I was thinking about this earlier today; Has anybody tried to figure out what CFM (or any other measurment of airflow for that matter) corresponds to what TP scale value? Obviously it'll change depending on which MAF you use but get base measurments using a specific MAF and you should be able to calculate the values for different ones, should you not?
This'd be really handy for sizing turbos and doing map expansions based on desired boost levels and turbo compressor maps. It seems to me you can infer it from datalogged runs using known boost pressures and rpm with a compressor map but it's accuracy depends on the accuracy of your calculations for determining the turbo's pressure ratio and flow rate. Not to mention this doesn't work at all for NA cars...
Anybody bothered to correlate TP with CFM?
Moderator: Matt
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Nissanclubman
- Posts: 27
- Joined: Sat Jun 20, 2009 11:40 pm
- Location: Gold Coast
MAF vs Fuelmap TP scale
I have recently tried to figure this out as well and this is what I have found out...sort of. May have some gaps however and am happy to be corrected.. 
MAF vs Fuel map TP scales and rpm. The output of the MAF is a measure of mass air that the ECU uses to reference the fuel MAP for the correction value to alter the injector pulse width from its base setting. Base setting comes fundamentally from the value stored in the K address register.
Here is the journey as I understand it. MAF volts output is 0-5v representing zero load to full-scale load measurable by the AFM. Read the volts out and divide this by 5. This gives you a load % number of full load. Because the AFM is non linier you have to reference the AFM Q curve. This map has 52 load points for full scale so multiply the % by 52 and this gives you the Q reference number on the AFM curve. The AFM full range value from the Q curve is from 65536 so read the appropriate value from the Q curve for the calculation above and divide it by 65536. This is a value that is corrected for the curve profile of the MAF installed. This will be a value between zero and one. Take this number and go the Fuel map. Notice the TP scale has 16 sections to it. Now multiply your earlier calculation by 16 and that will give you the TP load column to reference along the TP load scale. The value of the TP load is a K multiplier for the load measured by the AFM. Round this number up if it falls between values. Now look up the revs scale and where that crosses the TP load column will be the cell in the fuel map that is being used. The number in the cell is the correction value that is applied to the base injector pulse width which is correct by the number in the TP scale for the K number entered. The K number by the way should be the value to produce 14.7 AFR for a given make of MAF (and Q curve) and injector size with no corrections or multipliers applied………I think.. Cheers Richard
MAF vs Fuel map TP scales and rpm. The output of the MAF is a measure of mass air that the ECU uses to reference the fuel MAP for the correction value to alter the injector pulse width from its base setting. Base setting comes fundamentally from the value stored in the K address register.
Here is the journey as I understand it. MAF volts output is 0-5v representing zero load to full-scale load measurable by the AFM. Read the volts out and divide this by 5. This gives you a load % number of full load. Because the AFM is non linier you have to reference the AFM Q curve. This map has 52 load points for full scale so multiply the % by 52 and this gives you the Q reference number on the AFM curve. The AFM full range value from the Q curve is from 65536 so read the appropriate value from the Q curve for the calculation above and divide it by 65536. This is a value that is corrected for the curve profile of the MAF installed. This will be a value between zero and one. Take this number and go the Fuel map. Notice the TP scale has 16 sections to it. Now multiply your earlier calculation by 16 and that will give you the TP load column to reference along the TP load scale. The value of the TP load is a K multiplier for the load measured by the AFM. Round this number up if it falls between values. Now look up the revs scale and where that crosses the TP load column will be the cell in the fuel map that is being used. The number in the cell is the correction value that is applied to the base injector pulse width which is correct by the number in the TP scale for the K number entered. The K number by the way should be the value to produce 14.7 AFR for a given make of MAF (and Q curve) and injector size with no corrections or multipliers applied………I think.. Cheers Richard
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chris2712au
- Posts: 343
- Joined: Thu Jun 19, 2008 1:52 am
- Location: sydney australia
sounds fair enough... but what about the density of the air ? it is fair enough that the afm will measure a mass of air... Not the volume... so it automatically adjusts for temperature changes... these temperature changes effects the cfm your trying to calculate..
e.g there are 1000 liters of water in 1 mtr cubed which weighs a tonne...
tonne being the mass flow 1 cubic meter flow of 1000 liters of water..
If the water was to get hevier.. 1 liter was equal to 1 kg now it is 1.2 kg..
the new mass flow is 1200kgs for still 1 cubic meter of 1000 liters...
so if the weight (density of air ) is related to temperature then I think it would require consideration..
e.g there are 1000 liters of water in 1 mtr cubed which weighs a tonne...
tonne being the mass flow 1 cubic meter flow of 1000 liters of water..
If the water was to get hevier.. 1 liter was equal to 1 kg now it is 1.2 kg..
the new mass flow is 1200kgs for still 1 cubic meter of 1000 liters...
so if the weight (density of air ) is related to temperature then I think it would require consideration..
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Nissanclubman
- Posts: 27
- Joined: Sat Jun 20, 2009 11:40 pm
- Location: Gold Coast
Yes I think you are absolutely right. Temperature, pressure, moisture etc etc affects the mass of air and for a given steady state of engine revs this could change however the CFM of flow will remain the same.
AFR is about the ratio of fuel and air and given the relative changes of fuel as compared to that of air when considering the affects of temperature, pressure, moisture etc the self-compensating factor is mass not volume. So I suppose the question being asked, how to relate CFM for turbo sizing and desired boost, all which relate to a measure of flow volume (engine revs + environment pressure) to the TP scale of the fuel map, which is an engine load correction factor of injector pulse for a base stoichiometric AFR. AFR is based on Mass calculations and it would then suggest to convert CFM would require correction tables and sensors to included ambient pressure, temperature, moisture etc etc.
It seems a lot easier to just measure mass and be done with it…
It does however suggest that the most effective place to measure mass air therefore is after the turbo (where both temp and pressure are increased), after the intercooler (where temp is decreased), and after any methanol or water injection occurs (which removes temp and adds density).
The other point I suppose is that we have come to understand the affect of numbers relating to boost pressure better than those of mass air, which, by the way, is all we are really doing by changing boost. Performance on cold nights at 15psi compared to hot days at 15psi…we can all relate to that.
AFR is about the ratio of fuel and air and given the relative changes of fuel as compared to that of air when considering the affects of temperature, pressure, moisture etc the self-compensating factor is mass not volume. So I suppose the question being asked, how to relate CFM for turbo sizing and desired boost, all which relate to a measure of flow volume (engine revs + environment pressure) to the TP scale of the fuel map, which is an engine load correction factor of injector pulse for a base stoichiometric AFR. AFR is based on Mass calculations and it would then suggest to convert CFM would require correction tables and sensors to included ambient pressure, temperature, moisture etc etc.
It seems a lot easier to just measure mass and be done with it…
It does however suggest that the most effective place to measure mass air therefore is after the turbo (where both temp and pressure are increased), after the intercooler (where temp is decreased), and after any methanol or water injection occurs (which removes temp and adds density).
The other point I suppose is that we have come to understand the affect of numbers relating to boost pressure better than those of mass air, which, by the way, is all we are really doing by changing boost. Performance on cold nights at 15psi compared to hot days at 15psi…we can all relate to that.