Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Technical BBS > 1- Porsche Technical Forums > 911 / 930 Turbo & Super Charging Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Me like track days
 
Craig 930 RS's Avatar
 
Join Date: Sep 2003
Location: Kirkland, WA
Posts: 10,196
Question AFR - ideal 90 degrees IC outlet vs. 140 at the track - what's the diff. in density?

I've adjusted the AFRs so they are pretty slose to perfect - 12.0 to 12.3 all the way up...

Situation is under ideal dynamometer conditions - IC outlet temps a nice comfy 90 degrees.........all was good.

So what is the difference in air density - and thus the theoretical "textbook" difference in AFRs from this ideal, to 140 degrees on track under full on track use?
__________________
- Craig 3.4L, SC heads, 964 cams, B&B headers, K27 HF ZC turbo, Ruf IC. WUR & RPM switch, IA fuel head, Zork, G50/50 5 speed. 438 RWHP / 413 RWTQ -
"930 is the wild slut you sleep with who tries to kill you every time you "get it on" - Quote by Gabe
Movie: 930 on the dyno
Old 08-14-2007, 01:53 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Wo ist die Rennstrecke?
 
DonE's Avatar
 
Join Date: Mar 2004
Location: Brooks, GA
Posts: 1,211
You also need to know barometric pressure and humidity to figure air density. However, since it was hotter, the air will be less dense for the same amount of fuel - it will be richer. For example, on my EFI system I think I have -6% or -8% fuel correction for 140 degrees air temp to maintain same AFR.
Old 08-14-2007, 03:44 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered User
 
Join Date: Apr 2007
Posts: 394
Ive got a air density meter that i used for drag boats for about a year and a couple of times for a friends drag car . tells you air density, adlusted altitude, humidity , all sorts of useful Info. It is an ALTRONICS WEATHER STATION . I sent it back to factory a year or so ago and had curent upgrades done on it You can store a number of runs in it and for drag racing it will tell you what you should run acording to weather conditions. I have no use for it now. You can look @ their web site and if it is some thing you could use PM me .
Old 08-14-2007, 05:41 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Registered User
 
kellcats521's Avatar
 
Join Date: Feb 2005
Location: Atlanta Metro
Posts: 219
Very generally, Don E is correct in that higher temp = lower density which would lower the A/F ratio. Buuuuttttt, the increased air temp in the intake/heads can also raise head temps which can increase potential for detonation. Not sure if 140 is high enough to cause detonation though......
Old 08-14-2007, 06:37 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Me like track days
 
Craig 930 RS's Avatar
 
Join Date: Sep 2003
Location: Kirkland, WA
Posts: 10,196
Good stuff guys, appreciate it. Anything else? - please post
__________________
- Craig 3.4L, SC heads, 964 cams, B&B headers, K27 HF ZC turbo, Ruf IC. WUR & RPM switch, IA fuel head, Zork, G50/50 5 speed. 438 RWHP / 413 RWTQ -
"930 is the wild slut you sleep with who tries to kill you every time you "get it on" - Quote by Gabe
Movie: 930 on the dyno
Old 08-14-2007, 06:59 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered User
 
beepbeep's Avatar
 
Join Date: Jun 2001
Location: Sweden
Posts: 5,560
As I said countless times before, CIS only measures air flow trough it's barn door and injects fuel accordingly. What you do with air after that (compress it, heat it, dump it overboard) is nothing it cares about. With other words, changing discharge air temperature ex turbo won't do a squat to your AFR's as air is not added or dumped past CIS barn-door. Only thing that will happend is that your turbo will work harder and won't be able to pack as much air into engine as with cool temps.

With other words, only thing that really affects your AFR's is general air temps and humidity. As long as you didn't do your AFR tuning in -20 degrees, your 12.3 AFR will have more than enough margins to cover typical outside air temp swings expirienced at track.

Thus this whole discussion is quite moot.
__________________
Thank you for your time,
Old 08-15-2007, 08:38 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
Me like track days
 
Craig 930 RS's Avatar
 
Join Date: Sep 2003
Location: Kirkland, WA
Posts: 10,196
"Only thing that will happend is that your turbo will work harder and won't be able to pack as much air into engine as with cool temps."

.......which changes the AFRs, no?
__________________
- Craig 3.4L, SC heads, 964 cams, B&B headers, K27 HF ZC turbo, Ruf IC. WUR & RPM switch, IA fuel head, Zork, G50/50 5 speed. 438 RWHP / 413 RWTQ -
"930 is the wild slut you sleep with who tries to kill you every time you "get it on" - Quote by Gabe
Movie: 930 on the dyno
Old 08-15-2007, 08:50 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered User
 
beepbeep's Avatar
 
Join Date: Jun 2001
Location: Sweden
Posts: 5,560
Quote:
Originally Posted by Craig 930 RS View Post
"Only thing that will happend is that your turbo will work harder and won't be able to pack as much air into engine as with cool temps."

