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mark houghton's Avatar
 
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Location: Central Washington State
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Quote:
Originally Posted by john930 View Post
I attached my sensor at the end of the muffler and it is connected to a XD-16 gauge on the dash. This works well and was straightforward to connect.



John, I at one time considered doing the same with a direct-to-muffler mount, but my only concern was being able to keep the sensor vertical between 10:00 and 2:00...to keep the condensation from settling on and ruining it. So I opted with installing in (on top of) the tail pipe and snugged up against the muffler.

Fears about ambient air back-wash (10" for every 3" of pipe diameter...come on, who in a 930 has 30" of tailpipe available!?) are really only a concern at idle when not enough exhaust velocity exists. But we don't install AFR monitors to see what's happening at idle, so who really cares? With that said, mine reads just fine at idle regardless of being only about 10" from the outlet (maybe the fact that the pipe has about a 30 degree bend helps in this regard).
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 11-15-2009, 01:02 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #21 (permalink)
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I think the location depends on whether you are using the sensor for control, or to just monitor AFR. For control the optimum location is as close to the cylinders as possible. This means mounting it before the turbo. The problem with this is that exhaust pressure throws off the reading, which is why it is usually suggested to mount it after.

For EFI with the capability for it, you can correct the AFR reading for the pressure effect using this formula:

AFR(corrected) = (AFR(measured) + B x P) / (1 + C x P)

where: AFR(corrected) = the AFR corrected for exhaust pressure.
AFR(measured) = the AFR output by the AFM.
B = 0.009140 for AFR < 14.57 (rich).
B = 0.012100 for AFR >= 14.57 (lean).
C = 0.000627 for AFR < 14.57 (rich).
C = 0.000830 for AFR >= 14.57 (lean).
P = the exhaust pressure in mmHg above the pressure at which the sensor
was calibrated (using the CAL POT on the AFM while the sensor is held in air).
Valid for -152 mmHg < P < 532 mmHg.
Old 11-15-2009, 01:14 PM
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Mark ,I have over 30" of tail pipe on my 964 turbo and if you measure through that muffler to end you p;robably do also. I tune my afr @ idle as well @ higher rpm , as I am useing a digital wur and it makes a difference for me. I didn't say it should be 30" on a 930,just that is what the mfg. sugested to do . I had a zork on it for a while and had it mounted on top of it about 2" from the turbo as Brian Bodart sugested to do and it worked fine on boost but was not real reliable @ less than about 950 rpm.
Old 11-15-2009, 02:31 PM
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Quote:
Originally Posted by 964 T #304 View Post
Mark ,I have over 30" of tail pipe on my 964 turbo and if you measure through that muffler to end you p;robably do also. I tune my afr @ idle as well @ higher rpm , as I am useing a digital wur and it makes a difference for me. I didn't say it should be 30" on a 930,just that is what the mfg. sugested to do . I had a zork on it for a while and had it mounted on top of it about 2" from the turbo as Brian Bodart sugested to do and it worked fine on boost but was not real reliable @ less than about 950 rpm.
My misunderstanding...I thought it meant 30 inches before the open end of the tail pipe. If I include the length of the muffler....starting at the turbo...to the end of the tail pipe then you're right I probably have 30" as well (on my non-stock straight thru muffler). I realize it's just the manufacturers recommendation.

I also tune mine at both idle and on higher rpms/boost, at idle using both the AFR gauge and the old standby Gunson Gas tester for %CO. My whole point is that we should not discount mounting sensors (for monitoring purposes only) post-muffler and relatively close to the end of the tail pipe. Each application is indeed unique.
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Old 11-15-2009, 03:20 PM
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Here is what we do for the zorks. Not much to work with. Idle readings can be iffy.




I also have the O2 bung in the neck of the mufflers due to packaging and other considerations. A heat sink will protect your sensor while not affecting the readings.

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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
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Old 11-15-2009, 03:34 PM
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So hopefuly I can get some help here I've asked this quesiton on rennlist, I've got a B&B dual outlet which side do I put the sensor?
Old 11-18-2009, 11:35 AM
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Quote:
Originally Posted by 02fly View Post
So hopefuly I can get some help here I've asked this quesiton on rennlist, I've got a B&B dual outlet which side do I put the sensor?

Post a pic you might be able to mount it somewhere on the muffler itself.
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Old 11-18-2009, 05:28 PM
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So it's ok to place it in the muffler?
Old 11-18-2009, 05:33 PM
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You can put it in the exhaust any place you want so long as a few rules are met, one it must point up like at the 2 or 11 oclock position so moisture cant settle in the sensor it will destroy it and it must get full exhast flow no stagnant locations and it must be back from the exhaust oulet so outside air cant corrupt the reading. Thats why I said post a pic of your muffler and I am sure we can steer you in the right direction.
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
76 911 C.R.A.P. Gruppe #2 BIG time TURBO C.R.A.P. Bitz EFI/EDIS Now MegaSquirt 3
76 Blazer also restored by me
Old 11-18-2009, 06:49 PM
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I understand I will post a picture, I just have to take off the bumper which is no big deal I'll have to do to install the sensor. I just thought someone may know because for some reason one side of the exhaust is hotter than the other which must be due to baffling and I wasn't ready to do the job right now but I thought while it was brought up I'd ask. I really do thank you so please don't think I'm being an ass, I try and ask allot of question so I'm informed when I make a decision. So anyway I'll post some pics this weekend.
Thanks Don
Old 11-18-2009, 07:33 PM
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