The banner above is an advertisment - if it asks you to download software, please ignore.
Site News - 3/26 M Car Covers (by State of Nine) | 12/12 Make Amazon Pay Saabnet!
Date: Thu, 17 Feb 2005 20:35:04 +0000
From: Colin Stamp <col.dustbinnospamp.plus.com>
Subject: Re: new saab motor for 9-3 series


On 16 Feb 2005 23:34:50 GMT, Dave Hinz <DaveHinznospamcop.net> wrote: >On Wed, 16 Feb 2005 23:15:00 +0000, Colin Stamp <col.dustbinnospamp.plus.com> wrote: >>>> 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. > >Right, because there are definately engineering-specific meanings to >"step backwards", is that 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. > >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. >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? >>>>>> 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. > >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. 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? > >> That is the point which you seem to be >> missing here. The moving-mass forces have a fixed waveform for a fixed >> RPM. > >Yes. > >> 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) 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. > >> 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. > >>>>>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. > >> 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. >>>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. > >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. > >>>>>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. > >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. > >>>>>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. > >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. > 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'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! > >> 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. Cheers, Colin.

Return to Main Index
StateOfNine.com
SaabClub.com
Jak Stoll Performance
M Car Covers
Ad Available

The content on this site may not be republished without permission. Copyright © 1988-2024 - The Saab Network - saabnet.com.
For usage guidelines, see the Mission & Privacy Notice.
[Contact | Site Map | Saabnet.com on Facebook | Saabnet.com on Twitter | Shop Amazon via TSN | Site Donations]