Some theory questions
12-20-2005, 01:18 PM
#1
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Some theory questions Still warming up to doing some serious tuning, been reading like crazy, just wanted to step back for a minute from the mechanics...
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So with a given mechanical setup the objective is to get the most air and fuel combusted at the right time at the right AFR.
The LSx engine management system tries to adjust to a target AFR but it's imprecise because it's always playing catch up, correcting.
So the objective is to adjust the tables so it doesn't have to adjust, correct?
The only absolute measurement device that help us in our objective is MAF frequency. We adjust fuel and timing under various conditions with a fixed airflow (as measured by the MAF frequency). How well we do is measured by a wideband Oxygen sensor against RPM.
Is this about right?
So is there a calculation that uses IAT, MAF bore and MAF frequency to give us true airflow at any rpm point? That would be the first thing I would want to know.
The other thing would want to calculate is how much injector duration would be necessary to achieve a targeted AFR at a given rpm based on the airflow calculated above.
This would give us a basline tune. Tweaks would be made to this based upon a wideband. The first thing I would then tweak would be timing as we already got the AFR thing nailed (theoritically). Once we optimized timing then we could make minor changes to fuel to get things perfect.
Is that correct?
Sorry if some of this is "Duh, of course you moron" but I have a tendency to not see the forrest for the trees and I'm not an engineer.
Thanks
Pete
******
So with a given mechanical setup the objective is to get the most air and fuel combusted at the right time at the right AFR.
The LSx engine management system tries to adjust to a target AFR but it's imprecise because it's always playing catch up, correcting.
So the objective is to adjust the tables so it doesn't have to adjust, correct?
The only absolute measurement device that help us in our objective is MAF frequency. We adjust fuel and timing under various conditions with a fixed airflow (as measured by the MAF frequency). How well we do is measured by a wideband Oxygen sensor against RPM.
Is this about right?
So is there a calculation that uses IAT, MAF bore and MAF frequency to give us true airflow at any rpm point? That would be the first thing I would want to know.
The other thing would want to calculate is how much injector duration would be necessary to achieve a targeted AFR at a given rpm based on the airflow calculated above.
This would give us a basline tune. Tweaks would be made to this based upon a wideband. The first thing I would then tweak would be timing as we already got the AFR thing nailed (theoritically). Once we optimized timing then we could make minor changes to fuel to get things perfect.
Is that correct?
Sorry if some of this is "Duh, of course you moron" but I have a tendency to not see the forrest for the trees and I'm not an engineer.
Thanks
Pete
12-20-2005, 03:20 PM
#2
Interesting questions...
Being able to calculate mass airflow and ve would make short work of building baseline ve / MAF tables. The only problem is that there are a lot of little restrictions on the intake side that will introduce a %error to the calculated ideal values.
Poking around on the net a little, I found this page relating to turbocharged mitsu's that do not have the ability to log airflow data at wot.
http://www.stealth316.com/2-primer.htm
He uses the torque curve from a dyno run to come up with a theoretical ve at a given rpm. Not sure if that is accurate, it may end up introducing too large of a %error to the mix. His other page has some good airflow equations and java calculators as well.
http://www.stealth316.com/2-air-fuel-flow.htm
With all that said, there is a pid for MAF calculated airflow (in lb/min) which is based on iat and MAF freq. The only problem is that this assumes the MAF table is correct. If its off, then calc airflow will be off as well.
Once you know the mass of airflow at a given point in time, you can calculate idc pretty easily. Take your calc. airflow and divide by target afr. This will give you a calculated mass fuel flow. Divide this by 8 (# of injectors). This will give you the mass flowrate per injector at 100%. Take this # and divide by your actual injector flowrate and then multiply by 100. This will give you a calculated idc for the given airflow.
Being able to calculate mass airflow and ve would make short work of building baseline ve / MAF tables. The only problem is that there are a lot of little restrictions on the intake side that will introduce a %error to the calculated ideal values.
Poking around on the net a little, I found this page relating to turbocharged mitsu's that do not have the ability to log airflow data at wot.
http://www.stealth316.com/2-primer.htm
He uses the torque curve from a dyno run to come up with a theoretical ve at a given rpm. Not sure if that is accurate, it may end up introducing too large of a %error to the mix. His other page has some good airflow equations and java calculators as well.
http://www.stealth316.com/2-air-fuel-flow.htm
With all that said, there is a pid for MAF calculated airflow (in lb/min) which is based on iat and MAF freq. The only problem is that this assumes the MAF table is correct. If its off, then calc airflow will be off as well.
Once you know the mass of airflow at a given point in time, you can calculate idc pretty easily. Take your calc. airflow and divide by target afr. This will give you a calculated mass fuel flow. Divide this by 8 (# of injectors). This will give you the mass flowrate per injector at 100%. Take this # and divide by your actual injector flowrate and then multiply by 100. This will give you a calculated idc for the given airflow.
12-20-2005, 03:31 PM
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Thanks for that response.
Do we know the calabration for the MAF. ie what frequency is supposed correspond to a mass rate of air (for a particular size of MAF, 78, 90 etc)?
Would it make sense to flow bench a MAF and read the frequencies. ie control the flow of air through the MAF and find the corresponding frequencies?
Do we know the calabration for the MAF. ie what frequency is supposed correspond to a mass rate of air (for a particular size of MAF, 78, 90 etc)?
Would it make sense to flow bench a MAF and read the frequencies. ie control the flow of air through the MAF and find the corresponding frequencies?
12-20-2005, 03:44 PM
#4
The best we've been able to come up with is installing a wideband O2 sensor in the exhaust (with obviously more fidelity than the 0-1V narrowbands that become increasingly inaccurate with LT headers), and comparing the readings from it to what the computer is telling the engine to do. This can be used to adjust the existing VE tables as well as the MAF table with relative ease. The only trick is that you have to do this in open loop (so the narrowbands aren't contributing their 2-cents of worthless knowledge [worthless to me at least]).
