[durtkillon]

Having only worked on boxer engines in the past, I actually had to google why balance shafts were so critical for the 3.0 inline 4. I found this great little article that explains it well http://www.autozine.org/technical_school...mooth2.htm . So I now understand the concept of why an inline 4, particularly a large inline 4, can never be properly balanced without one. My question is why are the shafts mounted asymmetrically around the crank? In other words, why is one high and one low? It seems like putting them both low would be simpler. Better yet, why not mount them both below the crank and use a timing gear instead of a belt?

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[PorscheDude]

I would guess because the enigine is does not sit vertical in the chassis.

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[MikeVT]

Not sure but best guess would be a balance between "fit" keeping the engine as small and light as reasonable and still balance/smooth the engine out.

Adding a gear would limit positions and add weight and complexity. Gears need to run "wet"

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[RS Barn]

I don't know the reason behind location of balance shafts but I know th importance of them.
You can't believe the loss of power if they are timed incorectly.
I just had a car today that had belts installed by one of the top "racing Porsche" shops in the US that were way off.
Car felt like it would shake apart and didn't have any power.
Max reset balance belt and car had 50% more power and was smooth
Pete

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[stewhatch]

<!--quoteo(post=80751:date=Sep 26 2009, 06:07 PM:name=RS Barn)-->QUOTE (RS Barn @ Sep 26 2009, 06:07 PM) <{POST_SNAPBACK}><!--quotec-->I don't know the reason behind location of balance shafts but I know th importance of them.
You can't believe the loss of power if they are timed incorectly.
I just had a car today that had belts installed by one of the top "racing Porsche" shops in the US that were way off.
Car felt like it would shake apart and didn't have any power.
Max reset balance belt and car had 50% more power and was smooth
Pete<!--QuoteEnd--><!--QuoteEEnd-->
I think I know who "he" is. The same guys changed my belts / rollers two yrs ago and I had the same problem. Rollers were installed backwards and the new BS belt was shredded after a year. I also had to replace my motor mounts, although I cannot be 100% certain one caused the other. Needless to say, myself and two local 968'ers have taken our business to Pete.

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[durtkillon]

I was trying to understand the scope of a potential upcoming timing belt/balance shaft belt job and get my head around what had to be aligned. I wanted to learn how the belts were all tied together. In the learning process, I kept asking myself why Porsche would pick this big inline 4 design with 2 extra power eating shafts. They constantly do work just to keep the engine balanced. I think I read someplace that they actually paid royalties to Misubishi for this design. With all Porsche's Boxer experience, it's odd that they didn't stick with a horizontally opposed design. Boxers take the balancers out of the equation and lowers the cg of the car. I guess the answer may be the Porsche Boxster, but I truely love this 968 chassis.

I actaully have worked on one other inline 4, but it was a TDI and didn't have these balance shafts. It needed them, though.

RS BARN, I've seen some of the work you've done in this forum and on your website - very impressive stuff. Engine theory is right up your alley.

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[barth7]

I am surprised to hear that you lose a lot of power from incorrectly set up balance shafts. I thought it was common knowledge that you gain a few ponies through deletion of the shafts, at the expense of more vibration.

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[flash]

the shafts do draw a couple of horsepower when installed correctly, however, when they are disconnected, the engine is "fighting itself", and a lot of power is lost is that fight

i've been through this twice - the first time was with the DMF - not really a huge change, but noticeable

but when i just went through it again, this time with the light flywheel, it was nothing short of amazing at how hard the engine seemed to be working just to rev up normally

balance is critical to these engines - the huge displacement really makes a mess of things and needs to be countered

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[barth7]

<!--quoteo(post=80841:date=Sep 28 2009, 09:14 AM:name=flash)-->QUOTE (flash @ Sep 28 2009, 09:14 AM) <{POST_SNAPBACK}><!--quotec-->the shafts do draw a couple of horsepower when installed correctly, however, when they are disconnected, the engine is "fighting itself", and a lot of power is lost is that fight<!--QuoteEnd--><!--QuoteEEnd-->

Dyno charts? That is very interesting (I am currently running sans balance shaft belt for a short time).

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[flash]

i don't have any charts, but having been through it twice now, the first one being the drive from cleveland to los angeles, and having the belts done as soon as i got back, and the most recent one being a 1000 mile trip from home to paso robles and back, i can tell you for certain that i did not have the power i had before the belt went, disabling the balance shafts

it's big - i would have to guess in the neighborhood of at least 20 ft/lb

if you have the dmf, it is much harder to feel the change - that big heavy thing smoothes out a lot - but with the light flywheel, the revs are so fast that it becomes immediately evident that something is wrong - the car was very off - there is an entire thread about this experience - do a search and you'll find it

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[Johannvb]

<!--quoteo(post=80758:date=Sep 27 2009, 09:15 AM:name=durtkillon)-->QUOTE (durtkillon @ Sep 27 2009, 09:15 AM) <{POST_SNAPBACK}><!--quotec-->In the learning process, I kept asking myself why Porsche would pick this big inline 4 design with 2 extra power eating shafts.<!--QuoteEnd--><!--QuoteEEnd-->

I have actually wondered what it would take to fit a boxster/carrera engine to a 968. They fit LS1 engines, so surely a boxer should be an option?

