A/F Question. How does it change

[Mr Hemi Head]

with air temps? After installing a 6.5lb pulley and Roe PCM, street tuned for a flat 11.5 (3rd gear) when the air temps were around 80 deg.

On the dyno tomorrow at 10AM, temps will be closer to 55 deg.,relative humidity will be about the same (50%). Viper runs great so I won't mess with the tuning. Just want to compare W/B Commander with the Dynojet and get final HP numbers.

Will the A/F be higher or lower?


Rick

 

[99 R/T 10]

A/F should be higher with the more dense air. Maybe 11.8-12.0. My guess,

 

[KenH]

I have definitely seen mine lean-out as the air temp has dropped. The AIT sensor is suppose to help compensate for the air temp, but it does not seem to be enough. I'd guess that you will be up in the 12.0 - 12.5 range on your A/F.

It would be nice to come up with a simple rule of thumb for adjusting the fuel load based on basic air temp. Let us know how your A/F maps with a 30 degree drop.

 

[Schulmann]

Mr Hemi Head, don't forget to post your Dyno results to-morrow !

Even if the air temperature is 55F, likely your Viper will run hot and consequently rich on the Dyno. I am still waiting the temperature to drop below 40F before I go on the dyno. At least I will get closer numbers to the real world.

 

[Mr Hemi Head]

will post the results tomorrow.

I plan on taking exhaust manifold temp. readings as the car is warming up on the dyno (Raytek Autopro). What kind of values do you expect especially #10 cylinder?

Plan on data logging so I will have all the stats.

In a previous dyno pull the A/F was 10 between 3K and 5K rpms.During the drive home A/F was 11.5. Whats up with that?

Prior to the Dyno tomorrow I will attempt (depending on track conditions)some 3rd gear runs for comparison purposes.

 

[Schulmann]

It is normal that your car run so rich on the dyno but lean it out ! You can gain up to 10% more HP. Once the dyno session is over you will have to retune your viper for road use. With the methanol I usually try run in the 12.0-12.5 AFR.

It is a good idea to have a tool to measure the temperature. I always do that. Check also the header's temperature that can give a precious indication on your cylinders performance.

Usually on the highway my supercharger is around 110F. On the dyno its temperature can easily go up to 180F. That's a lot. The temperature depends on the number of dyno runs. Somebody just posted 3 days ago an information about 15 dyno runs. This guy has even tested a 10lb pulley during the same dyno session without gaining a single HP. In my opinion the cause was the temperature of the engin and the supercharger. Limit your dyno runs to 3 or 4, then take an hour break and cool down your engin. You can even spray water on the engin. The best is to have a top quality ventillation in the workshop.

The issue at cylinder #10 is a firmware issue. Once you upgrade the firmware in the VEC2 your troubles will go away like bad dream.

There is a guy in somewhere Texas, Bob Norwood, who tunes turbo Ferrari. He has done a lot of tests on his cars and blow up several of them. There is a book that talks about his work.

The book is: Jeff Hartman
"How to tune and modify Engin Managment System". There is some interesting information in that book about leaning out engins.

Another thing: If you will run on an exotic ignition timing, try to get some LV unleaded race fuel if you have time. It will not give you more HP but will insure a safe dyno session. Once the SC gets hot you have more chance to detonate.

 

[Mr Hemi Head]

Thanks Schulmann. Plan on three pulls tomorow, no tuning. Never been to this dyno facility so I'm not sure what the shop temps will be.

The Shop is Peter Farell Supercars and their specialty is turbo charging RX7s.
According to their web page they get 500+hp out of a rotary engine.

My Viper may have performance anxiety with all those RX7s.

 

[Anaconda]

If the air is cooler (and denser) wouldn't the computer realize that and adjust accordingly?

 

[Schulmann]

If the air is cooler (and denser) wouldn't the computer realize that and adjust accordingly?

I wish it would.

 

[davem]

The PCM does correct for inlet air temp. But now you are forcing 2x the air into the engine, so the correction is off by approx one half. The PCM calculates the mass of the air the engine is consuming based on manifold absolute pressure, inlet air temp, and volumetric effeciency (other stuff too, like engine temp). The calculations are all off due to the fact you are out of range of the manifold pressure the PCM, and are forcing at least twice the mass of air into the engine.

So the PCM does try to richen up based on cold days, but it only adds enough extra fuel on a cold day for a normally aspirated, unmodified engine. It needs to add approx 2x the cold enrichment. I usually tune 11.5 or richer on a hot day to make sure the car is safe when it's cold.

good luck,
Dave.

