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Inlet plenum and gas law calculations?

  1. Nov 2, 2005 #1
    Inlet plenum and gas law calculations???

    Not sure if this is the best pace to post this question but here goes......

    I want to get a custom inlet plenum made for my car, there are many already out there but they cost a fortune, but several people have made there own and the construction is fairly simple.

    The problem i have is trying to ensure that the design allows for an equal airflow to all cylinders.

    I would say that this is fairly straight forward, as the inlet is at one end, then all 4 of the cylinder intakes are spaced evenly away from it.

    My concern is that where the closest intake is to the inlet is cylinder No.1 and the furthest away is cylinder No.4, the inlet stroke is the same for all 4 cylinders, yet from the point of the inlet to each cylinder the air has effectively different flow lengths to each cylinder.

    The plenums you can buy, have a taper towards the Np.4 cylinder so as to reduce the volume and im guessing increasing the velocity so as to arrive at the cylinder the same as the other cylinders.

    But what is the calculation to work out the space required and the taper???

    any thoughts?

    I can provide a simple schematic of the plenum if it'll help........

  2. jcsd
  3. Nov 3, 2005 #2


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    I don't mean to get into auto tuning or anything, but the proper plenum design is largely based on the application. The length of the runners are absolutely crucial in the performance of the car. If they are too long, you will lose high end power, if they are too short, you may push your power band out of the range your engine can go.

    Maybe can you provide additional information about the application? Also, a possible link to the tapered manifold (I just know I've seen many without a taper in them, V6's even).

    I have a fantastic book at home to help work out volumes and things like that though.
  4. Nov 3, 2005 #3

    Thanks very much for the reply.

    The runners are unaffected, the plenum is a 'Dry' inlet i.e. Only air, fuel is added in the runners to the head.

    The basic design of the plenum is a rectangular box, which encases the four inlet runners. The inlet air into this plenum comes through a small slot at the bottom of the plenum i.e. at 90deg to the inlet runners.

    The better design has the airflow enter at the end of the plenum such that the airflow continues straight on over the top of inlet one, then two, three and finally the furthest away Four.

    I hope that makes sense, but im off now to get some pics and see if i can include them in this post to allow you to see exactly what im saying.

    Hopefully back soon with Pictures.

    Once again thanks for the reply :-)
  5. Nov 3, 2005 #4
    Ok here goes.......
    1. Attachment 1:
    This is the plenum chamber as standard. As you'll be able to see no doubt is the airflow coming in has to turn through a tight elbow, and this elbow is very restictive.
    2. Attachment 2:
    This is a simple schematic of the plenum, the rectangular box shows where the airflow enters from the elbow. You'll notice it is considerably smaller than the inlet.
    3. Attachment 3:
    This is how the expensive and much improved plenum is designed.
    You'll notice the airflow is direct from one end, and that the chamber itself is tapered to the rear where No.4 inlet is furthest away
    4. Attachment 4,5 & 6 (next post)
    No.1 is an expensive one to buy
    No.2 is a Homemade one (and worked extremely well)
    No.3 is also an expensive one to buy.
    Ive known of several people who have made them themselves, and to be honest i doubt they even bothered doing any calculations when designing it, however, if its not beyond my capabilities i'd prefer to go with Physics and Maths designing it for me :-)

    Attached Files:

    Last edited: Nov 3, 2005
  6. Nov 3, 2005 #5
    Thumbnails 4,5 & 6.........

    Attached Files:

  7. Nov 7, 2005 #6


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    My roomate is using that book I was talking about right now for a research paper. I will try to get a hold of it and re-read the section on intake systems.
  8. Nov 7, 2005 #7
    Thanks :-) Much appreciated....
  9. Nov 20, 2005 #8


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    Hi CosRush. You asked:
    I'll assume you're going with the tapered plenum design shown in attachment 3 above. I'd suggest that the plenum shouldn't introduce any additional pressure drop, the difference in pressure between each of the inlets should be zero. To do that, the plenum ID has to be as large as possible. At the very least, the ID of the plenum inlet must be more than the total outlet area (area of the 4 inlets added together). Also, the plenum can be tapered as shown, but I'd suggest making sure the cross sectional area at each location is significantly larger than where the air must eventually go. So at the half way point where the air has passed two inlets already, the cross sectional area of the plenum must still be much larger than the area of the two remaining inlets. I might suggest it being at least 50% larger in area, preferably more to reduce pressure loss.

    To test your plenum, you might try drilling a very small hole in each of the four air inlets that could later be plugged say with a pipe thread plug. The smaller the better. They go down to 1/16" if I'm not mistaken, though that's pretty rare. Then simply put a pressure gage on each air intake to determine if they are the same. If not, porting the ones with lower pressure to make larger ports might help.

    I also noticed in those pictures, the inlets were protruding into the plenum which is probably not the most efficient. Looking at restrictions to flow, such a design is generally going to have more restriction than one where the inlet ends at the plenum wall (ie: it goes between the engine inlet valve and plenum wall). What it looks as if is being done to reduce that restriction is to have a funnel type inlet, which should help significantly. I think the best design would be to have that funnel shape and also start the inlet at the wall of the plenum as opposed to sticking it into the middle of the plenum. Also, the funnel shape might best be made in the shape of a nozzle inlet such as used for venturies. Imagine a donut in which the ID of the donut is equal to the ID of a pipe. Now imagine the donut being stuck to the end of the pipe so you have a rounded inlet. That's about the best shape you can have to reduce restriction, and better for example than a conical shape.

    What I don't know a whole lot about is the dynamics of such a pipe. I suspect there is going to be some pressure pulsations going into and out of the inlets which are a function of the velocity of sound in air and the length of the inlets. Maybe someone else can help with that.
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