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F350Lawman's Avatar
 
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AFRs????

Just got the car back after some major work, including all new suspension, the Blownsix 1/2 bay cooler , the Leask solenoid, Zork tube and the XD16 afr guage. Obviuosly I am not giving you that much to go by, but how does my afr sound for a starting point??

Tonight in my first brief run (32F temps) I am seeing 13.2 afr cruising in 3rd or
4th at 60 and 85mph respectively, no boost and it drops to about 11-11.5 on boost. Seems to come back up to 12-12.5 if I hold full boost but I was at warp speed and couldn't REALLY repeat too many times.

Looks like the switch is working well, the drop comes on when revs hit the
4000s under boost. I have the 4200 chip from Brian in right now (might try the 4200 and 4600). I haven't gotten a chance to attach my laptop to the XD16 so I am just trying to keep one eye on the road and one on the guage.

Oh, and the Zork tube is making NICE flames, a great rumble, wicked whistle...and some nice grumbling and gurgling on decell. Suspension is stiff but I like it....feels like a slot car or go-kart!
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1987 930 - K27 7200 / DAS Full Cage / Blownsix Intercooler / BB Headers / Zork Tube / Tial 46mm / Adjustable WUR / RPM Activated Solenoid / Open Element Air Intake / Smart Racing 27mm Sways / 24 and 33mm Torsion Bars / Adj. Spring Plates / Wevo Strut Brace / Monoballs / Tow Hooks / Oil Cooler Scoop / Brey Krauss Harness Truss / Race Seats / XD-16 AFR Guage/ 17" Kodiak Racing Whees and Nitto NT-01 255/315s
Old 11-16-2007, 08:55 PM
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AFR's look good to me. Check the plugs to confirm.
Old 11-17-2007, 05:09 PM
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wait till you can graph it then analise the graph it's hard to watch the boost afr and the road and easy to miss if it drifts some.
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Old 11-17-2007, 06:57 PM
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Just watch it dosent go too lean before your fuel enrichment comes in via your switch,,if it dont go into 13's before your enrichment comes in you dont sound far away with your afr's,,try also keep it in the low 12's near the red line for safety!
Old 11-18-2007, 03:09 AM
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I don't like to see 12.5 up top as an AFR under WOT and boost. That is a little lean.
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Old 11-18-2007, 08:58 AM
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+1 on what Porschephd says.
I like to see high elevens all the way to the top under wot and ful boost, but low 12s are alright.
Off boost and light throttle shouldnt be very rich though- on a fuel injected car like one of the new 996s or 997, these motors run verrry lean (16-17:1) on light throttle. This is probley for epa and M.P.G reasons. When I go to tune my car in the spring, I am going to use my fathers 993 as an example for low rpm and cruising afr, I want to use an air-cooled car for comparison purpose.
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Old 11-18-2007, 11:16 AM
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I tried experimenting. I can't really get it lean boosting. I think it is getting too rich way up top. I don't think my turbo can keep up, I get a stumble at WOT near redline and holding full boost in 2.3 and 4th. I check the guage and it appears to be dropping into the 9s. I thought maybe I hit the overboost switch but I don't think so at 115-120mph in 4th WOT???

We made many changes, with the entire new suspension, seats,solenoid switch, B6 1/2 bay cooler inc. the C2 style recirc. valve, Zork so we'll see. My mechanic is going to play with the Leask WUR and check it out.

Right now I am taking it easy and just getting the feel of the car again. The only thing I would bet on is that we have PLENTY of fuel.
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1987 930 - K27 7200 / DAS Full Cage / Blownsix Intercooler / BB Headers / Zork Tube / Tial 46mm / Adjustable WUR / RPM Activated Solenoid / Open Element Air Intake / Smart Racing 27mm Sways / 24 and 33mm Torsion Bars / Adj. Spring Plates / Wevo Strut Brace / Monoballs / Tow Hooks / Oil Cooler Scoop / Brey Krauss Harness Truss / Race Seats / XD-16 AFR Guage/ 17" Kodiak Racing Whees and Nitto NT-01 255/315s
Old 11-18-2007, 04:54 PM
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Quote:
Originally Posted by Porschephd View Post
I don't like to see 12.5 up top as an AFR under WOT and boost. That is a little lean.
As a matter of curiosity, why do you consider 12.5 lean? Is it a matter of combustion chemistry, or detonation insurance, or...?

Thanks!
Pat
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Old 11-18-2007, 05:25 PM
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Stating 12.5 is a bit lean is a very broad statement for this list. For a stock motor (with stock compression and stock ignition, stock boost and stock fueling), 12.5 is probably what you are getting - maybe even 12.8 or more up top but it works. My motor has 8.0:1CR at .8 bar, so I am more comfortable with low 12's for detonation insurance - even though I have complete control over spark. Keep in mind that we are dealing with an air-cooled, turbo-charged motor, using late 70's engine management technology, so erring on the rich side is appropriate.
Old 11-19-2007, 05:37 PM
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As Don mentioned you increase your mods they also increase your combustion temperatures. The temperatures go up and you need to lower your AFRs to supply enough fuel to that range, but also cool the chamber down. Less fuel, more heat. On a stock motor the levels are so low that a 13 is probably ok. In fact most ran closer to 13 than 12.

