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Old 01-05-2009, 11:16 PM   #1
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Default AFR's and Powah

Ok, this is sorta weird. I was running 8 PSI boost at 12.5:1 AFR's for a couple days. The power was NICE. Now I'm at 10.7 PSI and running 11.5:1 AFR's and I could swear it's not much faster than 8 was at 12.5:1. Feels the same or a spec better yet it's 2.7 pounds more boost....

I'm gonna datalog it tomorrow and make sure I'm still getting 10.7 PSI boost (no boost gauge, I know, gay, but it's a sleeper). But I haven't touched the MBC so it should be the same as ambient temps haven't changed either. Just curious if anyone else has seen a noticeable power difference going from rich to lean or lean to rich? I don't have any dyno proof of course but it sure isn't impressive like it was. Or maybe I'm getting used to it. But I really don't think it's pulling like it should.

EDIT: I changed the numbers. Turns out I was running EIGHT pounds before, not 7. So not as big of a difference as I thought....

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Old 01-05-2009, 11:24 PM   #2
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rich is always a bit less power but safe. on my subaru going from 12:1 to 11.25 was a noticeable decrease in power. I really dont think anything is wrong with your car
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Old 01-05-2009, 11:49 PM   #3
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is timing optimized at both boost levels?
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Old 01-05-2009, 11:54 PM   #4
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Timing table stayed the same when I changed boost and wasn't optimized. Was just surprised that the difference in adding boost and changing AFR's to be richer made for a small HP gain. I edited my first post as I thought I was running 7 before, and that the change in boost was 4 psi, but it was actually only 3.

Here's the timing map I'm running.

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Old 01-06-2009, 12:06 AM   #5
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richer you go you trade horsepower for torque right?
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Old 01-06-2009, 12:44 AM   #6
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Originally Posted by johndoe View Post
richer you go you trade horsepower for torque right?
No. Richer you trade torque for safety.

The relationship between HP and torque is fixed. Specifically, HP = (Torque * RPM) / 5250.

For any given RPM, you cannot decrease one unit (HP or Torque) without also decreasing the other, nor increase one without the other. If one remains the same, then both remain the same.
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Old 01-06-2009, 02:42 AM   #7
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well u know what they say..... when its lean its mean.... but i keep it in the mid 11's to be safe that's what im at.... also you are pulling alot of timming no? at atmospheric im in 29 to 30 adv and for now since my car is only *** tuned (street stuff) im pulling about 1 deg per lbs and i think its safe.... im sure you could bump up that timing a lil. granted you have a big turbo i think you are pull a lil too much. it prob felt nice a 8lbs cuz you had more spark adv.. and that 12afr helped too... you dont really make power off of fuel.. ign is where it at..
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Old 01-06-2009, 10:08 AM   #8
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I built a conservative timing map, then tuned fuel on the street. After that I went to the dyno to tweak the IGN and then adjusted fuel. It took five (or six) pulls ($150) and I left with 30 more whp and 20 more ftlbs. The IGN map was the key, but I had no desire to try and tune the IGN on the street while trying to drive. Far easier on the dyno with two people IMO. Final AFR was 11.6-11.8 with 10* advance at peak torque, rising to 19* at 7k rpm on 93 pump.
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Old 01-06-2009, 10:09 AM   #9
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needs more advance and less fuel.
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Old 01-06-2009, 11:29 AM   #10
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Quote:
Originally Posted by patsmx5 View Post
Timing table stayed the same when I changed boost and wasn't optimized. Was just surprised that the difference in adding boost and changing AFR's to be richer made for a small HP gain. I edited my first post as I thought I was running 7 before, and that the change in boost was 4 psi, but it was actually only 3.
In the grand scheme of tuning turbo cars, the following will have the greatest effect on power in the order given:

1a. boost
1b. compression ratio (lets forget this for now)
3. timing
4. AFR

assuming you're only changing each by less than 10% and already in the ballpark of a decent tune.

in other words, adding 1.5 psi to a 15 psi car gains more power than adding 1 degree (to 10) of timing if you're below MBT and both increase (or decrease?) power more than changing AFR from 11:1 to 12.5:1
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Old 01-06-2009, 12:32 PM   #11
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Seems to rich to me Pat, im running 12:1 at 15psi.

IMHO anything below 12 is to rich, tune the spark to reduce knock, dont compensate with more fuel. If you have trouble controlling detonation with too much advance get water injection.

At least thats my take on it.
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Old 01-06-2009, 12:41 PM   #12
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i'm gonna disagree with sam, to a point. i'd rather have the best timing for power than the ideal AFR.

Why: because I think excessive spark retard causes as many problems as it solves. Start pulling timing and leaning things out, and you'll raise EGT and risk burning expensive parts.

