Technical Theory : Plenum Design

[pocket-rocket]

I would imagine the pulses stop outside the end of the runner because they act like an echo (not the ugly POS Toyota). If you yell in a hallway, it bounces around because it bounces off of the walls (runner). In an open field you have nothing to bounce off of and it looses it's strength (plenum). I'm just throwing something out there though. It isn't like I have done any research on it.

[Corpse]

"Also, the pulses don't really follow the same rules as air flow, since they're travelling at the speed of sound" - Yes but, the speed of sound is not the speed of sound exactly. Sound is traveling at the speed of air. That is why there is a sound barrier. Of course, as noticed with aircrafts, the speed of air can be surpassed and, by the fact it breaks windows and stuff I am assuming that goign past the natural speed of air somehow makes the air go insane, which could be a good thing.

Also, the speed that air travels at or sound is different at different air temperatures I believe. i read a bunch of stuff about this in the past. I remember doing some calculations and runner tuned volume changes per air temp. think of it like motor oil flowing, obviously high temp oil will flow much faster.

I need to get the calculations again but, i know for every 10 or 20 degrees the actual tuned length changes and the tuned RPM will be at a slightly different RPM.

"That's why idealy a really long runner to get the first pulse bounced back in at exactly the time the valve opens would be optimal, but good friggen luck getting tubes that long all under the hood. " - Ideally it would be best to have a full length runner. Which would be like 18 feet long, lol. That may be an overestimate for our engines as I'm not doing any math but, I have before and I know it was at least 9 feet.

About that third or fourth bounce, try 8-10 or more or less in some situations. You have to wonder what will really be better for the engine to brethe though, A 2 foot runner where it bounces say 2 times or a half foot runner where it bounces 8 times.

Obviously both designs would work but, I would not want the 2 foot runner for my engine. Of course, the half foot would leave little torque behind.

Also, I'm wondering what the difference would be between a unified plenum and single velocity stacks per cylinder. You must wonder, when the intake has the throttle 100%, how does the velocity stack work. Most of them have the TB's way down so, the wave would have to go past the throttle plate to reach the end of the velocity stack.

"The distance away from the runner that it changes direction is approx the same as the radius of the runner." - In a velocity stack setup this would mean the pulse would be going to the ambient air outside the engine and coming back in, I do not believe this would happen.

I wish I was a rocket scientist or something so I just knew everything instead of wondering about different design implementations.

"In an open field you have nothing to bounce off of and it looses it's strength (plenum)." - Yes but, that is do to your lungs being so small to the world. Get a plastic jug like a milk jug and yell into it, what happens? It comes back at you. Of course, this is a bad example do to the elasticity of the jug. Do it with a hard sealed container and you will see, it will be forced back. The smaller the container, the less breath it would be able to hold before becoming highly charged with air pressure. - IE: The smaller the place the air is forced into, the more pressure it is going to have in that spot, due to the fact you are essentially trying to compact the air and then the air is forced out at a high speed. With the milk jug, take your mouth off it when it is "charged", it will force air out at you.

I don't know how air in a runner would just reflect off of nothing. I will have to look into some wierd stuff now. Of course, by the intake pulse changing direction, there could be a "vacuum"-like scavenging effect in the runner, whereas the runner helps also to suck the charge back into it. I would assume this to happen as, why wouldn't it.

I am not trying to be an ass or anything, I'm trying to squeeze all the brain cells out of you guys.

-Dave

[XLR8ING]

I think even with velocity stacks, the resonation bounce back still applies. Guys have been using "tuned" velocity stack runners on some sort of set up for years. Like with single barrel carbs on each cylinder, and sidedraft peices as well. And of course fuel injection. This eventually will be what I want on my own car.
Why it reverses at the end, I don't know.
The different temp thing though is intriquing. Does air travel faster when hot, or cold?
Obviously we all know colder air is better for power due to density, but what about velocity?
If cold air is denser by mass, doesn't that theoretically make it heavier, and wouldn't it then slow down the velocity?

