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one of the great unwashed
 
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Boost Transition Flat Spot

Last question of the year...
I had to perform a few mods recently to the SC 3.0 BAE convert: new stainless steel airbox to replace the plastic one which exploded, and had BL fix my toasted 930 WUR. I put the WUR back in tonight, and took the car for a spin..not too hard, as it's a bit icy out. The car runs fine, til I hammer it at 2500 or so; when I get to 3500, which is my full 5 PSI boost, it gets (what I think is) real rich, like 10 or below on AFR (according to my AFR gauge), and bogs like crazy. If I feather it up to that RPM range, it's still there, but not as pronounced. As I have the BL RPM solenoid, set to 4600 RPM, I have taken boost enichment threshold out of the picture at the range where this problem exists. Up to, and when on boost, the car runs fine.

I will put the gauges on it tomorrow, but am left to wonder. The SS airbox is the type with the 2.7L runner diameters, so I had to get 2.7 rubber boots and stretch them over my early SC intake runners. Could I be inducing this enrichment a bit because of the restricted airfox flow capacity (which I don't think is much)? But, if I'm flowing less air, I'm also not moving the airflow plate as much, hence delivering less fuel. Or, does this sound like a control pressure adjustment issue?

I fabbed a different popoff plate from the one pictured, as I thought that may have been the problem. New plate is 3/8 6061 T6, with different springs, such that the 5 psi won't open it, but a backfire will. Also ditched the AAV in the photo; I determined I didn't need it.

TIA, and Happy New Year!
Pat

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Patrick E. Keefe
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Old 12-31-2007, 04:47 PM
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After thinking this over, and speaking with BL regarding this, I increased my warm control pressure to 3.8 bar, and the flat spot is almost gone. I was unaware that stock warm control pressure for the 930 is around 3.7 bar; the n/a SC was 3.2 bar, of course, no longer relevant. I think I need to get it to maybe 4.0 bar. Evidently, the IA mod to the fuel head really gets some serious mid range enrichment, which is quite non linear in this RPM/fuel plunger range.

So, it's much better, I'm not banging my chin on the steering wheel at 3500 RPM anymore. This is getting fairly sorted now. I can't imagine what real boost is like, vs my wimpy 5 PSI.
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Old 01-01-2008, 05:43 PM
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Sounds like a candidate for the Brian Leask RPM solenoid.
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Old 01-01-2008, 07:07 PM
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Actually, I think the opposite. Ditch the rpm solenoid and tune the WUR manually. You should not have the fuel needs that require that band-aid. The iA head is likely pig rich and will throw off all standard adjustments. Are you using an LM-1 to tune? You are in no-man's land and will need those reading to make your adjustments.
Your runner sized should not be critical. You are not choking off the air but rather forcing it in. Smaller or restricted runners will increase the inlet temperature and pressure somewhat but should not choke off flow.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 01-01-2008, 07:33 PM
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First of all, backfire of that magnitude is very uncommon on turbocharged engines as intake path between CIS and intake is much much longer.

Second, you have to use LM-1 or such to find out what is happening on boost transition. Playing it "by the ear" is not to do you any good. It might go rich or lean or you migh be loosing boost etc.

What about the hole in the red pipe going to intake? Is it plugged now?

I would close pop-off valve shut, pressurize whole system to check for leaks and then do LM-1 run (while logging revs too) to begin with. Otherwise you are just shooting in the dark.

Regards,
Goran
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Old 01-01-2008, 11:46 PM
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Well, the BL I referred to above is Brian Leask. I am currently running close to 4.0 bar control pressure warm, which has just about gotten rid of the flat spot. I do have one of Brians WUR and RPM Solenoid, set to 4600 RPM. It's kind of weird: in second gear, hard acceleration, it is still there badly. In third and fourth, it's not there. I am thinking about the rate of change in the enrichment circuit as a potential culprit.

Brian, I don't have an LM-1, but I do have an AEM AFR gauge dash mounted, with a Bosch wideband sensor before the turbo. BL and I discussed the IA head, and he confirmed that they do tend to run pig rich in the midrange. The adjustments I made to warm control pressure seems to have taken most of that out, but I can still get AFR to 10 or below, as noted prior, in a hard second gear run.

Goran, what you suggest regarding backfire is what I thought also, until I blew up my plastic airbox. That hole in the discharge pipe is the fitting for my water injector. And, I just last night read one of your posts regarding pressure checking of the system, which was very interesting, and I think I may do sometime in the naer future.

I'll tweak the idle mix a bit today, I braved the 30F temperatures and calls for snow flurries and took the car to work today.

