mass air flow
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Q45tech
- Moderator
- Posts: 14365
- Joined: Tue Apr 30, 2002 3:19 am
- Car: 1990 Q45 342,400 miles 22 years ownership with original engine
1995 G20t 5 speed 334,000 miles 16" 2002 wheels - 205/50/16 Sr20ve vvl
The major restriction to air flow is the intake valve diameter and lift and the curve [ram rate] of the intake cam. These have to be limited for a smooth idle and cruise efficiency.
Haven't measured a 3 liter but on a Q [4.5l] the TOTAL worst case restriction from the air intake horn thru MAF and past TB into plenum is 16-17" of water at 6700 rpm flowing around 220 grams per second.
Normally air flow bench experts use 25-28" water column when testing heads.........they maintain this flow restriction and look for tiny increases in flow volume at that vacuum...........because experience has shown that this is typical for the intake tract from the heads outward to the air horn.
28" is roughly 1 psi which means the entire system from the heads outward loses 6.6% of what is theoreticallly possible if the heads were just exposed to the atmosphere---- no runners, no plenum, no TB, no MAF, no air filter, no air ducts!
The maximum filling of the cylinder occurs at the torque peak rpm [by definition] and after that the valve and other restrictions and rpm limiting the filling time to create a slow decline in volumetric efficiency to redline.......[after the HP peak rpm the slope increases more rapidily] starving the cylinder of air.......say only 75% [redline] of what it was at torque peak.........vs 85% at HP peak.
A 3 liter engine only allows 3 liters of air [maybe only 2.5-2.7 liters] at the torque peak for every 2 rpms.
The theoretical limits on a street engine [with a safety reserve for longevity] are around 75 lb/ft per liter 225 lb/ft for a 3 liter.
The other problem is 10% of the old burned charge is still in the head space when the exhaust valve must be closed, so really only 90% of the cylinder can be filled with new freash air and the fuel volume takes up space so really only 80% new fresh air.
Haven't measured a 3 liter but on a Q [4.5l] the TOTAL worst case restriction from the air intake horn thru MAF and past TB into plenum is 16-17" of water at 6700 rpm flowing around 220 grams per second.
Normally air flow bench experts use 25-28" water column when testing heads.........they maintain this flow restriction and look for tiny increases in flow volume at that vacuum...........because experience has shown that this is typical for the intake tract from the heads outward to the air horn.
28" is roughly 1 psi which means the entire system from the heads outward loses 6.6% of what is theoreticallly possible if the heads were just exposed to the atmosphere---- no runners, no plenum, no TB, no MAF, no air filter, no air ducts!
The maximum filling of the cylinder occurs at the torque peak rpm [by definition] and after that the valve and other restrictions and rpm limiting the filling time to create a slow decline in volumetric efficiency to redline.......[after the HP peak rpm the slope increases more rapidily] starving the cylinder of air.......say only 75% [redline] of what it was at torque peak.........vs 85% at HP peak.
A 3 liter engine only allows 3 liters of air [maybe only 2.5-2.7 liters] at the torque peak for every 2 rpms.
The theoretical limits on a street engine [with a safety reserve for longevity] are around 75 lb/ft per liter 225 lb/ft for a 3 liter.
The other problem is 10% of the old burned charge is still in the head space when the exhaust valve must be closed, so really only 90% of the cylinder can be filled with new freash air and the fuel volume takes up space so really only 80% new fresh air.
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