If your setup has a plenum or a chamber that is shared between ALL the cylinders then it is likely that Speed Density is the best way to go for your engine, also if its boosted (Turbo or supercharger) then speed density is certainly the way to go!!! This means using the Manifold Air Pressure sensor (MAP) that's already built into the MegaSquirt ECU. To do this simply connect a piece of pipe from the sensor into the plenum onto the engine side of the throttle plate, NOT the ignition vacuum advance hole as this is sometimes a ported hole. The fuel pressure regulator's reference to plenum pressure is often the best place to "T" into. If the MAP signal is erratic at idle, etc, then it may need a small filter installed in-line to buffer the vacuum. Ensure the pipe is capable of a vacuum and will not collapse.

It is also a good idea to lay the vacuum pipe out so it has a loop in it, this stops any moisture getting to the sensor that may occur over night ,etc.

If you have Individual Throttle Bodies (ITB's) or a very lumpy cam that produces no vacuum at idle (this can soon be seen in MegaTune when the map sensor is plumbed in) say it has a higher KPa than 65 -75KPa, then you should go over to Alpha_N. This uses the TPS as the load reference rather than the manifold pressure.

With a boosted engine using Alpha_N, you MUST use the Hybrid_Alpha_N setting with MegaSquirt, because the throttle position bears little relationship to the amount of air going into the engine. Alpha-N is for naturally aspirated engines ONLY, the Hybrid Alpha-N adds the MAP value into the fueling algorithm, (obviously the map sensor will need to be connected to the inlet manifold, like the above drawing) so fueling is increased with boost pressure.
Please Note: There is no extra table or any other settings other than selecting the option in Advanced Settings. It is simply used to add the MAP value to the equation.
To use Hybrid Alpha_N simply set your Constants - Control Algorithm as Alpha_N and set MegaTune as Alpha_N.
Hybrid can then be used if it's set in Advanced settings.

(MAP - Speed Density) Setting the KPa values in the Tables

Note that if using a 16x16 table (MS2-Extra) the same theory applies, its just a little more spread out.
It is a good idea to set either the second or third row of KPa bins in the VE Table to the idle MAP value. So for example, if you idle at 35KPa set the first to say 20KPa (this can be used for fuel saving during over run, the second can then either be 35KPa or a value between 20 and 35 then the next could be 35. The reason is it is always easier to tune idle if the point of idle is on a cross hatch, then theres only really 2 points that influence it rather than 4, this may not be possible with a varying idle map, but its worth trying if you can.
The rest of the KPa values will depend on where your engine cruises and what boost pressure you run, remember 1PSI of boost is aproximately 7KPa (6.8), so for 8psi of boost your max KPa value will be 8psi x 7KPa = 56KPa above atmosphere, so 100KPa (atmosphere) + 56KPa of boost = 156KPa (round that off too 160KPa)
Lets take a NA engine into concideration; Idle is 30KPa and cruise is also around 30-50KPa. At cruising KPa there will need to be more rows than at any other to get the engine to maintain a nice smooth AFR through out that area. In a race engine this section wont matter so that rule doesn't apply. Wide open throttle on a NA will be 100KPa (depending on where you live but 100KPa should be fine) I would set this engine up like this:

KPa Scale for VE Table

Note that if using a 16x16 table (MS2-Extra) the same theory applies, its just a little more spread out.

MAP (Speed Density) or TPS (Alpha_N) Setting the RPM values in the Tables

It is a good idea to set the first RPM row so it is lower than idle, this allows you to set a richer area than at idle so if the engine starts to stall or slow then it picks up a little when it goes to the richer area. So, for example, if you idle at 800RPM then set the first row to 500 and the next to 700. The 700 RPM row will be the one that tunes the idle AFR, if it drops below 700 it will be richened by the 500RPM row, but only if it drops below 700, which is 100 below your idle speed. The next few rows should be around your cruiseing area, this again, allows for better tuning of the AFR at cruise allowing a smoother, more efficient drive and a better ride around town where slow engine speeds are necassary.

TPS - (Alpha_N) Setting the TPS values in the Tables

MS1-Extra ONLY for MS2-Extra see HERE

If you are using MS1-Extra then ensure you select Alpha_N during the install of MegaTune if you decide to go for that setup over speed density. Or use the Configurator to set up MegaTune for you:

To use the configurator, run MegaTune and select:

File - Configurator

Then select

Car1 - Settings.ini - Settings - Fueling Algorithm

Now select File - Save

If running Alpha_N you will need lots of bins set around your cruising area and two around the idle (foot off the throttle). In order to find the scale of your TPS, go to Tools, Calibrate TPS, and with your foot completely OFF the throttle hit Get Current. Remember this value(e.g. 33). Put your foot flat to the floor on the accelerator (engine not running!!) and hit Get Current on the Calibrate TPS dialog, remember this value too (e.g. 225). The first entry in the TPS scale should be just below your TPS value for the foot off the throttle test, so our example was 33, so set it to 31. This allows for any slight variations.

Please note: Later versions of MegaTune (+ 30th Oct 2007) I have changed the screen so it shows the RAW ADC value in the bottom left gauge when you have set MegaTune to Alpha_N mode.

The next few values should be very close to each other up to around 25% opening as this is the area that will need more tuning at low RPM. One thing you have to always remember with alpha-N is that you don't actually know where the effective WOT is anymore (i.e., when you have enough throttle that opening it further doesn't affect the amount of air being ingested). At low RPM WOT could be only 20% throttle. It is worth setting 5-8 of the total TPS bins (load on the VE Table) to cover the first 20% - 30% of the throttle opening to make tuning easier. This will help you end up with a driveable car, as most driving on the road is done in the first 30% of the throttle. The rest of the bins can fill in the gaps between 30% and WOT. (Wide Open Throttle).

TPS Scale for VE Table

At non idle areas of the VE table, you will generally run lean (low loads and speeds) or rich (high loads and speeds). High loads are associated with high kPa MAP (TPS ADC) values. Deciding exactly when (and how much) you should run rich is mostly a "seat of the pants" thing. When tuning, you will find the engine will surge (at low loads) and "coughs" at higher loads if it is run lean. Adjust the VE at the point where this happens so the this does not occur. Check the plugs for detonation (tiny black and white flecks) when tuning at high loads and rpms if you suspect detonation at all.

100 95 85 65 60 55 50 45 40 35 30 20 15 10 5 2	>85 will be wide open throttle (depending on engine speed) ------------------------------------------------------------------ 30- 60 would be for light to mid acceleration areas.   ----------------------------------------------------------------- 0 - 30 will be tuned for crusing, this needs quite a few rows to get a good smooth, economic cruise.
	500 700 1000 1300 1500 2000 2500 3000 ----- ----- ----- ----- -----

PDF file of this page