I dont think running 11ish AFRs is a crime if your spark is well tuned and you're running relatively high boost.
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Old 01-06-2009, 02:46 PM   #13
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^ im on the same level as u and cupcar
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Old 01-06-2009, 10:44 PM   #14
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Quote:
Originally Posted by y8s View Post
i'm gonna disagree with sam, to a point. i'd rather have the best timing for power than the ideal AFR.

Why: because I think excessive spark retard causes as many problems as it solves. Start pulling timing and leaning things out, and you'll raise EGT and risk burning expensive parts.

I dont think running 11ish AFRs is a crime if your spark is well tuned and you're running relatively high boost.
Think- that's a fact. Running too little advance puts more heat in the cooling system and more heat into the exhaust. And from this, there are numerous other problems.

I agree with what you say. I guess I'm tuning it a bit rich so I can get spark dialed in. I tune for 11.5, then if I tune spark and it begins running a bit leaner, I won't go too lean as to possibly induce knock.

I need to get a new fuel pump in here. I had to dial in a ton of fuel in the VE table to keep AFR's around 11.5:1 in boost over 160kpa. I think I have values around 130%.
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Old 01-07-2009, 11:12 AM   #15
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Quote:
Originally Posted by y8s View Post
i'm gonna disagree with sam, to a point. i'd rather have the best timing for power than the ideal AFR.

Why: because I think excessive spark retard causes as many problems as it solves. Start pulling timing and leaning things out, and you'll raise EGT and risk burning expensive parts.

I dont think running 11ish AFRs is a crime if your spark is well tuned and you're running relatively high boost.
I agree.

But I am not saying to retard the timing and lean out fuel, one or the other. Personally id rather run a more ideal AFR then run a more advanced spark where knock is more prominent. Tossing more fuel then you have to into the exhaust seems like a waste. Also running to rich can hurt power as well. So where is the happy medium? I just think running richer mixture to control detonation is impractical and at that point it makes sense to use water injection. I have noticed better engine performance from more ideal AFR mixtures then more spark, even though I am running a pretty conservative spark map.
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Old 01-07-2009, 11:27 AM   #16
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I could run 12.5 AFR at 16psi and not get knock BUT I made way more power with more IGN advance and more fuel, while the EGTs dropped. 11.6-8 was definitely the sweet spot for my car at 16psi after peak torque.
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Old 01-07-2009, 11:45 AM   #17
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Originally Posted by m2cupcar View Post
I could run 12.5 AFR at 16psi and not get knock BUT I made way more power with more IGN advance and more fuel, while the EGTs dropped. 11.6-8 was definitely the sweet spot for my car at 16psi after peak torque.
Ill trust your experience.

I'm no expert in tuning, but from the reading I have done, its always been like "dont run too rich".
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Old 01-07-2009, 11:59 AM   #18
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Ill trust your experience.

I'm no expert in tuning, but from the reading I have done, its always been like "dont run too rich".
the point rob and i were trying to make is that you get more power from the timing than you do from the ideal AFR. sure, dont run too rich but more importantly dont run too retarded.
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Old 01-07-2009, 01:16 PM   #19
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So where does the knock suppression of richer mixtures come from?

If the mixture gets ignited by the spark, a flame front spreads out from the spark plug. This burning mixture increases the pressure and temperature in the cylinder. At some time in the process the pressures and temperatures peak. The speed of the flame front is dependent on mixture density and AFR. A richer or leaner AFR than about 12-13 AFR burns slower. A denser mixture burns faster.

So with a turbo under boost the mixture density raises and results in a faster burning mixture. The closer the peak pressure is to TDC, the higher that peak pressure is, resulting in a high knock probability. Also there is less leverage on the crankshaft for the pressure to produce torque, and, therefore, less power.

Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard.

Optimum mix with “later” ignition can produce more power because more energy is released from the combustion of gasoline. Here’s why: When hydrocarbons like gasoline combust, the burn process actually happens in multiple stages. First the gasoline molecules are broken up into hydrogen and carbon. The hydrogen combines with oxygen from the air to form H2O (water) and the carbon molecules form CO. This process happens very fast at the front edge of the flame front. The second stage converts CO to CO2. This process is relatively slow and requires water molecules (from the first stage) for completion. If there is no more oxygen available (most of it consumed in the first stage), the second stage can't happen. But about 2/3 of the energy released from the burning of the carbon is released in the second stage. Therefore a richer mixture releases less energy, lowering peak pressures and temperatures, and produces less power. A secondary side effect is of course also a lowering of knock probability. It's like closing the throttle a little. A typical engine does not knock when running on part throttle because less energy and therefore lower pressures and temperatures are in the cylinder.

This is why running overly-rich mixtures can not only increase fuel consumption, but also cost power.
thug for life.
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Old 01-07-2009, 02:39 PM   #20
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thug for life.
Basically you put out the fire with too much fuel.
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