mass air strokers

[justinoshea]

Found this and it caught my interest;

"Mass Air fuel injection will become rich if you increase the displacement without re-programming. It will sense the extra air volume, but assume you are forcing it into the smaller engine."

My previous understanding was that mass air measures the intake air mass and adjusts fuel accordingly (among other things). so if you increase the volumetric efficiency of the engine with head/cam/intake (extra air) the computer adds additional fuel. Ford reccomends upgrading speed density to mass air for this reason, and sold a kit to do it. seems like adding cubes that pump extra air would be the same as seen by the mass air sensor and computer.

thoughts? technical babble?

 

[Broncobowsher]

Computer calculated cylinder pressure and thus ignition timing based on "load"
If the computer is told the engine is 302 cubic inches, it sees enough air to fill 151 cubic inches, it knows light load, plenty of spark advance.

Put your foot in it, it calculates 290 cubic inches of air then it knows you have no manifold vacuum and nearly max cylinder pressure.

Now put that computer on a 351
at 290 cubic inches of flow the computer thinks it is at full load, really you still have vacuum and more power left. I remember hearing that the EFI is programmed to 150% of engine capacity. Just to cover the possibilty that you run the engine at 1000' below sea level in -20° air where it is more dense then normal.

MAF does a real good job of keeping the air/fuel mix correct. But has to guess a lot on actual cylinder pressures and therefor ignition timing.
Early MAF ('93 and prior) systems would go into a fixed timing able at full throttle. This is where the tuner MAFs for mustangs came from. Screw the cylinder pressures and just get the air/fuel ratios right. As long as you are drag racing the fixed timing tables for full throttle will stay the same. Just bump the base timing to tune that part of it.

See any problems there? What works at the dragstrip isn't going to tune where you spend 99% of your time driving a Bronco. Unless you only race the Bronco.

A little more trivia, starting in '94 the computers got better. The baro sensor disappeared. Timing at full throttle was no longer a fixed table. Calculations were made based off cylinder load all the time. This screws up he basic thinking of how a racer tunes. So they figured out that they can stick an older computer into the '94/5 cars and keep there predictable tunes. Not ideal tunes, but predictable. And when drag racing started going from heads up to bracket racing, it was no longer about who got there first, it was who was the most predictable. I have seen a K-car run 22 second quarter miles and win because the car would turn the exact same time no matter what. There is a down side to bracket racing...

How is that for a mix of technical and babble?

 

[justinoshea]

How is that for a mix of technical and babble?

good, good, but I didn't see any irrelevant equations or overly complicated diagrams.

So a stroker at idle and part throttle cruz runs closed loop with the 02 sensors and mixture is 'ideal'-ish even if the ignition timing is off.
So a larger aftermarket MAF flows more air for the same voltage seen by the computer. the matching larger injectors supply more fuel. the computer doesn't really know that the engine received more air or fuel. it goes about it's timing scheme thinking the motor is stock.

 

[Broncobowsher]

At idle and steady state cruise the O2 should do it's normal job of fine tuning the mix. It is the transitions that the O2 is pointless. When it looks in one table, sprays fuel, checks O2, and by the time it figures out if there is anything it can change, it is in a different table due to a change in speed or load, O2 connot correct.

Timing is less then ideal with MAF strokers. Particurly part throttle driving. You may be throwing out even more then that if the computer starts thinking that you are at full throttle and goes open loop power mode, while at cruise.

Here is a fun one. Have not seen it on a pre'93 comuter, but on a '94 that had big injetors and a "tuned" MAF the computer kept throwing out a "MAF below minimum voltage" code. What they did was tune the MAF to trim the voltage to correct for the oversized injectors. The trouble was the MAF had to put out a super low voltage to trim enough fuel off the oversized injectors for it to idle. The computer thought the engine was running on half the amount of air it should have and thus half the injector pulse of a double sized injector. Now you have a computer that isn't happy with the MAF reading and throwing codes. Could only guess how the rest of the tune was.

The MAF knows exactly how much air has entered the engine. It is something like 20 cubic feet of air weighs in at one pound.

 

[justinoshea]

I recently dug up some old Tweecer docs that address the load, CID, spark issues. I am going to try to address my part throttle pinging and need for octane boost. many modern vehicles run fine on 91 octane fuel and have compression ratios higher than 10:1, such as my Lincoln LS.

IF you put larger injectors in (and matching MAF) the computer's calculated load will be low because the MAF shifted the curve down (for more capacity)
lets say 30#s instead of stock 19#s
19/30 = .63 so the max load the computer will calculate is going to be low (no more than ~60%). timing will be more advanced than it should be. not good.

lets say you stroked the motor to 347ci. it will screw things up in the other direction;
347/302 = 1.15 load will seem to be about 15% higher, as the MAF has more air flowing though it due to more cubes

The engine size scalar is used directly in the load equation, and hence the spark timing. both can be compensated for by just changing the engine displacement. certainly not the right way to do it, but....

so if you take your mustang or bronco to a 'professional tuner' and he is done in a couple hours instead of a couple weeks, this is probably how he did it.