To MAF or not to MAF

[techsol]

Hi all.

I'm getting read to go with the MS-II on a '74 454 corvette conversion. I'm dropping in a 7.4L vortex manifold with injector set up from a Mercruiser(99) with MPI set up.

I was wondering after sifting through all the on-board and other board discussions, if a MAF was worth any benefit or if its been done. Its a roller cam set up , about 450HP, being tweaked with an EDIS system while its coming together.

I was looking for opinions, hopefully learnied, on the value of a MAF or to stay with the MAP sensor.

All help valued and appreciated!

Dick

[FixItAgainTony]

If you did not see it, this thread has a good discussion on the topic:
viewtopic.php?p=187430&highlight=maf#187430

Note, that in the discussion above, the comment made by Jim that the restrictions of the throttle plate and the MAF are cummaltive. That is true, but it is also true that in normal operation (ie, all but but ~wide open throttle), the pressure drop across the MAF should be orders of magnitude less than the drop across the throttle plate => it really should not matter much, but it will be there. At wide open throttle, if not sized right, it could be a major limiter as noted in that thread. It would be interesting to measure the pressure drops with a MAP, I would guess that the air filter is a bigger restriction - still the less restriction, the better.

Charles.

[enyawix]

Just go mass air I have drag raced for a few years now. A large mass air will out flow the Intake manifold and cylinder heads. The real issue here is lack of good documentation. The manual and most of the post, are based on a stock low output engine. I am in the process of starting a MegaSquirt + Mass air Documentation project.

[Vicoor]

lets say the throttle drops 20"h2o pressure at full throttle and the maf drops 10"h2o. thats a 50% increase for the total.

why increase the restriction if you don't have to.

[enyawix]

lets say the throttle drops 20"h2o pressure at full throttle and the maf drops 10"h2o. thats a 50% increase for the total.

why increase the restriction if you don't have to.

How about a real world example? Do you have any dyno numbers, or time slips for what you are saying. Mass air 5.0 mustangs consistently made more power than speed density 5.0 mustangs. Ford used several computers on the 5.0 but, The A9L mass air computer was the computer to have, sometimes making 5hp to 10hp more than S.D with faster throttle response. Mass air also solves many drive ability issues caused by heavily modifying an engine.

[Vicoor]

No sir I do not have access to a dyno to validate these "theories"

All I know is what my flow bench tells me, Pressure drops are cumulative at a given airflow.

If you drop the pressure available to your cylinders then you limit the amount of potential power output.

Your examples of different ford configurations , are those ecms programmable? If so why megasquirt at all?

[enyawix]

My OEM computer is programmable, but it will not do COP, SFI, flex fuel, boost control, or data logging.

[thebigmacd]

lets say the throttle drops 20"h2o pressure at full throttle and the maf drops 10"h2o. thats a 50% increase for the total.

why increase the restriction if you don't have to.

There isn't 30" of pressure to drop when atmospheric pressure is 29.5". What you will end up with is a drop of marginally more than the original 20", and a similarly marginal drop in flow.

An engine's intake tract at a given RPM is basically a constant-pressure system, not a constant-volume. Hence, volumetric efficiency. The compression ratio of the engine determines the ideal, closed-valve pressure drop from ambient, and the characteristics of the intake tract affect how quickly that ideal pressure drop is equalized by incoming air.

Having two equal-sizes orifices in series does not halve the flow. Because flow is proportional to the square root of the pressure drop, having two equal-sizes orifices in series causes the flow to be reduced by only 29.3%. Include with that the effective orifice size of the intake runners/valve ports, and your losses due to a MAF are even less significant.

Take for example your 20" drop throttle and 10" drop MAF: at WOT they will simply divide proportionally the pressure that used to be across just the throttle.

Original flow through throttle at 100%VE = 100%
Original pressure drop at 100%VE = 100%^2 / K Factor = 100%
Therefore, K Factor = 100%
Now for same VE condition, add MAF in series
Total pressure drop still equals 100%
BUT, pressure drop across throttle = 2/3 total drop
Therefore new pressure drop = 66.66%
Reverse equation using K Factor of 100%: New flow = SQRT(66.66%* 100%)

New flow = 82.6%
This is only a 17.4% drop in flow, not a 33% drop as you would suggest. Size the throttle up to match the pressure drop of the MAF and you will have a 0% loss of flow over the throttle-only setup.

Granted, if your MAF flows less than the throttle you have an issue, but that's only caused by choosing the wrong MAF diameter.

If you are really concerned, you can replace (for example) a 3" throttle with a 4.2" throttle, throw on a 4.2" MAF and boom no losses. I am simplifying the whole thing but you get the idea.

[Vicoor]

I did not mention flow at all.

You are correct about the relationship between flow and pressure.

The example you gave seems to agree that pressure drops are cumulative.

and 20"h2o + 10"h2o = 2.20667737392Hg which is over 13 times less than your standard atmospheric pressure of 29.5"Hg

My pressure is 30.18"Hg right now