EFI Theory: Boost fueling

[CTX-SLPR]

Howdy,

I'll admit I'm cheating in the category I'm posting this but considering EFI controls the fueling vs accepting what the hardware gives you of a carb, I figure this section is likely more familiar with what I'm asking.

Basically I can't run EFI at Bonneville so I'm going for MFI since I think I understand it better. I understand that at 0 &75-100% throttle the engine's fuel demands can be roughly tied to rpm and TPS for an NA application. Something like an N-Alpha EFI setup. Now throw boost in and suddenly you are into somewhat rare territory.

My question is, does the extra fuel needed for boost come in as a relationship to boost only or does it ramp with rpm as well?

Equations (rough generalizations)
NA (Base) Fuel = TPS x RPM x constant

Purely boost dependent Fuel = Base Fuel + Boost x constant
RPM dependent boost Fuel = Base Fuel + Boost x RPM x constant?

Now granted this is completely ignoring temperature and A/F shifts as boost goes up but it's a starting point.

From talking to Hilborn, it sounds like it's the later of the two options as thier boost valve installs on the return line and restricts the return in proportion to boost but that might just be since they don't use seperate systems for the NA and boost compensation fuel.

Thanks,

[Barry Donovan]

I have set myself up mechanical for future boost..it is funny , the return setup, restrict it to gain more as an option. that is what I did with no instructions. at n/a I am the max fuel pump.. and it goes the other direction if to boost.

being there is no such thing as a constant in fuel...
a modern reading of at least o2 can get any theory going.

if to have to play with constants.. wherever the most common runtime is should have the strongest peaks. kinda like the return line.. a reversal. Gives parts a break for longevity.

[dieselgeek]

It ramps with BOTH, but it ramps a lot more with boost than RPM.

MFI with boost is going to be very difficult. If you want to try something "out there" that has the potential to work and would be legal for that class, running a PWM-controlled bypass valve would allow you to ramp pressure up and down with manifold pressure and RPM, and be legal as I understand it. You just can't use closed loop tuning and electronic injectors. You can run the ignition with a computer, and as I read it - you can run computer controlled pressure regulation, or for that matter a PWM fuel pump (or two or three) that vary pressure as you need.

This would be a fun project...

[CTX-SLPR]

It ramps with BOTH, but it ramps a lot more with boost than RPM.

MFI with boost is going to be very difficult. If you want to try something "out there" that has the potential to work and would be legal for that class, running a PWM-controlled bypass valve would allow you to ramp pressure up and down with manifold pressure and RPM, and be legal as I understand it. You just can't use closed loop tuning and electronic injectors. You can run the ignition with a computer, and as I read it - you can run computer controlled pressure regulation, or for that matter a PWM fuel pump (or two or three) that vary pressure as you need.

This would be a fun project...

DG,

I unfortunately can't run a computerized igntion in Classic (for some reason you can in Vintage as per what I read in the 2012 rulebook). I know I can't run anything with a feedback loop but an electrically controlled bypass might be legal and certainly easier than the totally hacked up Bosch K-Jetronic system I have been envisioning.
I really like the idea of the PWM fuel pump on the boost fuel supply (likely it's own set of nozzles) rather than running off of the crank driven pump. That way I can make it a non 1:1 increase as boost and rpm increase and I might be able to feed a IAT input into the curve (that could very well make it illegal though). If I can't run an electronic fuel bypass restrictor directly, there are legal MAP sensor controlled water injection systems that I could potentially use to hydrallicly control the bypass using the line pressure in the water line.

The purely mechanical solution I've dreamed up is to take a 6 cylinder Bosch K-Jetronic fuel distributor and mechanical MAF and basically convert the MAF to a boost sensor. The boost sensor would move the fuel distributor flow valve using a cam profile that I could modify to make the fuel curve as compared to boost "shaped" to richen up the mixture as boost went up and "hard code" in IAT corrections assuming my air-water intercooler is predictable. I was really wondering if I should remove the pressure regulator on this setup and run straight crank driven pump pressure or boost refference the regulator and run it with essentially constant differential pressure across the injector, but I'll take information and adapt what I can inside the rules. I know for a fact I can't run a feedback style boost controller. Basically just use a hobbes switch to "fire" the wastegate open using a clippard (or similar electro-pnuematic) valve cutting the pressure to the diaphram. Can easilly see that getting into a surge loop of open and shut without some intervening logic...