PaulW

<img src="http://www.audipages.com/Tech_Articles/enginemechanical/tb76.JPG">

So this is what I found out yesterday.

First of all, the Bently gives a "cold" and "warm" dimension for the damper. I always wondered what warm would be, but now I know, it's when you drive the car up to temperature, drive it in the shop, and then work on it and you are ready to tension the damper 4 hours later. The block is still warm. It's still warm after 6 hours too if it's 80F outside in the summer.

The Bently states the warm dimension is less than the cold dimension, but I have appeared to prove that wrong.

However, when I took my car apart yesterday, 45 minutes after I turned it off, I had the LH timing belt cover off and I measured the damper length. It was 5.970". When I adjusted it last Sunday, the car was really warm (only about 2.5 hours after shutdown on a hot day) and I tried to set it at the Bently warm setting, but could only get it to 5.280" or so because that's as tight as it would go!!!

So then I trended it for the next 1.5 hours, five minutes later it had relaxed 0.100", and then it slowed down, and it ended up being at 5.285" at the end of the 1.5 hours.....gee, that's right where I set it while the engine was warm on Sunday!

So I was confused, what should the "warm" reading be?

The Bently quotes for the following:

Engine Warm 4.98"-5.09" (126-129 mm)

Engine Cold 5.38"-5.49" (136-139 mm)

So I had to make a guess if the hot reading was either 1" off (a typo) or 10 mm off (they subtracted instead of adding to it).

Depending on how I looked at it, warm would either be 5.98"-6.09" or 5.77"-5.88" (146-156 mm) depending on which type of mistake they made.

But since everything in the Bently is in metric first, I doubt the 1" mistake is the problem. I figured I needed to add 10 mm to the cold reading to obtain a hot reading.

So then I waited and waited for the block to cool down some more. I wanted it to be as close to ambient as possible.

I then said that the block was less than the "warm" reading, but not at the cold reading, so I came up with:

5.57"-5.65" (141-143 mm)

It had been about six hours since shutdown, so I did an as-left reading of 5.64". It still has a tiny squeal on it....so maybe I was too conservitive.

So the Bently is wrong on the warm reading. It goes the opposite way of what really happens.

After doing about 10 TB's in my life, I also realized that you don't need "tight as a drum" belt tension, that doesn't work.

I have a friend at Bently, I will have to forward him this info.

Can you imagine how many hours I sat and thought about this?

It gave me time to put on my new inner fender that was to replace and old one that got rippped at a car wash because the belly pan was hanging (thanks Dealer!)....and a nice bike ride to the parts and hardware store too!

pw

Paldi

I'm thinking the tensioner gets longer? Would that be a problem if you are already adjusted too tight?

skiracer

tensioner works, it would seem to me that it should work the way the Bently has it- As the engine heats up, the distance between pulleys increases, putting tension on the TB. The tensioner would then compress (get shorter) to release some of the addtional tension on the belt. Look at the way the tensioner is set up- it uses two fulcrum points between the tensioning pulley and the damper (and yet another on the pulley!), and as the TB heats up the damper should shorten! However that assessment is based on limited photos of the setup, and the angles are close to going over center, so it is possible that the damper gets longer as the engine heats up. I still think the Bently may be correct, but your data is hard to argue with. The safe bet is to set the tension with the engine COLD!

PaulW

But normally it would be in a shop, I assumed 25C (77F) for "cold" measurement.

PaulW

Doug-

I couldn't figure out for the life of me how the damper even changes position relative to motor temperature.

Here's a pic of the tensioning device, you're right, it has many pivots on it, the pulley on the left side with orange also pivots:

<img src="http://www.audipages.com/Tech_Articles/enginemechanical/tb70.JPG">

Now that I look at that picture again, I captured where the ecentric pulley was adjusted too. My as-left yesterday was aligned to the left of the cam gear, not to the right....hmmmmm.

If the Bently was right, then you would be able to adjust the ecentric pulley to the 3 o'clock position (which is the tightest you can go to, and then cam position decreases again), but when you go there, the TB damper won't go to 5.00", the smallest I could get it to go was about 5.28", which is exactly where I found it.

Once the engine cools off, the damper starts getting to a smaller and smaller dimension.

I don't think it has much to do with the temperature of the belt, but temperature of the block and how the sprockets grow relative to each other. Do they get closer or further apart? I'm thinking further apart, but I can't explain it.

If you look at my graph again, you can see if you were to extend my as-found data down to 77F, the timing belt would be too tight by about 0.280"! Now that's why it's making noise at its present tension (or what I think).

<img src="http://www.audipages.com/Tech_Articles/enginemechanical/tb77.JPG">

Here's the cool down data:

Length - Time after Shutdown

5.970" - 11:15 (45 minutes after shutdown)

5.810" - 11:20

5.630" - 11:25

5.555" - 11:30

5.480" - 11:39

5.410" - 11:54

5.400" - 12:02

5.300" - 12:36

5.285" - 12:47

I thought A LOT about this......started plotting numbers in Excel....this is the only way that it could be. I've done 3 timing belts before, it's not like I don't know what I'm doing (however, I could still be wrong).

This all makes sense.

Once again, your advice is best, only do the TB tension on a motor at ambient temperature.

When we did yours, we pulled it in the garage, but outside was something like 58 or low 60's that day, it was probably close to 10 hours after we started work before we set it (after dinner if I recall).

When I did Floris' belt, his car was sitting in his garage overnight, so it was at ambient conditions.

When Scot did his, it was at ambient conditions too.

My problem is that I did mine on a hot day in a garage with little ventilation and the block wasn't cold when I set it.....

What do you think? Am I still wrong? Looking for feedback.

Plus, you cannot achieve the 5.00" dimension with the engine hot, it is impossible to set it to that dimension with the concentric pulley at max TB take up.

pw

ryoung

Isn't that small smooth pulley at 11:00 relative to the toothed crankshaft pulley free to pivot? It has an eccentric shaft and looks like it's held in place by the tensioner.

<img src="http://www.audipages.com/Tech_Articles/enginemechanical/tb70.JPG">

PaulW

...down bolt.

That's why I have a hard time figuring this whole thing out.

pw

ryoung

Are we talking about the same pulley?

skiracer

to obtain the correct damper length. The pulley at 1:30 is connected to the damper through the linkage to the right of it. notice that the 1:30 pulley's (that is the orange one Paul was refering to) hold down bolt is offset- there is a bushing under that bolt that allows the pulley to rotate freely under it. The small hole on the right side of the pulley is where the damper linkage hooks to it. With all of the nearly over-center pivots in the linkage, it is very difficult to say for sure which way the damper should go as the engine cools. Paul- as the engine cools, the block contracts and the pulleys should become closer together. The TB is made of non-metallic materials, and the coefficient of expansion for it is very small- likely insignificant over the temperature ranges we are talking about.

ttuling

... and the crankshaft at TDC?