coyoteboy
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| posted on 25/5/13 at 07:27 PM |
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Work with me on this because I'm failing to see the complication that requires floating of the pulleys (other than initial robotic assembly at
the factory).
Physical relationship between the two cams is set by a distance on the belt, say 50 teeth on the old belt. Tension in the belt at that point is a
factor of belt tensioner and cam resistance. If you replace the belt with another one of the same tooth pitch, and make the assumption they have some
quality control capability, you can expect the 50teeth on the new belt at the same tension to be the same distance as between the same two points on
the old belt when it was new too.
Same distance between the leading can and the crank too means they are locked together and therefore the only variable could be tooth to tooth length
of the belt. Are you suggesting the belts are that poorly matched that stretching with tensioner puts them wildly out? Or that variation of less than
a degree drastically affects the engine to the point of misfire?
PS did a belt change on a 16v Renault engine with friction can pulley (my first with that new feature), could not get it happy with the correct
locking tools, worked fine when I did it the old way. Possibly the cause of my dislike of locking kits.
Anyway, if this tension problem exists (which I'm not doubting it does to some unknown extent) wouldn't all engines with long runs and
keyed timing be impossible to time correctly and never have existed before? Keyed timing pulleys is exactly the same situation as not loosening the
floating pulleys, which has worked for decades worth of all manner of engines? I'm not questioning pointlessly, I just don't blindly
follow the herd without investigating the reasons for the stampede first.
[Edited on 25/5/13 by coyoteboy]
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froggy
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| posted on 26/5/13 at 12:02 AM |
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[IMG]http://i144.photobucket.com/albums/r187/froggy_0[IMG]
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mark chandler
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| posted on 26/5/13 at 08:09 AM |
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I cannot see why you worry about tension either using the belt cut system (very clever), tension is only an issue if you have the pulleys loose, they
are not so no need to worry.
Look at this another way, you have a correctly set up car and turn the engine the wrong way and the tensions will be all over the place, turn it the
right way and everything is restored because nothing fixed has changed.
Doing this 1/2 belt cut would have saved me near tears when I changed the belt on my 928 Porsche, I got lost on belt routing as well as tension on
that 
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40inches
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| posted on 26/5/13 at 10:12 AM |
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quote:
Originally posted by mark chandler
I cannot see why you worry about tension either using the belt cut system (very clever), tension is only an issue if you have the pulleys loose, they
are not so no need to worry.
Look at this another way, you have a correctly set up car and turn the engine the wrong way and the tensions will be all over the place, turn it the
right way and everything is restored because nothing fixed has changed.
Doing this 1/2 belt cut would have saved me near tears when I changed the belt on my 928 Porsche, I got lost on belt routing as well as tension on
that
This makes perfect sense to me. Does that make me a pain in the arse too?
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daviep
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| posted on 26/5/13 at 01:06 PM |
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quote:
Originally posted by 40inches
quote:
Originally posted by mark chandler
I cannot see why you worry about tension either using the belt cut system (very clever), tension is only an issue if you have the pulleys loose, they
are not so no need to worry.
Look at this another way, you have a correctly set up car and turn the engine the wrong way and the tensions will be all over the place, turn it the
right way and everything is restored because nothing fixed has changed.
Doing this 1/2 belt cut would have saved me near tears when I changed the belt on my 928 Porsche, I got lost on belt routing as well as tension on
that
This makes perfect sense to me. Does that make me a pain in the arse too?
Doesn't male you a pain in the arse, just means you don't understand fully yet.
Froggy and harv are 100% correct. I've gotten away wthout locking kits loads of time, I've also been caught once with a VW TDI engine
which hunted at idle after doing the timing belt, did the belt the correct way and problem sorted.
BTW Is the ABZ a CCW rotation engine?
Cheers
Davie
“A truly great library contains something in it to offend everyone.”
