Porsche 968 Cam Wheel

robert_hooper · 03-17-2015, 11:37 PM

Hi ALL, just after some help, where is the best place to get an adjustable cam wheel for a Porsche 968. Is it the same as the 944 one? Cheers Rob

flash968 · 03-18-2015, 06:03 AM

if you are talking about the tool to dial in a cam, i believe the 944 one works. pretty sure that's what we used on my engine. i didn't ask if it was the same tool though, and just assumed it was.

side note - it would have been all but impossible to do it in the car. not enough room.

robert_hooper · 03-19-2015, 04:24 PM

Hi,

I am after one of these for a 968....

I have custom cams and I want to ensure that we can dial these in correctly,

Cheers

Rob

flash968 · 03-19-2015, 04:38 PM

ah - yeah - i've seen it, but in 11 years i think i've only seen it used twice. might have been broadfoot or lindsey that had it.

bh912e · bh912e's Garage

I believe the stock one has adjustability.....

thomasryan · 03-19-2015, 05:37 PM

Bh is correct but the aftermarket unit may have greater range.

flash968 · 03-20-2015, 06:12 AM

it's not a case of greater range. what a gear or wheel like above will do is give you ability to tweak the belt relative to the cam lobes. while the OEM gear allows you to tweak things one cam relative to the other, it won't let you tweak just the one cam relative to the crank.

Jfrahm · 03-20-2015, 07:25 AM

I think to time the cams one-to-the-other you need adjustable cam gears (for the chain between the cams) or a belt conversion for the intake cam and two adjustable belt sprockets. Otherwise you only time the intake cam with the variocam fully erect and the exhaust cam timing depends on that. It's sort of backwards as the intake cam is a slave to the exhaust as it is driven off the exhaust cam, but the intake cam spec is what you shoot for.

As to what to do with custom cams, I'm not sure. You could superimpose the new profile on the old and try to adapt the factory procedure, with a different lift spec. If the new intake cam has more lift and duration it'd be pretty off if you timed it with the factory spec (calling for a particular amount of lift at exhaust TDC).

I would not expect an aftermarket cam sprocket to be needed, hopefully the adjustment range of the stocker is sufficient. If adjustment of the cams relative to one another is desired it would be interesting to consider machining and shimming the variocam unit to shift chain slack up or down.

-Joel.

flash968 · 03-20-2015, 07:41 AM

that's why the one gear is slotted. you adjust one cam to the other with that, and then the hydraulic variocam valve fills in the rest, and takes up the slack in the chain.

i agree though about wondering if it is necessary. with the adjustability of the existing setup, and the ability to adjust timing advance in the ECU, i wonder how much good could be done versus bad. sure, by tweaking the overlap you may get something, but the exhaust system is then going to be all that more critical. as it is, we've seen huge losses when you get it wrong, and not really any gains with anything. could be a very time consuming and expensive experiment, for very little potential gain, if you don't already have a template to follow.

Jfrahm · 03-20-2015, 07:59 AM

I'm sorry if only the exhaust cam is driven and has a slotted belt sprocket slotted how do you change the relative timing between cams?

flash968 · 03-20-2015, 08:56 AM

by changing the effective length of the chain. by pushing the valve out, you effectively shorten the distance between cams. this is exactly why the dial indicators are so important, and why the variorum has to be pressurized to set the cam timing.

the slots we already have allow altering the position of one cam relative to the other, but not the cam relative to the crank, as the keyway is locked in position relative to the lobes, and the driven gear is a single piece. by having the ability to rotate one cam, relative to the lobes, or in this case, the chain gear, you can alter where one cam is relative to the crank. it's similar to just installing an offset woodruff key, like we used to do in the mobs

Jfrahm · 03-20-2015, 12:21 PM

I understand how the variocam adjusts the intake cam timing, I thought you were implying that there was a phasing adjustment as well. 968 Variocam just moves between two fixed points so if you want to advance or retard the exhaust cam without taking the intake cam along for the ride you have to go beyond the stock setup.

When you say "you adjust one cam to the other with that" you imply there is a relative adjustment between the cams when they are actually fixed relative to one another. You adjust the intake cam relative to the crank with the slotted sprocket.

With custom cams there could be some left on the table due to a lack of ability to degree in the exhaust cam, but the intake is likely more sensitive so the first goal would be to dial in the intake cam and then testing advance and retard of the whole system. Personally I would mark up the stock sprocket and see if you can be happy within the limits of that adjustment. The lack of available adjustable sprockets is somewhat telling.

flash968 · 03-20-2015, 01:54 PM

that would be the case of the variocam valve pushed evenly on both sides. it does not, and in fact only moves the intake cam.

Jfrahm · 03-20-2015, 02:31 PM

You are confusing variocam operation with retiming/degreeing cams for performance. The variocam does not, as delivered, allow you to play around with the cam timing. It just shifts chain slack a fixed amount. The bean shaped holes in the belt sprocket allow you to shift the exhaust cam timing but the intake cam shifts the same amount due to the fixed sprocket and chain arrangement between them.

If the variocam unit acted up and down equally it'd just add tension to the cam and not shift timing, so naturally it only presses on the upper run of chain. Yes this changes the intake cam timing relative to the exhaust however it's not adjustable, it slams between two fixed states. If you wanted to try advancing or retarding just the exhaust cam 4 degrees you have no facility for that, stock. Probably no big deal with stock cams and maybe no big deal with performance cams as the exhaust cam timing is not as likely to change your life. However it is a reasonable thing to consider with aftermarket cams where you might want to dial in some midrange.

flash968 · 03-20-2015, 03:54 PM

the nominal position, or at rest position of the valve is determined by the rotational position of the exhaust cam. that establishes the chain position at the top and bottom pads. turn it one way, and the slack on the bottom gains, and the piston assembly lowers, because the vertical distance between the chain paths is maintained by the spring. turn it the other, and the slack on the top gains and the piston assembly goes up. then, when you activate the variocam, it moves at the top only.