Cam timing different on 1-3 & 4-6

[Mark McClure]

Hi,

I have just been doing a check on the lalve lash after a recent rebuild and decided to check the Cam timing. It appears that 1-3 is differenet from 4-6.

Now I have checked this without removing the chain covers and so without bracing the tensioners!

The cam is a webcam with a timing card of 5.2 to 4.8 mm.

I have
5.1 on 1-3 and
4.9 on 4-6.

Now these are both within spec but different to each other.........!

What is the effects of this? is it an issue ? do I need to fix it?

Thanks

Mark...........

[2.7RACER]

Mark,
When timing cams, you have a sprocket with 17 holes and a hub with 16 dwells. This gives you 272 possible cam timing positions, unless you jump the timing chains.
360 degrees divided by 272 equals 1.323 degrees per pin position. This is camshaft degrees, times 2 for crankshaft degrees is 2.646 degrees.
Where I'm going with all this?
5.1 on the left and 4.9 on the right may have been as close as possible by moving the locating pin into the various combinations of holes and dwells.
The difference between 4.9 and 5.1mm is less than 2 crankshaft degrees.
The question becomes, what were the cams set at rebuild?
If the numbers you see are different than the initial settings, you may want to look into what changed.
If these are indeed near where the cams were set, I wouldn't lose any sleep over it.
To get the right and left closer, you would jump the timing chain on one side 5 teeth at the cam sprocket then retime.
Some Porsche tech's don't understand how this works, so they accept the difference. It is within spec's, but it isn't as close as can be.
There are actually over 7,000 possible cam positions. Within one tenth of a crankshaft degree. Done by jumping the timing chain from tooth to tooth, then retiming.
All this for the anal amongst us.

[SP2]

Doug,

That is very interesting about the teeth and pin hole number differences. Thanks.

Isn't there a significant amount of play even when the pin is installed? I say that because when I timed my cams 3 weeks ago I noticed the consequences if I did not hold the camshaft steady with the special tool while tightening the large nut with the crowsfoot. I also realized that there is a certain point while tightening the large nut that the cam will not move anymore as I torque to spec.

[Mark McClure]

Doug,

Thanks for your reply on this. I now appreciate the amount of adjustment possible. I should have tried harder initially.

It is possible that the cams where timed to this initially. I used a degree wheel to set the timing and got the cams within 1.5 degrees of each other. I thought that this would have been fine. Unfortunately I did not record the overlap amount in the rebuild book.

OK so instead of tearing into things I will leave alone.

One of the reasons for looking closer at this after the rebuild was that the Dyno results are not as expected.........200rwhp @ 7,200.

10.5:1 comp 3.0
EBS valve springs
Webcam 512 camshaft
twin plug
Individual throttle bodies
ARP rod bolts
SSI
Monty
EMS Stinger ECU

I expected about 250rwhp.

Cheers

Mark.....

[2.7RACER]

Hi Mark,
'78 & '79 3.0's have larger intake ports than the later '80 thru '83 engines. 39mm for the early engines, 34mm for the later engines.
Could this be where some of your expected HP is bottled up?
I checked the Web Cam site looking for 512 cams.
I didn't find a 512, could they be 501/502? Plenty of cam if this is it.
The throttle bodies need to be 36mm to get the HP you are looking for.
I suppose you checked the A/F ratio while on the dyno.
I'm no expert, most of this I learned on this site. And trying to squeeze power out of my 2.7 at 5,000 ft.

[Eagledriver]

I wouldn't worry about that small of a timing difference. If anything it would "broaden" the torque curve slightly.

-Andy

[cstreit]

250 rwhp is pretty optimistic for that engine build. In similar engines I have built, with early SC port heads, I was getting around 220 rwhp at 7200 RPM. (twin plug, 10.5:1, pump gas).

240-260 Crank HP is a good target for a mild race engine such as this... You're at 235 so that's not too far off. You might try setting the cams as close as possible, retard them slightly if you want a higher peak number, and take a look at your dizzy curve and mixture.

I changed jets on my Webers the week before NASA Nationals last year and didn't have time to dyno it before we got there... I LOST 24 HP due to an overly rich mixture on my 3.3L NA engine.

[Walt Fricke]

Doug - That is really interesting. It had never occurred to me that by chain tooth skipping one could add a second vernier-like effect to the pin system. Makes sense, now that you mention it.

But one reason it wouldn't have occurred to me (the other being lack of technical imagination) is that this extra trouble will do no good practical good at all, I fear. As James notes, there is enough slop at a given pin setting that you can adjust the timing enough to get what you want anyway. I don't think I have ever run into a situation where, between an adjacent pair of holes, I couldn't get the setting I wanted. But should that happen I now know a way of dealing with it. 7,000 different settings? Of course that is tricky to translate into valve lift differences, given differing cam profiles, no?

If Mark relied only on the pin settings, he pretty much guaranteed that he would have this much side to side difference. I don't have any metric dial indicators (inch ones are dirt cheap, relatively speaking, but somehow not ones with fractions of MMs), so I set my cams in thousandths. I can usually get to within about two or three thou on either side of where I think I want to be on the first cam, and get the second to within about the same of wherever the first ends up. Kind of within the parallax or judgment error reading the timing mark against the case centerline (or the patience of the mechanic).

I thought there was a side to side tolerance as well as the overall setting range tolerance, but since I can't find it I must have been hallucinating. Makes sense to try to keep them as close to the same as you can, I'd think. Like keeping compression balanced and so on for smoothest running. But at some point small differences can't mean anything measurable in engine performance.

Walt Fricke

[2.7RACER]

Walt,
Mike at MBEngineering first brought this to my attention last July when he responded to a question regarding cam timing.
It seems the poster could set his timing just below or above the recommended spec's.
Mike responded just turn the cam sprocket 90 degrees.
His answer so intrigued me I spent the next several hours working out the math. What a surprise.
It goes back to the 27 tooth Cam sprocket driven by a 24 tooth sprocket at the intermediate shaft.
Further the 48 tooth intermediate gear is driven by a 27 tooth gear on the crankshaft.
As you know driving a even toothed gear with a odd toothed gear or vice versa is good engineering practice.
Same story with sprockets.
It's one of those things found in the details of a Porsche that continues to impress.
Might not be a big deal with a car engine, however that kind of attention to detail for the entire system make a superior car we know as Porsche.
Again thanks to Mike at MBEngineering for the info that led me on the path to understanding.
I'm still curious about whether the different SC port sizes are playing a role in the search for HP.

[2.7RACER]

As a follow up to setting cam timing, my feeling is to set cams a bit advanced inside the spec's as timing chains begin wearing from the start. This concept allows the cams to remain in the "sweet spot" for a longer period of time, rather than set them for the optimum and then see them retarded somewhat from the original setting several thousand miles later.
This is a technique I used for many years when timing chain and sprocket driven production machines. It gave longer intervals between service.
Now for a race car, it's one of the things learned on a dyno and optimized.

[cstreit]

Hmm, for some reason I thought he had said early port heads...

Mark, what year are those heads?

[ChrisBennet]

Checking the timing without real tension on the chain probably isn't too accurate.

you can get the cams timed side to side as close as you like (within the error of repeatability). You treat the sprock holes as short slots i.e. you can use the "slop" to get whatever position you want and then tighten the bolt or nut at the end of the cam to hold it in that position.
-Chris