Engine/Drivetrain Vibration Issues

[jondee86]

It seems to me that there are three separate and unrelated (in my
opinion) problems here.

1. The "clunking" from the rear differential. The CW&P will not clunk
if the backlash is set correctly, so the sound most likely relates to the
movement of the "ramps" as the LSD moves from the power-on to the
power-off position. Check with people who know about your LSD.

2. Bearing clearances in the engine. These have no affect on the
rotational inertia or power output of the engine, so are most unlikely
to have any affect elsewhere in the drive train.

3. A "rattling" sound or "vibration" while decelerating. Here it would
be useful to define if it is one or the other. For "rattling" I understand a
loud metallic sound, mainly heard rather than felt. For "vibration" a much
softer noise, and movement easily felt from anywhere inside the car.

I am making two posts here as my PC is crashing, and I lost the last
one

Cheers... jondee86

[jondee86]

From what you have said, the only things you changed (in the last
rebuild) that could alter or introduce a torsional resonance in the drive
train, are (a) the CW&P, and (b) the compression ratio.

By changing the diff ratio, you have not only lowered your overall
gearing for better acceleration, but you have increased the amount of
engine braking available. This will cause a change in gear tooth loading
under deceleration.

By raising the compression ratio (and I assume the ratio went up) the
strength of the cylinder compression impulses will have increased.

Resonance problems are difficult to pin down, and the solution is most
likely easier to find by trial and error, than by theorising

So, if the problem is one of "rattling", it would probably be worthwhile
swapping in another back axle assembly, or complete diff head if you
have one in the OEM ratio (4.3:1 ?).

If the problem is one of "vibration", then swapping back to the heavier
flywheel you had before, may be a solution.

Cheers... jondee86

PS: I would still like to know if you have fuel cut on overrun

[apardal]

jondee86, thank you very much for your thorough replies.

To explain better the sound, I should say that it is a sound similar to that made by the clutch release bearing when it goes faulty including vibration easily felt on the gear lever. I think I haven't written about this earlier but that was my main thought because I didn't put a lot of grease in it. One morning, a few weeks ago, I disassembled the gearbox and greased thoroughly the bearing and reassembled it again. The problem was still there (although the bearing was the same... it is 2 years old).

I've exchanged some mails with the guy that sold me the solid pinion spacer (Spence Weir) and he told me both the clunk and the vibration could come from incorrect LSD bearing preload. I don't think so, because I remember the procedure and I don't think I got it wrong. And that would also cause some whining under acceleration, I think.

The compression ratio was indeed increased, but not that much, because I was already running a decked head 0.6mm, decked block 0.1mm and 0.5mm head gasket on OEM pistons. By my calculations the CR was around 11.5:1. Now, with the wiseco pistons and a 0.8mm head gasket I'm around 12:1.

Tomorrow morning I'm going to align the wheels and steering since it wasn't aligned since I installed the Greddy Coilovers. That can cause a lot of strange vibrations, for sure... but I don't think it is the source of the problem... But I need to do that anyway.

I've talked to the crank balancing shop and they said (of course) that they have no prior balancing errors. That the machine is pretty accurate.

Tomorrow I'll have some news. Next week I'll disassemble the oil pan and measure (with plastigauge) crank/bearing oil clearances and I might measure rod/bearing oil clearances also while I'm at it.

About the fuel cut on overrun, I honestly don't know what that is, but I'm running stock ECU with an Apexi SAFC1 piggy back.

Again, thank you very much for your interest and answering all these questions.

Best regards

André Pardal

[jondee86]

Fuel cut is an economy feature on most modern ECU's. If high manifold
vacuum, closed throttle and rpm above approx 1500 all exist at the same
time, the ECU will shut off the injectors. These three conditions are typical
of closed throttle overrun (deceleration).

It is a standard feature of all 4AGE ECU's as far as I know. But I don't
know if the SAFC will override this feature. If you have a Air/Fuel ratio
gauge, you can tell when fuel cut occurs, as the A/F ratio will become
very great >60:1 or more.

The difference between fueling and not fueling on overrun has an affect
on gear tooth loadings, and hence the possibility of "chattering" of the
gear teeth.

Does the rattling happen only in one gear, or can it be heard in several ??

Cheers... jondee86

[apardal]

The rattling is heard on all gears.

About the fuel cut, my AFR only measures between 10 and 16, above 16 it will show "Air", and that's what I read on decelerating off throttle.

Yesterday I went to a friend mechanic and he jacked my right rear wheel and asked me to gear the car and start rolling until I reach 3000 rpm to listen underneath the car. We could hear the rattling but couldn't really say where it came from. He told me it might be unbalanced flywheel, but he gave me a good way to find out.

He told me to disconnect the driveshaft (after the center bearing) and start the engine throughout the gears to look for rattles. If there are none, it most likely comes from the rear end. If there are, then it can come from the flywheel or the crank bearings that I installed wrong.

Best regards

[jondee86]

If the rattling is heard in all gears, and even while the engine is running
in gear at constant 3000 rpm without deceleration load, then it is not
likely to be a torsional resonance It will be a simple vibration of loose
parts.

I had a rattle that started at 6000 rpm as if a switch had been turned on
Sounded like all the valve shims had fallen out !!! But in the end
it was found to be the clutch hydraulic line banging against the firewall.

As you say, when you start stiffening the car up, many things can cause
vibrations. Out of balance wheels, rigid or super-stiff engine mounts or
transmission mounts, spigot bearing missing from crankshaft etc. You will
have to use a process of elimination, checking every loose or flexible
metal part, that could possibly vibrate against another metal part

Cheers... jondee86

PS: I hope you you had both rear wheels off the ground when you were
running the car in gear looking for the rattle !!!

[apardal]

Jon, why do you hope I had both rear wheels off the ground?

This weekend I'll start the process of elimination! I have my exhaust broken which can be another source of rattles and vibrations...

Thank you very much for your knowledgeable help!

Best regards

[Rasteirinho]

Throw that shitty car to the garbage and get a decent car like a Yugo.

At least the vibrations are normal and not a problem...

[jondee86]

With a mechanical type LSD installed; if the engine was to misfire and
cause a sudden acceleration/deceleration load on the back axle, there
is a possibility that the LSD will momentarily lock up !!!

The car will then fall off the jack... and when the spinning wheel hits
the floor... woo hoo

Cheers... jondee86