Really quick question about cam design 911SC/ Carrera

[theiceman]

I have helped rebuild a couple of engines including i did my own and had the special tools for holding the cam and tightening the nut. These were engines up to SC. The timing is pretty straight forward with a bit of fiddling.
Yesterday i was helping a friend time a Carrera engine which had the bolt configuration to hold the sprocket. We could not figure out how to counter hold the cam with this configuration.
We all know there is a fair bit of variation of timing even with the pin in the right location so having it move while tightening is not ideal . Being able to counter hold the cam made it easier.. we were eventually able to get it very close to spec ( within 2 thou ) but it seemed arduous compared to the previous method.

Is there some technique we missed or some special tool we should have had ?

Thanks

[boyt911sc]

Clive,

Aside for cylinders #1 & #4 rockers, install the rest of the rockers (intake and exhaust). The rockers would produce the frictional resistance on the camshafts’ lobes to prevent them from slipping. Keep us posted.

Tony

[gled49]

New cam tools needed for cam bolt system.

[theiceman]

Tony you are absolutely correct.. we did it with just 1 and 4 in. We did eventually get it very close.
but what a pain in the ass.....

oh on a related question. when i rebuilt my engine. the over lap on my sc is 1.00 mm i think .. when i did my before check it was about 1/2 and about .5mm on both sides.. i didnt think much about it and moved on..
When i was helping Ed yesterday he happened to tell me that his " before" check was approximately half of what it should be.. i thought now thats interesting. Could this be due to wear of components like chain stretch etc ??? my car had on about 130K mikes when i did it.

[theiceman]

Originally Posted by gled49

New cam tools needed for cam bolt system.

Do you have a link to said tools and how they work ?
Eventually we just used an open ended wrench and locked the crank..

[boyt911sc]

Originally Posted by theiceman

Do you have a link to said tools and how they work ?
Eventually we just used an open ended wrench and locked the crank..

Using the correct tools make your work much easier and convenient. These are the common camshafts tools people used:

P-202................Camshaft counter-holding tool (‘78~’81 SC).
P-203................46-mm camshaft nut tool.

P-9191..............Camshaft counter-holding tool (‘82~’83)SC/(‘84~’89)Carrera 3.2.
P-237................Same as P-9191.








Sometimes you have to use what tools are available at hand. But if you are doing this job regularly, using the correct tools make a lot of difference.

Tony

[theiceman]

so Tony if i am understanding this correctly.... the one for the later cars , the holding tool for the later design, those protrutions go in the holes where you choose the correct location for the timing " pin" ?

My Buddy ed is very resourceful and built a tool for the earlier engine. It worked absolutely fantastic.

[winders]

https://www.stomskiracing.com/collections/all/tools

[theiceman]

Thanks Winders.

From looking at the pic in the link you sent it does look indeed like the protrusions go in the cam timing sprocket holes as i thought

[tholyoak]

Yes, the pins engage the sprocket holes.
Many years ago I made one from a pipe fitting and some bolts. Someone on the board did something similar
https://forums.pelicanparts.com/911-engine-rebuilding-forum/741749-homemade-p237-camshaft-securing-bar-tool.html

Worked great in a pinch until I bought the actual tool.

-Todd

[winders]

Originally Posted by theiceman

Thanks Winders.

From looking at the pic in the link you sent it does look indeed like the protrusions go in the cam timing sprocket holes as i thought

You're welcome. William Knight and I used that tool to time the cams when we put heads I had rebuilt on my race engine last year. Easy to use and worked perfectly. Not cheap but worth the price!

[KTL]

For the later style cams with the bolt instead of the big giant nut (and also big nut cam has the snout to grab onto) you can snug/bottom-out the bolt in the cam with the pin in place and the counterholding tool. Bolt need not be super tight. Just snug enough to overcome the resistance to independent rotation, that is keeping cam and the sprocket "connected."

To set your timing, pull the pin and use the snugged bolt to run the cam timing up to where you want it on your dial indicator while you hold the sprocket with the counterholding tool. This is assuming you already have the crank in close vicinity to the TDC overlap point on the crank pulley (this is Z1 +360 degrees). Then hold the cam at your desired/set lift spec and turn the crank forward or backward to TDC overlap on the crank pulley. Then install the pin, remove the bolt and install the big washer with the bolt, tighten it up. Rotate the engine around back to TDC overlap and check your lift setting on the dial indicator to see if it's right where you want it.

