new to this still. what is speed density mean??

Not mass air.

Speed density only has a predetermined fuel and spark functions. So if you are running a high lift cam or really big heads you'll run into promblems.

Mass-air adjusts the fuel/spark by how much air is entering. I think

This is how I always thought about it I'm sure there's alot more details.

TrophyHead said:

Speed density only has a predetermined fuel and spark functions. So if you are running a high lift cam or really big heads you'll run into promblems.

Mass-air adjusts the fuel/spark by how much air is entering. I think

This is how I always thought about it I'm sure there's alot more details.

Click to expand...

Thats correct.

SD has preset air/fuel ratios, MAF adjusts according to what the MAF sensor is reading.

right on. maf is not as precise since it flies by the seat of its pants, whereas the sd has pre-programmed maps, as trophyhead mentioned.

maf has idle surge problems, whereas sd doesnt. sd makes more power than maf, stock vs stock. sd uses vac to determine things, so if a lumpy cam or something is tried, the computer doesnt know what is wrong. a new chip can solve many such problems. quite a few run decent power sd cars.

im sd and like it.

A speed density system uses input from the throttle position sensor, the MAP sensor (engine vacuum), air and coolant temps, and rpm to calculate how much air is entering the engine. Based on that it determines how much fuel to inject and what the timing ought to be. Mass air systems have a meter that attempts to actually measure how much air flow is entering the engine, and with temperature and pressure (BAP sensor) data, it can determine the mass of air entering. It uses that to determine how much fuel to inject.

In terms of performance, both systems are comparable; however, the mass air system will tolerate a wider range of modifications than the SD system will. The mass air system is also better at maintaining fuel economy and emissions control over a wider range of driving environments (variations in temp., elevation, etc.). I've read posts about idle quality/surge issues with both systems - they are almost always due to components in the system being defective, vacuum leaks, or because people have made changes to the system beyond the range of the system's ability to cope -- and the mass air system will cope with a wider range of changes than the SD system will. As for stock vs. stock performance, the change from SD to mass air for non-CA cars occurred from 88 to 89. There was also a camshaft change in the HO line that year that accounted for a 3-5 HP difference - the SD camshaft makes more power than the mass air cam does. I believe that difference often gets attributed to the efi systems. It's my opinion that there's virtually no difference in peak power output between the systems if everything else is equal.

Hissin said - "maf is not as precise since it flies by the seat of its pants, whereas the sd has pre-programmed maps" -- not sure I agree, although I'm not entirely certain what he meant. I would say mass air is more precise as it actually measures the amount of air entering, rather than trying to calculate it based on various sensor readings as the SD system does. Seems to me that any system that is able to more accurately respond to ambient conditions, and is better able to control emissions output is the more precise system. But it's probably just semantics.

Michael, thanks for setting me straight on the idle surge (i thought it was more predominant in maf cars).

what i was trying to get at, in regard to your last paragraph, was that i had read that maf is not as precise as sd. thinking was, that maf really takes an average of the air going through it. it also deals with the wire heating and cooling depending upon the quantity of air goin through it. whereas sd has already calculated what should be occuring from pre-programmed tables. but mod the sd without a chip, and it gets bad.

i think the maf stuff applies (or makes a difference) more when mods have been made.

this is what i have read and think i kinda' know. as always, feel free to correct me (i'd appreciate it, as opposed to running around spouting untruths and being more foolish than normal).

EFI systems have gotten more sophisticated over the years, not less; more precise, not less. The mass air systems evolved from the SD systems, not the other way around. Actually measuring the volume of air entering the engine real time is considered more precise than calculating the amount of air entering the engine based on measured parameters like rpm, throttle opening, pressure, temp, etc. I think a basic logic check applies here - why would the manufacturer replace SD with a more complex, more expensive, less precise system? They wouldn't - they'd replace it with a more precise system. As emissions regulations became more stringent, they had to have a system that controlled more precisely for longer periods of time (over more miles on the car). That's why they moved to mass air. It just so happens that the same flexibility that allows the mass air system to deal with the wear and tear that occurs with mileage - while still meeting emissions requirements, also allows it to deal with mods that take an engine further from it's stock operating parameters - something the SD system is much more limited in. That's my understanding/opinion about it all.

alrighty, i have my tail between my legs; i stand corrected. thanks Guru.

BTW, the part about "basic logic" and what follows really made me feel dumb. J/K. what you said does seem logical, but a lot of things dont (to me), so i am more open (read:gullible) to believe things i read which state otherwise.
in kindergarten, i always did try to jamb the square peg in the round hole (also used to do the maze puzzles by drawing a line out of the starting point and around the peripheral border to the finish). teachers thought i was retarded, now i wonder if they werent right..... LOL.

seriously, i appreciate the lesson. just trying to learn.

lol i actually do searches for michaels posts. very interesting and packed with info, keep up the good work mike =)

Well, don't take it too far - the SD system is a perfectly good efi system as you know from your car. However, as corp average fuel economy (CAFE) guidelines and controlled emissions levels became tougher to meet, engineers needed more control -- and the mass air system gave it to them. Nothing wrong with SD though - provided you don't ask it to operate outside the parameters it was designed for.