BlaCkPlaGUE

I remember reading in maximum boost that you should design your forced induction system with a plenum 3 times the volume of your engine displacement. I can't remember if he explained why as well, im at work, bored out of my mind, and I don't have my book with me.

How come I only see plenums on EFI setups, and not carbed engines? Does the extra volume of displacement in the intake help with atomization of fuel with the injectors?

Whats the point exactly with having a plenum, what would happen if you didn't have a plenum with a turbo or non turbo'd engine?

thx

silvr94r2

iTrader: (3)

I could be a little off but from what i understand it's to equal out the airflow into each cylinder or rotor. You have different pulses and the extra space inside the plenum allows each cylinder or rotor to get exactly the amount of air it needs without starving another. That's why you multiply by displacement.....too small will choke it and too big will kill response.

TT_Rex_7

Corkys book states that then plenum should be several times larger than a single cylinders displacement, or 50-70% of the motors total displacment. However, i've also heard it's best to make the plenum 100% of the motors displacement. Personally, i'd make it between 80-100%. The exact size all depends on what you tune the runners for.

As far as "why" a plenum...The plenum has a couple main functions.

1. The plenum "collects" air and keeps it inside to allow the motor to get air from it. As someone already stated, to small, it'll choke the engine, to large, it'll hurt responce. If you look at an FD manifold, there's no plenum. I have no clue as to why it's like that though.

2. It alows something for the air waves to bounce off of, and back into the motor. When you tune the runners, you tune them to make max power at a certain RPM. What happens when air is rushing in, and the valve/rotor closes, all the air just slaps right into the valve/rotor. When this happens, it bounces back towards the plenum. If you tuned the length of the runners right, at that specific RPM, the waves that bounced off the valve/rotors, should go back towards the plenum, bounce off it, and hit the rotor/valve RIGHT when it opens. By doing this, it forces more air into the motor. Usually, you can't utilize the first "bounce" for longer runners. Reason being, they'd be like 10ft. long. So what you do is divide by 4, 6, 8, ect. By dividing by 4, the waves would bounce off the plenum and valve/rotor twice, and come back to the valve/rotor right when it opens again. It'll also bring the length of the runners down, so you can actually fit them inside the motor compartment.

It has been argued that the length of the runners doesn't matter near as much for a turbocharged motor, as it does an N/A motor. However, that only applies to longer runners. No matter if the motor is N/A, or turbocharged, really short runners will only become a benefit in the higher RPM range. Normally, if you make the runners longer, to help with low end, it'll hurt the top end power. However, a turbocharger can normally over come this. This is due to constant pressure being against the valves/rotor.

To sum the runner length up, shorter runners will shift the power band in the higher RPM, and longer runners will shift the power band in the lower RPM. With longer runners, the turbocharger will keep the top end for suffering. Also, make sure you don't make the runners to short. If you make them to short, you could shift the power so high in the RPM, that your motor can't utilize it. (motor can't rev that high)

I know this is long, but I hope it helps!

-Alex

BlaCkPlaGUE

Long? That was great, thank you! both of you. I had NO idea that air waves would bounce off of a closed rotor/valve and help push more air in with a properly designed plenum. Its just amazing as to how much R&D goes into a gasoline engine, rotory or piston alike, just amazing. Understanding it all can really really have some excellent advantages when your tuning on the dyno!
I did understand the length of runners though, the longer the better for low end, wich im not sure why, but the shorter the better for high end rpm, wich that part I do understand. Less surface area for the air to pass through, the less resistance, the more top end power.
Im still unclears as to why creating more resistance helps with low end power.. that part has always confused me.

This place is a serious wealth of knowledge, I really don't think any other message board is as good as this, for engine talk of any kind.

thanks!

Crusader_9x

From what i know/have heard about runners, the longer runners keep up the velocity of the air in the lower rpms and that somehow helps with torque.

I get the runner lengths for tuning theorys, but what about runner diameter? Will going up or down help out with high end hp or low end torque at all? (This is for turbos by the way) Or is it just keep em short and straight for high rpm hp?

rotarygod

iTrader: (1)

The RX-8 does not have a plenum either.

SPOautos

Waves travel up and down the runners. When a intake port is closed it sets off a wave that travels to the nearest crossection (which is basically a opening like a plentum) then the wave turns around and goes right back to the intake port. The waves move at the speed of sound. The obvect is to time to wave so that it meets back at the intake port just as its opening and it will help assit with moving more air into the port. The lower the rpms the longer your wave needs to take to get back and forth. The higher the rpms the faster the wave needs to get back and forth. With a long runner the wave will travel for a longer period of time which is idea for low rpms. With short runners the wave will travel for a shorted period of time which is ideal for high rpms.

If you want some more info to chew on do a google for Helmholtz theory regarding intake and exhaust applications.

This is of course only one reason and thoery. Just thought I'd give you some info to chew on.

Stephen

TT_Rex_7

If you understand everything else, then this should be pretty easy to understand also.

In a simple form, the smaller the dia., the more velocity. Meaning the air will travel faster through it. The larger the dia., the slower the air will travel through it.

The best way to figure out what dia. you need, is to figure out how much your air your motor takes in at the tuned RPM. (same RPM as you tuned the length to) After you figure that out, find out how much each dia. of tubing will flow. Once you have those 2 answers, find out what dia. tubing will flow about the same as how much air your motor will take in. Always make sure the runners will flow a tad bit more than what your motor will actually take in. By doing this, it gives you some wiggle room for upgrades and so on.

