930 Turbo basic operation of components request.

[jwmarc]

To any and all.....

I have a 930 turbo and would ask the assistance of the board to provide me with a little tutorial on the operational elements. I have read a book ( read not studied) about turbos.. Corky Bells Maximum Boost.

So here goes.. As air enters the air cleaner housing and moves the CIS air/volume plate ( feedback to the FD head) it passes out the bottom and into the ducking which attachs to the BOV plenum housing. This plenum is massive and I removed it recently to replace the plastic O rings which muffle the slamming of the internal cylinder piston part. The air flow continues to the air inlet side of the turbo and passes ithrough the compressive outlet which then passes into the intercooler and then exits into the throttle body/ intake/cylinder manifold. When not under boost does the air bypass the turbo/ through the BOV plenum which feeds directly into the throttle body/intake manifold ? when boost spools up the air takes the longer route through the turbo/intercooler route.... The BOV operates when I punch the throttle I can hear the ram hitting the plastic O rings. At what point does the BOV operate when running and under boost? Can someone describe the Porsche BOV implementation?

I have heard/read that you can do away with this massive plenum and BOV by installing something like a KoKeln intercooler. Does the after market approach detract from the legacy engineering...Why did Porsche make such a huge assembly for this arrangement? Was it the state of the art in 1987?

The boost pressure switch that connects to the yellow over boost relay switch and cuts out the fuel pumps. At what pressure level - what PSI ? ( it sits in the intake manifold or BOV plenum..I cant remember). Could someone explain the operation...... At this point I would like to acknowledge Mr. John Walker who responded to a post about fuel cut-off and directed me to the overboost relay as a culprit... sure enough under a microscope there where cracks and fissures in the solder welds which I re-soldered and havent had any more random fuel cut-outs and side of raod events...Mr. Walker thank you again.

AFR is a function of the fuel metering spec associated with the FD head and air volume plate. The frequency valve dumps more fuel based on lambda feedback. What are the contolling conditions of this? I am comfortable with the CIS operation basics but had a question about the fuel enrichment relay... is this the power source to the frequncy valve? is it the control for the cold start injector? Does the CIS controller have anything specific to Turbo based CIS Porsches?

I know I have thrown allot out ..I will welcome responses., and will appreciate your advise....

[jwmarc]

Ok I answered one of my questions the overboost sender unit is in the BOV plenum.

[RarlyL8]

I thought someone would chime in by now. I will try to answer some of your questions.
The BOV bypasses the throttle during closed throttle events. This prevents the rotating turbo from pounding the system with pressure pulses while winding down.
The C2T uses a long neck intercooler and a stand-alone BOV. The large assembly of the 930 is eliminated. You can reproduce this with your engine. Porsche needed to sell as many 930 engines as possible to maximize R&D costs, one reason they stayed the same for so many years.
The boost pressure switch cuts off the pumps at ~1.1Bar. That is why it is marginal to run a 1.0Bar spring with a stock switch. Any leaks and the switch becomes over sensitive due to a faulse signal.
I'm not familiar with the fuel enrichment relay. The WUR governs the mixture beyond the static set of the fuel head. The cold start valve is electricly innervated and timed during cold start events.

[jwmarc]

Rarlyl8... thank you for your response. You are quite prolific in your contributions to this board and I always enjoy your write ups.

So the C2T has no large plenun for the BOV but rather has a stand alone that fits into the long neck intercooler ( I am assuming that this would be the most appropriate location for it ..ahead of the throttle/intake maniifold. So Kokeln or a newer Porsche intercooler/BOV assembly could be substituted... Where does the pressure switch get relocated to?..

A 1 bar spring in the waste gate as you say could affect the tolerances on the Boost pressure switch. Where would one get a pressure switch that would negate this and what spec would you say would be appropriate --- 1.2 to 1.3 maybe?

Lastly the BOV...so the huge spring loaded ram in the plenum....I am trying to understand the flow here... the BOV plenum fits between the throttle/intake mainfold and the neck of the intercooler as well as sits in the path of the air flow which exits the CIS box and feeds the input side of the turbo.. so when the throttle plate is closed..deceleration.. compressed air being pushed out the intercooler neck circulates in the plenum and the BOV opens up to create a loop... intake air out of the CIS box through the turbo through the intercooler ( throttle plate is closed) exit left through the BOV and back into the input side of the turbo....

The BOV is normally closed under no load ( air circulates cis through bov plenum to throttle/intake manifold) and when high engine vacum pulls the plunger/spring open the air is routed through the turbo/intercooler route to the throttle/intake manifold... do I have this right?

Thanks....

[beepbeep]

Yepp, you are right. Porsche was very early in implementeing turbo and probably the first manufacturer to implement BOV so their first attempt to build BOV created somewhat overbuilt and unnecessary assembly.

Basically, you have a vacuum-operated cylinder that moves and opens the chanell that short-circuits the turbo and re-routes air back into turbo intake on decell. It can be replaced with el-cheapo Bosch plastic BOV and it will work just as fine. Later (on 964:s) Porsche modified it and used ordinary BOV as seen on most turbocharged cars nowadays.

Frequency valve is pulse-width modulated by lambda brain to hold >controll pressure< to certain value. Lower controll pressure means more fuel. Input into box is lambda sensor and output is pulse-width to power frequency valve. If lamba goes up or down from 1.0 it will trigger a signal which alters pulse width and thus control pressure until lambda goes to 1.0 again. If you stomp on it brain is bypassed and motor goes open-loop.

Few pics of BOV-assembly (taken when I changed gaskets to get rid of "clonk-clonk" sound on deccel):

[jwmarc]

Isnt the frequency valve like an electronic fuel injector. To control FD head pressure I have to change pressure resistence on ram movement--as you say low pressure easier movement more gas....so the Frequency valve bleeds off pressure as it relates to a pulse width as controlled by the ecu value intrepreted from the lambda output voltage. NO gas is introduced into the intake by frquency valve then is what you are saying.

If it were like a electronic fuel injector this orifice opening, if it is aligned with the constant pressure modification on the fuel head, would bleed pressure and enrich by increasing volume of gas flow.

Am I there?

Also you have an older 930 by the looks of your BOV plenum.. I see the over boost pressure switch next to the spring housing and the red rubber boot turbo pressure dash sending unit.. what is the function of the large vacum diaphram next to the boost sending unit?

[RarlyL8]

I am not familiar with all the workings of the frequency valve. My engine does not have the Lambda addition. From what I understand of it the frequency valve simply changes the mixter to help pass smog (read leans out at idle).
The large diaphragm is the decel valve. The function of that unit is to bypass air around the throttle body during close throttle events allowing the engine to slowly idle down instead of very rappidly doing so which may overshoot and cause the engine to stumble. This unit tends to "hang" the idle when it malfunctions.