Home porting? Like Home perm?

[Grady Clay]

John,

“Not smart enough….” I don’t think so. I’ve been reading your posts. Your banter with snowman has enlightened many Pelicans.

Regarding the animation:
http://www.optimum-power.com/RET/unrestricted-9000-pressure.gif

I Notice there are almost two pressure cycles in the intake after the intake valve closes. I interpret this as reflected pressure pulses from the intake valve closing to whatever discontinuity upstream in the intake (like the transition from the intake runner to the outside world.)
This clearly has significant “intake ram effect”, note how high the intake pressure is just before intake valve closure.

Before I spend too much time on this animation, what engine is this duplicating? Is it a computer simulation or direct measurements? What engine dimensions (bore, stroke, rod length, CR, valve timing, intake and exhaust runner length, RPM, etc.)? Can I download the actual data points?
There is potentially some really useful information here. Has anyone done this for our nice little hemi cross-flow head?

Best,
Grady

[jluetjen]

Grady;
Basically, the example was created as means to demonstrate the software's capabilities for modeling, even in the difficult case of an intake restricter. In two words, as a sales tool for the software. If my memory is correct, it is for basically a 1 liter 4-cylinder motorcycle engine. They demonstrate it at 2 or 3 engine speeds with and without the restrictor. In all cases there is an intake plenum. I believe that they have coorelated the software since they are selling it as an engineering tool, but of course they didn't provide examples. I did link the original web site at the bottom of my first post with the animations.

In our case most early 911's are not restricted and don't have plenums, although the venturi on the carb'd models can act like a restrictor if the are undersized enough. CIS engines are a whole 'nother can of worms as far as I can tell.

[Steve@Rennsport]

Hi John:

Just as a data point about plenums,......the OEM MFI air cleaner assembly acts as a resonance chamber (just like on all the 3.6's) and these engines lose power with individual air cleaner assemblies on each stack.

Sounds nice tho,....

[Grady Clay]

Steve,

You are right on. The plates above the stacks on 906, 911R and later are reverberation plates. They reflect the intake pulse back to the valve.
This is why an early 911S needs to have the air filter assembly.

Best,
Grady

[snowman]

A comment about the shape of the port for a street car. I still think what I said about the narrowing of the ports is the best way to go on a street car. A race car is a whole nother story, as it will not idle or run well at lower RPM like a street car HAS to do.

The picture dosen't show both sides of the port. The port size dosen't vary as much as it would appear on this photo.

Finally as I have repeatedly said, the Porsches are so good, you don't really have to do anything to them (unless its a race engine). This pretty well agrees with your statement on the subject.

[snowman]

An analogy to how porting works, my own words:

Envision groups of cars, traveling on an eight lane highway, all even and going in a straight line and the highway is a race track to boot.

A sharp left hander approaches. Most of the cars bunch up on the inside of the turn, the outside ones go on their way with much room to spare.

A tire, part of the inside wall falls out onto the track. The inside cars have to slow further, and because the tire is obstructing the way, the next line of cars has to make way for them to alternately merge into the flow (really slowing things down a whole bunch)

Another serario, a tire falls onto the outside of the turn, the cars there have ample room to go smoothly around the tire and no significant flow interruption results.

This is a whole lot like air going around a corner. Small bumps on the outside are detramental, but not a whole lot, small bumps on the inside of the turn are major obsticals to the flow.

I beleive this analogy is highly accurate. The opening and closing of the valves could also be analogis to putting on the brakes, the explanation a whole lot more complex.

I will continue to attempt to explain , and I suspect that the exact same reasons are responsible for what we observe. Traffic, at rush hour, on a CA freeway, goes like heck, stops, then goes like heck. If you were to look at it at a distance you would see a series of bunches of cars, all stopped at the same interval, and inbetween cars going like heck, just to stop again. This is a real life standing wave. The same thing happens in the intake. When a valve closes, EVERYTHING STOPS moving, IT HAS TO, or the valve must fail. Think of a bunch of cars coming upon a landslide across the road. everything stops. The result is a whole bunch of cars piling up in a bunch. Being a race, as many cars as possible turn around and go back the other way. As they do they start piling up with oncomming traffic, untill they come to a total stop, creating another bunch of traffic, that is totally stopped.. This bunch acts like the bunch before it and creates another bunch the same distance in back of it and so on.

The valve opens, the traffic clears, for a while, but still flows with bunches that were formed while stopped. if the valve closes at the right time the following bunch will fall thru and create a surplus of cars on the far side of the valve (tuned induction), but only if all conditions are perfact (ie at a single RPM or multiple of it). IF the valve closes at an inopportune time it will stop the entire next bunch, totally, and acutally suck power out of the flow, ie a suck out, or dead spot) Thats my first order cut at how things really happen.

PS

Since cars behave with some intelligence (except in LA traffic and NASCAR racing) the same example should be invisioned with parallel groups of evenly spaced ping pong balls. The inside corner gets very congested and any, ANY obstruction really makes a mess of the flow. On the outside a small obstruction only makes a small wrinkle in the flow as the spacing between balls is large and consequently there is lots of room to go around anything that gets in the way.

PPS
Wayne, I really wish spell check would work with Netscape.