Simple EFI ?

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a start, then look into the Megasquirt injection computerkit at If it was as simple as just turning on the injectors you could do with a square wave generator and an amplifier to run the injector coils, but it's much more complicated than that. For one, you need to vary the injector pulsewidth based not only on TPS, but manifold pressure (or vacuum) since both sensors indicate in part how much actual air mass is entering the engine. At WOT you can have a wide range of air mass flow, so running a single injector pulsewidth will result in wildly varying air/fuel (A/F) ratios, a bad thing for most engines. Another variable is that as manifold vacuum varies the pressure drop across the injector varies as well, and that directly affects the fuel flow through the injector, so you need some way to compensate. Older cars do it with a fuel pressure regulator connected to manifold vacuum such that the fuel pressure rises and drops in synch with the manifold pressure. Newer cars accomplish that function in the ECM by modifying the injector pulse width.

JazzMan

Why do you want ELECTRONIC fuel injection? Guys around here use a manually adjustable system, works fine. No need for fancy electronics. Look at the old Hilborne system. Most of the local guys that build their own FI use a copycat system much like the Hilborne.

I agree with Don about machanical FI being the way to go but I think you all forgot that he said the engine is two stroke! I think this would be HUGE engineering problem, (intake port position, exhuast port location, etc.) The injectors would have to mounted to the head and crank position factored in somehow. How is this done on a two stroke diesel? OK! now your brains are racing!! Man I Love this stuff!!

Rick

Two cycle diesels have camshafts and exhaust valves and injectors operated by a cam lobe. At least the Detroit Diesel does. There are valveless two cycle diesels such as the Stuart Turner, Cerlist and the rare 51 Series Detroits but they are museum pieces now. There are probably a couple of Fodens still in England and maybe in the odd English doubledecker in America (most had Gardners and have ben refitted with Cummins or Detroits here) and I have no idea what they use.

Anyway, FI is a waste of time in this application, crude metering devices like the Posa-Fuel or Ellison spray carbs would work as well. But the Hilborn is a crude system indeed-the RSA Bendix is far better.

Perhaps if I shed a little more light on what I'm up to . I build and race PRO 350cch outboard boats . I'm allowed to do ANYTHING to these EXCEPT run fuel that is not liquid at room temp and supercharge . My current set ups use carbs on long manifolds . This means the motor is mounted high on the tower ( high CG = "interesting " handling issues ) The idea behind this lowering the CG with out having to go to longer runners where the charge will really fall out of suspension. The charge is initiated right at the port and should get well mixed by the time it's blown through the transfer ports. Means I can place the butterfly TPS pretty much where ever I like ( dry would be good ) with out having any pooling issues . At

12,000 rpm I think the injector is going to be spewing pretty much all the time and having it "timed " to the port opening would not really be a necessity . Mechanichal FI would require driving the pump with all the attendant bits and pieces . This a "racing only " application on a 2 cylinder 350cc motor that will spend most of it's run time at WOT . "Drivability" is not too much of an issue . I want to keep this as simple as possible . I appreciate any input being as this is not a car . As far as mounting the injectors , fuel rail and so forth , that should be "a piece of cake " considering some of the stuff I've pulled off in the past . Thanks , Will

Well... That could give me a head ache . During my oridginal R&D I found some really interesting things happened concerning manifold length vs the choice of tuned pipe . There were certain rpm ranges wre you would get a "coupling " effect . Hmmm... Will

Why not use throttle body injection. Most of the homemade FI I have seen at our tracks were tbi, though one is homemade port injection.

One big thing, though, that I forgot to mention. All these homemade FIs were on cars running methanol. The old fashioned mechanical fuel injection systems ran VERY rich at part throttle. This is acceptable with alcohol- engines will burn a VERY rich mixture of alcohol. When these systems are tried on gasoline they don't work well at all. Lots of folks tried Hilborne injectors on small offies for road racing. Was not that successful- in fact, it was very hard to get Offies to run on gasoline, carb OR injection. So I would only try homemade FI on alcohol.

Well, alcohol is liquid at room temperature- more so than gasoline, so I would say go with mechanical injection and methanol.

Thanks to all for your thoughts . Lot's of "food for thought". I'll have to digest for a bit . Would like to stay away from alcohol due to the , roughly , twice as much weight issue . It "seemed" like a pretty simple at the time but I can see there are some issues that need to be "re-thunk" ( so to speak) . Thanks again , Will

The Stu Hilborn mechanical constant flow EFI and its emulators, were used on gasoline burning drag racing flathead Fords and other engines before they were put on Offies. They were used successfully with gasoline in circle track, road racing and even in limited street applications. Continental aircraft engines use a fuel injection system that is substantially similar to the Hilborn as well. Later in the turbo Offy and DFX era they went to Lycoming Bendix RSA injection, a more sanitary version of L-Jetronic, which gave a lot better results.

Most Offy teams ran not straight methanol, but "secret" blended fuels with all kinds of stuff including gasoline, benzol, acetone, nitrobenzine, nitromethane, and other solvents, until all USAC racers were put on straight methanol for safety.

The Offy ran fine on gasoline if you dropped the compression ratio and ground cams for gas and used a spark plug that was appropriate, not the Indy recessed gap affairs. A battery ignition or a mag with centrifugal advance was also a necessity. The sporty car guys didn't know how to set up an unknown engine and the Meyer Drake people considered them pains in the ass and sissy rich kids and were disinclined to provide much help.