I'm assembling the pieces that I'll need to rebuild my 1973 2.0L engine that will retain the stock FI. I'm wondering about cam choices and why I would want to choose a particular one. The only change I've made from stock at this point is that I've got Mahle Euro pistons & cylinders, but they are stock size so there is no increase in displacement.

I hear a lot about the Web Cam 73 and here's the info I've been able to find on how it compares to stock:

Type/Lift/Duration/Duration @ .050
Stock/.392/?/210
73/.426/262/224

Am I correct that Duration is how many degrees both intake and exhaust are open? Or is that called Overlap? If it is called Overlap, does anyone know the specs on the two cams?

What is the difference between Duration and Duration @ .050?

Lift is pretty obvious - the 73 is going to open farther. I assume this means it can deliver more fuel mixture in the same amount of open time compared to the stock lift.

My car is a daily driver that will probably never see anything more exciting than a PCA autocross. So how do I choose a cam and why do I want one or the other?

Staying with Djet?
The Web 73 will work and increase the high rpm hp alittle and lower the bottom end torque.
It will either idle ok or hardly at all depending on the year which I see you have a 73. That year is tough to get to idle right as they don't idle as well as the later cars.
Stock works fine

Geoff

Thanks for the advice Geoff. Just for my own education, can someone point me to a resource for the answers to my questions about what these numbers mean?

Duration is the amount of time that the valves are open. Sometimes we speak of a split duration camshaft. This means the intake valve and the exhaust valve have different amounts of "duration" Typically cams for our stockish engines tend to have the same duration for intake and exhaust. It is widely accepted that our cars benefit from having more exhaust duration than stock (more crankshaft rotation that the valve is open)



B

And that is the problem with using the Djet as the longer duration changes the vacuum signature/VE curve. This ECU is hard wired for a certain engine specs and if you change it much you have some problems. Some have very little problems with the Web 73 others do. Why? It depends on the ECU year near as I can figure out. The 73 is a bastard year where Porsche cobbled together the 2.0L Djet as they could because they couldn't get a new 2.0L ECU from Bosch fast enough. They actually rushed the whole 2.0L engine project to get it to market after the six was dropped. They knew they needed a second more powerful engine for the 914 other than the 80hp 1.7L. The problem with the six was it was just too expensive.

Geoff

http://www.compcams.com/Technical/TimingTutorial/

And overlap is the period in which both intake and exhaust valves are open. Race cams tend to have more overlap (more cylinder scavenging by the incoming intake charge), street cams tend to have less. It's overlap that usually causes difficulty with OEM EFI systems, as this increases the amount of "pulsing" in the intake manifold, which confuses MPI sensors in D-Jet, and air flappers in K and L-Jet. Overlap depends not only on duration, but also the lobe centers (where "peak" of the cam occurs relative to the crank). On a single cam engine, the lobe centers are fixed by the cam. On a twin-cam engine, they can be adjusted independently.

Duration at 0.050, btw, means the number of crank degrees that the valve is 0.050 off its seat, rather than the whole time the valve is off the seat. I've also seen numbers quoted as 0.040 off the seat, which will make a cam appear to have longer duration than the same cam quoted at 0.050. These numbers are usually quoted because when you measure the cam on the engine itself, it moves so little and so slowly at the beginning of the movement that it's often hard to tell exactly when the valve starts moving. It's easy to make an error of 10 crank degrees or more if you try to use 0 instead of 0.050. By the time the valve is open 0.050, it's moving fast enough that you can pin it down to one crank degree pretty easily.

The MPS needs a strong 15hg of vacuum at idle as that is what is hardwired in the ECU. Old worn out engines have weaker vacuum and pulses because the valves leak. The result is a overly rich idle as the MPS reads 15hg is idle and 0hg is WOT so if you are only pulling 10-12 hg at idle you are running richer.
You are right about the pulses too as that confuses the MPS as its needs a steady vacuum not one with pulses. The pulses will flex the diaphram ever so slighty which is not good.
Geoff

You do realize that 0.040" is about 0.1mm, right? That's why you'll often see European (and Asian) cars quoted as having X duration at 0.040" lift.

--DD

Actually, 0.040 is close to 1mm, not 0.1mm. (1mm = 0.039 inches)

I'd like to buy the 0.040 is close to metric theory for non-US cams, except they also all seem to quote lift in inches, too. 0.050 v. 0.040 is just a convention. I'm used to seeing 0.040 on motorcycle cams, for example. My point is that you need to pay attention to that value when comparing cams. I have seen some places quote duration at 1mm, perversely British car places, which are usually selling cams for cars that use inch-dimensioned parts and fasteners (unless they're so old they're using Whitworth).

Can you get a stock cam still?

Yup. Web Cam sells a stock grind for $133 (compared to $158 for the #73).

Thanks to all for the advice on this. The article at CompCams is very good. I had previously looked at How Stuff Works and they have a pretty good article, but not as detailed.

What surprised me was that I expected to see the amount of overlap as part of the specs for a cam, yet no one ever discusses it. Perhaps there is a relationship between overlap and lift/duration so that you don't need to spec overlap, but I was thinking that there might be 2 cams that were identical in both of these but had different overlaps.

Overlap is easily calculated with any "cam card" just look at the opening and closing events and do the math. The difference is "overlap"

This page at Web Cam is typical of the info that's out there. I don't see any events to help with calculating overlap. http://www.webcamshafts.com/volkswagen-auto.html#1