VE question
Thread Starter
Racer
Joined: Apr 2003
Posts: 260
Likes: 0
From: Palm Bay Fl
VE question
I calculated my VE based on the MAF/TEMP/RPM data (actually just used the ambient air temp, not the IAT) and my VE numbers are way below what the memory map says.
Anyone else experience this?
The PCM map shows they should be on the order of 84% whereas I'm getting around 70% in my calculations.
What's up with that?
-Dave C.
Anyone else experience this?
The PCM map shows they should be on the order of 84% whereas I'm getting around 70% in my calculations.
What's up with that?
-Dave C.
Burning Brakes
Joined: May 1999
Posts: 760
Likes: 1
From: Greenwich, CT
Re: VE question (Dave C. '97 Z28)
FWIW:
Try using MAT rather than ambient T (in *K or *R) and check the density differnce when calculating VE. Also, did you derate the air density for the observed MAP when you did the VE calc for that point? Those two factors could probably make up the difference..
HTH
Thread Starter
Racer
Joined: Apr 2003
Posts: 260
Likes: 0
From: Palm Bay Fl
Re: VE question (DOCTOR J)
Ah ha... That may be it. I never corrected for the drop in air density in the manifold.
But wouldn't the result be the same? I mean, the air outside is flowing throught the MAF at the outside density, which is where the measurement is being taken...but I guess if I corrected for the exact barometric pressure outside somehow maybe it could make a difference.
Also, I tried the MAT and the results changed by a percent or two... not too much.
I was thinking that maybe my MAF calibration is way off. At 5300RPM I am getting a measurement of about 200-210g/s, whereas in my Camaro it was regularly 240-250g/s.
Is 210g/s normal for a 5300RPM LT-1 vette motor (stock internally) @ ~90'F air temp?
-Dave C.
[Modified by Dave C. '97 Z28, 3:49 PM 8/14/2003]
But wouldn't the result be the same? I mean, the air outside is flowing throught the MAF at the outside density, which is where the measurement is being taken...but I guess if I corrected for the exact barometric pressure outside somehow maybe it could make a difference.
Also, I tried the MAT and the results changed by a percent or two... not too much.
I was thinking that maybe my MAF calibration is way off. At 5300RPM I am getting a measurement of about 200-210g/s, whereas in my Camaro it was regularly 240-250g/s.
Is 210g/s normal for a 5300RPM LT-1 vette motor (stock internally) @ ~90'F air temp?
-Dave C.
[Modified by Dave C. '97 Z28, 3:49 PM 8/14/2003]
Burning Brakes
Joined: May 1999
Posts: 760
Likes: 1
From: Greenwich, CT
Re: VE question (Dave C. '97 Z28)
This will not answer your Q directly, rather it's some methodology to review the scale of the delta you are looking at:
This is where I'd start the calculation: VE = actual air flow / theoretical air flow -
1. Theoretical flow is calculated from the physical size of the cylinders, adjusted with PV/T = nR. Note you need to use the P & T occuring at the cylinder inlet. Dunno about yours, on mine I'd expect to see MAT ~150 *F in summer - which would result in more like a 10% density drop compared to ambient. There would also be some residual delta P across the intake tract, after the plenum.
2. Actual air flow is either measured with the same corrections, or estimated from a fuel balance.
Re MAF accuracy:
The MAF is estimating mass flow so it doesn't require the same corrections (although relative humidity is problematic). I don't have any first hand experience with MAF calibration; in general MAF's have an exponential transfer function, with something like 0.3 for the flow exponent.
If you are interested, there is a GM SAE paper posted here that discusses MAF design, modeling, and calibration errors: http://www.efi-tuning.org/forum/inde...pe=post&id=722 (air meter ref)
HTH
Thread Starter
Racer
Joined: Apr 2003
Posts: 260
Likes: 0
From: Palm Bay Fl
Re: VE question (DOCTOR J)
Thanks for the responses and link... it certainly gives me stuff to think about.
-Dave C.
Also, Can anyone with an LT-1 MAF car tell me what thier MAF reads at WOT around 5300-5500RPM? Just trying to see if my MAF reading is similar.
[Modified by Dave C. '97 Z28, 2:14 PM 8/15/2003]
-Dave C.
Also, Can anyone with an LT-1 MAF car tell me what thier MAF reads at WOT around 5300-5500RPM? Just trying to see if my MAF reading is similar.
[Modified by Dave C. '97 Z28, 2:14 PM 8/15/2003]
