Cam Timing Confusion

neilca · 12-19-2004, 05:06 AM

First let me say that timing cams is not new to me. I have been timing cams on different motors for 20 years. Then I try a Porsche 6. I read Wayne's book on cam timing, machined a Z block, even bought a metric dial indicator.

Normally, I would time a cam using the centerline method. The Porsche procedure intriqued me so I used it. I don't trust factory timing marks so I set up a degree wheel on the motor.

I timed the motor and I did something wrong because both the number 1 cylinder and number 4 cylinder had the intake valves opening at the same time. Knowing that the two cylinders have to have different timing events, I changed the number 4 cylinder to opening the intake on the compression stroke of the number 1 cylinder. This looks like a very easy mistake to make. Am I correct in my procedure? Is this a common problem?

Thanks,
neilca

cgarr · cgarr's Garage

I only did this myself for the first time on my 2.7 I had dots on my cams to make the initial "close" setting then used the dial to fine tune it. I think the cams I had use a .020 to .040 overlap but every cam and engine is different. I got them within 5 thou of each other but then everthing changes if your valves are not set to spec.

Craig

Dan J · 12-19-2004, 07:02 AM

Neilca,
I think you inverted the 4,5,6, cam before you started. You need to start over. With the crank at Z1 both cams should have the keyways on top. This is the basic timing position for Cyl 1 firing position cyl 4 overlap position. You will then have to time the cams to their spec. Are you clear on how to measure the intake lift on overlap?
Dan J

Nitrometano · Nitrometano's Garage

Dan J is correct. That problem happened once to me.

Wayne 962 · Wayne 962's Garage

Someone called me with this problem this week (first time in two years, since the book has been out).

Start with the cams with the two dots facing up. At TDC for cylinder number one, the dot on the left side should be facing up. At TDC for cylinder number four, the dot should be facing downwards. The spot where you measure the overlap for cylinder number one, is very close to TDC for cylinder number four.

-Wayne

Dan J · 12-19-2004, 01:27 PM

Wayne,
TDC for cyl 1 and 4 is the same. With the dots (keyway) up ypu're at firing position on #1 and overlap position on #4. With dots down vice versa. The spot where you measure is TDC
Dan J

Wayne 962 · Wayne 962's Garage

TDC for cyl 1 and 4 is only the same if you're simply thinking of crankshaft orientation. In reality, TDC for cylinder number one is the same as overlapping TDC for cylinder four.

TDC for cylinder one is the point on the 720 degree crankshaft rotation where the plug is fired. TDC for cylinder four is 360 crankshaft degrees off of TDC for cylinder one. This is where you install the rocker and adjust the clearance. The timing point for cylinder number four is approximately 360 + 180 crankshaft degrees or 540 degrees off of TDC for cylinder number one (or about 180 degrees off of TDC for cylinder number four).

Does this clarify things a bit?

-Wayne

Dan J · 12-20-2004, 04:19 PM

Wayne,
I was under the impression that TDC stood for Top Dead Center.
Dan J

Wayne 962 · Wayne 962's Garage

Just to make sure I wasn't going insane, I Googled, the definition of TDC and found it in several places:

Definition: The point at which the piston reaches the top of its travel on the compression stroke. (http://autorepair.about.com/library/glossary/bldef-762.htm)

TDC for #1 and TDC for #4 aren't the same, because one is on the compression stroke, and one is on the exhaust stroke.

-Wayne

neilca · 12-21-2004, 07:04 AM

Well I am glad to see I am not the only one confused over this. My degree wheel has TDC in one place. I set it for when the piston is at the top. The Porsche six is some what unique in that the cams can be set 360 degrees out. Heads up all those timing cams on these motors.

Thanks guys,
neilca

Henry Schmidt · Henry Schmidt's Garage

This is the definition as stated in THE AUTO DICTIONARY by John Edwards. Every cylinder has two TDC. TDC refers only to the location of the piston. When a specific job is being performed, more information is necessary. For example. TDC #1, valves closed. TDC #4 overlap and so on.
When you begin the cam timing process on a Porsche 911 flat six the left cam #1,2,3 cylinder should be on TDC #1 valves closed and the right side cam should be at TDC #4 overlap.

Wayne 962 · Wayne 962's Garage

The Bosch Automotive Handbook refers to TDC on the compression stroke and overlap TDC (or gas-exchange TDC, as they call it) on the 2nd half of the four-stroke cycle. (see page 374).

As such, I follow Bosch's nomenclature (in the Engine Rebuild Book) and refer to:

TDC - means the Top Dead Center on the compression stroke.
Overlap TDC - Top Dead Center on the exhaust stroke.

Henry's right - you set the clearance for rocker number one when the dot on the camshaft is facing upwards. You set the clearance for rocker number four when the dot is facing downwards.

The measurement you take with the dial gauge for number one will be close to where the dot is facing downwards. The measuremend you take with the dial gauge for number four will be close to where the dot is facing upwards.

In response to the original question. The only way that you can have number one and number four open at the same time, is if you have the camshafts off by 180 degrees. In this case, the dots would be opposite of each other (one would be up, one would be down).

On all four-stroke engines, the piston will encounter TDC twice during a cycle (one at TDC-compression, and one at TDC-exhaust). The camshafts spin at 1/2 the speed of the crankshafts, so one rotation of the camshaft = 2 rotations of the crankshaft.

I hope this clarifies things a bit...

-Wayne