Honda and Acura Car Forums banner
1 - 20 of 23 Posts

· Registered
204 Posts
Discussion Starter · #1 ·
I read on the following link that you can't properly dyno an OBDII car. It meantions something about the ECU adapted to different areas of the map depending on your driving style.

Check the link out:

This is an excerpt from the link
See, previous to OBD-II, adaptive information was stored in
battery backed SRAM. Unplug the DME to swap chips and woosh
all adaptive factors set to "nominal" (1 for mult. and 0 for add.)
Then came Kalifornia, and KARB, and OBD-II.. and EEPROM for
storage.. this means you can't just "erase" it anymore because
the last thing the DME does during shutdown is write the SRAM
locations to EEPROM.. and guess what the first thing it does is.

Yup... zero SRAM, run a galpat memory check, and reload the
needed ram locations from EEPROM.

The only way to clear the adaptive locations is via diagnostic
commands down the serial port (even desoldering the main
FLASH memory program store doesn't do it, because there is a
separate EEPROM elsewhere)

So if you can't clear it.. you can't do an apples to apples

What's worse is the "quasi-tuners" out there who have no idea
how it all works, spouting nonsense..

Either you trust the person your buying it from, or don't
buy it. It really is like "Natural Flavors" in soda...

Either you trust the manufacturer, or you put the can back
on the shelf, because it's not easy to quantify the contents.

I'll take my "Natural Flavors" in Surge or Mountain Dew flavors ;)

(Unless you carry Jones Soda, and then I want rasberry!... YUM!)

Now.. back to adaption.. since you can't clear it
you can't get a valid comparison.. want to try something..

Take your car.. OBD-II.. chipped or stock.. drive it one week
like a MADMAN and dyno it.. then drive it for another week
very meekly.. and dyno it.. compare the results.. and they
WILL NOT be anywhere NEAR the same.. and we didn't change the
car at ALL..

Why?? (Prof. Shark asks the class)

Exactly.. the car ADAPTED to different areas of the maps
and either enrichened or enleaned the overall mixture.

Now.. in whatever state the car is in, you can be rather
assured that ON AVERAGE the chipped car is making significantly
more power than the stock car.

Adaption works like this.. at lower loads and rpms it watches
the AVERAGE correction (realize we swing rich and lean of
stoichiometric continuously, except for WOT)... and it adds or
subtracts a bit of fuel until the low speed fuel mixture is
swinging evenly around Lambda = 1.0 / AFR 14.7:1..

Now as the rev range increases.. we switch to a multiplicative
correction which adjusts (if you will) the SLOPE of the fuel
curve by multiplying the STORED values by a factor (based on 1.00)

Depending upon how good the manufacturer tuned the low end and lower
midrange, the environmental conditions, the fuel you are using,
and the flavor of soda I have in my fridge (*grin*) .. well really
what AREAS of that lower speed area you are in most of the time
during the drive cycles.. that determines WHAT those two magic
numbers will be.

The kicker.. and bummer.. is that even though lambda control isn't
active, and adaption isn't HAPPENING at high throttle angles, those
two numbers ARE STILL USED IN THE CALCULATION regardless as an
"overall correction" to fueling.

So.. let's say that at 20% throttle and 3000 rpms the manufacturer
is a bit rich.. the system adapts lean.. if you drove at very light
load/throttle for a while at 3000.. you might see your peak HP
drop 10HP on the dyno.. once the car adapts

Conversely, let's say the manufacturer is a bit lean at 2000 and 50%
and you spend your week in LA traffic jams always gassing it in 4th
and not downshifting.. car goes rich.. now you dyno your car and
BOOM magic horsepower have appeared (well.. not really)

Simply put.. while any chip manufacturer can tell you that 90+% of the
time your chipped car will have more HP than the stock one, unless you
have cleared the adaption BOTH times when you run the cars you can't
quantify it at all.

· Registered
1,104 Posts
That's all very nice... Though, I'm not sure that it's true.

I thought that the computer had enough fuel and ignition maps to keep the entire range of settings. So if you caned your car around a track at WOT, the car would keep advancing timing, until it heard knock, then back off. It would then store that setting.

If you then drove it for a month at a nice sedate 30% throttle acceleration, blah blah blah, then it would store that profile down. The second you slam WOT, it would revert back to the map it learnt a month ago - on the provisio that it would still detect for knock and advance or retard timing accordingly.

Also, the bit about wiping them may be true for BMWs, but I've been reassured many times, that pulling the fuse to the ECU for 30 seconds or more, wipes any "memory" it had, and has to relearn where the knock point is.