 

[Schulmann]

Dave,

I agree with you and this is the reason why dyno tuning is not accurate.
Espectially the roe supercharger gets very hot under idling during long periode.

But I have something to point out:

Cold air means higher boost (as much as 1 psi)
Under higher boost the VEC2 adds more fuel.
So the compensation should not be as bad as it is.


But under vacuum the problem is real.
I just did 2 posts the last week about this issue but not a single person answered. The temperature dropped from 70F to 28F in a night. I was unable to start my engin. Once I cranked it it was running lean like 22 AFR at idle. That's the lean burn limit of any type of fuel. I had to compensate with the VEC2.

My PCM is running rich in hot weather and lean in cold. Tell my why ? This problem is very obvious on days when temperature varies a lot espetially in sprint and in autumn.

Anyhow once the engin is running the PCM gets used the temperature by updating its fuel map after 4-5 hours.

 

[Cris]

The answer depends. It depends on how you are adding the extra fuel. If by larger injectors with the stock cal and some delta adjustments by a VEC or equivalent, if by adding fuel pressure or by creating a completely new calibration.

For mine, The Paxton Gen III setup, extra fuel is delivered via additional pressure and using the stock calibtration for fuel injector pulse width. In this case the inlet air temp sensor is positioned after the supercharger and intercooler, thus it sees the air temperature entering the engine. In this case the engine controller adjusts the fuel injector pulse width (and base timing) based on the inlet air temp and other factors. A change in inlet air temp is met with a change in pulse width. Since the injector flow is proportional to time and pressure, the change in pulse width nets the desired change in fuel. Thus the Paxton system is relatively unaffected by temperature.

On the other hand if you have a Roe system the inlet air temp sensor is in the air cleaner before the supercharger. And there is no intercooler. The engine controller will see ambient temp changes but does not know about the higher than normal inlet temps due to the supercharger (both pressurization and conductive heat transfer). It adjusts the fuel injector pulse width based on the inlet air temp. The VEC then adds a set amount of pulse width based on its cal factor, again not noting the true inlet air temp. The delta offset from the VEC is temperature adjustment free, and the base cal from the PCM is only adjusted for the ambient temp. Thus the a/f will not be well controlled for increases in temperature that are not directly proportional to the ambient air temp change.

The same applies to ignition timing as well. The combination of these two means that the Roe must be calibrated very safe so that in cool conditions that it avoids high a/f ratio and resulting detonation. That results in a very rich and less than optimium spark timing when in warmer realms.

 

[Schulmann]

It would be an interesting test to move the air temperature sensor to somewhere else. I don't know how it would impact the overall effectiveness.

 

[Jack B]

Schulman:

There have been numerous post relative to quirk in the viper PCM. Lets just discuss warmer days. Like many other people on this board I do a lot of logging. When the temperature goes up the air is less dense, therefore, the a/f should go richer automatically if the PCM did not make a correction . The obvious is that the PCM will pull fuel and the a/f will remain constant. Here is the issue, somewhere around 80 degrees the PCM reacts out of proportion to the temperature, however, instead of pulling fuel it adds fuel to an already rich mixture. In other words, it does the opposite of what it should be doing.

I cannot comment on the cooler temps, but, I personally know of multiple cars where the PCM seems to react contrary to logic at about 80 degrees. If I tune for under 75 degrees, the car will go almost a full point richer when the IAT is in the eighties. To further complicate the problem, if you are in traffic the viper will typically have IAT's in the 90-100's. If you now go wot you are extremely rich.

I am going to do a little tuning before I put the car away for the winter. I will find out how the car reacts on the cold end.

 

[Mr Hemi Head]

Jack brings up an interesting observation. The IAT is "running the show" based on airbox temps.

Does W/M, cooling the air after the IAT, complicate tuning?

Like Schulmann suggests, maybe a different type of sensor located after the W/M nozzles.

 

[Schulmann]

As JackB points out even if we relocate the IAT sensor above 80F its algorithme is diffective. It will likely worsten the situation.

Is there a fix for this PCM issue ?
Can a dealer reprogram the PCM so this trouble goes away ?

I talked to a local dealer Sunday about this problem. He had no idea about what I was talking about. He even didn't know that 00/01/02 PCMs have ignition issues.

 

[Mr Hemi Head]

Someone on the West Coast was reflashing the PCM that deleted the adaptives. Would that help?
The AEM system is looking like the next option.

 

[KenH]

I am revisiting this to see if anyone has any updated info on tuning the Roe SC over a temperature range.

