peejay

I'm contemplating fabricating my own sheetmetal intake. (Why? I don't like what's on the market, I'm cheap, I don't like to buy things "off the shelf", and mainly because I can)

I can find all manner of wildly conflicting formulae for calculating these dimensions, and of course they are for four-stroke boingers, which I would imagine would be a bit different than a rotary's requirements.

Or should I just toss it all and just make simple, say, 3" stubs to the plenum which would be sized, say, the same volume as 1.5 rotor chambers? (for example roughly one liter for a 13B)

pp13bnos

iTrader: (1)

I'm guessing your looking to make more peak power right? If thats the case, the shorter the better...but i have no idea on how short.

I have seen some pp engines with the TB mounted right onto the engine...no intake manifold at all. CJ

j9fd3s

iTrader: (3)

look at the factory: the 787 has what look like standard ram tubes, with sliding throttles.

mike

peejay

I want to know HOW to calculate it, so later when I decide what I want to do, I'll be able to do it

enfini3

trial and error my friend, it is going to depened on the port of the motor, duration, rpm peak. let use no how it turns out.

REVHED

Peejay, see if you can find Scientific Design of Exhaust and Intake Systems by P. Smith. It's a good read and will tell you all you need to know.

peejay

I guess what I want to clarify is, since intake events occur twice as often for a given RPM, shouldn't I be doubling the RPM and intake duration that I calculate for?

Also, how would I figure for a bridge or peripheral port engine, seeing as their intake ports are never actually closed at any time?

WackyRotary

Ha? What do you mean, once the apex seal crosses the PPort, then its "closed" in a matter of speaking, since the motion of the rotor controls opening and closing for that cycle for that rotor face and it goes into compresion. They have massive overlap as you well know, which accounts for there poor low end and awsome highend performance, depending on how large you go of course. Bridgeports just happen to have more overlap then a standard or mild port, that why we get that pleasent eye turning idle sound. I love how my bridgy sounds. I actually don't like the attention it gets.

rotarygod

iTrader: (1)

I have a custom built aluminum manifold on my 2nd gen. It has a 100 cu in plenum volume. The intake runners are slightly larger than the lowers but there is a taper down to their size. This is done to broaden the torque curve since the air accelerates as the volume decreases. I also have a 75mm Mustang throttlebody on it with my blowoff valve machined about a quarter of an inch in front of the throttle plate. This engine was designed for big power but still be street drivable. I'll have to post some pictures later this week. The mathmatical formulas are hard to apply to a rotary. Keep in mind that on a piston engine the air is stagnant longer in the intake runners than they are in a rotary. On our engines the intake ports are only closed for a very short amount of time. On a piston engine, unless it is a 2 cycle, the air in the ports is not moving until every other time the piston is at the top. Because of this the effect of plenum volume changes in relation to piston vs. rotary. A bigger plenum volume will raise the torque peak. Port runner length is also dependent on port runner size. The effects are due to a volume/velocity relationship. Theoretically peak power (n/a) is achieved at whatever rpm the engine is at when air velocity entering the engine is at .6 mach (60% the speed of sound). This is however offset by runner length as a runner of too much length can kill power. Its almost too complicated for me to get into without writing a huge book (already several out there on this subject) or giving a night school class. I'm afraid something I write will sound contradictory to something else. Lets just say my setup was trial and error and it works damn well! Oh btw a pp engines intake is always open. When the apex seal crosses the port it is briefly open to 2 chambers. Therefore it never closes.

Fred

peejay

But also in a manner of speaking, the port is never closed. The next intake event begins before the previous one ends.

Sterling

Just trying to understand about the PP...
My understanding is that as the charge is compressed, there is a slight "blow-back" in the runner until the seal sweeps that port closed. But is there an equal and opposite "suction" (so to speak) when the other side of that apex seal reveals the rotor face that's just been exhausted, and is increasing in volume?
On a full bridge port, can the port timing be made such that these "pulling and pushing" forces are made to be exactly equal at peak VE so that the flow through the runners is virtually uninterupted?

peejay, I'm also gonna make an intake manifold for my Eaton project. It will consist of plates of aluminum stacked up and bolted together. They will each have four holes for the runners; the shapes of each changing to make the 90* bend upwards.

The advantage here is that the inside diameter can be graduated easily, or changes can be made at a later date.

The dis advantage is that there needs to be gasketing ,material in between each plate. (However, I'm hoping that that "ribbed...for her pleasure." effect will have an advantage in "turbulating" the mixture.