· Registered
29 Posts
on all computers(obd0 obd2 obd3 etc) the maps for Air to fuel are preset. Depending on the amount of air, it is switched to a preset fuel. Its the fuel curve. This is why you reprogram an ecu or "chip it". This changes those preset fuel curves to a more agressive level.

· Registered
29 Posts
of course. When they dyno your car, they will do 3 runs. The resason that they need to do three is because your car WILL be a bit different in each run but it has nothing to do with your ecu. In each run you will see about a 5-10 hp differance, this is basicaly a differance in launch. They will take the average for the three(or the best of the three depending on the shop) and thats your resault.

The New BMW(and from what ive been reading the new IS430) are the only "smart chip" ecu's i know of. Even them its not a matter of hurting preformance, but rather for fuel economy. It judges your you average throttle position for 30 sec. and then adjusts your fuel to compinsate, but at WOT its smart enought to max your safe levels.

Good luck on your dyno, let us know.

· Registered
1,104 Posts
I did a bit more research on this one, since you really got me curious. And I found out that you're right - to a point.

OBD-II computers have two states. Closed loop and Open loop. That probably means jack all right now, but I'll explain in a second.

When you are cruising, your RPM and MPH doesn't change. In fact, the air temperature doesn't change, the fuel quality doesn't change and not a lot changes at all. Under these conditions, the computer has enough time to read things like airflow, air temperature, exhaust temperature, oxygen sensor and it puts it all together and tries to run at the most fuel efficient settings possible. It'll advance timing, lean out the fuel mixture, whatever, until it gets the cleanest exhaust emissions. This will be stored in the ECU map as described in the article you published. It will then be used for the base to start analysing and correcting again next time you drive.

So in that regards, the article is right. The ECU has a memory and is self correcting. That's called closed loop, because the process is circular: Read environmental variables, make adjustments, read new environmental variables, make more adjustments, read adjusted environmental variables... and so forth.

The ECU also has what's called an Open Loop. This is where the computer ignores the environmental variables and uses a predefined set of maps. This is because the computer can't keep up. For example, if you are at wide open throttle at 3000rpm while launching. The computer reads the O2 sensor, decides that you're too rich. Adjusts the fuel ratio to be leaner. By this time, the engine is already at 4000rpm and a whole new ball game applies. You may actually be running too lean here and running even leaner can destroy your engine.

So the factory does the tests, puts in a wide margin of error in the preset map and conditions they determine to be Open Loop, the computer defaults to static settings.

When does an ECU go to open loop? It depends on the car. For S2000s, it is when the throttle position sensor is at 25% or greater, or when the RPM gets about 5700 (All of VTEC)

We all know (or should know) that dyno runs are done completely at WOT. So with the exception of the very low RPMs, anything done on a dyno should be in Open Loop. That means that results should be consistent and dependant only on environmental variables such as ambient air temperature, humidity and fuel quality.

Is there a way you can tune closed loop? Yes, you need to alter the sensor signals back to the original ECU. But realistically, this shouldn't be necessary, because

1) You don't race on <25% throttle
2) The computer should be making the best choice for you anyway

· Retired *********
4,663 Posts
Sunder, you got it backwards.

Closed loop: Based on the O2 sensor, the ECM determines the air/fuel ratio and controls the amount of injected fuel.

Open loop: Ignoring the O2 sensor output, the ECM/PCM refers to signals from the throttle posion sensor, MAP sensor, and other various sensors to control the amount of injected fuel.

The reason for the ECU to run in closed loop mode is to give it a chance to update the LTFT (Long Term Fuel Trim). The STFT (Short Term Fuel Trim) runs in real-time and records the changes the ECU makes to the fuel injector pulse width to the keep A/F ratio around 14.7:1 ( the chemically correct ratio of air to fuel for complete combustion to take place) while in closed loop mode. Over time the ECU will look at the STFT to determine if the engine is running rich or lean overall. The ECU uses the info from the STFT to update the LTFT and allow for changes occuring in the fuel supply system and environmental conditions over a period of time.

The ECU only runs in closed loop mode when the engine is under little or no load i.e. when crusing at a constant speed or at low (warm) idle.

In both open and closed loop the ECU uses the LTFT to apply a correction to the base fuel maps. The max amount the LTFT can adjust the fuel maps is approx. +- 30%.

Resetting the ECU will erase the current LTFT. The ECU will use the base fuel maps (0% correction from LTFT) until it has enough info to update the LTFT.

· Registered
1,104 Posts
2000 EBP Si said:
Sunder, you got it backwards.

Closed loop: Based on the O2 sensor,

Open loop: Ignoring the O2 sensor output, .

Umm, isn't that what I just said?

The problem with the way you're describing it, is that in effect V-AFCs and similar fuel tuners then wouldn't work. After the first "honeymoon" period, the computer would have relearnt the signal tampering the fuel controller is doing update it's LTFT so that it's back to stock...