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[flash]

lots of complications to every design - bigger engines would have required more substantial frames, suspensions, and brakes - boxer engines would have involved some pretty serious balance changes in the car

what never ceases to amaze me is how people find the oil consumption and cylinder wear of the flat opposed engines acceptable - while it made sense in the VW, and the idea was simplicity and minimal expense, as that concept progressed, and they continue to try to derive more and more power from it, the limitations begin to show themselves very quickly

i think we have seen the beginning of the end of this design in normally aspirated form

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[PorscheDude]

A boxer engine is very wide.

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[flash]

yeah - it would have meant a LOT of redesign of the front end, and a complete departure from the originating concept of the car

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[Johannvb]

<!--quoteo(post=80879:date=Sep 28 2009, 09:18 PM:name=PorscheDude)-->QUOTE (PorscheDude @ Sep 28 2009, 09:18 PM) <{POST_SNAPBACK}><!--quotec-->A boxer engine is very wide.<!--QuoteEnd--><!--QuoteEEnd-->

Hmmm...I haven't looked at dimensions, but my thought what that if a Subaru has a flat four, surely a flat 6 should fit into a 968. Not like I would ever do it, but I do have a bit of Dr. Frankentein in me.

I still think a modern tip in a turbo charged 968 with paddle shift could be a lot of fun.

Ok, I'm done being of topic now. back to balance shafts [img]style_emoticons/<#EMO_DIR#>/wink.gif[/img]

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[Ryan]

Flash, Porsche has been getting more power out of that design every year for 46 years. I would hardly say the design ran out of promise very quickly.

I think the original engine case may have run out of displacement potential due to cylinder placement , but that can change.

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[jlm968]

Seems I recall that there were lightened balance shafts available somewhere. Are they still available? Worthwhile?

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[flash]

what i said was that the design begins to show its limitations quickly, not they quickly ran out of power - first they had to go water cooled - now they are having to deal with even more issues in the design as they try to keep up with the other manufacturers who are making more and more power - there is very real talk about this being the last generation of the flat opposed engine for porsche

as for lightened balance shafts, since it is the weight that makes them work, i'm not sure how that would work - i suppose that if you did the math on crank weight reduction and such, that you might find a lightweight balance shaft mass that would cancel the harmonics - lots of homework though

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[jlm968]

I think the mass of the shaft itself would be lighter, perhaps made of aluminum, and the balancing lobes still made of iron. Something like that?

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[JH6]

<!--quoteo(post=80704:date=Sep 26 2009, 09:35 AM:name=durtkillon)-->QUOTE (durtkillon @ Sep 26 2009, 09:35 AM) <{POST_SNAPBACK}><!--quotec-->Having only worked on boxer engines in the past, I actually had to google why balance shafts were so critical for the 3.0 inline 4. I found this great little article that explains it well http://www.autozine.org/technical_school...mooth2.htm . So I now understand the concept of why an inline 4, particularly a large inline 4, can never be properly balanced without one. My question is why are the shafts mounted asymmetrically around the crank? In other words, why is one high and one low? It seems like putting them both low would be simpler. Better yet, why not mount them both below the crank and use a timing gear instead of a belt?<!--QuoteEnd--><!--QuoteEEnd-->


I went through the tread and it looks like no one knows the answer. I remember the principle from University so I did a little search in my Library and re-typed an article regarding the balancer shaft configuration of our engines. It is very close in comparison, only Porsche design differs in using the double sided belt so doesn't need the spur gears. It is rather long so if you are not interested feel free to skip it. [img]style_emoticons/<#EMO_DIR#>/smile.gif[/img] I think the article explains it all so enjoy the reading.

<!--sizeo:3--><!--/sizeo--><b>Front-toothed belt-drive parallel twin countershaft harmonic secondary force balancer</b><!--sizec--><!--/sizec-->

A belt-drive twin-countershaft secondary force balancer (Fig. 3.63) incorporates long twin countershafts with the right-hand countershaft being directly driven by the belt via a pulley, whereas the left-hand countershaft is indirectly driven by a pair of similar-sized spur gears which drive this shaft in the opposite rotational direction to the directly driven shaft. The belt pulleys are chosen to give a 2:1 speed step-up and a jockey pulley is included to provide the correct tension to the belt drive.
Thus, the balance countershafts rotate counter to each other at twice the crankshaft speed and are so phased that they counteract the positive secondary forces at either dead centre, and the negative secondary forces at mid-stroke. This is achieved by the weights facing in the opposite direction to those secondary forces when in the vertical plane, and then for both balance-weights to face either inward or outward and thereby oppose each other when they have moved to one of the two horizontal positions.
An important feature of this secondary force balancer is that the countershafts are both mounted the same distance out from the cylinder's line-of-stroke but the left-hand countershaft is mounted considerably higher than the right-hand countershaft, which is itself positioned just above the crankshaft centre (Fig. 3.63).

   

The object of positioning the balance counter-shafts at different heights is so that when the countershaft weights are in horizontal plane, the height offset produces a balance-countershaft torque which opposes the fluctuating torque generated by all the piston secondary forces which occur at both dead centers and mid-stroke, otherwise this secondary inertia torque will promote engine roll (Fig. 3.64).

   

In addition to neutralizing the secondary torque fluctuations the offset balance-countershaft torque partially counteracts the explosion impulse torque produced over a wide speed and load range.
Measurements of secondary force vibration (Fig. 3.65) at the gearbox mounting rise at an increasing rate with engine speed, but when twin countershafts are incorporated these secondary force vibrations are considerably reduced, thus producing a much smoother running engine, particularly in the upper speed range.

   

Best regards

Jan

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