joey_turbo

iTrader: (26)

Not a spelling mistake, I didn't mean Warble.

I was wondering, When I first got my Scoob's I could almost feel the car physically wobble/shake when idling. It seems to more of a controlled shake rather then something erratic. And the rev's don't hunt at all, when it does it, the revs are very stable.

Is this how they are supposed to be?

My mate reckons he can actually feel the pistons moving from side to side in this car, as opposed to a straight engine. But he's the type of bloke that says he can feel his car is faster just from removing a large bird crap from the paint

Any input appreciated.

Cocker

iTrader: (3)

mine does the same - i put it down to the pistons going side to side aswell!

TonyBurns

iTrader: (3)

My UK classic did the same, though my MY03 JDM didnt do it, id say it was more to do with the exhaust than the pistons

Tony

joey_turbo

iTrader: (26)

Thats good to know.
Mine is a classic also, a MY99 UK. I've heard about the MAF problems, but as it wasn't hunting or revs fluctuating I ruled that out.
And to be honest, it was only more noticable when I sealed a leaking backbox flange.

Mr Footlong

iTrader: (7)

Same here, had 3 classics and only the current one with a rebuilt block does exactly the same thing on idle. Well, I don't feel the car wobbling, but on idle you can just feel something. No problem with revs etc, just feel like something goes off balance slightly every couple of secs or so. No problems when driving. Has done it since I have owned her and isn't a problem as far as I am aware.

marmski

I get the same... first thing i noticed when i first got in mine.

Im guessing it must be a un-equal headers thing if the JDM's dont do it.

The rookie

iTrader: (1)

Its the torsional effect of the flat four pistons and there inertia, obviously engine mounts as well as piston weights effect how much you will feel it.

Simon

GeeDee

iTrader: (2)

Call me sceptical, but surely you can not feel the actual pistons moving.

At tickover, the engine is running at around 7-900 rpm which is 12-15 revs per second which is twice that many piston movements owing to it being a 4 stroke! I also assume (not sure of the flat 4 firing configuration) the firing pattern means that, as a piston moves in one direction, another piston will move in the opposite direction. I’m pretty certain that frequency could not be transferred to the car.

However, I do accept that a resonant frequency could be hit and the car was vibrating in sympathy at a lower frequency.

The rookie

iTrader: (1)

Eh, no 4 strokes make 1/2 the power strokes, not double the rpms, so 700rpm is 350 power enets per cylinder per minute, but still 12 revs per second (ish) and yes you will feel it, the G on the piston is very high and stopping it at TDC/BDC will move the engine each time.

Simon

[Davey]

ofcourse you cant and they even move against each other in pairs so very well ballenced. I.e. the rear bank both hit TDC at the same time .. and ANY engine that the piston movement can be felt has a very very badly ballenced crank and will shake its self to pieces

The rookie

iTrader: (1)

Except of course the pistons AREN'T directly opposite each other, so they do create a turning moment on the engine and they DO make it wobble. You cam't feel the pistons, but can sure as heck feel the effect they have on the engine (the mounts mean the frequancy of wobble will be a function of engine speed and mount stiffness/damping - stiffer mounts will mean the engine will feel harsher but probably not allow the wobble.)

The turning moment on the engine is proportional to the distance between (not quite) opposing piston centre lines and piston weight, max piston speed and engine speed.

Source Bosch Automotive handbook v5

Simon

[Davey]

I suppose that is true, you'll get an oscilating effect but the front bank should counteract this surely.

borat52

I think its more of a twisting movement than outright wobble, as they are not directly opposing each other there is an overall turning force created when they move in and then the opposite direction when they move back out. this makes the engine twist slightly on each half turn of the crank which would be felt as a vibration through the car as the engine is bolted to it.

But having thought about it placing the front and rear pistons with a degree of thought should result in this being cancelled. Not sure of the placing myself though.

joey_turbo

iTrader: (26)

Notice how I've stayed away from all the technical talk.
Well thanks to all that replied, my question is well and truly answered.

GeeDee

iTrader: (2)

Quote:

Originally Posted by The rookie

Eh, no 4 strokes make 1/2 the power strokes, not double the rpms, so 700rpm is 350 power enets per cylinder per minute, but still 12 revs per second (ish) and yes you will feel it, the G on the piston is very high and stopping it at TDC/BDC will move the engine each time.

Simon

First of all, I was assuming 1 revolution of the engine comprises spark to spark and therefore 2 complete revolutions of the crankshaft - thinking about it now, I guess it has to be crank revs. I’m sure someone with specific knowledge will confirm one way or the other.

However, since you refer to power strokes, as you intimate 1 Rev will give two power strokes (half of 4 pistons) and so 25 ish explosions per second. I still can not believe this can be directly transferred to the chassis through rubber engine mounts so I stand by the rest of my previous supposition.

Also, as others have intimated when one piston hits TDC/BDC there is another one creating a virtually equal opposing force albeit slightly offset which will create a twisting force in the vertical axis of the engine.

The flywheel also helps damp the pulses of power and so if a lightweight flywheel has been fitted the engine would be "more wobbly".

The engine will also twist through the front back axis when power is either applied or reduced owing to an opposing force being required to offset the twisting of the crankshaft which I assume does not have an opposing shaft (some cars do) but this will not be felt as vibration.

vindaloo

Mine's done the wobbly thing but only after a recent service. As part of the service, slightly colder running spark plugs were used. I put it down to the plug change initially but TBH, I can't recall the car doing it recently. I'll have to go all paranoid and try and feel it doing it now, on the way home - LOL.

J.