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Date:         Wed, 24 Apr 2002 23:00:58 EDT
Reply-To:     FrankGRUN@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Frank Grunthaner <FrankGRUN@AOL.COM>
Subject:      Vibration Issues for TIICO and I4 Engine Conversions
Content-Type: text/plain; charset="US-ASCII"

As I am oft noted as saying this topic has been well discussed in the archives, as has the now ongoing thread on diesel engine conversions. Now I don't object to any technical discussion on this list, I only wish the discussion would start with some knowledge of what's gone before.

But back to vibration issues. There are several issues with the TIICO system and the various inline 4 configurations, in that there are so many different ways VW has placed accessories on these engines. There is an exhaust contribution, there can be an accessory contribution and there is an engine mount part of the problem. The contribution of harmonic balancers has been well treated lately. These largely address the vibration nodes along the crank and bearing lifetime. I-4 donor engines without AC typically have hard pulley systems, those with AC often have balancers and those with AC and PS always do. If you listen to the pulley end of the block with and without the harmonic balancer on you will hear the difference (mechanics stethoscope). If you change the alternator belt tension you will hear them sing differently.

On Exhaust Contributions: See my posts in the archives, no time to repeat, now.

On the Accessory contributions: There are at least two issues. (Disclaimer: I have never seen a TIICO setup other than with pictures provided by listees over the years. I don't know how the AC and Alt package is still mounted). First off, just as has been recently alluded to on the list, the I4 engine has several inherently unbalanced vibration modes. They occur in all non-balance-shaft-compensated I4 engines. With the same crankshaft design as VW and in the displacement range of over 1.6L, these vibrations occur in the 3 to 4K rpm range. The amplitude is a function of displacement. 1.6L weaker than 1.8, weaker than 2.0 , etc. By 2.3L, the I4 is so rough balance shafts become a necessity. To see the details of vibration mode tuning, see the German design articles I have posted to Alistair Bell's website on the design of the Porsche 944 and 944T.

The location of various components and the nature of the connection to the center of vibration (crank centerline) determined whether or not a resonance can be established. This magnifies the amplitude of the net vibration by a factor related to the Q of the resonant system. This is my obtuse way of saying, look at the cited bridge collapse site. So poorly designed exhaust pipes can provide the resonant mode. Here the combination of heat and vibration will cleave metal at its weakest point (of course). The developing exhaust system crack amplifies the vibronic energy again, for a faster complete break than the starting event. Long time to start, quick to finish. For accessories, the further the centerline of the component is from the crank C/L, the more vibrationally susceptible it is.

So in the few TIICO pixs I have seen, the alternator is mounted on an outrigger further out from the AC unit relative to the crank. But typically, this should not be enough to trigger vibration. Now the archival poster noted that removal of the clamp to the head (AC?) decreased the vibration. Also that soft mounts had the same effect. I have found that the issue is related to the axial planarity of the pulley system. If the alternator pulley is slightly (few tenths of a degree) misaligned, the belt sets up a resonant chatter (its already oscillating from side to side with the misalignment). The experimentalists can duplicate this at some peril by removing the adjustment brace and using a long handle wrench to twist the top of the alternator (grab the flange on the top and twist slightly) You will feel the vibration onset. Careful with the wrench, easy to damage you or the engine severely. The long bolt plate that I see in the TIICO images seems too thin for the job.

In my conversion, I have the alternator in the original Golf position relative to the AC compressor (between the AC and the head. Several times during the conversion first year, the retaining bolt for the alt belt adjustment snapped off. Finally I went in with a long aircraft quality bolt and did a double brace --- that is two alternator arms, one in the front and one on the back of the alternator bracket. The serrated edge adjuster is on the front and the unserrated piece on the rear. I machined a large aluminum spacer for the proper gap, then used double nuts to lock the shaft. Never a broken bracket since and no alt related vibration.

Engine Mounts: As Alistair noted he and I have been going over the issue of active and passive vibration isolation from the I4. In studying the problem (only looking at the 82 style diesel mounts), I have been very impressed at the level of sophistication used by VW for these things. With the possible exception of sorbothane compounds (didn't exist when they designed these things), the VW approach seems as good as one can do with passive isolation technology. But Porsche (first) then many others including MBz, Audi, VW, Volvo and SAAB designed dynamic fluid filled mounts that act as active isolators or shock absorbers between the engine and the frame. I have now tested mounts from the Porsche 944T, the Audi 5000, the MBz 280, the Volvo 740 series, the Volvo V6 engine and the SAAB 2.0L 4 cylinder turbocharged engine. I also checked out the cost of these mounts (new). I have found that all work well, but the Porsche and SAAB units work really well against the 3 to 4K frequency band. I have modified the diesel engine cradle (as I'm building up the Turbo Audi 3A engine), to accept the SAAB fluid isolated mounts. I am including the heat shield mounts that thermally isolate the mount from exhaust temperatures. Also included the SS strap from SAAB to keep the engine in place should the mount fail. I'll post drawings for those interested when I'm finally done. In my testing there is a more than 10 dB isolation of the 1.8L RV engine vibrational spectrum from the frame using a dual trace oscilloscope and knock sensors for inputs.

Sorry for the random blither but I'm in the middle of Mars proposals. Hope you can make sense out of this.

Frank Grunthaner


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