Pat
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Old 01-02-2008, 04:58 AM
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For accurate results you are meant to mount the WB sensor after the turbo due to back pressure.
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Old 01-02-2008, 05:51 AM
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I've heard that rumor also, Nathan. I'n my case, I dont have enough wire or a good mounting location. I also figured that whatever gas is before the turbo is the same gas after the turbo, just a little cooler. I suppose people mount them after the turbo for longevity of the sensor, in addition to the reason you describe. There will still be a pressure loss in the system somewhere regardless of where it's mounted.
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Old 01-02-2008, 08:39 AM
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Pat, it is most helpful to have an A/F readout to do your adjustments. You need to see specific points and exact numbers.
The LM-1 will give you the ability to track and graph your ratios while also measuring RPM and boost levels. You will be able to address specific areas needing adjustment. The results, as you know, will be optimal fuel mileage, driveability, and power.
You might unplug the RPM switch and try to tune the WUR without it. You can always plug it back in if the threshold range of adjustability is too narrow.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-02-2008, 08:55 AM
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Brian:
My problem is that I am not into the RPM range where the switch is active. The switch is normally open, but powered closed, which I verified. So, I have nothing related to the WUR boost threshold functionality affecting the AFR at thes RPM range where the trouble exists. When I did get the revs up into the above 4600 RPM range, the car ran fine.

I think my AEM has the wiring for connecting up an RS-232, such that I can probably log some data on my laptop via hyperterminal. I'll have to look into this, as what you say is a pretty good idea. I generally just memorize readings and RPM readings, but my feeble mind does not have the storage capacity of a disk drive.

Pat
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Old 01-02-2008, 09:03 AM
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Where is your boost threshold in 4th gear?
Where does full boost come in 4th gear?
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-02-2008, 10:03 AM
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Right around 3500 is full boost, and I'm making a couple PSI in the mid 2000's.
I'll verify when I leave work, if I can get some open highway.
Pat
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Old 01-02-2008, 11:14 AM
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Rarlyl8:
I'll go with about 2600 being threshold and 3500 for full boost in 4th. It changes a bit, as the flat spot comes in at the instant my AFR gauge reads 10. At this point I'm sure the actual AFR is way below ten. The harder I push the throttle between threshold and full boost, the worse it is. On WOT, i can only make like 4 PSI before it drops to 10 AFR.

I'm starting to think the IA head makes it way too rich. I have it so lean (like mid 13's to mid 14's) at low end n/a operation, I don't know if I can turn the control pressure up any further.
Pat
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Old 01-02-2008, 01:12 PM
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Pat, i think you should boost that thing to 8lbs. you apparently are giving it enough fuel to do so.
I was running 8 on my setup with the 7th injector enrichment and had no problems doing so.
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Old 01-02-2008, 01:12 PM
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Hey Chappy, Happy New Year!
I've got too much fuel, I think. I have run it at 7 PSI, I just can't get through the midrange.

Get your engine straightened out yet?
Pat
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Old 01-02-2008, 02:50 PM
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I am waiting for the long steep hill outside my house to clear off. I cant tow my car up the hill to the shop until some of the ice melts off, looks like i can do it next week, were expecting a warm up.
The guys at the shop wont come to my place to checkout the leaks, the car is up on my lift and it would be so easy for them to check it out here. So I am waiting for the January thaw, then i will get the car to them and back here so i can drop the engine strip it down to the short block and bring it back to them to fix.
Spring is just around the big corner so hopefully i will be back on the road when the snow is gone.
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Old 01-02-2008, 03:05 PM
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I kind of figured the modded fuel head might be too much for your application.
What is your fuel pressure? Maybe there is some room to turn it down a bit and readjust. If not you might fit a stock fuel head and see where you are at using 5psi.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 01-02-2008, 07:47 PM
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Brian:
That is exactly what I was thinking. I am going to try to turn down the fuel pressure. I have 72 PSI now. However, I don't want to lose the top end pressure. Next option is finding a stock, rebuilt fuel head off an SC. I am also thinking sequential pumping, but that becomes complicated.
Pat
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Old 01-03-2008, 04:45 AM
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72PSI isn't high and you probably need the volume. A stock fuel head bumped to 85 or 90 psi might be the ticket. It's been a long time sense I messed with an SC head, I don't recall the specs and ranges.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-03-2008, 05:09 AM
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72 PSI is spec for an SC head, which I think is all I can get from the SC pump. At 72, I just about run out of fuel at high RPM, according to the dyno sheeets. So, I have too much in mid range, and barely enough at top end, hence the consideration of series sequential pumps. I'm just trying to figure out how to turn the second pump on.
Pat
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Old 01-03-2008, 06:55 AM
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