Kyrasis6

Hey guys, some time in the future I plan on doing some fabrication to my intake, havn't decided exactly how much I will do on this engine but at the moment I'm tossing around the idea of making a UIM and plenum. I was going to make the plenum cylindrical in design so that I can easily make a piston which moves to change plenum volume at different rpms.

For the best pulse tuning do I need to isolate the plenum for the primary runners from the secondary runners? Also what kind of volumes have people tried with the plenum?

Thanks,
K6

Kyrasis6

lol, no replies?

monkhommey

this might help you a little.

http://sdsefi.com/techinta.htm

i dont know what your trying to accomplish with the piston idea to change the volume of the pleneum..

rotarygod

iTrader: (1)

What I've done in the past is to design a manifold in which the plenum unbolts. Then design a few different plenums and bolt them on. See which one works best for you. You can use formulas to get you in the ballpark. I've done this. The problem with it though is that formulas are used assuming full loads and on the street, a plenum or manifold that is great at full load isn't necessarily good for daily driving at part load. Different porting style also will require different intake designs so don't think one plenum on one ported engine will necessarily work the same on another.

The best advice I can give is to design a modular system. You can learn this way and if you ever change engines or porting styles in the future, you can easily change the intake design to adapt.

Kyrasis6

So what type of engine setup did you try this configuration with and what type of intake design worked best with that particular setup?

rotarygod

iTrader: (1)

I originally built a single 75mm Mustang TB based upper manifold for a TII. It had a plenum that was 200 sq inches in size. The runners were about the same length as stock. The car ran fine. It was quick. I felt the throttlebody was way too large for street use though. I should have started with a 60-65mm tb. Although the stock tb has 3 plates, remember that only 1 of them feeds 2 ports. That means high velocity through these runners and ultimately an imbalance in flow capability leading to the primaries maxing out first. With forced induction you can mess up pretty bad and still get good results. The people making lots of power with a stock manifold and tb could really improve top end by installing a single plate tb. Since there is so much tb area, part throttle during regular daily driving is kind of a pain to deal with. I didn't personally like it but then again there are also people out there that actually feel a peripheral port is a streetable engine so different tastes I guess. I wasn't a fan. I had pictures of this manifold posted here a few years ago. I had also modified a couple of stock manifolds and posted pics but was met with a handful of nonbelievers trying to tell me what did and didn't work on my car. I don't put much faith in dyno runs. They are full throttle 2D pictures of a 3D powerband (which doesn't mean much) and aren't terribly useful for real world road driving results. I didn't have paper evidence of more power so people insulted me for it. It's not my fault they don't know how to test things. As a result I never posted anything about any later design studies and I never will. At least not on this forum.

I also had a custom built intake on a streetported 6 port motor. Keep in mind I have a Megasquirt and can tune any combo. Years ago I had an old Haltech and even played around on the stock ecu just to see what would happen. If you are going to build a manifold, you can get some results with a stock ecu but my advice is don't try. Get a standalone. This manifold I played with a bit. I had different runner lengths and different plenums. It was made from steel pipe and sheets so the design wasn't the lightest or the prettiest. It was just an experiment to see what would happen. I've found that shorter runners obviously make more top end power but to a point. When you get in between resonance peaks, the opposite can sometimes be true. For my personal taste, I liked runners that were about 19" long or so. That gave pretty good power everywhere with a peak near 7500 rpm or so. The reality is that the gearing on my car would be better setup for a power peak closer to 6500 with a shift point just above 7K. This would mean longer runners but 7500 rpm is more fun!

Larger plenums typically give you better top end horsepower but at the expense of throttle response. A smaller plenum was better at lower rpms but could hurt upper range power. I tried a very small plenum but flow through it was terrible as it was nothing more than a 3" pipe with runners popping out the side. I don't remember the exact size but it was well under 80 cu inches. I tried a 200 cu in plenum that was similar to my TII plenum and I tried a larger 300 cu in unit. After a certain size, there isn't much point in going larger. I wish I'd have played with these more but I get impatient and move on to other projects. Out of the 3 plenums I made, the 200 worked nicest on the street but it's not to say it couldn't have been better. For that one I had a stock Mustang tb out of a junkyard and I'm going to guess it was a 65mm. Better drivability than the 75mm on the turbo engine.

