Alan

Ken - are you saying there is a direct path to the heads also?

Alan

the flyin' scotsman

Alan......I may have confused matters somewhat as the picture I posted was only for the early 87 MY with no-cats.

Your GTS does not have the pipes going into the cylinder heads only the diverter valve, hose to air box and metal pipe to cats as well the incoming hose to diverter from the air pump.

Alan

Actually I'm still not seeing the feed to the heads there on the '87 photo? I previously assumed a no-cat car would not have an air pump...

Ken - I'm actualy looking at a vacuum pump in place of the air pump to evac. the crank via a separator - then ingest the air via a low back pressure path on the output side (TB port or Airbox).

See Colin's Vac Pump Thread

I'd actually likely put this pump after the Provent since the Provent has a vac limit valve on the output port for closed systems that will limit the max crankcase vacuum.

Alan

Hilton

Its the steel pipe with bolt heads along it just beneath the exhaust ports on the heads. On a cat car, those ports are just blocked off, except on 89 where the ports on 4/8 have exhaust temp sensors, and 90+ where the sensors are in 3/7; for the exhaust monitoring system designed to prevent cat fires.

Corresponds to the item 29 manifold in this PET pic, altho the actual manifold pipe isn't straight in between the ports.

Alan

Hilton - OK I guess I was being dense - esp. since you fully described it before (!) - but I just wasn't seeing it.

So on non-cat cars - why the air injection into the exhaust ports - better post combustion, cooling...? I had no idea this existed, seems to be complicated plumbing to implement - does it make much difference?

Alan

Alan

BTW in follow-up to this - do these exhaust air injection ports exist on all engines? are they just capped with bolts & crush washers on (Cat) cars that didn't need injection there? OR are they not drilled/tapped out for cat cars?

Has anyone used these ports for exhaust pressure/EGT monitoring?

Alan

Hilton

They exist on all late 32V engines (87-95) - the 16V cars had a different air-injection setup (not sure about S3 32V heads?)

For 89-95, two of them have EGT sensors (4/8 on MY89 cat-equipped cars, 3/7 after that) for use by the ignition monitoring system. The other ports are capped in those years.

All non-cat S4's (87-89) have the manifold setup discussed above. I've removed this on 2 of my 928's and capped all the ports, although my current '87 I've kept it all and rebuilt the smog pump with new bearings, just for the hell of it

I can go check the thread if you want - I have the banjo bolts which came out of the heads in the garage somewhere.

Rob Edwards

Alan-

DUnno specifically about EGT monitoring but I recall Todd T using these ports to plug 8 WBO2's into one of his engines for monitoring purposes.

https://rennlist.com/forums/928-foru...ds-garage.html




Posterity: A pretty picture of the diverter valve setup.

Alan

Hilton thanks.

Shark2626

I am gutting the cats on an s4 and I’m just wondering if this exhaust gas that comes back to the engine via
the valve shown in the image is filtered in some way.

My concern is that there may be some small debris in the tubes mounted on the cats that I cannot see.

TIA

Alan

Its air injection into the exhaust - not the other way. If you are gutting the cats anyway - I don't see it serves any purpose. It probably doesn't pump much air at this point anyway.

The feed to the intake is for the fresh pumped air (diverted) when it is not being injected - no path back from the cats. I'd remove the air pump too.

Alan

Shark2626

Thank you, Alan.