I have been reading thru a bunch of the threads in here on EFI and there is a lot of good interesting stuff. It has got me to thinking (dangerous on a good day) Having never played with FI I have a question, am I correct in determining from the info in here that the injectors fire on an Even rail, Odd rail fashion? If so, doesen't that mean it is squirting go juice at other times than the intake stroke?
And this is aparently more efficient than a Carb? I am confused.

 

Sixfooter,
It is a little dangerous to lump all fuel injection systems into one group. Some systems have the ability to inject sequentially, ie when that cylinder is on its intake stroke. Many systems though fire on a batch method. This could mean any batch depending on the system design. One bank, evens, odds, etc.

While sequential injection is probably more efficient overall, it's not what makes fuel injection more efficient. Many other factors come into play. Also, a poorly setup system will be just as inefficient as a poorly adjusted carb. Some of the big advantages of fuel injection that I see (with a properly setup system) are:

1. Better cold starts
2. Ability to compensate for altitude and ambient temp
3. Ability to go closed loop with an O2 sensor
4. Eliminate fuel on manifold walls with port injection system
5. FI is not bothered by acceleration (as carbs can be with fuel sloshing in the bowls)

There are many more depending on the system. For example, the Megasquirt system I am installing has the ability to perform as a rev limiter, traction control, boost control, and just about anything else you can imagine.

Ken

 

A carburetor has fuel available to all the cylinders all the time.

 

Many aftermarket systems are batchfire. Either all injectors at once or one bank at a time. Efficiency wise the difference between batch and sequential is minimal. For instance, the 1993 LT1 Camaro was batch fire and the 94 was sequential. Both made 275 HP and both had identical fuel mileage. The reason GM switched to sequential was emissions. Timing the injector to squirt onto the opening intake valve kept the fuel atomized better and lowered emissions a hair. Batch fire usually squirts onto a closed valve where the fuel will sit until the valve opens.

 

Ahh, thats makin it more clear, I was lookin into the Megasquirt unit when I realized what I was looking at. According to Zwede it doesn't seem to matter that the fuel sits on a closed intake valve until it opens, by my counr by then it will actually have 3 squirts sittin there. Well, it apears to work.
Another question, can you run an O2 sensor closed loop with Duals? Only on one side?

Thanks for the info BB and Zwede

Markdtn, while the carb has fual available all the time, its only on the way to the cylendar while the intake valve is open and the piston is on the way down, correct?

 

I remember being shocked when I first learned how most fuel injection systems worked. I always assumed, probably like most people, that EFI would be sequential. It just doesn't seem to make sense to have batch fire systems. Why bother locating the injectors so that they are pointing at the intake valve if half the time they are firing when it's closed? It seems you could get the same effect just having injectors pointing somewhere into the manifold plenum area and atomizing fuel.

Oh well, batch systems seem to work pretty well anyway. Their biggest advantages, I think, are tuning and reliability...

 

A good many of the systems put the O2 sensor in the collector of one exhaust bank. The assumption is that the exhaust mixture of half the pistons is pretty representative of the entire engine.

 

That's not quite accurate. More accurately, each batch of injectors (a GM PFI V8 fires in two four-cylinder batches about 180 degrees out from each other) fires at an average of the valve openings. It'll hit at least one near perfectly, while the others are partially or fully closed. When you're turning 2500 RPM, it's just the tiniest fraction of a second. We're talking about 41 2/3 RPS!

If I remember a GM-ECM email discussion correctly, at about 3200 RPM, the injectors essentially become sprayers, anyway.

 

Zman back around '92 or so I had a '87 vette, a friend had a TBI equipped 455 Olds drag car, and I got curious about putting DPFI on my 455 Pontiac/GTO/Lemans convertible....I did it myself in the garage, having the intake modded/machined for the injectors....

I made the HUGE error of not aiming the injectors down toward the intake valves...the engine would NOT idle, and indeed backfire through the Tbody just like an ill times carb setup....but it would run at 2k and higher...pretty good....

so that finally persuaded me to alter the setup and get the injectors aimed toward the valves, then everything settled down and I picked up about 3-4 mpg and this was open loop, analogue computer, old Caddy system from the mid-late 70's...built by Bendix....
If you take a logical look at the timing of these systems, you find it's not important much at all as previously stated, between batch and sequential...think a minit.....600 rpm is ten rp second...or 5 firings per second....meaning that fuel is shot with a pulse width of maybe 2 ms at idle, and then sits there for about a MAX time of .2 second...or 200 milliseconds...that is not enough time for it to do much of anything but get ingested by valve opening....and the head passage way is very short comparted to any induction system passageway....

GENE

 

What you discovered the hard way was probably fuel robbing. By not aiming the injectors at the valves the fuel would go out to the plenum and be ingested by another cylinder. You had some pig rich and some dead lean. No wonder it wouldn't idle.

 

6. Doesn't mind in the least if the vehicle is inverted for extended periods.

I know this because of an incident with a snowmobile....uh....nevermind....

 

Yes, all GM I know of before 1996 do it this way.

Depends on how rich you are running. On a single plane intake anyway, cylinders share fuel so in a sense there is always fuel heading in that direction.

Also consider the length of the intake runners. 3,5,4, and 6 are shorter than 1,7,2,and 8. With any port injection, you are going to get more consistant amounts of fuel to each cylinder.

 

Sixfooter,
What can be done with the MS system is basically limitless. That is the main reason I am going to use it. They are working on not only having two O2 sensors, but two wideband O2 sensors. However, you can run the system with only one O2 sensor. You should try to mount it as close to the merge as possible, so it heats up quickly and still sees all the cylinders from the bank. You could put in two sensors, and then just switch the wiring between them as well. This would be a rough way to see if the engine is getting equal amounts of fuel to both banks.

Ken

 

Holley did some tests on their dyno with 8 wideband O2 sensors. They found that many dual plane carb intakes have really bad distribution issues. They found some cylinders running a full ratio leaner at full throttle. So some cylinders at 12.5:1 and some at 13.5:1!

Single plane carb intakes were better at about 1/2 ratio variation with the best race style single planes coming in at .3-.4 variation.

Port EFI was the best at no more than 0.1 variation.