What I'd really like to play with is the DCOE or IDA style throttlebodies. I'd like to experiment with runner length both before and after the throttleplates. I'd also like to try different sized plenums in front of the tb as well. I think that would hold some good potential. On a turbo engine I really want to play with a dual taoered plenum design. There isn't much information on them out there but they were used for decades on Audi Quattro rally cars and the R8 race car that was retired last year. Unique design worth studying.

The problem is that there isn't necessarily a "best" design as there is always more than one way to do it. There are some general rules that apply though which point you in the right direction. Make a modular system and try them out. That's the best way to see what happens. It's so hard to convey in words. You just need to feel it. At the end of the day, it doesn't matter how long the runners are, or how crazy it looks as long as it works like you wanted it to. I think too many people design intakes around what looks nice and easy first and then hope it works. I'd rather find out what works best and then figure out a way to make it look nice.

If you do and you decide to experiment and post your work, be prepared for people asking you for dyno charts as proof of concept. You can have an engine that makes more power everywhere at part throttle levels such as daily driving but slightly less at full throttle. If you post a full throttle dyno run, what are people going to say about your results? See the issue? My advice is to try out different things and then use what you want. Don't get caught up in trying to figure out which one makes the most power on paper. Get out a stopwatch and do acceleration runs. Even a gtech is good for comparisons. Use what you feel is best. Forget the dyno. The car with the most peak power isn't always the fastest car anyways.

Kyrasis6

So do you have any theories as to why a larger plenum seemed to have more high end for you and a smaller plenum more low end? That doesn't seem to make any sence to me because a larger plenum would require more time for the intake pulses to return to the runner which would mean more power at lower rpms.

Kyrasis6

It would be an attempt to accomplish something similar to VDI, changing the plenum volume to help change the RPM band of the intake. It would never be as effective as VDI but should be simple to make. The big question is can I get it to change the volume of the plenum enough to make a practical difference.

ikari899

the larger plenum acts as a large capacitor would in a electronic circuit. which also explains why the large and small plenums act as they do. the size of the plenum does not change the rpm at which the tunned runners effect the motor. only runner diameter and length do that, as far as the intake manifold is concerned at least.

oh and dont get stuck into the idea that a larger plenum always means more power at higher rpm. if you check into helmholtz resonance stuff you will see some VERY small plenum sizes, comparatively, can and will make large power increases at high rpms.

and i completely agree with rotarygod on the modular design. if can do it, do it. but most people dont have the time, money, skill what ever it takes to do some thing like that. so just do the math, make one, and then move on. you are only going to get so much of a gain out of it. but if you REALLY want the best you are going to have to do testing. as i have learned in FSAE simulations and calculations are always going to be off, they can only point you in the right direction, and testing is the only real way to get good designs.

ikari899

you might have more luck with a dual runner setup if you can fit it in there. they are rather easy to actuate and design, in comparison to a variable displacement plenum.

or you can go 787B style and go with continuously variable runner lengths.

ziig

iTrader: (1)

I aggree with RotaryGod as to your findings....if you post it be prepared for critics......as for plenums I also agree with what was said....try differnt sizes and lengths........dont trust flow benches and dyno reading..nothing can sub for good ole test and tunning on a track or the road...but if you use basic aerodynamic principle such as Bernoulli Theory and such you will be able to get close to what what you want to acheive before actually building .......

peejay

I wonder how well the smaller plenums would have worked with a larger throttle body. Normally, plenum size and throttle body size are inverse in optimization, and if you think about it a moment it makes perfect sense.