Now, if you go to small, you can loose power. The reason being, is that if the runner dia. you choose, flows less than how much air your motor brings in, it'll create a restriction. If you have a choice between 2 dia. runners, and one flows 100cfm less than what your motor can bring in, and the second one can flow 100cfm more than what your motor can bring in, choose the largest of the 2.

You'll also loose power if you go to large. Reason being, the air will move slower through the runners. So always go with a runner size that will flow just a little bit more than what your motor will bring in.

I made everything as simple as I could to understand. If how I worded something confuses you, just say, and i'll try and re word it so that everyone can understand.

Anywho, hope this helps!!

-Alex

silvr94r2

iTrader: (3)

There are so many variables that come into play when designing a manifold. For the Virtual Works Supra manifold we spent ALOT of time on the flowbench testing runner length,size, and taper. We also tested different velocity stack designs. You'd be suprised how much one little tiny thing affects flow. We found out sooooo much and that's why our manifold flows better than any other one out there.

It's really neat messing with the clay and hearing the difference in the air. Noise is bad news when optimum airflow is your goal. You want to keep the velocity up without creating too much noise.

The one we just did for my car has the same exact plenum design as the Supra. We just shrunk it down and changed the angle a little which actually helped airflow. The runners are pretty much stock length.....they just look really long because of their design. Throttle bodies affect overall manifold design as well. Where they're placed and the angle all come into play.

It'll make you crazy trying to come up with a design from scratch. But it's also alot of fun to see how things really work.

TT_Rex_7

The things I don't like about your plenum is the diffrent distances from the end of the runners to the right side of the plenum. If you look at the runner closest to the firewall, the wave will have less of a distance to travel to bounce off of the plenum, than the runner closest to the alt. It won't effect much at all, but I like to try and keep everything as equal as possible. Also, it appears that the 2 rear runners will have more air going to them, than the 2 front. However, I could be wrong on that. I'd be interested in looking at the flow graph for each runner.

Corky states in his book that a plenum design, like the one found on the Hogans 3 rotor manifold, is the best. However, I feel one like yours, without the slant/slope/whatever on the right side of the plenum, is better. With a design like the one on Hogans, the sec. runners won't get as much air as the primarys. However, with the runners being so short on the Hogans manifold, you really don't have much of a choice but to go with that style. If Hogans went with one like yours, the piping would go right through the alt. (not possible, unless relocated)

-Alex

TT_Rex_7

What I forgot to mention in the post above, was the things I didn't like are directed towards N/A, or while your not in boost. Once in boost, those things don't effect everything near as much as when your N/A, or not in boost.

Also, i'm sure that Corkys "ideal" plenum design is directed towards the times your in boost.

-Alex

silvr94r2

iTrader: (3)

On the flowbench we tried out different plenum designs and this one had the most equal distribution. We didn't think it would by the looks of it but the #'s don't lie.

Alot of a manifolds design comes down to fitment. This setup worked out best for us as far as fitment goes and visual appeal. We wanted something to fill up some space and still put out the #'s.

I can tell you that it has almost perfect distribution.....within 1%.

I'm not trying to make this into a selling post for my manifold because it's not even for sale so i hope it's not taken as that. I'm just giving examples from my experience.

TT_Rex_7

Very interesting. From the looks of it, you'd think the 2 runners closest to the firewall would get more air than the 2 closest to the alt. As you stated though, the numbers don't like!

That's totally true! It can be a PITA trying to fit the length and plenum design that you wan't. Unfortunately, you have to sacrifice a little of both to get it all to fit.

During both boost and vac., or just boost?

I got a couple questions about your manifold though...Did you only fabricate the UIM, and use the factory LIM, or did you use the GZ/Xcessive LIM? If you used the factory LIM, did you test it on the flow bench with your UIM, or did you only test your UIM? The reason I ask is because the factory LIM feeds more air to the rear rotor than the front, (not equal length) so even if your UIM is perfect, the LIM would change all that. Of course, if your using the GZ/Xcessive LIM, that wouldn't be a problem.

-Alex

Edit: BTW, what RPM did you tune the length to?

silvr94r2

iTrader: (3)

Yes i'm using the GZ lower which flows much better than stock. It actually flowed better than my ported and Extrudehoned LIM. I had Extrudehone match the front and rear rotors to within 1% of each other and the GZ still flowed better.

If you look inside my mani from the throttle body you can see all four ports perfectly. I think the angle in the pic might be throwing it off a little.

We didn't try to tune the rpm range as much as we tried to get the most flow and the least amount of noise. We figured that the stock intake can support a certain amount of hp so we didn't try and remake the wheel. The runners were kept pretty much the same size with just a little length added to try and pick up some torque down low.

TT_Rex_7

Yea it's definitely a nice peice! Hell, have you ever seen their Supra LIM/UIM? http://www.xcessivemotorsports.com/m...ml?id=7&step=2

After taking a closer look at the picture, your probably right. The picture is taking facing down at an angle, with the person standing closer to the driver side, and tilted. lol

How much did noise effect things? Did excessive noise make a drastic change to the airflow, or was it just mild?

-Alex

TT_Rex_7

Any idea on why that is?! That just sounds strange not to have a plenum.

-Alex

rotarygod

iTrader: (1)

No idea. It seems to work though. Look at some of the other systems such as individual runner systems with multiple throttlebodies or even Weber type systems that don't connect each rotor to the other one. There's more than 1 way to do it.

TT_Rex_7

Do you by any chance know the effects of a plenum vs no plenum? I'm thinking no plenum will have a faster response, and more power up top?!

-Alex