(Didja like that one..."turbulating"?)

j9fd3s

iTrader: (3)

i'm trying to make this make sense in my head.
2 questions: so peak power is effected more by runner velocity?
and calculating the velocity, is dependent on ve right?

mike

rotarygod

iTrader: (1)

You pretty much hit it on the head. However its gets more confusing in that the runner length plays an important role in this. This will require a little bit of thought to make sense of but here goes anyways. The longer the runner of an equal size, the slower the air will move through it. Think hard about that one. A longer tube has more wall area that will slow the air down. There are surface frictional losses where the air touches the runner walls. The engine has to work harder to suck in the same amount of air. The only way I can really give an example is by using drinking straws. Cut one to an inch long and blow through it. Remember how much effort it takes. Now join several of them together end to end and try the same thing. The internal diameter of the straws is the same but now it is harder to breath through. Same thing with an engine. The intake tuning is determined by a volume/velocity relationship not just one of them. The longer the runner the lower in the power band the power peak will be and vice versa. The larger the runner for any give length the higher the power peak will be. Determining how to put these two together is the hard part. Optimally you would want your intake runners to decrease in area as it gets closer to the engine. Mine do this to a point. This helps keep the velocity high through the runners. From the beginning of the runners to the end they should lose their area anywhere between 10% and 20%. I've heard conflicting info here. Another benefit of tapered runners is from pressure wave tuning. When a port closes a pressure wave is sent backwards through the runners at the speed of sound. This is an acoustical phenomenon. The conical shape of a tapered runner will broaden the useful range of this wave. You need to understand audio hornspeaker design to fully grasp this. If your runner length is such that this wave arrives at another port just as it is about to close you will receive a beneficial ramcharging effect. I knew this would get damn complictaed and hard to describe. The real expert on this is Paul Yaw but he is hard to get a hold of. I just started reading books on the subject and studied different manifold designs on all kinds of engines. My knowledge of acoustics has also helped. Hopefully I haven't confused you even more.

Fred

j9fd3s

iTrader: (3)

i get the idea, actually i think the facotry does, or has done the very same thing. like the cosmo, its got a huge plenum and the runners i think taper in the lower intake manifold, they are just tuning for a different goal.

mike

pp13bnos

iTrader: (1)

I was thinking about making a shorter (cut the middle out of a 86-88 manifold, and weld it back together.) intake for my n/a 90....i wonder how much i should shorten it?

Very nicely written Fred.

CJ

rotarygod

iTrader: (1)

Thanks CJ. I almost tried that on my '88 n/a but decided to go crazy and build one instead. I was going to use the stock lower manifold and the plenum. The plenum would be mounted straight above the lower by using an adapter a couple of inches long. The tb would have to point the other direction. I never tried it and always wondered how it would work. Is this what you mean? I know the power and torque peak will rise in the rpm range and ultimate power will go up. I just don't know by how much. Let me know how it works if you try it.

Fred

pp13bnos

iTrader: (1)

I thought about making an adapter to just fit the lim, to the uim too...but decided it would be to much of a pain to change the throttle cabel direction, find a new place for the other injectors and some other minor junk....

I too might just end up having to have a custom middle intake made...because (I don't have one to look at so i'm not %100 shure on this.) it would be impossiable to seal up each individual runner, once it was cut in half. Am i making sence here? CJ

rotarygod

iTrader: (1)

Yep that would be damn near impossible without cutting the runners off of the flanges. A new middle section probably won't do too much. You may shift your hp/torque peak up a couple hundred rpm or so but probably won't do that much for driving purposes. The benefit you will get is better airflow due to the lack of secondary injector locations blocking some of the flow. You could now put them in somewhere properly. A project I am about to try is on my GSL-SE intake. I am going to physically cut the plenum chamber in half vertically and hollow it out completely. No more double layer crap. Then I am going to weld it back together and do the finish work to make it look stock. I should pick up a little horsepower on the top end. Someone is probably going to say that I won't because there is only so much throttle plate area. In the current setup the velocity through the middle ports is faster due to 2 ports sharing 1 throttle plate vs the outer ports having a plate each (shared between them). By giving the air unrestricted access to any port, velocity will remain more equal between all of the runners and should be beneficial on the top end since the velocity will be increased on the outers which are larger. The low end power should decrease though. It can be done to any gen plenum chamber. I'll have to post the results when I do it. I think there would be more benefit in this rather than shortening the middle a small amount. Just a thought.

Fred

j9fd3s

iTrader: (3)

i was thinking yesterday to make a plenum box and bolt it right to the gsl-se lower intake, maybe with a bigger throttle body (86-88 na?). too bad the cars still got its afm...

mike