M20 intake manifold design thread

[B C]

A few of us here are interested in designing an intake manifold for an M20, and a few have already started working on them. 

SOME DISCLAIMERS

UNLESS YOU ARE A NERD, THIS WILL BE AN IMMENSELY BORING THREAD FOR YOU.
IF THIS THREAD SEEMS BORING ALREADY, CLICK HERE  https://user.xmission.com/~emailbox/trivia.htm

1. I do not know everything, especially when it comes to unsteady flow dynamics.

2. I have done a lot of research on intake design over the last 13 years in my free time, but I am still nowhere near an authority/expert on the subject. It is just one that fascinates me and I may have an unhealthy level of interest in it.

3. I DO know that even when playing with advanced maths and analysis, that there are still 9302934021 things that aren't being accounted for, or cannot be predicted. If you have done well, and get lucky, you may find yourself in the ballpark of where you need to be. Real world testing, when possible, trumps mathematical analysis all day everyday and 3.14 times on Sunday

4. Even when testing a manifold design on a flow bench, there are still at least 2814731 things that aren't being accounted for, or cannot be tested.

5. The only 100% accurate testing and analysis is performed with a manifold installed on a car with proper measurement equipment (dyno, etc)
    

6. The time, cost, and knowledge involved in thoroughly developing the ideal intake manifold is unrealistic for anything short of an F1 team, and even then, they have to draw the line somewhere.

7. Assumptions must be made. Whether they are good or not, you must make assumptions. If and when the end product doesnt work as intended, you must re-visit your assumptions and make adjustments.

Now that is through,   this thread is for us to exchange thoughts and theories about taming the crab under the hood. 

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[B C]

I will avoid doing elaborate maths if at all possible, for the sake of my time, and your sanity, but if you would like to punch out some elaborate maths, please feel free to do so.

This is from an old Porsche performance handbook showing a chart of ideal runner length for a given RPM. 

 

This is an extremely simplified view at a very complex topic and might not even hold true for old Porsches, let alone an E30,  but I find it interesting how the chart matches up well with some numbers I ran through an intake length calculator. 

http://www.velocity-of-sound.com/velocity_of_sound/calculator3.htm

I went out and quickly measured the M20 manifold I have and got 1.37" diameter for the ports. They are a bit taller (1.41" or so) in the other dimension, but if making a tubular manifold, 1.37" ID tubing is readily available. 

I used values from this thread  http://www.e30tech.com/forum/showthread.php?t=52085 in the intake length calculator.

Assuming it is correct,

The stock M20B25 cam has 256 degrees of duration
The intake runner length is 360mm / 14.17"

Many race engine manifolds are designed for 3rd harmonic resonance, race engines also run higher rpm, therefore 4th harmonic may make for a more reasonable manifold length on a street car engine. 

It would appear that the M20 manifold was designed for 4th harmonic resonance, if at all (which I strongly believe it is).

I entered the cam duration (256) and harmonic (4) and entered rpm values until I converged upon 14.17" as a suggested runner length. 
For the info I entered, this suggests that the M20 manifold would be designed for 5400 rpm. 
 

I would have imagined the target rpm would be 4250 where the torque peak is, but perhaps designing it for a higher rpm allows for a broader powerband.

Now back to the old Porsche schematic. 14" runner length ends up about 6500 rpm on the graph.  ~6500 rpm on an old low resolution graph vs 5400 rpm from a random online calculator. Using the online calculator  7200 rpm is what is required to get 14.17"  for the 3rd harmonic. 

So if the online calculator can be trusted, and if the M20 manifold was designed with harmonics in mind (which I believe it is), It would be operating on the 4th harmonic.

More ramblings to come...  feel free to add your ramblings so we can start a massive e-fight

[KaiserRolls]

male cats are called Toms, or Gibs - if neutered. 

also. PVC intake

harmonics be dammed 

[B C]

Chris sent me a picture of a different M20 pvc manifold. What a bunch of trolls.

[Jeff]

Do they help in any aspect?

[B C]

smooth surface finish, but plenty of ledges for air to "trip" on and become more turbulent. 
Probably weighs the same as the cast aluminum manifold to boot. 

Looks like garbage.  Someone needs to start an  "E30 engine bays that look like crap" thread.

[Rowleym]

I really like where this is going! We have a unique opportunity here I have all the tools to build an awesome manifold for us, but we simply need to design it! I think it would be cool to post all the files publicly as well so people could have them built. I can only fathom building 5 or so at first  

We can do this at an extremely affordable cost too! I could even build a die to form the velocity stacks, to save mucho moneys(or not). 
There are many ways we can make this really affordable! I want to kill as many crab manifolds as I can....

[B C]

Right now im  on board with what Jeff is doing. Lop the flanges off, weld on some 6061 tubing for the runners and probably pipe, plate, or a combination of both for the plenum. Super simple stuff.

[Jeff]

I like simple. I have to sit the flange in half so I can cast it to make duplicates for different manifolds. We need to figure out the optimum runner length and plenium size. Also shape or the plenium has factor too round oval square triangle what makes what difference. The volume inside the plenium is also needed

[B C]

My design priorities are

-Equal cylinder filling

-Minimal flow loss

-Plenum size and runner length beneficial for low rpm off-boost performance if possible. 

-Manufacturability

-Low weight

-Acoustics

-Engine bay cleanup/ease of maintenance

[Rowleym]

Snap said:

My design priorities are

-Equal cylinder filling

-Minimal flow loss

-Plenum size and runner length beneficial for low rpm off-boost performance if possible. 

-Manufacturability

-Low weight

-Acoustics

-Engine bay cleanup/ease of maintenance

Perfect! Exactly what I'm after.

Can't wait to see some numbers so I can order the runners that's all I need to build 3 or 4 of them.

[B C]

If at all possible I would like to use the stock throttle cable as well

[Rowleym]

Snap said:

If at all possible I would like to use the stock throttle cable as well

No front(angled) entry?

[B C]

I havent looked into generic throttle cables much but yes id like the tb to be front mounted

[Jeff]

Why front mounted vs side mount?

[Rowleym]

Jeff said:

Why front mounted vs side mount?

I personally like it for packaging reasons, and long radius(and short in length)  intercooler piping

[Jeff]

I think there needs to be two different ones designed here 1 for the guy that is going n/a and one for the turbocharged monsters

[Earl]

^Agreed=) There should be an m42 manifold made too=D

[B C]

Dear Earl,

2 nights ago I had a dream that I swapped a rebuilt M42 into my car (with my current turbo and such).

It sucked. true story. 

sincerely,

Snapple

[Rowleym]

Jeff said:

I think there needs to be two different ones designed here 1 for the guy that is going n/a and one for the turbocharged monsters

I would hope so! They are two totally different beasts(not exactly)

If i wasn't FI I would definitely use ITB's or triple webbers or something