Cam timing - what's "close enough"?

[bkdamkr]

I've just timed the cams in my 73 911S (2.4L MFI) and would like a guru's opinion on what is "close enough".

The spec for the intake lift (during the timing procedure) is 5.0 to 5.4mm, with 5.2 being optimum and matching the two cams being more important than the actual number. I have one timed to 5.25mm and the other to 5.15mm.

I'm reluctant to mess with the timing any more because (a) my measurement & crank mark alignment procedures really don't seem to be repeatable enough to distinguish between these values with any sort of statistical significance, and (b) adjusting the timing is a real pain, and I'm not sure that I could do better, even if my measurements were good enough to distinguish the changes. The lift values change a LOT with even tiny movements of the cam (or the crank, when connected).

So - my thought is to pronounce this "good enough" and move on. However, if this is too much difference, then I'll have to try to do better, with both the timing and the measurement. Not a pleasant prospect, but I want to do it right.

Comments?

Thanks,
Brian

[sayah]

If the engine is out of the car and apart, I would attempt to get both sides as close as possible: your effort with a motor on a stand will be nothing compaired to the hassel in correcting the timing with the motor in the car.

Ask me how I know.

[john walker's workshop]

you should be able to get closer than a 10th. you know, you can rock the cam and sprocket with the pin in the same hole and get more movement than that. in other words, loosen the nut and turn the cam one way or the other until you get closer, then tighten the nut and do the reading again.

[Henry Schmidt]

BrianYou would not be able to see the difference on a dyno. You are plenty close enough. If you want, play with it some and you'll find you can get it closer and the more you do it the earlier it gets.
In our early racing days, we timed a set of cams at the track by eye, (no dial indicator) put the car back on the track and noticed no performance difference. When we got back to the shop we checked the cam timing and found we were .45 of from one cam to the next. Don't get me wrong, cam timing as close as possible is our goal but try not to get carried away with the concept of perfect.
Perfect doesn't exist.

[garibaldi]

Do yourslef a favor, bolt a degree wheel on the crank and use that alaong with your indicator.

You can get the timing nearly perfect on a 911- the problem is that once the damn thing is running, its moving around and changing. But its nice to know that you did all that you can.

[bkdamkr]

Thanks for all the advice. I checked and re-checked again this morning. One side consistently reads between 5.26 and 5.32mm lift at overlap, and the other between 5.21 and 5.27mm. I really don't think I can get it any closer, unless by accident. I'm just not that good at it.
Brian

[Porschephd]

This is a well debated subject and with all due respect I disagree it is not that important. I too have been doing this for a while. Cam timining difference like this will change AFRs from side to side. Up to a point. Now on some motors that may not make a difference, but on some of the ones we build one point is a difference between lasting or putting a hole in it. It also makes for a smooth idle and better throttle response. Same theory and the reason we sync webers. You want both sides to draw the same. The owners may not notice the difference but I sure do and take cam timing to .01 if I can. Porsche felt by the 993 that it wasn't a big deal and used simple lock in tools. We have had to correct more timing errors and strange idling problems then I would like to admit. The common issue with a 993 idling poor is an uneven cam timing. This and a 1.25 will buy a cup of coffee and that's about it.

[hobieboy]

On a related note - when I timed my cams, they were dead on before I start the engine. Checked that at least 6 times, before & after installing the engine to the car.

After 500 miles, I checked the valve & cam timing again. The valves were all properly adjusted but the cam timing on both sides changed. And we talking going from 1.52 to 1.4x. I think that might be because of chain stretch/slack? To redo timing with engine in the car is really a pain, but should I?

Thanks...

[Tinker]

Patrick,

If you are a perfectionist, knock yourself out. The Porsche spec book gives a range and I believe yours are close.

Remember the 930 has some of the mildest cams ever used in 911 series. As Henry mentioned and I have heard, some dealer mechanics use to time those mid 70's CIS motors by "eyeing it". They also had pretty mild cams.

Is each bank similar in overlap? If it were my motor, I would drive it a few thousand miles and check it again during a valve adjustment.

Tinker

[john walker's workshop]

dealer mechanics do a lot of things by eyeball.

[TimT]

Your 0.05 out of tolerance (edit: out of "ideal") on each bank, thanks pretty damn close!!

One trick I llearned is to set the cam to spec, then tighten the cam nut and quickly measure how much your setting changed.

Then you can "correct" for the effect of the torque on the cam timing..

kind of anal but it allowed me to get duplicate the reading bank to bank...

then again this all goes out the window when you fire the engine up LOL

[hobieboy]

Thanks for the reassurance folks... makes me feel more secure now (since I haven't redone the timing since).

thanks...

[bkdamkr]

I started this thread, so I'll close the loop on my saga. I've learned a lot along the way, and got a lot of practice at timing my cams.

One thing I learned is that the delta in "lift @ overlap" associated with moving the pin one hole is about 0.3mm - on my '73S, that is.

The next thing I learned is that there is about 0.4mm of play in the lift I can get w/o moving the pin.

The next thing I learned is that it's damn near impossible to nail any particular spot in that range - other than the endpoint you get by tightening the big cam nut - if you're doing it by yourself. And only marginally easier if you've got a second pair of hands.

I also learned that my engine either has noticeable carbone build-up (likely) or has had machining work done (not very likely, but possible), because my piston-to-valve clearances are VERY tight. At the "retarded" (lift @ overlap ~5.1mm) end of my achievable timing range, I have about 1.5-2mm of intake clearance and only about 0.75mm of clearance on the exhaust side. At the other end, the "advanced" end (lift @ overlap ~5.4mm), I still have about 1-1.25mm lift on the intake side and about 1.5mm on the exhaust side. I opted for the latter, because the minimum clearance point is when the exhaust valve is closing, hence more prone to float. Obviously, I could get even more exhaust clearance by further advancing the cam timing, but that would reduce intake clearance further, and the next repeatably achievable step in timing would give me a lift @ overlap value of ~5.7mm - pretty far out of the 5.0-5.4 range.

Oh - as for "close enough", my two cams are about 0.05mm apart in the lift @ overlap value.

Cheers and thanks for all the advice.
Brian

[Wayne 962]

As the engine is started and run under temperature, other factors will cause the timing to change from side to side. Porsche indicated a range for a reason - within that range is within tolerance. I tell people to try to get it as close as possible, but not to worry about it too much if it's well within the range specified by Porsche...

-Wayne