930/16 Engine cam timing.........

[boyt911sc]

I'm doing a cam timing set-up for an '81SC stock engine (930/16) and do have a few questions. From the Bentley SC service manual the over-lap tolerance for this particular type of engine range from 1.4 - 1.7 mm or mean of 1.55 mm. At the moment I have these numbers:

Cam (1-2-3)............1.40 mm (low end)
Cam (4-5-6)............1.55 mm (median)

Considering the the Bentley manuals have numerous 'typos', what is the correct or recommended valve overlap tolerance (cam timing)? Should I shoot for 1.40 or 1.55? Do I have the correct range? Any advise or suggestion highly appreciated. Thanks.

Tony

[proporsche]

Hi Tony
Factory camshaft timming for 911 SC 1981 ---930/16 is 1,4-1,7..........
You can go as far 1,6 it will give a nice torgue...
proporsche

[boyt911sc]

Originally Posted by proporsche

Hi Tony
Factory camshaft timming for 911 SC 1981 ---930/16 is 1,4-1,7..........
You can go as far 1,6 it will give a nice torgue...
proporsche

proporsche,

Could you share some thoughts about the difference in the engine performance if the cam is set at 1.4 (low) vs. 1.7 (high)? In summary, what would be an engine set at 1.4 mm compared to another engine set at 1.7 mm for engine characteristics (torque) at different engine speed. Thanks.

Tony

[Flat6pac]

The 1.7 gives the torque and power response at lower speeds, the US stop light to stop light. The 1.4 will give the power and torque line at the higher speed such as cruising the autoBahn.
Look at the different settings from the 76, 3.0 through the 89 Carrera. All the cams are set at different positions but all the cams are the same.
Bruce

[boyt911sc]

Originally Posted by Flat6pac

The 1.7 gives the torque and power response at lower speeds, the US stop light to stop light. The 1.4 will give the power and torque line at the higher speed such as cruising the autoBahn.
Look at the different settings from the 76, 3.0 through the 89 Carrera. All the cams are set at different positions but all the cams are the same.
Bruce

Bruce,

So if I understand it correctly, a track car is better set @ 1.4 than @1.7? And a daily car @ 1.7? The reason I'm asking is that I have two (2) late SC engines I'm about to set the cam timing. One is a daily car and the other is a dedicated track car. What would you suggest? Thanks.

Tony

[CaptainCalf]

Tony, I think the main thing to do here is to get the two cams timed as closely to the same mark as possible wheither it's 1.4 or 1.7mm. I think Bently's says they should be within +/- 6 thou".

My Super Cup cams are timed at 2.0mm, so based on what you guys are discussing it should be a torque monster!

[boyt911sc]

I spent a few hours last night and about half a day today to get these numbers and still not closer to resolving my predicament. Here are the numbers:

'81SC (903/16) engine: Spec. 1.40 - 1.7 mm (valve tolerance per Bentley manual)

Left cam: Multiple readings and took average.
Adjustment #1............1.65 mm.
Adjustment #2............1.37 mm. (pin moved 1 hole CCW).
Adjustment #3............1.65 mm. (pin moved back to original position).
Adjustment #4............1.36 mm. (pin moved 1 hole CCW).
Adjustment #5............1.65 mm. (original location)

Right cam: Multiple readings and took average.
Adjustment (A).............1.55 mm. (same as the value for the median-spec.)
Adjustment (B).............1.24 mm. (too low)

While I could get the right cam @ 1.55 mm, I'm not having any luck getting the reading close to the median for the left cam. Adjustment #4 @ 1.36 mm is below the spec. while adjustment #5 @ 1.65 mm is very close to the end of the upper range number. The best set-up I could obtain is a 1.65 mm (left) and 1.55 mm (right). Are these acceptable settings (numbers)?

I would appreciate any suggestion or recommendation so I could proceed with the project. Thanks.

Tony

[CaptainCalf]

Tony, if I understand you right you've got them within 4 thou", so you're within the Bentley's spec of +/- 6 thou" you are done my friend!

[CaptainCalf]

here's where I found specs in my Bentley's on page 152-8.

[boyt911sc]

Originally Posted by CaptainCalf

here's where I found specs in my Bentley's on page 152-8.

Captain,

Thanks for the effort. BTW, those spec you provided is for 3.2 Carrera engines. I'm working on 3.0 liter (930/16) type engine for '81SC. Right now, I got the cam timing under control. Both left and right sides have very close measurements. I will post the numbers later.

Tony

[CaptainCalf]

LOL, yep the Bentley Manual is for a 3.2L and as far as I know they never printed a 930 manual prolly cuz the engines are basically the same for 3.0 & 3.3L 930s and the 3.2 Carreras. The displacement is different and the cams profiles are all different, but I've found that about 80% of the 3.2 Bentley manual applies directly to my 3.3L 930

IMHO the 3.2 tolerance spec 0.3mm given in Bentley is a good road map for anybody timing cams of any profile on our motors.

