Some Stage 3 MAF math - (say that three times fast (discussion))
12-08-2005, 10:07 PM
#74
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Based upon your initial question I thought you realized the theoretical quandry you posed...
Calculations with error compounded by further calculations/instrumentation with even more error, etc...
It's definitly not super-precise, but given the same car with the same equipment it works well to chart changes on that specific vehicle.
Anyhow, I think a poor air inlet system is the cause of most metering problems anyhow:
Two things I immmediately look for in an air inlet system:
1: Stable readings at idle. I like to maintain ~.1 fluctuations, just like stock.
2: Smooth MAF curve: No chop or random dips up and down or curve abberations which affect afr.
The "hacked airbox" intakes I've made all pass this with no problem. All of the open air intakes I've tested except one have been very bad in idle and a bit choppy in the curve.
It's definitly not super-precise, but given the same car with the same equipment it works well to chart changes on that specific vehicle.
Anyhow, I think a poor air inlet system is the cause of most metering problems anyhow:
Two things I immmediately look for in an air inlet system:
1: Stable readings at idle. I like to maintain ~.1 fluctuations, just like stock.
2: Smooth MAF curve: No chop or random dips up and down or curve abberations which affect afr.
The "hacked airbox" intakes I've made all pass this with no problem. All of the open air intakes I've tested except one have been very bad in idle and a bit choppy in the curve.
12-08-2005, 10:21 PM
#76
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You're welcome Cable. [more]
In regard to your compressor map test rig question:
A test stand is created, and the turbo is fitted with a slew of instruments. The instrumentation records shaft rpm, inlet temp, inlet pressure, outlet pressure, outlet temp, and compressor flow.
The process involves keeping the rpm constant via mechanical means (not actual exhaust gas), then reducing the air flow (using a throttle body type valve on the outlet) until the outlet pressure and flow become unsteady and unstable in general. That's what the SURGE point is, and it's plotted as the line that defines the minimum usable flow for that shaft rpm. Below that point, you're moving into a pulsing flow that is useless.
Then, they simply increase the air flow on the outlet side while measuring total pressure (on inlet and outlet). That's how the pressure ratio becomes relative to the plotted curves you see, since the curve is just a calculated pressure ratio based on total pressure. The lines you see are just the values plotted at different shaft rpm, usually in 10k increments or so. Pretty simple process really. The flow rates on the X-axis are usually in mass units. Since an engine is a volumetric pumping device...we have to convert to useful units when running further calcs.
There's a speed correction note on most comp maps, but don't worry about it...it's just for really cold conditions (aviation conditions, high alt.).
A test stand is created, and the turbo is fitted with a slew of instruments. The instrumentation records shaft rpm, inlet temp, inlet pressure, outlet pressure, outlet temp, and compressor flow.
The process involves keeping the rpm constant via mechanical means (not actual exhaust gas), then reducing the air flow (using a throttle body type valve on the outlet) until the outlet pressure and flow become unsteady and unstable in general. That's what the SURGE point is, and it's plotted as the line that defines the minimum usable flow for that shaft rpm. Below that point, you're moving into a pulsing flow that is useless.
Then, they simply increase the air flow on the outlet side while measuring total pressure (on inlet and outlet). That's how the pressure ratio becomes relative to the plotted curves you see, since the curve is just a calculated pressure ratio based on total pressure. The lines you see are just the values plotted at different shaft rpm, usually in 10k increments or so. Pretty simple process really. The flow rates on the X-axis are usually in mass units. Since an engine is a volumetric pumping device...we have to convert to useful units when running further calcs.
There's a speed correction note on most comp maps, but don't worry about it...it's just for really cold conditions (aviation conditions, high alt.).
