20/21 cam emissions and interference

[mike sampsel]

Does going from stock cam to 20/21 type (webcam) make meeting emissions more challenging? Suspect it might, and would be a small factor in these versus finding good stock ones. Still going with CIS.

Also, I’ll assume properly timed 20/21 cams “eliminates” chances for valve to piston interference? I’ll keep my rpm limited. I’ll be using JE pistons with 9.5 compression (98 mm short stroke).

[KTL]

You shouldn't have any problems with piston-to-valve clearance because the JE pistons have sizeable valve reliefs cut in them. But the cams are designed to work with original Porsche pistons and even those don't have issues with valve clearance.

I don't think you'll have emissions issues because the cam is certainly a bit more aggressive in terms of duration and lift (especially more peak lift on exhaust) but the lobe center is a still a "smog cam" in the 112-113 range.

[stownsen914]

I’m sure KTL is correct, but most builders will tell you to check critical clearances anyway to be safe especially for a non-stock build.

[mike sampsel]

Thanks guys, I’ll likely check on clearance on the valves to pistons. But if I get rushed for time I might not be totally meticulous when checking clearance.

[VFR750]

Do check the clearance.

I had too little and needed the valve pockets deepened. $90 at local machine shop. So it’s a pretty easy thing to fix.

[mike sampsel]

Originally Posted by VFR750

Do check the clearance.

I had too little and needed the valve pockets deepened. $90 at local machine shop. So it’s a pretty easy thing to fix.

Thanks, were your pistons from JE? They have some deep pockets already

[VFR750]

Yes they are JE pistons.

I installed a DC40-102 cam and it was just a little too much. So it was a good thing I checked.

And pretty easy to fix.

Shop did it for $90 and the piston weights were reduced an equal amount. This proved the machinist knew what he was doing.

No issues ever since.

[mike sampsel]

Yes good you checked, what is the timing intake valve overlap for these DC40-102 cams?
The 20/21 cam is 2 mm. 1 mm more than stock. And the JE pistons valve recess is deeper than my Mahle piston.

[VFR750]

4.5mm.

I also had a DC19/20 at 2.4mm with no problems.

So the likelyhood of you having a problem is low