elisowski

What are your RWHP numbers and corresponding airflow (AFGS) readings?

95Blackhawk

Did you ask this in the LT1/4 forum area already? There is no relationship because MAF sensors are not linear.

elisowski

I asked it wherever I thought someone would answer it (I can't remember all the places, but in at least 3 different boards).

Just a tool to play with and see how much inlet restriction you have.

It appears that most people pull 1 RWHP per AFGS.

Others report closer to 1.1 RWHP per AFGS.

I pulled the air filter at the track and went from 358 AFGS to 447 AFGS and 730 cfm to 890 cfm and picked up 3 tenths. Could not get the AFR below 13.5:1 with tuning above 6000 rpm. So now I know I need a bigger fuel pump. And I may be able to squeeze a few more hundreths or even a tenth out of it after I tune it for the new fuel pump.

95Blackhawk

The relationship of AFGS to HP is not just that it is nonlinear but that it changes based upon your reading. It is a curve that does not stay constant so others input is only relevant for that particular AFGS reading while on a dyno at that particular HP...specific to them and their car. Other than that, you can ignore it.

Why do you need a bigger fuel pump? Are your injectors stock? If so, look there first.

elisowski

I have 42 lb lucas injectors.

The pump would only put out 30 psi from 6000-7000 rpm. It should have put out 43 psi across the board at 95-100 MAP, just like it does when you unhook the vacuum line when you set the pressure regulator.

I had increased my PE (power enrichment) vs RPM tables to the maxed out value of 50% in the 6400 and 6800 rpm cells using LT1 edit.
The lowest AFR I could get was 13.5:1.

I installed a 225 lph pump today and I had 43 psi at WOT all the time. Also, the AFR dropped to 11:1 when I got over 6000 rpm. So I know it was the pump.

Now I have to retune all of my PE tables so that I get 12.5:1 AFR at WOT and I should be good to go.

And your right about the AFGS readings being different at different dynos tracks, streets, altitudes etc. The temperature and pressure at the time of the readings will change the density of the air and change the AFGS readings considerably. As far as I know the PCM does not adjust the AFGS readings based on altitude, but the fuel tables do compensate for the temperature of the engine coolant temperature. And, even though it reads the IAT, I do not think that it compensates the fuel mixture based on the incoming air temperature (IAT).

It would be nice to know how the PCM calculates AFGS using the MAF.

So, you're right. A car pulling 447 AFGS with an IAT of 140 will produce less HP than when it pulls 447 AFGS at 120 degree IAT. And it will produce less HP with the same AFGS at a higher elevation.


I found a really cool hot-air-balloon web site that had all of the pertinent info on it when I wanted to know how to convert AFGS to CFM.
http://www.overflite.com/thermo.html

Can you shed some light on how the AFGS are calculated?

95Blackhawk

You are missing the point.

A MAF compensates for altitude, temp, etc. automatically. How? I can't remember the details since it has been about 4 years since I read up on that. It has to do with air flowing over a thermocouple and maintaining a certain resistance of that thermocouple. Hence, lower flow, lighter air (altitude) automatically have to be compensated for.

Anyway enuf about bull**** I don't remember. The MAF curve is very obvious if you have ever taken the points from TC or LT1Edit and plotted them out. It shows the curve "curves" and is not linear. This then shows that at low flow, there is more accuracy, exactly where you would need it in order to maintain a stoich. AFR more easily. However, at higher flow, it is not as accurate. But on top of this, the curve changes based upon where you are on the curve. Now damnit, I have to start talking about derivatives and calculus and shiite like that! I am too drunk to do that at this point...

elisowski

I read your sig and see that you do your own tuning.

How do you arrive at your perfect timing for maximim power?

Do you keep adding to the whole table until you see knock then back off, or do you add total advance at the top end until your mph slows in the quarter?

95Blackhawk

We all know pretty close to where we need to be for timing. I started a few degrees below that and did a dyno run. I then increased the timing across the board and did another run. In my case, I was able to see that increasing timing created no more torque in the lower RPM range but at the upper, adding timing helped. Unfortunately, I ran out of time to do more iterations but I started to get the idea.

Looking at HP only (which I don't like to do), a 4 degree advance in timing (going from 32* to 36* as read by Datamaster) gave me about 5-7 HP at the top end. In my mind, I was getting "close enough". Personally, I would not want to add more timing to eek out of few more HP.

Injuneer

The "curve" is simply the translation of MAF output frequency to air flow grams per second. That really doesn't have anything to do with the relationship of MAF GPS to HP. I think what the original poster is getting at is what mass air flow is required to produce a specific HP. Can he correlate AF GPS to HP. Since he's after dyno results, it would appear he is looking for the relationship between mass air flow and peak HP. And there is a nominal relationship. But there are a lot of variables.