Additionally lets not forget that you can have a 11.5 AFR and still detonate based on timing. AFRs are only half the equation.
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Old 11-19-2007, 06:30 PM
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Info regarding AFRs from MoTec

http://www.motec.com.au/pdf/30JUNE2007.pdf
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Old 11-19-2007, 06:43 PM
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Another fact often overlooked is EGT exhaust gas temp. If you have a bunch of timing, say north of 20 degrees on a high boost motor, even with an 11.5:1 or simaler afr- there is still chance for detonation because the cylinder pressure is so great that it heats everything up too much. Therefor turning things like ground straps on spark plugs and slight burrs in the combustion chamber into mini glow plugs and igniting the air/fuel at the wrong time.
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Old 11-19-2007, 07:54 PM
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I'm not sure I completely agree. First, stock ignition is anywhere between 24 and 29 degrees at redline. A stock motor with that timing and 11.5 AFR will last forever.

When I last had my engine out, and on a DTS engine dyno, the EGTs stayed constant (at one bar) between 18 and 28 degrees timing on top (about 1575 degrees). When we retarded the spark below 15 at WOT, the temps started to rise and the torque really fell off.

In addition, the EGT's really did not move up all that much (past 1575) even at 13.0 at WOT. As a result, I scrapped the idea of installing EGT probes and running the info to my ECU because I didn't think it would give me any better indication of tune compared to AFR's.

My timing is now (approx) 8 at idle (-50kpa), 36 at 2500rpm (0 kpa) and 18 at 6500 rpm (100kpa)
Old 11-20-2007, 04:18 PM
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Yea, but the stock motors run small amounts of boost on pretty small turbos. I run my car at 22 PSI via GT-37R. I know everyone is going to say "Oh My *od" but I beat the crap out of my motor for 10k now and have 0 problems with leakdown. At the boost level I run, 27, 28, and 29 degrees gets a little hot even with 11.5:1. But we all have our diffrent uses for our cars..

1575 degrees EGT sounds damn good and the engine should last forever like that- low heat. But how high do you get the timing under full boost @ 1 bar? You can get away with a little more than me because your running less boost. Thats where my trade off is- less timing.
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Old 11-20-2007, 08:15 PM
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You are in a completely different league than 99.9% of us if you're using 22 psi. I wouldn't know how to respond to anyone telling me they are running that kind of boost on these motors and expecting them to last. It's taken me years of experimenting with my car, and many, many dollars to dial mine in at 1.0 bar. Good job on engine management.

Under full boost (.8 or 1.0 bar), total timing for my twin plugged motor is 18 degrees.
Old 11-21-2007, 04:31 PM
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Here's some trivia...

I just checked our 1944 model P-51 manual and converted inches of mercury to psi. In short bursts, we can run 33 psi but max is 29 psi, but CR is 6:1. This is on a 27 liter, supercharged 1675hp motor.
Old 11-21-2007, 04:48 PM
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Did the P-51 run water injection?
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Old 11-21-2007, 04:58 PM
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Quote:
Originally Posted by patkeefe View Post
Did the P-51 run water injection?
Not this model (P51D). When the supercharger kicks into high speed, the mixture automatically richens through connection with the throttle cable and you really need to watch water temps.
Old 11-21-2007, 05:24 PM
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Hello!

Few things worth considering:

- Twin plug engines need less ignition advance to reach optimal BMEP. They "lighten the candle on both sides" which combusts mixture quicker.
- Boost is not always equal power. Point is to extract as much power as possible, not to run as much boost as possible. More boost equals more stress, more backpressure etc.
- Aircooled engines have finicky heads that aren't cooled well and need to run richer than watercooled pentroof-ones.
- Even if it doesn't knock, more boost will make heads "walk" more, eventually leading to sealing problems.


To conclude: My opinion is that aircooled 930 engines with stock internals that are expected to hold for more than track work should run not more than 1.0 bar with intercooling. AFR's should be 14.0 for steady load with no boost to conserve fuel (= driving at 55mph) and drop to between 12.8-12.4 when full boost is reached (depending on fuel quality and ambient heat). When tose AFR's are reached ignition advance should be tweaked to reach optimum torque untill you reach knock limit and backed down few degrees for error margins. If you don't feel confident with "chasing knock limit" you can dial ignition to preset value know to work well with your engine combo.
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Old 11-22-2007, 02:02 AM
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Excellent thread guys. Just recently installed an Innovate LC-1/XD-16 system in my mostly stock car and I'm trying to maximize fuel economy vs. engine longevity vs. fun factor. I have the Andial fuel enrichment system which I know some don't like (sorry Don ) but it strikes me as added insurance.

But my car is very much what Steve (Porsche PHD) and Don describe - a street 930 with the standard external modifications (intercooler, K27, GHL header and straight cat/muffler). I don't run hard boost as long as you guys do - part of the emphasis on street car I guess.

My goal is to get the car to try running around low 14's or high 13's at constant rpm in 3rd or 4th - basically cruising range. Then when the boost comes in between the stock CIS and the Andial the A/F drops to low 11's. Problem is I'm still learning how much to trust the Innovate, how accurate the readings are, etc. So I do tend to run a bit rich. After all, you know you are too lean when your car dies at a red light when you take your foot off the gas. It's tough to explain this to car guys who don't run air-cooled turbos. To them "stoich" is the ideal - to us gas is for burning and paradoxically for cooling.

Great thread for us newbie tuners.

BTW. Don - don't tell me you have a P-51 in addition to the stable of horses?
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Last edited by BlkBird; 11-25-2007 at 08:00 PM..
Old 11-25-2007, 07:56 PM
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