[bszopi]

I hate my internet connection at work... I tried posting something, but it won't let me. Probably too long, so I'll redo it later. But anyways, I'm glad to see more people FINALLY getting into this conversation!!! Oh, and yes, the speed of sound does vary with temp... from ~1100 to 1300 fps.

[bszopi]

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe.html

[Corpse]

hey Brad, that's the page I got the temp vs. speed info from back in the day, now I don't have to look for that page, sweet.

At 200 Deg F Sound/Air travels at 869.8666666666667 MPH. At 100 deg F it is 793.9555555555555 MPH. That is a 75.9111111111112 MPH difference which means for a different temp the runner would have to be a different length. Otherwise, it will tune up at a slightly different RPM.

There was something else also that effects it... I know I was thinking back in the day that all the manifolds on the market advertise this and that about their RPM range but, how do they know what temp air is going to be used? They must just go for average temp in a N/A street engine or something.

As for, I think someone mentioned that, the tuning of the intake runner only is effected by the intake valve and not the exhaust.

The exhuast timing does also play a role do to the situation that it leaves the cylinder in. If the cylinder was still charged it would hinder the intake runner's tune and work against it. If it closed and left a vacuum in the cylinder due to the exhaust scavenging, there would be even more scavenging and it would be more effective than just your intake runner being tuned.

The above is especially critcal in turbo setups, if you do not have the valves right you shouldn't waste your time tuning the intake runners too much as the cylinder may still have pressure when the intake valve opens.

EDIT: I forgot to also mention, the amount of fuel in the air and also how atomized it is also effects the flow speed of the air in the runner. You must also add in resistance from the runner walls and from turbulence.

Realisitcally, only NASA could make the perfect IC engine.

EDIT AGAIN: Also, I am friends with a ford guy that I got a 60's FI intake with individual velocity stacks and injectors. (For 428 FE Engine, Algon Intake) I am very familiar with this unit and I am aware that the scavenging effect is present but, i am wondering how it would even be possible, unless it is just slightly hindered by losing some momentum to the "ambient air".

It is all a bunch of very scientific math.

-Dave

[bszopi]

Ok, haven't messed with this plan in awhile, but I just started an AutoCAD class a week ago, so decided to get back into it alittle bit. I took some measurements and drew up a base plate to mount to the lower manifold. Just wondering if anyone wants to check the dimensions for me. I think they are right, but I did it late at night after a weekend of being sick with strep-throat.

http://www.60degreev6.com/hlperf/3400plenum/IM1.jpg

Once I get the dimensions right, I might make the plate a little smoother, perhaps flowing more around the ports and such. I would like to have part of this done to show off at the 60V6 meet, but I'm not going to guarantee anything!

[bszopi]

Decided to move this to the performance section since it seems no one ever looks in the articles section...

[Corpse]

I'll look into it when i get a chance. Just, haven't been around much lately...

-Dave

[bszopi]

Chris White was gonna measure it as well if he got a chance. But the more the better. I measured to match the stock openings, but I'll be moving it out for the thickness of the runners before I send it off to get cut (unlile Colin did... ).

[phantom505]

If you still need measurements my top end is coming off my car this weekend. Just let me know...

[v6h.o.]

what do you need measurements for bradly?

[bszopi]

I just want someone (or several people) to check my measurements. I am almost sure they are correct, but I don't want to assume my measurements are correct, go get the pieces made, and then find out that I was off by a millimeter or so. Once I get the measurements verified (or changed), then anyone can use this for whatever they want (phenolic plenum spacers, custom gaskets, whatever). I just want as accurate of a drawing as possible.

[v6h.o.]

I just want someone (or several people) to check my measurements. I am almost sure they are correct, but I don't want to assume my measurements are correct, go get the pieces made, and then find out that I was off by a millimeter or so. Once I get the measurements verified (or changed), then anyone can use this for whatever they want (phenolic plenum spacers, custom gaskets, whatever). I just want as accurate of a drawing as possible.

you still didnt answer the question... LOL, measure what part? and which part of a part? :P

[bszopi]

You obviously missed the link a few posts back....

http://www.60degreev6.com/hlperf/3400plenum/IM1.jpg