Crankshaft Hub Bolt Loose? Torque spec?, Sigh... Options

[GregAmy]

Is it a known issue for the fan hub bolt to loosen?

Backstory: car started running like hell last Spring on my Microsquirt conversion. Wouldn't rev, #3 CHT going vertical, very hot exhaust. WTF, over? I checked some basic stuff, nothing found.

Drove it again a week later and it seemed to be getting worse. Checked the ECU logs and all seemed normal, TPS, MPS, all working as expected. I even went back to a tune that worked great in November (only difference was cold startup stuff, which I can test/adjust in the Spring when it's colder), but still even worse. Checked timing with an adjustable light at full advance, spot on for what the ignition map was calling for.

I parked the car. Life was busy this year and I just didn't have time to deal with it. But I trotted it out again in September, and it was just as bad. Since I had problems with the IGN4VW coil a year or so ago (mounted in a hot place and was cutting out), I replaced the coil with one from NAPA.

Suddenly the car would not start at all. And when I looked at the ECU logs during cranking, I was seeing 0 RPMs. Bad crank position sensor?

Today I put the car on the lift and visually inspected the crank position sensor (as well as I could, anyway; the toothed gear replaces the A/C spacer) and the spacing of the CPS tip to the teeth was clearly too large; whereas it should be about 50-thou, it was actually around 1/8"-3/16". I pried on the sensor bracket with a screwdriver, and it was solidly mounted, no looseness.

I know I spent a LOT of time getting that sensor set just right during the install (you can't get it it with the engine installed). So I knew something was up. The sensor was solidly mounted, the toothed wheel was not loose, so there was only one thing to check...

..and yup, the hub bolt was loose, allowing the hub to walk outward. Sigh...

I tightened it down - best spec I can find in the interwebs is 23ft-lbs - and of course the car fired right up and drives fine. I hope this didn't cause any other damage...

Is this common? I've never seen it before. The engine was built in California and I truly don't recall who installed the hub, them or me. It's torqued down right now and I'm going to check it on a regular basis (I thought about pulling the bolt and putting Blue Loc-Tite on it, but if I lost that bolt in there then I'd be toast and would have to pull the engine to get it back. Arthritis in the thumbs and tennis elbow so NFW I'm tempting that Fate...)

Oh, and then I broke the door cam actuator when I was putting it back in the garage..it just never ends, does it...? - GA

[Superhawk996]

@technicalninja

Found some interesting data for the FA engine



Harmonic damper is tuned for 2nd order - for about a 150hz resonance.

I’ve overlaid in red what the crankshaft resonance would probably look like without a damper.

When a tuned mass damper (harmonic damper) is used it cannot reduce the energy input; it can only redistribute the energy to other frequencies. When it does this it usually creates two side bands - one above and one below the resonant peak. I’ve artfully (IMG:style_emoticons/default/laugh.gif) illustrated this in green.

These plots appear to have been created from data supplied by Fluiddampr so there is the comparison of OEM Harmonic Damper to their product.

While you can see they (Fluiddampr) knock the resonance down further, they do this by pushing the energy out to higher frequency (RPM) range. This would not be a good thing for extended racing / track day use where the engine would spend more time at those higher RPMs.

The bottom line: FA motor making a whole lot more HP and Torque than our measly T4s and it would appear that perhaps the Subaru cranks has some propensity for a 150Hz resonance.

Another interesting dilemma is that just because it has this resonant frequency doesn’t mean that it is a durability issue. It just may be that Subaru is using the damper to calm NVH and make the car more enjoyable to drive. Without access to Subaru durability or warranty data we can’t know for sure which reason it is that Subaru is using the harmonic damper.

Obviously, we would need similar data for the VW T4 to draw any conclusions about whether the T4 would benefit from a harmonic damper and of course that data would be needed to tune it appropriately.

Source:
https://www.ft86club.com/forums/showthread.php?t=139127


PS: Thanks to you I’ve learned a little more about subi’s than I knew a few days ago (IMG:style_emoticons/default/beerchug.gif)