Opelotus

In the process of making my own PCV system modification along the lines of what 928 Motorsports sells. First things I notice about the setup on the '84 - oil filler/crankcase vent cap has a very short neck on it (tubular extension which fits into the oil separator/strainer insert). It also has a very small orifice in it, maybe ~1/8" or less? Pulling the cap off with the engine running does not cause stumbling, etc. Compared this cap to the one I took of an M28/03 (78 or 79 motor) - older cap has no restriction/small orifice - it is a straight 3/4" hole all the way thru. The neck is also longer than the '84s, which would position the filler cap's PCV hose farther from the hole in the top of the crank vent casting, hopefully reducing oil in the intake. What does the '78/'79 do differently downstream compared to the '84? There must be some sort of flow control somewhere.

Also, it is curious that I cannot match up the part #'s of either cap with anything in the 928 parts catalog, Pelican Parts list, etc. (I don't have PET).
'84 part # = 928 107 710 03
'78/'79 part # = 928 107 710 01

I was hoping to swap the oil filler caps to see if oil consumption/oil in intake would be affected, but without the orifice in the older cap, I am changing too many variables at once. Suggestions?

Opelotus

bump

danglerb

Maybe a picture?

I'm thinking about finding a way to use the oil temp gauge cap some Harley's use.

pjg

From Pet 7.1

Top diagram is the 84, bottom is 79.

Opelotus

Yes, those would be the correct parts - but how come the part #s differ?
It looks like the 79 oil cap is routed to the airbox, with another small hose coming off it?

Optimator

The 78-79 cap is routed to the air box. The smaller tube that you see is actually a coolant line that is routed inside a length of the breather hose to act as a sort of heater. Seriously weird Rube Goldberg stuff! Starting in 80, this setup was dispensed with, and the cap vent was routed into the intake manifolding, immediately upstream from the throttle.

Can you please post a photo of the 1/8" orifice that you found on the 84 cap? My car blows way too much oil through that vent cap, even though the engine does not seem to have a serious blowby problem. I wonder if a restricter was added early on to address such a problem.

Opelotus

Sure, I will take a picture tomorrow evening. Are you thinking of swapping in a later cap? You could try adding a baffle/separation system as well. That is what I am in the process of doing, deciding which cap will work better.

So it sounds like the 78-79 cars didn't need a restriction in the PCV system because it was routed directly to the airbox, and therefore not exposed to full manifold vacuum. I am sure if I put the older cap on the '84 I would have a major vacuum leak. Wonder why the later cars still used such a large diameter PCV hose if the cap orifice was so small.

Optimator

Yes, I'm considering swapping to the later cap. I already have a slotted baffle plate that goes between the block and filler housing, but it doesn't seem to help.

On 80-83 CIS systems, the breather hose connects to the air guide housing just before the throttle body, so it is not exposed to manifold vacuum. I suspect the 80-84 L systems are the same. If I understand correctly the restrictor you are describing, I have no idea why they kept the large hose diameter, unless the goal was to maintain backward compatibility with the older cars.

Opelotus

Ok, here is a pic comparing the two caps - left is '78/'79, right is '84. The orifice in the '84 cap is actually 1/4", the '78/'79 has no orifice so it is 5/8" straight through. '84 cap's neck is also half the height of the other cap. Would the longer neck work better? Or does positioning the neck opening closer to the oil puddle ontop of the strainer make it more likely for the engine to inhale oil?