Help me solve this kpa discrepancy
03-11-2008, 09:48 AM
#1
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Help me solve this kpa discrepancy Working on an auto heads/cam vette.
New Vararam
New cam
ported heads
fast 90/90
78mm granatelli MAF
Key on/engine off MAF reading was 102kpa on the track but at WOT we were getting 98kpa on our logs
Same thing on the dyno we were getting 4kpa less at WOT then with key on/engine off.
Any ideas what is causing this?
Does this suggest a restriction?
Do we need a larger MAF?
How much loss of power is 4kpa?
Thanks
New Vararam
New cam
ported heads
fast 90/90
78mm granatelli MAF
Key on/engine off MAF reading was 102kpa on the track but at WOT we were getting 98kpa on our logs
Same thing on the dyno we were getting 4kpa less at WOT then with key on/engine off.
Any ideas what is causing this?
Does this suggest a restriction?
Do we need a larger MAF?
How much loss of power is 4kpa?
Thanks
03-11-2008, 10:42 AM
#2
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Your kPa reading at lower RPM ought to be close to
ambient (KOEO) and fade as airflow runs up and drops
pressure along the intake tract. That seems like a big
drop with the large TB / manifold. However a good
manifold will increase the MAP drop, because it lets
the motor draw more air against the front end's
restriction.
On the rollers you might just take off the air bridge
and filter, see what that shows you. Maybe try a
Z06 MAF as possibly better than the 78mm G thang.
But I would not expect much there and it might
undo some prior MAF tuning effort.
4kPa/102kPa is about 4% left on the table. So a
400FWHP motor is paying ~16HP in excess taxes
up top.
ambient (KOEO) and fade as airflow runs up and drops
pressure along the intake tract. That seems like a big
drop with the large TB / manifold. However a good
manifold will increase the MAP drop, because it lets
the motor draw more air against the front end's
restriction.
On the rollers you might just take off the air bridge
and filter, see what that shows you. Maybe try a
Z06 MAF as possibly better than the 78mm G thang.
But I would not expect much there and it might
undo some prior MAF tuning effort.
4kPa/102kPa is about 4% left on the table. So a
400FWHP motor is paying ~16HP in excess taxes
up top.
03-11-2008, 09:40 PM
#3
Just like Jimmy blue said, I'd step up the MAF size to at least an 85mm maf.
Think of it like like an air compressor hose or even a vacum cleaner hose. No matter how powerful or big all the lines are, you're only going to flow as much as your smallest restriction, which just might be the maf. I noticed that I went from 95kPa to 99kPa when I swapped from a stock descreened maf and tsp lid to an 85mm maf and 85mm Fast Toys lid. But my KOEO kPa reading was 100, so I'm pretty efficient on the intake side right now.
Think of it like like an air compressor hose or even a vacum cleaner hose. No matter how powerful or big all the lines are, you're only going to flow as much as your smallest restriction, which just might be the maf. I noticed that I went from 95kPa to 99kPa when I swapped from a stock descreened maf and tsp lid to an 85mm maf and 85mm Fast Toys lid. But my KOEO kPa reading was 100, so I'm pretty efficient on the intake side right now.
