Dyno Correction Factor - Does it mean anything for a Computer Controlled Engine??? - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 9 Old 04-14-2002, 10:41 PM Thread Starter
 
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Dyno Correction Factor - Does it mean anything for a Computer Controlled Engine???

I did a search and didn't find any posts relating directly to this matter. Maybe someone can help me work through this...at least in theory.

Say you dyno on two different days that have substantially different ambient conditions. As conventional correction factors such as SAE, STD, DIN, etc all are based on various ambient conditions, the chosen correction factor takes into account ambient conditions such as humidity, temperature and absolute pressure.

From what I've read, these correction factors are very accurate for a car with mechanical or engine speed designated advance and a carb as timing is largely unaffected by ambient conditions and fuel delivery is largely dicated by the ambient conditions.

However, for an engine controlled by a computer, the computer is also making timing and fuel changes based on these same parameters in an attempt to optimizing emission, performance, fuel economy, ??? for current conditions. So, it seems to me that dynoing on days with significantly different ambient conditions may lead to data (between the two days) that isn't comparable even even when corrected.

For a 4.6 Cobra, here are a few parameters that are based on/affected by ambient conditions:

Volumetric efficiency manifold fill
MAP Slope
numerous timing parameters
spark barameteric pressure bias

It seems to me that tuning on a day with exceptionally good or bad air may lead to a tune that is less than desirable for 'normal' conditions or real ambient conditions outside a climate controlled dyno room.

Anyone have any thoughts on this???

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post #2 of 9 Old 04-15-2002, 12:42 AM
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I think the EEC can only compensate so much. It can't reproduce the ideal conditions under which max performance is achieved.

Everyone has seen their car slow down 3/10ths on a really hot, muggy day. Sub-optimal air is just that...it's crappy air. The EEC has a tendency to attempt a balance (in stock form at least) of emissions, efficiency, and performance (in that order). Basically, any correction factor assumes there is a condition(s) that can be taken into account, which may or may not vary from one combination to another. (vague) A correction factor doesn't consider the EEC's attempts to reach it's goals - it merely allows for sub-optimal conditions at the time and tries to estimate the output at "standardized" conditions. In a nutshell, the correction factors can produce a number "close" to standard conditions, but not "dead on" with a computer controlled car. Correction factors are, however, still useful.

Very, very thoughtful observation, though. Makes me realize the futility of tuning for 3-9 horsepower.


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post #3 of 9 Old 04-15-2002, 01:49 AM Thread Starter
 
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Quote:
Originally posted by lo pony
The EEC has a tendency to attempt a balance (in stock form at least) of emissions, efficiency, and performance (in that order).

Makes me realize the futility of tuning for 3-9 horsepower.
Lo Pony - Thanks for the comments. Your two comments quoted above are exactly what I've been thinking about. I've been discussing this topic with a friend and he thought the computer addressed ambient conditions in the exact order that you mentioned - emissions, efficiency, and performance.

I'm wondering if an engine with carb and mechanical distributor sees less of a differential in performance at the strip between good air and poor air than does a Ford EEC controlled engine? Perhaps, someone here can answer this question.

It would seem that an EFI engine 'could' see a computer-induced 'power loss' in poor air (i.e. more than just that attributed to poor air as the computer is sacrificing power for emissions) and a computer induced 'power gain' in good air (i.e. from optimized fuel/timing not practical in lesser conditions).

Seems like unless it's a same day evaluation or different days/same conditions, the results may not say much. In fact, there seems to be the chance of detuning the car relative to typical ambient conditions.
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post #4 of 9 Old 04-15-2002, 02:34 AM
 
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A very interesting theory - something I never really put together until I read this.

Here are some thoughts. First, a computer controlled engine can only do so much, yet it can do a heck of a lot more than nothing. We've all driven non-controlled carb equipped cars (or at least us old farts). The change with air density, esp altitude, is dramatic with those cars.

Think about this though - one thing a modern FI engine can't do is increase compression ratio - which, all else being equal, is a huge factor in hp output. Ok, so the engine gets cold, dense air. The computer can detect this with the MAF and adjust A/F and timing. But it's goal is still to maintain 14.7:1 given the O2 sensor feedback. If the conditions persist long enough, it may affect WOT output a bit as the long term trim values are used in open loop calculations.