HP is produced by the heat released when burning gasoline. The relationship of fuel mass to air mass, required to produced peak HP is dependant on specific engine characteristics, but it typically falls in a fairly narrow range.... say 12.8-13.0:1. Narrow, but variable.

Then the amount of fuel energy that is converted to mechanical energy (thermal efficiency) at the crank is fairly variable. It will relate to combustion chamber temperatures, coolant temperatures, and exhaust temperatures - all will vary from engine to engine.

Then there's the issue of valve overlap. How much of the air/fuel mix that enters the cylinders passes through the exhaust valve before it closes..... yet another variable.

If you collected enough valid data points, you could construct a fitted curve of AF GPS vs. HP, and then evalute the range of variability of the data around the curve, and determine a confidence level. But at that point, its little more than an exercise in statistical analysis.

The mass air flow sensor measures the mass of air passing the cross-sectional area of the heated wires. It extrapolates the full-face flow from the small amount of air that actually touches and removes heat from the air. The calibration curve in the PCM will reflect the data collected on an engine dyno for the specific configuration of the meter... and that includes the inlet and outlet ducting, which will affect the velocity distribution across the full flow area.

Since its measuring MASS air flow, it has, in effect compensated for the temperature and pressure of the incoming air. Those are not variables.

If all you are interested in is the air flow restriction in the intake system, looking at the MAP vs. barometer will suffice.

Another major fallacy in the approach you are suggesting is looking at rear wheel HP vs. AF GPS. How do you plan to account for the fact that drivetrain losses can range from 12% - 25%? That alone will more than account for your suggested range of 1.0 - 1.1 rwHP/GPS. You have to work in flywheel HP, not rwHP. That's the biggest mistake most people make when talking about "HP".

elisowski

Could you exlain this relationship further?

elisowski

That's the problem. I'm really not sure where we need the timing to be. My last car was a big block Olds. Thought back then was to have all the advance in by 2500 (32-40 deg total).

Then advance the timing 2 degrees at a time until the mph in the 1/4 slowed, then jump back 2 degrees.

Then, add 2 jet sizes to the mains and rears until the mph slowed and/or the plugs read rich.

There are many ignition curves (for each 5 MAP) and many data points (rpms) along each curve to edit.

Are we still looking at 36 total advance? Where do you start? and finish?
Without a dyno, do we increase advance until knock starts pulling timing?

95Blackhawk

I am looking at total advance as read by Datamaster.

I started the analysis at the start of the pull. For me, it was 2000 RPM. Next time I will start lower. I ended at the top of my HP curve...around 5500 rpm.

I would not time based on what you are thinking. The best way is to dyno. Max torque at any given RPM may be well below the point of knock and not close. My car, for example, produced the same torque in the 2000-3500 rpm range at both 32* and 36*. There was no reason for me to use the 36* if I got the same results, so I didn't.

I hope someone can chime in here with more ideas.

Injuneer

The only thing that pushes air into the cylinder as the piston drops is the barometric air pressure. You want to have the pressure in the cylinder equal to, or as close as possible to barometric pressure. The higher the pressure, the denser the air, the more fuel you can burn, the more power you make.

But, flowing air loses pressure. You have barometric pressure at the outside surface of your air filter. As the air passes through the filter, it loses pressure. As the air flows through the ducting, it loses pressure due to friction on the walls of the duct. When the air turns a corner, it loses pressure due to friction and turbulence. When the duct suddenly expands or contracts, you lose pressure. When you pass though a restriction like the MAF sensor or the throttle body, you lose pressure. Pressure loss is dependant largely on velocity. As the velocity of air flow increases, the pressure loss increases with the square of the velocity increase.

In effect, if you look at the barometric pressure (one of the readouts from your scanner), and then subtract the MAP at WOT/peak mass air flow, you have the actual pressure loss in your intake system. That is the sum of all the little bits and pieces between the outside of the air filter, and the inlet to the manfold's runners. For example, you may see the barometric pressure at 100kPa, and MAP at 94kPa. That means you lost 6kPa in the intake system. The mass of air in the cylinder will be proportional to the pressure ratio. If you only have 94kPa in the intake, but started with 100kPa at the filter, you have lost 6% of the air mass, and you will make 6% less power.

It will never be possible to have the MAP = barometric pressure, but you could get very close if all you had between the outside air and the intake manifold was the throttle body. No loss for the filter, MAF, ducting, etc. Of course all this assume the air temperature would be the same, since air temperature has a similar affect on the mass of air the ends up in the cylinders.

The above description does not address the issues of "ram air" increasing the pressure available, or of inlet port pulse tuning actually adding to the pressure in the cylinder. Its actually possible to get cylinder pressure higher than barometric pressure. But that would just complicate things at this point. And obviously, all this applies to naturally aspirated engines, not forced induction.

elisowski

thanks to both of you guys.

It really explains a couple of things I had been wondering about.