Regarding the before check of timing, I wouldn't be surprised if somebody played with the cam timing to advance or retard it to shift the power curve a little bit. But also keep in mind that if somebody is careless when setting the timing, you can lose some lift pretty easy if you're not paying attention to holding things while you snug stuff down. It's easy to lose a tenth or two of a millimeter!

Lastly, the lift amount that you read is also influenced by chain tension. When you see some of the techniques used to crank up a lot of chain tension, like a clamp cranked on the tensioner lever arm, it makes me wonder how close that is to mimicking the tension applied by the actual chain tensioner itself. You can see the effect of chain tension simply by firmly pressing on the chain with your fingers and watch the dial indicator move. The thread below also touches on the effect of tension

Question about cam timing nuances...

[theiceman]

all good info ..

[mikedsilva]

Originally Posted by KTL

Lastly, the lift amount that you read is also influenced by chain tension. When you see some of the techniques used to crank up a lot of chain tension, like a clamp cranked on the tensioner lever arm, it makes me wonder how close that is to mimicking the tension applied by the actual chain tensioner itself. You can see the effect of chain tension simply by firmly pressing on the chain with your fingers and watch the dial indicator move.

Yes, good point.
Using Stomski chain tensioning tools, you can greatly affect the timing... which leads me to ask myself, how tight should I set the tool? how can you ensure that left and right are set equally?
And then, when the engine is running, is the tension the same? I have found that when using the fixed Stomski tool, and then setting the timing, that when installing the oil fed tensioner and rechecking the timing, gives a slightly different result.
You have to ask then, when running, does the oil pressure increase the pressure or not?

I have read here, that the oil pressure is simply to dampen and support the spring tension in the tensioner.. and that it doesn't increase the tension in the chain.

I don't know what is right. And perhaps the differences are too small to perceive... I have set timing using both tensioners and the engines run run and as expected.

[targa72e]

The pressure fed tensioners will see what ever pressure the engine oil pressure is at as the oil line connection has a direct connection to the tensioner bore. The bore of the tensioner is just a little more than .5" so (.25x.25)x 3.14 = .196 Sq inches. If at high RPM and oil is at 80 PSI tensioner pushes with .196 x 80= 15.68lbs (+ unknow tension or internal spring) on the chain idler.

john

[KTL]

To reinforce what John said above, the engine tech document linked below describes how the later 964 timing chain system sees varying tension based on oil pressure. See page 19 (published page 23)

https://drive.google.com/file/d/0B7Bo-8bAu9G9NUY3YUJQcjFJTW8/view?usp=drivesdk&resourcekey=0-wMe-9TYRISmq7Hi3nqVJpw

I know that doesn’t answer mikedsilva’s question regarding what tension level should be used to set the timing. Seems to me we would plan for worst case scenario. Is that low chain tension or high tension?

[Walt Fricke]

John - at least before the 964 style tensioning system, there is a relief valve in the reservoir section of the pressure fed tensioner. If you hunt around you can find where some one here measured the relief pressure. The pressure from the feed oil on the piston in the working cylinder is a whole lot less than the supply oil pressure. One assumes that is because Porsche wanted a rather consistent pressure here, since the oil is just there to dampen chain vibrations while the spring applies a constant pressure. The actual pressure on the chain is also going to vary some based on what the chain is doing due to its harmonics, isn't it? The pressure chamber's controlled leakage out the tip can't reduce things much, can it?

My take here is that the "how much tension to use when setting the timing" is a bit of overthinking in terms of practical effect.

[dannobee]

Originally Posted by targa72e

The pressure fed tensioners will see what ever pressure the engine oil pressure is at as the oil line connection has a direct connection to the tensioner bore. The bore of the tensioner is just a little more than .5" so (.5x.5)x 3.14 = .785 Sq inches. If at high RPM and oil is at 80 PSI tensioner pushes with .785 x 80= 62lbs (+ unknow tension or internal spring) on the chain idler.

john

Check your formula. Area of a circle is radius squared x pi OR diameter squared x .785.

If diameter is 0.5, then either 0.25 x 0.25 x 3.1416 OR 0.5 x 0.5 x 0.785 = 0.196 sq in. 80psi x 0.196 = 15.68 lbs.

[targa72e]

Thanks, Dumb math mistake. Fixed.

john