[alfa] re: the workings of V6 tensioners

[Joe Elliott <jee@xxxxxxxxxxxxxx>]

The hydraulic tensioner has two functions, which are largely 
independent of one another.  One, as you've figured out, is that it 
slackens the belt in response to high oil pressure.  This was done 
because oil pressure is proportional to engine speed, and the belt 
doesn't need to be as tight at high engine speeds.  This whole 
hydraulic aspect of the hydraulic tensioner has nothing to do with 
thermal expansion, the real justification for having a dynamic 
tensioner.  This more important function is the job of the two 
springs.  (Yes, there are three springs, but one is there for 
pretensioning during installation and doesn't do anything once the 
tensioner body is locked down.)  The springs just apply a roughly 
constant force on the tensioner pulley, so as the engine warms up and 
expands, the tensioner pulley can move to accomodate that expansion 
while being pressed into the belt with roughly constant force the 
whole time.  Thus, belt tension remains roughly constant whether the 
engine is cold or hot or in between.  And unlike (IIRC) the 
mechanical tensioner, the hydraulic tensioner has *two* springs, so 
that if one were to break, your engine wouldn't immediately 
self-destruct.

You're right that relatively high belt tension is key during startup 
(notice how many mechanical tensioner failures occur at startup). 
The mechanical tensioner really doesn't do anything about this, but 
as long as its response to engine temperature is *exactly* 
proportional to the thermal expansion of the engine, that's not 
really a big deal.  The hydraulic aspect of the thermal tensioner 
actually does account for this, because until the engine fires and 
attains idle speed, the oil pressure is next to nothing (that's why 
the majority of engine wear over a car's life happens in the first 
30sec following a cold start).  With little or no oil pressure, the 
hydraulic tensioner is as tight as it can get, thus assuring adequate 
belt tension on startup.

The drawback to the hydraulic tensioner (besides the propensity to 
leak oil) is that oil pressure is also proportional to engine 
temperature.  So in addition to making the belt considerably tighter 
at startup and somewhat loose at high revs, the hydraulic mechanism 
makes the belt tension slightly lower when the engine is cold.  I'm 
not sure how significant this effect is, but it's there, and there is 
also a similar effect from the springs.  (Hooke's Law, F = k*x, 
dictates that the springs will provide slightly more belt tension 
when they are slightly more compressed when the engine is hot--that's 
why I say they keep the tension *roughly* constant.)  So while these 
effects probably aren't significant, they're there, and they're not 
desirable.

The one drawback to my non-hydraulic hydraulic tensioner is that it 
always applies the slightly higher tension that the designers 
intended for startup.  Considering how frightened some people are by 
the timing belt design on the Alfa V6 anyway, this may not be a bad 
thing.  If I were less lazy though, I'd take some measurements and 
replace at least one of the springs with a slightly weaker one.  I 
don't worry about the higher tension provided by my tensioner though, 
because I know from experience that while scary, the Tom Zat 
tensioner works, and when an engine with a Zat tensioner warms up, 
the belt is *WAY* too tight.  Despite that, I've never heard of 
anyone having a belt break with a Zat tensioner--I get the impression 
that tensioners failing and releasing tension trigger the demise of 
many more of these engines than broken belts do.  The Zat experience 
is why I'm comfortable modifying the tensioner the way I do--mine 
won't leak, and while it runs the tension a little tight, it's 
nowhere near as tight as the Zat tensioner on a warm engine, nor will 
it ever get as dangerously loose as a Zat tensioner on a cold engine 
can.  And unlike the mechanical tensioner, it responds *directly* to 
thermal expansion of the engine, and relies on *two* fairly robust 
springs (I've never seen or heard of one breaking) as opposed to 
*one* seemingly fragile spring.  I also get the impression that the 
bearings on the hydraulic tensioners last a lot longer than those on 
the hydraulic tensioner.

Joe Elliott
'82 GTV-6



> Date: Wed, 15 Sep 2004 12:39:41 +0000
> From: f.rastinejad@domain.elided
> Subject: [alfa] the workings of V6 tensioners
>
> I have tried to understand the oil-fed tensioner, but one thing just 
> doesn't seem right.  I accept that the high oil pressure functions 
> at high revs to loosen the tension, and that when starting the 
> engine (zero oil pressure) the tension is at its highest as one 
> would want.  However, with the cold engine, the oil pressure is 
> always high (at least for a few minutes after start-up) causing the 
> belt tension to get loose.  I would think that during this time one 
> would need higher tension since the aluminum has not yet expanded. 
> So does it stand to reason that the oil fed tensioner can't possibly 
> be there to adjust for expansion of the aluminum block as stated by 
> some?
>
> On the other hand, the "replacement" tensioner, being thermally 
> controlled, can compensate for the thermal expansion, but when one 
> tries to start a hot engine the tension is low just as we need the 
> belt to be tight.
>
> Am I missing any key facts?
>
> - -Farzan (never had a belt slip when starting from cold or hot!)
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