Making a new intake manfold... injector aiming question

[SQLGUY]

Hi folks,

I've got a 1982 Yamaha SECA 750 that I converted to fuel injection with a bunch of junkyard parts and Microsquirt. One thing that bugged me, though, was the fairly lumpy idle I had, regardless of trigger wheel, semi-sequential injection, or whatever.

My setup uses a single, 1997 Ford Escort, throttle body (so I can take advantage of PWM idle air control). My previous intake manifold (and my new one as welll) was made from 1 1/4" copper pipe fittings and other copper and brass pieces. Because of the way things seemed to fit together at the time, and because I was making the manifold to "plug in" where the carburetors used to go, I had some relatively long pipes with two 90 degree bends for cylinders 1 and 4, and some much shorter pipes with two 45 degree bends for cylinders 2 and 3. Here are some pictures of the old manifold:




Recently I decided to build a new manifold and fuel rail to see if that helps with the lumpy idle. For the new one, I made flanges to connect directly to the cylinder head, eliminating the "carb holders", and I arranged the pipes so that the 1/4 pipes are not much longer than the 2/3 pipes and are somewhat straighter, with all four pipes having only 45 degree bends. Here are some picture of its current progress (it will be much prettier once done and painted).




Now to the question: from the opening of the intake port to the intake valve is actually not a straight shot. The 1/4 intake valves are about 30 degrees to the outside of the port, and the 2/4 intake valves are about 15 degrees to the outside. Of course, the valves are also below the ports. Should I try to aim the injectors directly at the valves (i.e. 30 or 15 degrees off the straight ahead) or should I just aim them downish and leave them all in parallel facing straight ahead? Of course, the latter makes fuel rail and injector socket work easier, but I can do the angled setup as well.

Thanks,
Paul

[SQLGUY]

I just found a surprisingly appropriate study on this topic: http://www.yamaha-motor.co.jp/global/ab ... 4gr03e.pdf

Sounds like their conclusion is that I should aim the injectors at the intake valves, and also ensure that I'm using synchronized injection that does not spray on an open valve.

[SQLGUY]

Well, I did a bit more work on the manifold today. The valves are far enough back in the heads, with the ports going so far "in" before turning down, that I actually don't see a way to hit the head of the valve with the spray regardless. Maybe a bit of the edge of the spray pattern will hit the valve head, but most of the spray is going to have to do wall wetting for the port. Still, I'm going to aim the injectors the 30 or 15 degrees off-axis so that, at least, I'll be mostly wetting the "floor" of the port and not too much of the sides.

The ports are rough-cast... I'm tempted to grind/polish the walls a bit while I'm in there to give the fuel less surface area to stick to. Any thoughts on that?

Thanks,
Paul

[SQLGUY]

Well, thought I'd share in case someone else is looking for the same answers....

From what I'm finding, it sounds like, counterintuitive though it might be, the rough surface of the intake ports is preferable to a smooth surface. The explanation, if I understand it correctly, is that fuel is going to stick to the surface either way, but the rough surface breaks up the pool and makes it easier for the incoming air to vaporize it.

Since my engine was designed for carburetors, there's no way I could really aim the injectors directly at the intake valves without some significant, and probably impractical, head modifications (i.e. I would need to drill the head so that the injectors could be mounted directly in it.)