So how does that work? I'm pretty sure that on open loop, no memorised data is used and only factory programmed information is used.

· Retired *********
4,663 Posts
Sunder said:
Umm, isn't that what I just said?

The problem with the way you're describing it, is that in effect V-AFCs and similar fuel tuners then wouldn't work. After the first "honeymoon" period, the computer would have relearnt the signal tampering the fuel controller is doing update it's LTFT so that it's back to stock...

So how does that work? I'm pretty sure that on open loop, no memorised data is used and only factory programmed information is used.
I think I just misread you post, sorry :)

Anyway, fuel controllers work because they mod fuel delivery at a certian RPM. The LTFT is not RPM based, the correction covers the entire fuel map. If you increase fuel between 3K+4K RPM, the LTFT will cause the ECU to decrease fuel across the entire RPM range. The only real job of the LTFT is to correct for changes in the engine and fuel system over time.

The problem with fuel controllers is that the ECU will always try to average out the changes you have made.

The LTFT corrects the fuel maps all the time, open or closed loop. For example: let's say that you are driving on the highway (in closed loop mode) and your fuel pump goes on the fritz. As fuel pressure slowly drops from 40 psi to 30 psi, the ECU slowly increases the fuel injector pulse to keep the A/F ratio at the predetermined point. These changes to the STFT are averaged into the LTFT. You decide to pass another car and you step on the gas causing the ECU to go into open loop mode. If the ECU didn't use the LTFT to adjust the fuel maps in open loop mode, your engine would immediately run lean and would probably fry a piston or worse.

Here is a pic I borrowed form Hondata.

The red line chart is the voltage reading from the O2 sensor, the ECU is trying to keep the output around 0.60 volts.

The blue line chart is the STFT. You can see the ECU lean the mixture out to to keep the O2 sensor output around 0.60 volts.

The purple line chart is the LTFT. Notice how the amount of correction is an average of the STFT.

Edit (I forgot to include this in the original post): the charts are from a ECU running in closed loop and cover 4 sec. of time.

· Registered
33 Posts
I cant seem to find the link where I read this but....This link was discussing this subject but in much less detail. It was focusing on "resetting" the ecu. You guys stated that Fuel Controllers are hindered by the averageing of the changes that are made. If im understanding you its because of the adjusting fuel maps. This site was refering to a honda prelude and wiring in a switch that has the same effect as removing the 7.5 radio fuse. This switch would allow you to reset the ecu and not have to reset your radio and things like that. Is that an easy way to get around the adjusted fuel maps by using this switch ever so often and esentially keeping the ecu from remembering anything or is that just bogus? I have been meaning to ask this very topic but I was beaten to it.

· Registered
1,104 Posts
mls said:
I dont need to read the article to tell you that it is bullshit that an OBD-2 car can not be dynoed properly.
Well, in that case you'll remain ignorant for the rest of your life.

Keep an open mind.

This switch would allow you to reset the ecu and not have to reset your radio and things like that. Is that an easy way to get around the adjusted fuel maps by using this switch ever so often and esentially keeping the ecu from remembering anything or is that just bogus?
I've been thinking about that... The computer only alters the signal in open loop (Which is why in the VAFC you have a lo and a hi setting). At that point the computer isn't learning so the modifications you made, are not actually going to impact the LTFT.

So, the changes you make should be permanent, as long as you set your "lo" settings all to neutral.

But yes, you could wipe your ecu, tune your car, then every now and again rewipe your ecu to get the best settings.

Problem is, as one person pointed out - what if there is genuinely a condition, like a blocked filter, that changes your car's requirements?

· Registered
2 Posts
05 tC is right basicaly this is what I have found also.

Us poor slobs with the 7thgenCivic OBD2c try to add more air to our system.
What we get is more air across the RPM range but what the ECU registers is more air across the low to mid range. This is due to the fact that under 80% throttle or closed loop the ECU is seeing more air and upping the fuel. All well and good you say. But try and get up to higher RPMs without going into open loop or 80% or over. As soon as you hit 80% throttle the last known LTFT (Long Term Fuel trim) is locked in and the map is adjusted accordingly.
So if you need to lean off a bit up top this will not hapen as the ECU has locked in +10 because you are pushing more air at 3000RPM. :bash

The only way to fix this is to tune out all the the STFT (Short Term Fuel Trim) + and - and get LTFT as close to 0+- as possible. :hug Good luck.

I have been doing this over the last 6 months and it is hard work. Fuel changes and Air Temp Changes really F*ck it up.
When it is right it kicks ass though :byye
1 - 20 of 23 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.