > simple. OK, I'll say that the 4 with balance shafts can be smoother than the 6 without balance shafts. Throw 4 balance shafts into the 6, then the 6 will probably be smoother, because then _all things are more equal_, and then, at that point, the fact that 6 is bigger than 4 comes into play. >>Not hardly. No symmetry at all in a 1. > > There's no symmetry at-all in an I2 either, though there could be if > it was made with the pistons 180 degrees out of phase. They're not > made like that though, because that would compromise the combustion > force problem because of uneven firing. The moving-mass forces aren't > as important as the combustion forces in engine vibration terms - > particularly in engines making high specific powers. I don't know that a 1 or 2 anything is really in-scope, because they present unique challenges as has been mentioned. a I-3 two stroke would be in scope for Saabs, and there is quite a bit of tortional vibration at idle, but that's more the nature of a 2-stroke's scavenging than anythign else. >> >>> 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. >> >>At least in a 4, they're all going in the same direction. Heck of >>a lot easier to deal with one vector than two intersecting ones. I think that's pretty key there. >>> 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. >> >>Never said it did. > > I'm sorry. When you said "If the vibration never gets anywhere", I > went and assumed you wanted to consider the possibility of a perfect > engine mount. Silly me. If you can't feel it, does it matter if there's vibration? Sometimes, "good enough", is. >> Are we talking about vibration in a car, or not? > > I'm talking about engines in isolation, in order to keep the debate > down to manageable proportions. I'm also talking only about one aspect > of the engine - vibration. You keep trying to drag cars into it. > > >>> Yep. you're getting nowhere alright ;o) >> >>Pot. Kettle. Black. > > Indeed. The kettle was forced to make a counter-claim of blackness > against the pot, in response to a pre-emptive strike. I'll see your pre-emptive strike, and raise you a dose of gratuitous sillyness. >>> 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? >> >>Depends on the engine mount. Depends on the four. Depends on the six. >>Depends on the priorities of the engineers (or, usually, their managers). >>If the engine and mount are from GM, I'd suspect it more than an engine >>and mount from Saab. For starters, the mount in the Saab was designed >>for that specific engine in that specific car. The GM engine mount, >>for all I know, is "whatever we have already with a 4" bolt spacing". > > You're absolutely right. It all becomes a completely unmanageable mess > when you start trying to include engine mounts, and it could well > obscure the fundamental differences between engine types that we're > trying to explore. Hang on... That's what I've been saying all along! No, that's what *I* have been saying all along. I think. Damn, now I think I have you confused with me. Or the other way around; yes, that's more likely. Seriously though, let's take a different look at this. What, other than internal stresses, would account for the Saab High Milage list over at (evilplace) having a whole lot more 4's than 6's? Market share is part of it, but I wonder if it's disproportionately skewed towards the I4 during the years when both were offered. I'd bet a pint that it is. >>> 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. >> >>Can be. Is it? > > Who knows? Who cares? Engine mounts are irrelevant to whether one > engine vibrates more than another. Not if you're saying one is more "harsh" than another; since that's a human-based measurement, the only vibration that matters in that context is what the human can observe in normal operation of the vehicle.
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