I wonder because I was thinking about making my own manifold and use a 75mm throttle body I grabbed from a 6 liter Chevy. (The IACs fail, and the only way to get one is to buy the whole throttle body assembly, so they essentially are free to me)

I strongly feel that independent throttle plate is the best way for engines like the peripheral port for throttle response purposes. I'll give it a shot with the 75mm anyway just for S&Gs, just like I will for the stock 12A

Kyrasis6

When you have a throttle body that is too large your response gets very touchy and engine load peaks out before you reach WOT.

I think the reason why rotarygod liked the smaller plenum volume is because at part throttle you are at a lower volumetric efficiency. If it where a full race application I think the smaller plenum would have worked better for high end and the larger plenum for low end because it would see the maximum volume of air on a regular basis.

I think I'll finish the build keeping the stock manifold setup, get it tweaked and broke in, then do some timed runs and some dyno runs to get a control baseline, then I will make 3 different plenums with 2 different runner designs and see what the characteristics of each are with timed runs and dyno runs.

The variable displacement plenum will be very easy to make and even a constantly variable version would be fairly simple with the manufacturing resources I have at hand. Basically I'll just make 2 pistons with a worm gear machined into them to move them in and out, they will be concaved on one end which should keep the reflective pulses stronger. I'm not sure if I will try a dual runner setup, I havn't seen VDI in person to see exactly how it works, I would probably go for something more like what was on the 3000GT which used a butterfly to close off one set of runners and open up a shorter set. A combination of the two should really give it some kick.

ikari899

yeah by dual runner thats what im talking about. what the 3000gt does and the civic gsr does. shorter and longer runners. way easier to calculate out then variable displacement shenanigans...

peejay

I was going to argue that VE stays the same and the throttle only affects manifold pressure (the volume going into the chamber is unchanged, the pressure and therefore mass is what is changing) but then I realized that exhaust reversion effects are nonlinear to load, and that's the perfect way of describing why bridge/peripheral ports suck on the street: VE changes with manifold pressure.

peejay

The only problem with dual runner setups is they sacrifice power because of all of the internal surface area. That's why the 4AGE intake manifold sucks so hard, it has eight narrow runners and all that surface area inside. It's also why Honda did *not* use the dual runner intake manifold on the ITR engine.

ikari899

yup :P. but he seems set on doing some thing fancy that moves in the intake manifold and that was the only one i thought of that is really easy to do.

oh and its not that the variable volume plenum is hard to make its going to be very hard to tune it so it makes more power if any.

make one 787B style with the sliding runners, shouldnt be that hard to build or tune. i just dont think you will end up with more power from the variable volume plenum...

Kyrasis6

"more power" probably not, the variable displacement plenum would probably have more of an effect on moving the power band around and if the results are measureable it should be no more difficult to tune than anything else on the engine. I work for a race team and deal with this stuff every day on piston engines fabricating, design, and tuning are not what concerns me. What I'm interested in is what other people have found from practice to get a better starting place with my first rotary.

I read in another post where people in racing circuits that must run the stock manifold cut the plenum in half, cut out the "baffles" and then weld it back together and get posative results for their application. Would this be because of better pulse tuning in the plenum or from the primary runners only having air provided by the one throttle plate in throttle body?

Kenku

I know a guy who had that done... it's pretty much purely for the additional flow, as it's also required to keep the stock size throttle butterflies. The stock manifold runners are too damned long to be able to do more or less anything useful with pulse tuning, and in EProd rules you can't shorten them.

rotarygod

iTrader: (1)

I posted a thread on that a few years ago. I had cut apart both an n/a and a turbo manifold, hollowed them out and welded them back together. The S5 n/a intake I did had a 75mm Mustang tb. It was interesting but the tb was admittedly too large. I still have the TII plenum. It uses the stock tb.

ikari899

when i say tunning i mean getting the correct change in volume vs rpm.
and more power as in larger area under the curve, is what i was thinking...

good luck on what ever you pick and post up some pictures!