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coyoteboy
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| posted on 26/5/13 at 07:15 PM |
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Still not seeing the complication that requires floating of the pulleys (assuming you don't HAVE to float the pulleys to get the belt on, of
course) and no-one seems to be able to explain it, though they're good at posting funny images 
Assuming a keyed crank, keyless (but un-removed) cams, there's no need at all that I can see. The belt tooth spacing has to be accurate (or else
you'd have belt failure in no time due to the teeth not matching the pulley) and there's fixed tooth-size relationships between the
various rotary parts. Rather than just making smart remarks, if someone knows better and can explain it - do so. If you can't explain it, you
obviously don't understand it either? All of the failures with keyless pulleys I've read about (done by others) have come about because
they didn't realise the pulley floated OR through the mistake that caught me out on my first attempt - they used the locking tool to lock the
cams and crank but the crank locking tool doesn't lock the crank from rotating clockwise, meaning a slight shift occurs which knocks the timing
out a degree or two.
FWIW the engine is a clockwise rotating engine, when looked at from the timing belt end.
quote:
Look at this another way, you have a correctly set up car and turn the engine the wrong way and the tensions will be all over the place, turn it the
right way and everything is restored because nothing fixed has changed.
Exactly, so long as the tooth-tooth distance hasn't changed and the tension at the longest stretch between cam and crank is the same after a
couple of rotations, the relationship between cam and crank remains the same. It's easy to check with matchmarks?
I believe this discussion isn't really relevant to the ABZ if it has the keyed crank as it is effectively a "normal" engine at that
point, I'll only know if I pull the pulley off and find it's keyless, but even then I could matchmark crank and pulley 
[Edited on 26/5/13 by coyoteboy]
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daviep
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| posted on 27/5/13 at 12:41 AM |
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If you want accurate timing you need to have some method of being able to make sure any slack in the belt is taken up with the cam(s) in the correct
position in relation to the crank.
I can't provide a definitive explanation of why this happens but I can say for definate that if you lock the cams and crank and fit a new belt
there will certainly be some slack in the belt between the crank and the cams which is on the untensioned side of the belt. There are two ways to get
rid of this slack.
1: The correct procedure is to slacken the cam pulley(s) and allow the tensioner to take up the slack with the cam(s) and crank still in the correct
position, then retighten the cam sprocket(s)
2: The wrong way is to remove one of the locking pins and allow either the crank or cam(s) to rotate so that any slack is moved to the tensioner side
of the belt. Doing this means that with the belt correctly tensioned the crank and cam are no longer correctly aligned.
I certainly don't share your confidence in the manufacturing tolerances of timing belts.
I'm not trying to make smart remarks and I have used various methods when fitting belts but it depends on how precise you want /need to be,
marking the belt and pulleys on a pinto or similar works fine, doing the same on a modern engine with crank and cam sensors doesn't.
Cheers
Davie
“A truly great library contains something in it to offend everyone.”
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coyoteboy
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| posted on 27/5/13 at 06:27 PM |
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Just to note, I wasn't claiming you were being smart arsed - that was aimed elsewhere, and in jest. I can accept when I'm being an idiot,
I just don't follow the herd without seeing the evidence of the threat myself.
If the tolerances of the belt tooth-tooth lengths were that bad the belt would rip itself to pieces on the pulleys. And to get a belt that is the same
overall length with the same tooth count, the average must be pretty damn close (+- 0.1mm or so). There might be fractions of a degree difference
(which I suppose could affect the ECU reading the cam/crank if sufficient separation isn't available on the tooth outputs) but that in itself
won't affect the running of the engine (assuming the ECU doesn't have a hiccup) - hell, on a V8 there will be a good degrees worth of
stretch in the belt on its longest run anyway I'd wager. I've done quite a few timing belts in my time, through all manner of means, and
never known one that had a problem when the matchmarks lined up afterward. Floating pulleys makes perfect sense in a high speed factory production
environment - automated assembly is a synch when you're not having to line up keyways with a robot, but I genuinely don't think it's
for any reason outside of that. I do beleive that if you let it slip a few degrees during assembly you're in trouble and the ECU will have a
fit, but I don't believe the variation in belt dimensions would be even close to that and my previous experience gives me (possibly false)
confidence.
I'm willing to put my trust in the knowledge that the belts are bloody accurate and tooth counts negate any issue, when I pull it apart, put it
back and fire it up and ram 32 valves through 8 pistons you can chuckle at me, but to be honest a new engine is cheaper than buying the official tools
and the cheapo tools won't hold it to the accuracy you claim is needed
[Edited on 27/5/13 by coyoteboy]
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