Minor change in timing switching from mechanical to hydraulic/pressure fed tensioners
 

Hi all,

The rebuild has been going well for the past week. I timed the motor today, it was really not hard at all. To recap, I'm using stock 8.5:1 USA 3.2L Motronic pistons, and a 3.2L camshaft reground to 964 profile.

As per Wayne's book, I used mechanical (Stomski) chain tensioners while setting the timing on #4 and #1, and I had the tensioner screw turned out pretty far to put a bunch of preload on the chains. I timed both sides to between 1.25mm and 1.27mm of overlap, consistently.

When I replaced the mechanical tensioners with my original Carrera pressure fed ones, the timing seems to have changed mildly, on both sides. I proceeded carefully, and without letting the idler gear drop down or skip a tooth. However, my dial indicator is now showing about 1.35mm of overlap on both sides. This appears to be because the chain is a bit looser with the pressure fed tensioners than it was with the mechanical Stomski's. You can see the piston move in and out a little bit as you rotate the crankshaft through 720 degrees.

Do I need to re-do my timing? Is this an issue? Will it revert to the originally measured timing when the Carrera tensioners get fed oil and "pump up?" I did have to compress them to remove them originally and put a cotter pin in, but they were still tight feeling, i.e. they can not be easily compressed by hand.

Thanks for any advice. I'm looking forward to getting the fuel injection back on and getting the rest of the motor buttoned up!

Preston,
The running tension on the timing chains is not tight and should not be tight.
A chain that is set up tight with a mechanical tensioner will simply wear sooner.
The extra tension does not reflect the running tension seen with the Carrera tensioners.
Installing the Carrera tensioners and seeing the dial reading go up indicates to me too much tension was used with the mechanical tensioner.
If the chains were run at such a high tension, the intermediate shaft bearings, the idler bearings, cam bearings and sprockets would all be seeing loads they were never designed for.
If you want the cams running at 1.25/1.27mm, then yes reset the timng.

That's what I was thinking. So I wonder why the rebuild book doesn't recommend timing the motor w/carrera chain tensioners in place? Only because it was written from the perspective of upgrading an older engine with the mechanical/spring tensioners to carrera tensioners, and assuming those new carrera tensioners had never been primed with oil, thus were "extra" compressible?

And I should have searched more thoroughly, a similar issue has been recently discussed. BTW I do have new chains in place.

John Walker also recommended timing with "normal" tension on the chains.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/377931-timing-chain-tension.html?highlight=timing+964

I've worked with roller chains all my life. Chain driven packaging machines. Big chains, small chains, long and short. Heavy loads, light loads.
Chains like to run snug, not tight. Chains also like regular oiling or better continuous oiling.
The pressure fed "Carrera" idler is far superior to any mechanical idler because it is firm, not hard.
The Carrera tensioner also responds to chain wear (stretch).
A mechanical idler needs constant service to maintain any semblance of proper takeup.
The reason is a roller chain transmitting any kind of load will wear right from the beginning.
High loads, long chain runs, serpentine chain travel, all contribute to chain link bearing wear.
Porsche significantly improved the design with the 964 engine by eliminating the idler sprocket and going with the gently curved moving chain guide.
I'd love to use a design like that on the earlier engines. Better for the chains, less serpentine path and of course quieter.

All of the above is true but your numbers are fine as they are. Why reset when 1.35mm is right in the heart of the spec and if anything a slightly advanced cam (your case) is supposed to be a performance improvement.

-Andy

Understand that slight advanced timing is supped to be OK and a minor improvement, but I rest both sides today. Left side was 1.20 - 1.22mm, right is now 1.18 - 1.20mm. Getting them the "next sprocket over" lead to timing in the 1.3Xmm side on the left that was right near high tolerance.

Got the rest of the rockers in and the cam box covers on. Fuel injection and oil system this week.

The readings you get when timing the motor will vary depending on how tight the tensioner is. In my experience the reading will get more consistent as the chain gets tighter - but only to a point.

I performing multiple rotations and write down the readings. I increase the tension a little, and repeat the multiple rotations/reading until I see that the numbers aren't getting more consistent. That's when I stop increasing the tension.

I try to get the timing the close to equal side to side and get the absolute reading "in the ball park" of where I want it. The above meathod allows me to get consistent repeatable results for a single side. My reasonging (and I could be wrong) is this will produce the same tension on the left chain as the right chain so I'll end up comparing apples to apples.

In a running motor, I think the tension on the chain is constantly changing. Think of a bicycle chain; the tension on the chain is not a function of the tensioner, it's a function of how hard you are pressing on the pedals.


If you spend a Saturday experimenting with the cam timing you can find out all kinds of stuff. For example: If you goob on the cam lube, the timing will change as the cam lube smoothes out into a thin film.

-Chris