But it's fleeting. A really hot day does the opposite - even worse because the air density is less AND the air temp is up. So the PCM pulls fuel and timing back out.

Now back to the theory. If the engine was baselined for programming at the same conditions as the SAE correction factor table, then all is well as neither makes a correction. Beyond that - I'm not sure we can know. If I were to design a very high output engine (e.g. high compression, high rpm) I might take even greater steps to protect that engine under extreme conditions - I may pull more timing back and watch air temps and knock sensors like a hawk. In most cases like this, detonation is the enemy. I'll let the feedback system take car of the mixture (and hence emissions), but I'll cut power back much farther than the SAE correction. So I'll net even LESS corrected hp - as there is effectively double correction.

A less powerful engine may have enough inherent margin such that I don't need to reduce output as much. So it's corrected numbers, as you suggested, may be higher. But it's all relative, as this engine would produce less hp anyway - hence you couldn't tell just by the results.

Bottom line: the PCM cares most about emissions (as mentioned) - but primarily during closed loop. In doing so, it corrects for the conditions at hand. We can't know what impact a set of conditions would have on the output, unless a Ford PCM expert wants to chime in, or someone takes a Mod motor and slaps a carb on it. (ugh)

How's that for a long winded, verbose, non-answer?
Heck, I was just typing as I was thinking, and it's late. So take this all with a grain of salt, or a bit of memory.

- Dave (SCOA #1474)
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post #5 of 9 Old 04-15-2002, 02:54 AM
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Dave - you beat me to it!

What's been said here about EEC-governed cars vs. mechanically-goverend is absolutely correct to the "T".

I remember back in the "good ole days", we bracket raced a nearly stock 5-oh. We won some money that season, so we were evidently doing something right. We could control the ET of the car by the temperature of the intake plenum. We could toss a bag of ice on the plenum and get is as cold (and consistent) as possible. And it was VERY consistent! If we monitored the coolant temp, we even knew how much to change the dial-in if circumstances necessitated. But, in Alabammy in September, when the temp is 95 degrees and the humidity is the same %, we'd see a horrendous drop in the car's performance no matter what we did. It might vary as much as 3/10ths sec. in the EIGHTH! We could, however, still make the car consistent (even though it was slower) with the bag of ice.

This was with speed-density. It's even gotten trickier since MAF.

With MAF, Ford can meet emissions for a variety of conditions, but the performance criteria haven't changed! And the importance of performance has even lessened over the years to Ford! Automakers' priorities must change as the FDA mandates. No matter what anyone says, MAF was invented solely from an emissions point of view, and not performance. Think of it - speed density and MAF both attempt to do the same thing...adjust fuel according to what goes into the engine. One does its measurements at the exhaust, and the other at the intake. How redundant it is to have both systems in place at one time??!!

So basically, yes - you get an over-correction by the EEC on performance to keep emmissions and economy in line. If you try to allow for this to get increased performance, without considering a wide range of operating conditions, you "over correct" an existing over-correction.

Makes you wonder how ANYONE can design an efficient EEC program at all (although, it IS done).

The serendipity involved with racing.

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Last edited by lo pony; 04-15-2002 at 02:57 AM.
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post #6 of 9 Old 04-15-2002, 03:18 AM Thread Starter
 
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Quote:
Originally posted by Bower

How's that for a long winded, verbose, non-answer?
Heck, I was just typing as I was thinking, and it's late. So take this all with a grain of salt, or a bit of memory.

I think you make many good points. And, this is a tech forum. Long-winded, vebose non-answers that have great content are always welcome, IMO.


Quote:
Originally posted by Bower
I'll let the feedback system take car of the mixture (and hence emissions), but I'll cut power back much farther than the SAE correction. So I'll net even LESS corrected hp - as there is effectively double correction.
This is a much more concise description of what I was thinking. Essentially, a loss in power associated with less oxygen for combustion and less power due to the computer compensating to maintain emissions/fuel economy.