In my case, I literally have to retune the car for about every 10 degree change in air temp. A 10 degree swing seems like it is enough to cause my conservative 11.5 A/F tune to swing to high 10's or low 12's depending on which way the temperature is going. When it goes rich, the car no longer runs very well at WOT and starts cutting out at arounjd 10.8 A/F. Not sure if my setup is unusually touchy, or if the water/methanol injection aggravates this or what. It seems to be especially touchy up in the 80-100 degree temp range. Is everyone that monitors their A/F seeing the same problem? Other than having 5 programs and continually swapping them in and out of the VEC2, has anyone come up with a more clever way to deal with this issue?

BTW, anyone know whatever happened with the in cabin card reader for the VEC2?

 

[Qualitywires.com]

If the air is dense from temp dropping, that would cause you to run a bit on the leaner side. The air dense air once heated up will expand.

 

[Schulmann]

I know that Sean was working on a fix to take into account the temperature change. However it is not so easy to build a good algorithm. And anyhow it is not a major trouble.

The only issue is when the temperature changes more than 20 Celcius (or 30-40F). Othewise the AFR change is minimal.


Once my intake temperature dropped from 120F to 20F in a night ... I had hard time to start my engin and I had to give a little more fuel with my VEC2. Here in Canada temperature can change really quickly as much as 60F ! For instance this saturday my intake was 160F and two days before it was 100F. My AFR changed from 12.1 to 11.3. If I am not racing I don't care about. Loosing 2-5% hp on the street is not a big issue.


However I noticed that ground gets very weak when underhood temperature is above 160F. In this case AFR drops well below 11.0 ! So maybe your AFR drop is due to a grounding issue. I installed a 1200W stereo and got strange issues. After 3 weeks of searching I noticed that my Viper had a grounding issue. Not only the stereo but the engin too. I also posted a thread on this but nobody answered. Usually ground is 0.3 ohm. When engin runs it gets up to 2-3 ohm. When running hard and under hood temperature gets above 160F, the ground can weaken to 20-30 ohm !!!! I just reworked my entire grounding on my Viper and this Saturday with the high temperatures my engin was running unbelivable stable.

I added 4 new grounds to my Viper:
- Vec2 got its own ground and its own +12V
- Engin got 2 new 4 gage grounds
- My two WBC got a new ground
- My PCM got a new ground


These days I worked a lot on my ignition. I got a couple of very good programs for 91 and 94 oct fuel. I leaned out my AFR to 12.0-12.3.

 

[KenH]

Thanks for the info Schulmann. Unfortunately, my car doesn't just lose a little bit of power, it starts cutting out pretty heavily at WOT until I retune the car when the temp goes up. Your idea on grounds getting weak when the engine is hot is an interesting one. I'll definitely look into that.

 

[Steve 00RT/10]

While I am new to this game, we too get big temp ranges. 30-100F or so. I installed the Roe the end of March--beginning of April. My first 'good to go' completed card was 4/20/06, done in 50 degree temps. and snow on the ground. I noticed a difference at 75F or so and a big difference above 90F-------running way too rich on the WBO meter. I now have 3 cards. 55,75,90+ degrees. Even at 75 degrees, I was running solid mid 10s on a card that was zeroed in at 11.5 in April. To me, the drop off in power was noticeable below 11 AFR. I will likely need one more card for the late fall rides when we start out at 30 degrees in the morning. I'll probably just overwrite the 90 degree card. Sean is working on a VEC2 addition for this. After having done a good bit of actual real road logging, I don't see how it would be possible to have a one card fits all approach---especially tuning with a dyno at only one ambient temp. I don't have water/****.....maybe it's less of an issue with the injection.

Steve

 

[KenH]

I too have a cold program which I was able to run from about 40-60. I then did a warm program that seemed to work fine from about 70-85, but wouldn't work at 90. I then did one on Saturday for 90 which ran pretty good and now it is 100 today and the car is once again not wanting to run right (goes rich and starts cutting out). I agree that the one card fits all doesn't seem possible unless you always run within a 20-30 degree temp range or don't pay much attention to how your car is running.

Anyone changing more than just Fuel Load/RPM when compensating for temperature? I noticed that at high temp, even though the A/F was getting very rich, I started to notice detonation, probably due to the excessive heat soak on the SC. I would think that the water/**** injection would take care of this, but perhaps not, so I have also started to back timing out.

Schulmann, what are you doing to provide additional grounding on the VEC2? Are you hooking more ground wires into the wiring harness? If so, what pins?

