Cam overlap and EFI engines

[olie05]

I was talking with someone earlier about different engines and somehow the topic of a big cam on a fuel injected car came up.

 

He insisted that overlap is not so important on fuel injected engines, because they don't need the added intake velocity to suck fuel out of the carburetor. I was under the impression that it was always better to move air through the engine, and the more air the better.

 

could someone please clear this up? I'm curious now.

 

-Oliver

[strotter]

Not true - overlap counts w/ efi. From thirdgen.org,

 

Cam overlap is the period of time, measured in crankshaft degrees, in which both the intake and exhaust valves are open. At low RPM, this allows some of the fresh intake charge (i.e. unburned fuel and air) to leave the combustion chamber via the exhaust. It can cause a car to smell like fuel when idling, depending on how much overlap there is.

 

Now, keep in mind that there is an oxygen sensor that measures the amount of oxygen present in the exhaust stream. When it detects the unburned intake charge from the high-overlap cam, it tells the ECM that there is a lean condition (assuming closed loop is enabled). The Integrator, INT, then changes to compensate for this "phantom lean condition." As the INT moves, it updates the BLM, essentially dumping extra, unneeded fuel into the engine because the ECM thinks it's needed. Run like this for a little while, let the learn feature kick in, and pretty soon your car will smell like a gas station AND it'll cause all sorts of unusual characteristics at idle.

 

Plenty of other good stuff there regarding EFI tuning in the DIY PROM forum.

[BRAAP]

Olie,

Well this “someone” is misinformed and now is spreading that misinformation. This instance of spreading this misinformation … (I was writing up a humorous slam, but in the end decided to give this poor misinformed “someone” a break, unless he insists on spreading more of something he really doesn’t have a firm grasp on…)

 

 

EFI is merely a “different” way to introduce fuel to the air stream vs a carburetor, and as such, the cams specs needed for a given power band will, for all intensive purposes, be identical for a carbureted engine or an EFI engine, with both running the same intake manifold, exhaust etc.

 

I could easily spend 4-8 hours writing up one of my REALLY LONG posts with all the details of how a carb works and explain how that “sucking velocity” works, (air flow through the carb generates a decrease in pressure across the discharge nozzle because of the venturi, thank you Mr. Bernoulli), I just don’t’ have the time this week.

 

Hope that helps.

[mom'sZ]

I could easily spend 4-8 hours writing up one of my REALLY LONG posts ... I just don’t’ have the time this week.

Whew... that was close

 

....joking Paul JOKING....

 

Oliver: I concure with Paul, this guy is... well incorrect

[thehelix112]

I'm going to disagree with the previous 3 posters here.

 

I (think I) understand how a carby works, I understand how EFI works.

 

At a very theoretical level, if the airflow into the engine is being resisted by anything (like the inertia of the fuel it is being used to move), its velocity will be different than if it is being accelerated (like if 60psi's worth of fuel is being sprayed into it in the directionish of flow).

 

Whether or not this is in anyway significant I will not comment on.

 

Dave

[datsunlover]

It was always my understanding that an FI car would have trouble with a big cam (high lift and overlap) and the ecu would get 'confused' and not be able to run the motor properly. Maybe newer and/or aftermarket FI systems can, but I KNOW that there are diffrent cam specs listed for say, a 1st gen SBC/BBC for carb or FI. If there are diffrent listings, there must be some reasoning for it. Hell, when I got my colt cams .510/280 cam , I was told it would NOT run properly if I ran FI.

[RTz]

It was always my understanding that an FI car would have trouble with a big cam (high lift and overlap) and the ecu would get 'confused' and not be able to run the motor properly.

 

This can be true with the popular MAF/AFM, or speed density systems (MAP).

 

Alpha-N solves this problem by using only TPS (for load) and RPM. This can be a favorable solution with aggressive cams. Even the OEM's have used it successfully. The biggest problem with Alpha-N is that it won't work in a boosted application. That left some of the aftermarket systems with the chore of designing a 'blended' system. This allows you to run Alpha-N to a predetermined threshold, then MAP of MAF from there.

[BRAAP]

Hellix point has value and is worth investigating. As for how much affect on the power being developed does that concept alone, have? Theoretically speaking, it should have some influence, my guess is that its influence is so minute that it would even be hard to see on a dyno.

Even at that, carburetors do offer some spectacular fuel atomization characteristics, and at least back in the mid ‘90’s, EFI still wasn’t atomizing the fuel as nicely as carbs so that might ”make up” for the losses incurred by the air stream having to accelerate the fuel and any losses of air flow induced by the venturis. I’m just throwing that out there to chew on, I honestly have no idea.

 

 

As for LSA specs for Carbed or EFI engines, lets through out this hypothetical question for the engine builders amongst us .

 

If you were setting out to build up a high performance engine, hot street or even a full tilt race engine, whether it be an L-6 or a V-8, and you were to use a specific intake manifold that could accept a carb/s or EFI, (that is to eliminate any variables in intake manifold design), in the cam specs phase of designing this high output power plant, would you alter the Lobe Separation Angle, (valve overlap), whether it is carbonated or Electronically Fuel Infected?

 

 

Rusch Motorsports would not alter Lobe Separation Angles based on how the fuel is being introduced to the air stream. Other factors in the design and intended usage of the power plant affect what LSA would be chosen such as intake design, Exhaust design, rpm range the engine is to be used (that is bigge), but “how” the fuel is introduced into the air stream is not one of those factors we would use to determine LSA specs for cam shaft selection.