.......which changes the AFRs, no?
Nope. You see, turbo sucks air trough the barn door, then compresses it. So regardless of how much it heats up the air, it always has to suck it trought the barn-door. Rembemer, air drawn trough the barn-door is uncompressed and proper amount of fuel dispatched. If turbo heats it up it'll expand and turbo will have harder time to cram as much of it into engine. But amount of fuel measured proportional to air drawn trough the CIS will stay fairly constant.

That's why it's called Air Fuel Ratio...you draw x amounts of molecules of air trough CIS, it squirts X amount of fuel into engine. If it heats up air afterwards, it's turbochargers problem...fuel for that air will be delivered. Turbo just won't be able to suck as much if it gets heated, thus power-loss.

Granted, CIS has no provision to compensate for outside air temperature, thus margins in it's AFR's.
__________________
Thank you for your time,
Old 08-15-2007, 09:12 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Me like track days
 
Craig 930 RS's Avatar
 
Join Date: Sep 2003
Location: Kirkland, WA
Posts: 10,196
We're talking about air density into the *engine* - it decreases with heat compression, regardless of CIS or XYZ -
__________________
- Craig 3.4L, SC heads, 964 cams, B&B headers, K27 HF ZC turbo, Ruf IC. WUR & RPM switch, IA fuel head, Zork, G50/50 5 speed. 438 RWHP / 413 RWTQ -
"930 is the wild slut you sleep with who tries to kill you every time you "get it on" - Quote by Gabe
Movie: 930 on the dyno
Old 08-15-2007, 09:22 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Registered User
 
beepbeep's Avatar
 
Join Date: Jun 2001
Location: Sweden
Posts: 5,560
Quote:
Originally Posted by Craig 930 RS View Post
We're talking about air density into the *engine* - it decreases with heat compression, regardless of CIS or XYZ -

You don't get it. It's a closed system. Everyting that has been drawn into engine passed barn-door. And everything that passed barn door has been accounted for and corresponding fuel injected. Air (=oxygen molecules) annot magically dissapear or get bigger between engine and CIS just beacuse turbo compressed them.

If you pushed 13 g of air into engine, CIS will inject 1 gram of fuel. AFR=13. If turbo starts finding it hard to push that air into engine (beacuse it expanded) it will just inhale less, and CIS will push less fuel into engine. Status Quo.


Example: You have a bakery. There is a stream of dough passing trough a tube and flow-sensor of some sort. For every 14kg/sec of dough it passes out 1kg/sec of sugar. After that device, dough flows trough warmed tube where it expands. Dough cannot escape anywhere else, it must go trough tube. When it comes out of the tube, sugar is added.

Imagine someone turns up the heat in the tube after flow sensor and dough starts swelling more than usual, making it harder for pump to push it trough the tube. What happends is that flow slows below 14kg/sec, device senses it and starts adding less sugger. In the end, the dough is just as sweet as usual, but there is less dough coming out of the pipe.

Trust me on this
__________________
Thank you for your time,
Old 08-15-2007, 11:37 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
Registered User
 
kellcats521's Avatar
 
Join Date: Feb 2005
Location: Atlanta Metro
Posts: 219
Quote:
Originally Posted by beepbeep View Post
You don't get it. It's a closed system. Everyting that has been drawn into engine passed barn-door. And everything that passed barn door has been accounted for and corresponding fuel injected. Air (=oxygen molecules) annot magically dissapear or get bigger between engine and CIS just beacuse turbo compressed them.

If you pushed 13 g of air into engine, CIS will inject 1 gram of fuel. AFR=13. If turbo starts finding it hard to push that air into engine (beacuse it expanded) it will just inhale less, and CIS will push less fuel into engine. Status Quo.


Example: You have a bakery. There is a stream of dough passing trough a tube and flow-sensor of some sort. For every 14kg/sec of dough it passes out 1kg/sec of sugar. After that device, dough flows trough warmed tube where it expands. Dough cannot escape anywhere else, it must go trough tube. When it comes out of the tube, sugar is added.

Imagine someone turns up the heat in the tube after flow sensor and dough starts swelling more than usual, making it harder for pump to push it trough the tube. What happends is that flow slows below 14kg/sec, device senses it and starts adding less sugger. In the end, the dough is just as sweet as usual, but there is less dough coming out of the pipe.

Trust me on this
I know this is not entirely true, as CIS has real issues when exposed to elevation changes, just as it does when exposed to temperature changes. IMO (this may not be the exact explaination) the negative intake pressure (either from the engine itself or from the compressor inlet) draws air across the CIS plate causing a deflection based MAINLY on the degree of negative pressure applied to the plate. If the air moving across the plate is less dense (i.e. at high altitude or elevated temp), it will have less oxygen, and the motor will run richer.

There may be more to this, but I know for a FACT that CIS needs to be tuned to intake temp and altitude. Porsche designed some margin in the WUR (in my experience in working on other, non-Porsche CIS cars, we called the WUR a control pressure regulator (CPR), which is a MUCH better description of what this device is) to account for some temp and altitude variation, but I have had to 'adjust' the CPR on other cars due to temp AND altitude change.