 

[Schulmann]

I am quite sure that reworking your grounds will help.
My Viper is really feeling different since I modified my electrical system. Even some guys noted that my exhaust at full throttle sounds smoother.


Actually all my ground were attached to the same point on the chassis. It was really stupid! I had 8 ground wires at the same point. Attaching more than 2-3 ground wires to the same point is a bad thing in the case of a forced induction engin. Now, the VEC2 has its own ground on the chassis. Nothing else is attached to this point. Nearly all my electrical devices got their own grounding points. That's a lot of new holes.

Guess what ... I can idle a half an hour in a traffic jam and everything works fine eventhought the underhood temperature is 160F ! At WOT my engin runs a little richer 11.5 instead of 12.1 but it is still good !

 

[Jack B]

I have dozens of logs that show the a/f as it relates to air temperature. This is with the true air temperature, not, what the oem positioned IAT sensor shows, the IAT indicates higher than ambient air temp. These are also at a constant coolant temperature, this is important since the coolant temp also changes the injector pulse.

There is a quirk in the vipers PCM. The PCM should offset the injector pulse for temperature differences and it does not do this accurately. As an example, going from 70 degrees-F to 90 degrees-F results in a 4% difference in air density, in other words the car should run 4% richer if the PCM was not in the equation.

With a perfect PCM it should keep the a/f constant at wot. Perhaps it is just that the PCM's offset is for a stock car and the algorithem adjusts for the stock injector pulse and with a modded engine the pulse is not enough. My car always shows a difference in a/f similar to or greater to the change in air density. There does not seem to be as much change from 40 to 70 degrees as there is at greater than 70 degrees.

I have very accurate data up to about 75-80 degrees, I am just accumulating the data for greater than 80 degrees and will have some good offsets for the VEC at the end of the year. Since I have optimized the placement of the IAT sensor it now reads the true air temperature, this has narrowed the variance in a/f at different air temperatures.

 

[99 R/T 10]

I am quite sure that reworking your grounds will help.
My Viper is really feeling different since I modified my electrical system. Even some guys noted that my exhaust at full throttle sounds smoother.


Actually all my ground were attached to the same point on the chassis. It was really stupid! I had 8 ground wires at the same point. Attaching more than 2-3 ground wires to the same point is a bad thing in the case of a forced induction engin. Now, the VEC2 has its own ground on the chassis. Nothing else is attached to this point. Nearly all my electrical devices got their own grounding points. That's a lot of new holes.

Guess what ... I can idle a half an hour in a traffic jam and everything works fine eventhought the underhood temperature is 160F ! At WOT my engin runs a little richer 11.5 instead of 12.1 but it is still good !

Hey Schulmann,
Can you post some pictures on how you accomplished the multiple grounding? Example, Vec 2's ground.

 

[Ulysses]

Actually all my ground were attached to the same point on the chassis. It was really stupid! I had 8 ground wires at the same point. Attaching more than 2-3 ground wires to the same point is a bad thing in the case of a forced induction engin.

Why is that bad?

 

[Schulmann]

Actually all my ground were attached to the same point on the chassis. It was really stupid! I had 8 ground wires at the same point. Attaching more than 2-3 ground wires to the same point is a bad thing in the case of a forced induction engin.

Why is that bad?

Resistance goes up to 60 ohm ... computers (PCM and VEC2) don't like that.

 

[Schulmann]

There is a quirk in the vipers PCM. The PCM should offset the injector pulse for temperature differences and it does not do this accurately. As an example, going from 70 degrees-F to 90 degrees-F results in a 4% difference in air density, in other words the car should run 4% richer if the PCM was not in the equation.

The problem is in open loop operations: startup and acceleration.
Startup: If temperature change is very important it can be really hard to start the engin no matter where the IAT sensor is located.


Acceleration: This is not so bad but things can be improved. A 4% change in the AFR for street application is viable and even 8% change is acceptable. However for racing I would definitely build a program to the right temperature, fuel etc ... ( and this is what I do).



In closed loop the PCM uses the Oxygen sensors to regulate the AFR and it does this process perfectly. PCM will adjust the AFR in the limit of its short and long term adaptives.

 

[Ulysses]

Actually all my ground were attached to the same point on the chassis. It was really stupid! I had 8 ground wires at the same point. Attaching more than 2-3 ground wires to the same point is a bad thing in the case of a forced induction engin.

Why is that bad?

Resistance goes up to 60 ohm ... computers (PCM and VEC2) don't like that.

You're saying that one grounding point makes more resistance? Could it be the way they are branched out, each connected in series on one line and separated by some distance?