As a SOTP example, when I had the SC on my car, it would be an absolute beast in cold weather (35 degrees and low humidity) compared to the same set-up on a 70-75 degree day with typical humidity. Seemingly, a much greater differential than the correction factor would dictate. Let's say the corection factor equates to 5% more HP. That's about 23HP for an engine making 450 rwhp at typical ambient conditions. I'm not sure I could even feel another 23 peak rwhp on top of 450 rwhp
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post #7 of 9 Old 04-15-2002, 03:26 AM Thread Starter
 
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Quote:
Originally posted by lo pony
With MAF, Ford can meet emissions for a variety of conditions, but the performance criteria haven't changed! And the importance of performance has even lessened over the years to Ford! Automakers' priorities must change as the FDA mandates.
Again, excellent points.

Although, it's 2:30 am CST and I'm laughin' my a$$ off at your reference to the 'FDA'. We might be better off with the Food & Drug Administration (FDA) regulating emissions than the EPA
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post #8 of 9 Old 04-15-2002, 10:59 AM
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Yeah, I just noticed that myself.

Sometimes work and play overlap, even unintentionally.

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post #9 of 9 Old 04-15-2002, 11:16 AM
 
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Quote:
Originally posted by lo pony
.......

Think of it - speed density and MAF both attempt to do the same thing...adjust fuel according to what goes into the engine. One does its measurements at the exhaust, and the other at the intake. How redundant it is to have both systems in place at one time??!!

.......
Ah, SD systems. But there is a difference - esp for those of us that mod our cars. Speed density PRESUMES the air flow and volumetric efficiency in the engine. In this case, it's all preprogrammed maps and tables, with the O2 sensor providing feedback during closed loop. Now, add free flowing intake/exhaust, and those pre-programmed values are just plain wrong. In cruise, closed loop conditions, the O2 and the MAP will keep everything in line, mostly. But cold temp operation, WOT, and many transient states will result in lousy drivability - or worse, holed pistons.

I, too, often wondered what the MAF is really for (in non-SD systems) - given the O2 sensor feedback. But I've figured that the O2 is after the fact, while the MAF is giving current, real time information to the PCM. Compensates for that humid, hot day - or the cool dry morning, and for altitude. The O2 is used to *trim* the result for the next engine cycle. Plus, let's not forget about ignition. With the MAF (along with air temp and throttle position), the PCM can control the ignition curve based on air flow and rpm.

But the best part about MAF systems is that if you mod your car, the PCM will learn about the new airflow capabilities. Within limits, of course - as you can increase airflow to beyond the measurement limits of the stock MAF (those with blowers need a re-calibrated MAF). Once the PCM learns, the long term trim values are alterted - adding more fuel as needed to correct for that really cool exhaust you just put on.

But here's the downside: some aftermarket vendors use that to their advantage. Take our beloved Cobra. It's programmed to run a bit rich at WOT and high rpm - probably intended to add margin in case of detonation (recall that the knock sensor reacts AFTER the knock - not a good thing all the time). So, Ford chose to sacrifice a few hp in favor of safer engine operation.

Ok - let's say I install a Ronco muffler bearing. I do a dyno run before the install and just after. Low and behold, I pick up 10hp!! And looky here - my A/F mixture leaned out a bit at high rpm. Cool! Ah, the catch. Once the PCM learns about this mod, the long term trim will correct, and eventually the mixture will go back to the pre-programmed values (e.g. - the PCM is programmed to go xx% richer than the trim values at a certain rpm and throttle). So the results are fleeting - esp if there is no real change to the airflow. Aftermaket MAF vendors are famous for this slight of hand. If there is no improvement in airflow, then there cannot be hp increases other than for a short time.

But, beyond that example - improve breathing, and the PCM will add more fuel as needed. More air, more fuel, more power.

No more need to futz with carb jetting or timing!

It's still amazing to me - we get 320hp from 280 cubes. With great mileage, and exhaust so clean you could almost breath it (well, if you like CO2 and water). Isn't electronics grand?!!

Good thread guys - got the brain working for once.

-Dave
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