What are the disadvantages and advantages of each? I know how the MAF works but I don't have a clue how the speed density unit works. Thanks in advance.

 

The MAF meassures weight of air entering the intake; SD relys on a Manifold Air Pressure (MAP) sensor as well as other sensors to calculate the weitht of incomming air.

The advantage of a MAF is that it does a supurb job of optimizing your engine with mods, right up to it's measurement limit with the stock chip. The disadvantage is that it restricts the maximum amount of air that can flow into your intake.


The advantage of SD is that it offers no restriction to air flow and I don't think there's an upper limit on it's measurement capability.
The disadvantage is that it usually requires a chip change after a relatively small change in air flow and it simply is not as exact at measuring/calculating air flow into the motor. Remember the General only used SD for a few years and then went to an improved MAF.

 

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another difference is cost. at least to the auto manufacturer, it is cheaper to design in a MAP and a manifold air temp (MAT) sensor for a speed density setup.

there is also a third option known as alpha-N. this estimates the air going in the engine by mainly using the throttle angle (alpha) and the engine speed (N). this has the advantage of the elimination of sensors, and is very responsive if designed correctly. this is why it is used in Formula 1 and (i think) Indy cars too. the downside is that you'd never pass emissions, at least with present technologies. (hey, is that a downside???)

 

Thanks a lot for the replies. :cool:

 

Does anybody know what the limit of the MAF is measured in HP ?... I do know of a motor that makes roughly 475 flywheel HP running the stock gutted MAF.... but I'm trying to figure out if you can make it up to 500 or even 550 with a gutted MAF.

Thanks
Beach Bum

 

typically its around 450-475 with a stock maf

 

Depends on the CPU and MAF combo. 87-89 cpu is limited to 255 grams per sec. MAF actually goes higher in the cpu table but it will only read 255 because the cpu programmers limited the MAF gps value to 8 bits. 94 and up use a different MAF and have more gps capability. Stock tables are set to 424 gps but its probably possible to increase that number if you know the MAF output freq curve beyond 424 gps and adjust the MAF cal tables.

424 gps translates to 3360 lbs/hr of air. Using fuel to air ratio of 12.5 gives 270 lbs of fuel per hour. Using a BSFC of .5 ( I think thats 12.5:1) = 540 hp.

The older MAF is limited to 330 to 350 depending upon the WOT Airfuel ratio used. A lot of guys are able to get higher hp with the older computer setup by manually tuning the engine on a dyno and a wideband O2 sensor. Changes in air density will affect the performance of the engine in the WOT RPM ranges over the MAF flow limitation.

The computer on a MAF car allows you to set the desired air to fuel ratio in the tables. I have heard from a guy that does a lot of tuning that the actual ratio comes out about 5% leaner than the programmed value. This probably varies with variations in injector or and MAF accuracy.

The racing throttle angle and rpm sounds like a good idea. Holley projection 4s use that method and its supposed to be pretty good.

Speed density uses the MAP (absolute manf pressure), IAT (inlet air temp), engine size and the VE tp compute the air flow. The only limit you have is the maximum size of the number used in the computer to do the calculation.

 

I' ve noticed that the s/d setup is a little more sensitive to roller rocker arms' clack. Knock sensor?

 

I talked to TPIS about this a few years back and they told me that they could burn a chip for maf and make 500hp at that point the GM computer was maxed out if you want more power than that you have to go with SD I'am using the accel DFI unit. The general switched from maf to sd because it was cheaper to make. :chevy

 

the MAP might pick up pressure fluctuations from a noisy valve, but unlikely. even if it did, MAP information is usually only read by the ECM at certain crank angles, so it normally wouldn't "see" the noise.

the knock sensor on the other hand, can be very sensitive to valve noise. tuning knock control systems is a bit of a black art. pretty much any physical variant of the engine OR TRANSMISSION(!) requires independent calibration sessions.

 

Thanks for the info guys !!

cheers,
Beach

 

Beach...e-mail Phil(LT401) He's worked on MAF on his brothers car

 

Thanks Smallblock... I've already talked to Phil many times about it... in fact the 475 HP example I gave above is Phil's brother. He's not positive, but he too thinks you can get around 500 or so out of a MAF.... I'm planning on some modifications this winter that should get me kinda close to that #... I hope,.... and certainly don't want the fruits of my labor to be wasted because the MAF is at its limit.

later
beach

 

The SD system is a lot easier to tune because you don't have to fart around with scaling tables. Also, consider that when you are scaling a table up, it makes the area between each value larger, and thus less accurate. I did an SD conversion earlier this year and it's pretty simple to tune.

 

Actually MAF is pretty easy to tune as there is really not much tuning to it at all. With a high flow unit it is wonderfully adaptable and as long as you are in the ballpark you will be okay.

SD is easier to tune *the exact way you want it*, but isn't as adjustable for weather conditions, and takes much more time to get dialed in. If you don't have a decent base program it is also a good bit harder to get set up proberly.

 

Chris I have to disagree with you on both of these comments. The stock 87-89 ECM internally limits the airflow to 255 gps. Changing the MAF will not fix the problem. You cannot scale around it the problem either. The scaling table actually has space to go up to 350 gps but the number 350 will not fit into the 1 byte memory space allocated for it. The ECM also uses a 2nd scaling factor for each MAF table but it cannot be used to over come the 255 gps limit in the last table. :(

Speed density can handle weather changes because it uses the absoute manifold pressure which is the outside air pressure when the motor is at WOT. The actual air mass is calculated using air pressure, inlet air temperature, engine size and engine volumetric efficiency at the rpm the calculation is made for. Humidity is another story. I don't think MAF or SD can measure humidity.

 

I agree, the 87-89 MAF does limit power production - which is why I qualified my statement with a hi-flow unit. I believe the original poster had a 93 (LT1) so I have some basis for that . The LT1 MAF in stock form can handle up to about 500rwhp NA, and 450rwhp on boost, and does a very good job of it. With a MAF switch (Z06, calibrated ported unit, whatever) that limit can be extended. Once you get into the 600+rwhp range you *do* need to go speed density, but in all honestly there aren't that many cars making that kind of power - and anyone who can manage that power should have the knowhow to realize they need an aftermarket DFI anyway.

The LT1 has a 16 bit MAF transfer lookup table, but the number of samples is also greatly increased, so you will max out from 450-470 g/sec depending on which function you look at. More than enough for 90% of the "street cars" out there.


SD can work well, I never disagreed with that, I simply said that it is much more difficult to tune. And yes, it can correct for different weather conditions - but this is yet another set of tables that you have to tune also, and it does take a good bit of work to get them nailed down.

It's really the difference between having a a good "relative" measurement (since I doubt anyone here has actually measured VE scientifically it is all a guess, making the entire equation relative) VS. an absolute measurment (thus theory can be more easily applied and initial tables calculated). You can of course do this for both, but the MAF setup is just more accurate as a means of determining airflow into the engine. From the literature Delphi puts out concerning the LT1/LS1 style MAF sensor it is supposto correct for humidity also, though I am not quite sure how this works. (remember, it is not a simple hot wire setup though, but a hot/cold setup).

 

:confused: :confused:

 

With my 396, I have experiance with both. I am currently running SD and I like it. Throttle response is very quick. Almost explosive. Starts alot faster also. The SD is easier to tune for modified motors IF and only IF you do all the testing and tuning yourself. It's jsut about impossible for a mail order tuner to get a SD program right on a modified motor. Starting in 94, Vettes has a MAF and a MAP sensor. They combined both for the best of both worlds. For your 93, I would keep what you have and maybe look into doing your own tuning. LT1 Edit is a nice program I hear.

Jason