Based on the tolerance of 0.3mm/25.4 = 0.012" or a 12 thou" range your results of 1.65 & 1.55mm or 0.1mm = 0.004 thou" apart is within this +/- 6 thou" tolerance, so that's why I said you're done!

Here's a pic of mine and I used a 2 dial gauge setup which I found makes it a little easier to dial them in at the same time...mine are Super Cup high lift cams set at 2.0mm overlap and I was able to get them within 0.09mm of each other. Like I said in the PM you'll have a hard time getting them much closer with the stock cam sprocket setup. However, there are adjustable race cam sprockets available that are more adjustable so you can set them both exactly the same, but that may be a little overkill for a street build

[JFairman]

1.25mm intake valve lift spec at #1 and #4 TDC intake and exhaust valve overlap is for 964 cams.

I thought 3.2 Carrera cams are the same profile as SC cams and 1.25mm would be timing those too late but I may be wrong and shouldn't say anything.

[CaptainCalf]

Originally Posted by JFairman

1.25mm intake valve lift spec at #1 and #4 TDC intake and exhaust valve overlap is for 964 cams.

I thought 3.2 Carrera cams are the same profile as SC cams and 1.25mm would be timing those too late but I may be wrong and shouldn't say anything.

I just looked it up on John's site and the 964 cams are set at 1.26mm at overlap while the SC & Carrera are 1.4-1.7mm. Not the 1st time Bentley was wrong

[JFairman]

1.25mm and 1.26mm is so close you'd never feel the difference and if you put a motor together with new chains and sprockets, timed the cams at 1.26mm the small increase in timing chain length as the new chains wear in will probably retard the cam timing to 1.25mm or less.

When I timed my cams it wasn't easy to get them the same and I have one dial gauge and Z block so I had to switch it back and forth between 1 and 4.
Anyway, there was enough play or clearance slop around the little pin that you could move the cam sprocket fore and aft a tiny bit changing the cam timimg a little before torqeing the cam bolt down locking it in place.

I tightened that bolt with the pin in slightly different positions in the same hole a bunch of times, then turned the crank over a couple times and remeasured the 1 and 4 intake valve lift at TDC before the intake stroke until I got it as close as I could.

I'm enjoying this thread and your progress with pictures, good luck with everything

[bullethole]

On the topic of timing cams. Where do you get a dial indicator with a long shaft. I have one of the extenders, but I've never liked the way it fit on my digital indicator and would rather get one with a single shaft.

[CaptainCalf]

Originally Posted by bullethole

On the topic of timing cams. Where do you get a dial indicator with a long shaft. I have one of the extenders, but I've never liked the way it fit on my digital indicator and would rather get one with a single shaft.

I bought mine offa Fleybay!

[totle]

Stick with 1.65 on 1-2-3.
You need to loosen cam sprocket on 4-5-6 and turn it and insert again, and then dial in again to get to 1,65.
You will need to try several times to make it correct.
There is many nice threads at technical forum that describes SC cam timing

[boyt911sc]

Not all 3.0 liter SC's use same cam timing set-up. Based from my limited experience in engine rebuilding, the '78-'79 (USA) with 930/04 type engine use 1.0 mm valve over lap value while the later SC's with 930/07 and 930/16 engines use larger valve over lap value of 1.55 mm. See Bentley SC manual page 152-10. I have no experience to contradict these numbers.

While I struggled to get a very close readings for the left and right sides by moving the pin left or right, they were not good enough to fine tune the measurement. I never realized until Flat6PAC (Bruce) come to my rescue. He educated me about his method in fine tuning the measurement. And took me just under an hour to dial these numbers and was totally happy. My earlier attempts took me more than 6 hours and abandoned those settings.

Cam timing numbers (yesterday):
Left side = 1.65 mm
Right side = 1.55 mm
(these were the best combination I obtained by moving the pin left or right).

Cam timing (Bruce's technique):
Left side =1.45/1.46/1.44 (ave. =1.45 mm)
Right side =1.45/1.44/1.45 (ave. =1.45 mm)
(Took me an hour work to dial these numbers).

And three (3) hours later, I got the engine running after a second crank. The sound of a smooth running engine after an engine rebuild is like some sweet music to your ears. BTW, a member did his top end rebuild for this engine and was hesitant to do the cam timing. I offered to do the cam timing for a fellow enthusiast. But it needed cam sprocket alignment, electrical wiring, and other minor things. So I offered to do the tune-up and start before the pick-up.

Tony

[boyt911sc]

Wished the owner spent more time cleaning the engine during the rebuild. Anyway, the owner who I never met before until he showed up at my place should be excited to get his motor back into in his car. It is running now and all it needs is a chassis for home. Plug and drive........



Tony