Tuned Port Injection Exhaust Scavenging Question

[apehead]

Here is an article http://www.superchevy.com/technical/eng ... index.html that outlines the basic principles of a zero back-pressure, tuned length exhaust system. Here is a quote:

At this time a few numbers will put the value of exhaust pressure wave tuning into perspective. Air flows from point A to point B by virtue of the pressure difference between those two points. The piston traveling down the bore on the intake stroke causes the pressure difference we normally associate with induction. The better the head flows the less suction it takes to fill (or nearly fill) the cylinder. For a highly developed two-valve race engine the pressure difference between the intake port and the cylinder caused by the piston motion down the bore, should not exceed about 10-12 inches of water (about 0.5 psi). Anything much higher than this indicates inadequate flowing heads. For more cost-conscious motors, such as most of us would be building, about 20-25 inches of water (about 1 psi) is about the limit if decent power (relative to the budget available) is to be achieved. From this we can say that, at most, the piston traveling down the bore exerts a suction of 1 psi on the intake port Fig. 3.

The exhaust system on a well-tuned race engine can exert a partial vacuum as high as 6-7 psi at the exhaust valve at and around TDC. Because this occurs during the overlap period, as much as 4-5 psi of this partial vacuum is communicated via the open intake valve to the intake port. Given these numbers you can see the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore. The only conclusion we can draw from this is that the exhaust is the principal means of induction, not the piston moving down the bore. The result of these exhaust-induced pressure differences are that the intake port velocity can be as much as 100 ft./sec. (almost 70 mph) even though the piston is parked at TDC! In practice then, you can see the exhaust phenomena makes a race engine a five-cycle unit with two consecutive induction events.

In a future build, I would like to take full advantage of this system, but I have a concern. I plan on using MSII and GM's TPI (Tuned Port Injection) system, which is a batch fire MPI system. I also need a cam with some overlap to take advantage of the scavenging. I am not sure exactly which cam, as I still need to determine a few things (deck height, valve relief, etc.), but let's say about four degrees of overlap for the sake of argument. I hope that the "push" of the tuned intake runner combined with the "pull" of the exhaust will be sufficient to fill the the 64cc chamber completely with fresh charge. My concern is that it will work "too" well, and end up sucking the bulk of the injected fuel charge out the exhaust valve. This wouldn't be an issue in some sort of "wet manifold" setup, as the charge lost would be replaced by more air/fuel emulsion, and it wouldn't matter in a sequential setup, as the injection events could be timed to occur after the scavenge events. But for a batch MPI system, all of the injectors are fired at once, and only once per cycle (right?), so there is only a finite charge, and it is waiting right behind the intake valve, so it would probably be first to be sucked out of the runner during scavenge. Is this a valid concern?

[Matt Cramer]

Four degrees of overlap would be a very mild cam with little scavenging. If you have a much wilder cam, you can have a bit of difficulty getting a good idle, but no more so than with a carb in my experience. Just takes some careful tuning.

[apehead]

O.K., say I ended up choosing a cam with a larger degree of overlap to maximize scavenging. Would it cause a lean condition or noticeably increased fuel consumption due to the characteristics of an MPI system such as the TPI with MS-II?

[375instroke]

I see what you are thinking, but I don't think you can do what you want. All general use automotive engines have overlap. Here's an example of the smallest cam that Comp Cams has in it's catalog. It's a .425"/206°. Measured at .020" lift, the intake opens 20°BTDC, and the exhaust closes 12°ATDC, so there is 32° of overlap when measured at .020" of lifter lift, so there is actually more overlap, but it's negligible. Generally, there is so little flow at minimal valve lift, that volumetric efficiency is actually increased by allowing the valves to start opening earlier, and staying open later, than having them open and close at TDC or BDC. The burnt exhaust gases in the cylinder are at much higher pressures than atmospheric. The exhaust valve opens much before BDC on the power stroke, to allow the gases to escape, but after most of the energy has been captured by the piston being forced down. Near the top of the exhaust stroke, the intake valve starts opening, allowing it to be fully open by the time the piston is at it's fastest movement on the intake stroke. The exhaust gasses flowing out the exhaust system have momentum, and that allows them to continue in the direction, and not flow into the intake system. A similar situation happens to the intake charge, which continues into the cylinder, even though the piston is moving upwards on the compression stroke. These effects of momentum on the exhaust and intake chatges are RPM dependent. They have increasing affect as the engine revs higher, and is why a bigger cam is used on a higher revving engine. Having the valves open longer allows the intake and exhaust charges to start moving backwards, because they don't have enough momentum, and the slower rotating engine gives them more time to turn around. One final thing. Even if you use sequentially firing injectors, the injectors are actually open longer than the intake valve is at higher RPMs.