EFI without MAP...

[FloridaRustang]

OK, crazy idea, but let's assume I'm building an EFI system without a MAP sensor.

To elaborate, assuming that using the "gas" pedal varies the maximum lift of the valves rather than the opening of a throttle body butterfly, there's no real pressure upwind of the valves to give a reading on any MAP sensor. A carby worked in this situation because it relies on air speed, not vacuum.

My present line of thought goes like this:

For a given A/F ratio, an engine will settle at some optimum RPM level for the present maximum valve lift and air temperature.

At any given time, the RPM level is higher than, equal to, or lower than the optimum level.

What to do?
If higher, lean out
If equal to, you're right on schedule
If lower, richen up

Or is there a better way? MAF sensor? Or is my assumption off, that the MAP sensor won't function in this arrangement?

 

[BB6Mustang]

For a given A/F ratio, an engine will settle at some optimum RPM level for the present maximum valve lift and air temperature.

How are you defining "optimum RPM"? Load on the engine has to be accounted for. RPMs will be lower going uphill than on flat ground because of the higher loading on the engine.

There are three basic types of EFI: MAP, MAF, and Alpha-N.
MAP uses a manifold pressure (engine load) vs. RPM table to lookup the engine VE and then calculates fuel from that.
MAF uses an air metering device (hot wire, vane, flapper, and others) to measure the amount of air passing through and then calculates the required fuel. Of the three this is most similar to a carb.
Alpha-N uses a TPS (throttle position) vs. RPM table to lookup the VE value and then the calculates fuel. Alpha-N is typically only used on race cars with wild cams.

MAF and Alpha-N should both work in your application.

So, is this just a hypothetical question, or do you really have a throttle-less engine? I've heard that BMW was experimenting with solenoid valve/cam-less/throttle-less engine, but AFAIK they don't have a production version yet.

 

[FloridaRustang]

Sort of both. I've been playing with the Pattakon thing. (http://www.pattakon.com if you haven't seen it.)

It's an extremely simple mechanical method of varying valve lift. They're running it on a daily driver Renault 4-cylinder in Athens, Greece.

BMW's idea revolves around electronically controlled valves. It's a 42V system and very complex. And I'd imagine it runs rather warm, too.

Mechanically modifying the lobe centers is easy enough, but you cannot increase duration, though that's not too big a deal when you can vary lift.

The real roadblock at the moment is that our six is pushrod, but I've even seen OHC heads from Oz, so it's very possible. Pattakon built up their stuff with plate metal right on top of the stock head, including the cam bearings, so I figure that could be done on our six heads.

MAF won't work properly, from what I've read, at higher speeds because of airflow pulses and reversion. MAP sensors compensate in those cases, IIRC. That puts it right out for this application.

I didn't realize that Alpha-N worked like that. I always look at things like these as mathematical problems anyway. That seems to be the way to go. The lookups could obviously be modified for intake temp and further adjusted based on exhaust readings.

 

[MustangSix]

The earliest Holley Projection systems were rpm/throttle position only. The interpolated load by looking at those two sensors.

SDS also has a tuning strategy for engines with huge cams and little manifold vacuum as well. Both of these work, but with limitations.

 

[BB6Mustang]

Your right, MAF doesn't handle reversion well, but that is usually only a problem at low RPMs with a big cam with a lot of overlap (like over 280° duration and less than 110° LSA). Plus you can significantly reduce the pulsation and reversion that the MAF sees by putting a large plenum between the MAF and intake runners. I really think a MAF could work in this situation. Besides you will probably have far more problems trying to making an Alpha-N system work properly.

I'll be very interested in any progress you make. Keep us informed.

 

[rbohm]

8) unless you are building a turbo motor, a 1 bar map sensor is what is used on all normaly aspirated speed density systems. if you want a turbo motor you need either a 2 bar or 3 bar map sensor to allow for the boost. the map sensor reads vacuum or the lack of pressure in the intake tract. while the alpha-n efi systems work, speed density is much better as it is more flexable.