Fuel injection technical questions... (TBI vs. Multi-point)

[MaineShark]

And odd thing that I read from some guys having an online discussion about superchargers, and whether to run a carb or EFI. The claim was made that the wet flow of a carb (or TBI) will help cool the charge more than the dry flow of a multi-point system. Basically, if I follow the details correctly, the wet flow allows more time for the fuel to evaporate, reducing the temperature of the charge air. Obviously, the gross heat (thermodynamics here) of the charge will remain constant, but that heat is not evenly distributed, so I think the idea is that the fuel will have more time to atomize, and will spend more time in contact with the air, so that greater heat transfer will take place between the fuel droplets and the air.

A couple issues with this. First, in order for it to work out, physically, it requires that the fuel be lower temperature than the air. I've never measured fuel temp or inlet air temp, so I don't know if this is the case. Second, even if it were the case in a supercharged system (hotter-than-normal air temp), would it still be the case in a naturally-aspirated system?

Thoughts?

Joe

 

[AKRAY4PLAY]

Joe,
interesting thoughts! fuel atomizes easier when hot. for all intents and purposes, the air and fuel temp are close to the same, unless you just filled the tank or the temp swing from morning to afternoon is huge. i don't think the small amount of energy absorbed by the fuel during vaporization will amount to a measurable sum. and after compression the fuel and air will be heated together. going carbed or tbi, will allow some lubrication of the supercharger and possibly extend its life. mpi will suffer because of the higher intake pressure, unless you raise the fuel pressure of the mpi system to account for the difference. basically you are looking at 5hp out of 500. it may be worth it to some, and no bother to others, but it is interesting, Brian

 

[MaineShark]

Yeah, that's what I thought, but I wasn't sure.

Although I don't think they were considering the fuel as a lube, since they were talking about blow-through (blower ahead of carb).

I'm seriously thinking about some form of fuel injection, since I live in a fairly hilly area, and am looking at moving into an even more mountainous area, so I want some form of quality altitude compensation.

The way I see it, there are three ways to inject, primarily:

TBI, with low pressure fuel, injected ahead of throttle body, for wet manifold flow. MPFI, with high pressure fuel, injected right before intake valve, for dry flow. And plenum injection, with high pressure fuel, injected right after the throttle body, for wet manifold flow.

Now, there are some advantages and disadvantages.

High pressure systems (MPFI and plenum) have better fuel atomization, but also require more specialized plumbing, as well as the increased risk of fuel leaks.

TBI is essentially a bolt-on mod, which should perform as well or better than a carb, but with the more advanced computer features.

Wet flow may have some advantages in heat transfer (as those guys were debating), and may give the fuel more time to vaporize.

MPFI precisely meters each cylinder's fuel. This can be both an advantage and a disadvantage. On the one hand, control is generally good. On the other hand, as the injectors wear/clog, a lean condition can be introduced in one cylinder, without seriously changing the O2 sensor readings, so the ECU doesn't compensate... lean=boom.

I'm not quite sure what I want to do, exactly. I've put the 406 project on hold for a while, since I want to take my time and do it right. But I need to get something more advanced than the Q-Jet on this engine (which is unfortunate - other than the inability to compensate for air/fuel ratios, I really like the Q-Jet).

I'm sort of leaning toward modifying a carb-type intake (wet flow design) to have injectors (say, four large ones) just after the throttle body, aimed at the air passage pairs. Mostly, I'd just like to do this since it's rare, and I like being different.

It does mean I'll need to deal with high-pressure fuel, unlike TBI. I'm thinking of having a multi-stage fuel system (low pressure from fuel tank to a surge tank, and the high pressure pump from the surge tank to the fuel rails), which should solve some problems (fuel starvation on turns, pressure-induced fuel leaks), but may cause others (packaging issues, fuel heating, and noise).

Joe