Pat K
87 930
Old 08-15-2007, 12:12 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
Registered User
 
Join Date: May 2002
Location: Up North
Posts: 1,434
Pat,
It is my understanding that one of the "side benefit" of CIS is self-altitude adjusting... Depending how much air (mass) is drawn through the throttle plate (barn door), corresponding amount of fuel will be added. So, it there's less air, because of heat, less throttle application, less air density because of barometric/humidity changes, etc., the plate will be pushed open less, hence less fuel.
__________________
87 930 K27HFS/B&B/Twin-Plug... Megasquirted
Old 08-15-2007, 04:35 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #12 (permalink)
Me like track days
 
Craig 930 RS's Avatar
 
Join Date: Sep 2003
Location: Kirkland, WA
Posts: 10,196
Goran.....

CIS automatically maxes out fuel delivery upon full boost.
"That's all there is.....there ain't no more"
So your air HAS changed once it passes the metering and through the turbo

Beyond this, controlling the AFR is dependent:
On air density via Intercooler cooling
The ultimate amount of fuel added under full boost -managed/added by the WUR, with additional added by the IA fuel head if necessary.
__________________
- Craig 3.4L, SC heads, 964 cams, B&B headers, K27 HF ZC turbo, Ruf IC. WUR & RPM switch, IA fuel head, Zork, G50/50 5 speed. 438 RWHP / 413 RWTQ -
"930 is the wild slut you sleep with who tries to kill you every time you "get it on" - Quote by Gabe
Movie: 930 on the dyno
Old 08-15-2007, 05:50 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)
Wo ist die Rennstrecke?
 
DonE's Avatar
 
Join Date: Mar 2004
Location: Brooks, GA
Posts: 1,211
Quote:
Originally Posted by hobieboy View Post
Pat,
It is my understanding that one of the "side benefit" of CIS is self-altitude adjusting... Depending how much air (mass) is drawn through the throttle plate (barn door), corresponding amount of fuel will be added. So, it there's less air, because of heat, less throttle application, less air density because of barometric/humidity changes, etc., the plate will be pushed open less, hence less fuel.
I heard that....

When fine tuning my CIS car, I had to tune it twice per year - summer (less fuel) and winter (more fuel). The temp at which I had to change the WUR pressures was about 55F or so. Below that, I needed to decrease WUR pressures to increase fuel flow. Also, didn't Porsche make a WUR that took pressure (aka, air density) into account in later CIS models? My 79 model did not.

So, although the CIS "barn door" generally works well, it is NOT a mass air flow sensor.
Old 08-15-2007, 06:30 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)
Registered User
 
E-man930's Avatar
 
Join Date: May 2005
Location: Atlanta GA
Posts: 624
Good book to have if you are running a CIS based 930...

Last edited by E-man930; 08-15-2007 at 06:43 PM..
Old 08-15-2007, 06:38 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #15 (permalink)
Registered User
 
E-man930's Avatar
 
Join Date: May 2005
Location: Atlanta GA
Posts: 624
Old 08-15-2007, 06:44 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #16 (permalink)
Me like track days
 
Craig 930 RS's Avatar
 
Join Date: Sep 2003
Location: Kirkland, WA
Posts: 10,196
Since you have not made it clear who or what you refer to....

Care to shed light on this vague assertion?

Edit: Oh I see you changed the post from:
someome needs to take a few refresher courses in physics and chemistry... and it's not Goran... If you really want to get on target, pick up a thermal-dynamics textbook.
__________________
- Craig 3.4L, SC heads, 964 cams, B&B headers, K27 HF ZC turbo, Ruf IC. WUR & RPM switch, IA fuel head, Zork, G50/50 5 speed. 438 RWHP / 413 RWTQ -
"930 is the wild slut you sleep with who tries to kill you every time you "get it on" - Quote by Gabe
Movie: 930 on the dyno
Old 08-15-2007, 06:44 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #17 (permalink)
Registered User
 
E-man930's Avatar
 
Join Date: May 2005
Location: Atlanta GA
Posts: 624
yeah sorry - I was letting the bad voice coming from my left shoulder take control of my thoughts. I changed the post to reflect the true nature of this board - 930 owners and enthusiasts helping each other and learning while they go.
Old 08-15-2007, 06:50 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #18 (permalink)
Registered User
 
E-man930's Avatar
 
Join Date: May 2005
Location: Atlanta GA
Posts: 624
I apologize for my remark...
(sorry tired and reading boring regulatory state laws...)
Old 08-15-2007, 06:51 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #19 (permalink)
Registered User
 
kellcats521's Avatar
 
Join Date: Feb 2005
Location: Atlanta Metro
Posts: 219
Quote:
Originally Posted by E-man930 View Post
yeah sorry - I was letting the bad voice coming from my left shoulder take control of my thoughts. I changed the post to reflect the true nature of this board - 930 owners and enthusiasts helping each other and learning while they go.
If I was the target, I have this book -

And, it does mention the altitude compensation function of the later model WURs (Probst also calls the WUR a CPR). The book does not discuss the effect of air density variation on mixture specifically, but it again does specify that a late model CPR adjusts for this, which, if it didn't matter for the CIS system, would not be needed.

Pat K
87 930
Old 08-15-2007, 08:06 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #20 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 08:10 AM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.