Been kinda slow in here latley so I thought I would post some questions and ya'll could exercise your brains and teach me something

I have been thinking(and searching) for new knowledge on engine design and theory. I found some very interestin stuff that just seemed to make for more questions. One of which was intake manifold design and theory.

I understand the basic theorys but applying them has got me bamboozeled.

1. Wh do you measure runner lenghth? I read where Larry Meaux was talking about measuring the floor of the port in the head, and the roof, add them and dividing by 2 to get a mean port lenght. Does this continue into the intake manifold? where does this end in a single palne intake?

2. When using a single plane in a EFI app, there is a myraid of ways to regulate air flow. But some of these would seem to add to plenum volume and adjust runner length. Like an elbow, wouldn't the entire volume of the elbow be added plenum volume? Would the power curve be affected between a elbow or a 4 hole mounted to the top of the intake? How so??

3. Longer runner equals better torwue, short runner is better for higher rpm, but I hear people talking about tunning this to a specific RPM. Wouldn't this be dependant on intake runner cross section and flow as well? How do you figure this magical length?

Sorry its kinda a novel but just some questions that have been burning the brain latley.

 

I'm sure you will get a lot of people to respond.

1. Two aspects need to be looked into when doing manifold design. I guess you could call them particle and wave theory (like light). You need to flow the raw air through the intake (particle). Period. But since the valve is pulsing the air, it has a resonance (wave) much like a pipe organ. If you figure out the right length to tune the intake to a certain RPM, then you can gain better chamber fill because of the standing waves in the runner.

2. The intake systems are interesting. Think of what you can tune when you start to look at it as a sound system. Chamber + Runner = ported speaker box! Chamber + Runner + plenum + throttle body = 5th order bandpass (three chamber bandpass box)! So when you start to add volume or length to the intake manifold, you are changing a very complex acoustic system.

3. Much like answer 1, longer runners make deeper tones. Think of a pipe organ. There is an optimum CA (cross area) but that has more to do with trying to flow the raw air through than tuning. You tune for CFM first, then tuning length.

I'm not a guru by any means, but I understand the principles.

 

Thanks alot.

I under stand the concepts but I wanted to get how you figure, what length, volume ect. I kinda understand the wave theory, but not really how it works, but I can believe it. But how(mathmatically) do you come up with the answers?

 

Its complicated. A little much so to get real numbers. You get guidelines and then tune from there. If you go straight off of particle flow...

Ensure that you don't exceed your mach index (0.6 of 764mph),
Velocity stack,
Trial and error with bowl sizes and porting.

For wave tuning,

Use the guides to get you close, then guess! After you do the above for simple airflow, getting it to actually resonate is difficult. All of the books I've read suggest making adjustable runner pieces and experiment with a dyno.

 

For virtually ALL the answers, read Gordon P. Blair's book: Design and Simulation of Four-Stroke Engines

http://www.motolit.com/desandsimoff.html

More than you wanted to know, but very informative, and a not-to-difficult read....

My short answer to your questoin is: use a very good engine simulation program! You'll know why after enjoying GPB's math.

 

You can also build an intake to take advantave of harmonics created the closing of the intake valves.
If you could use the 1st harmonic wave you would get a 10% boost in power free. It would be free after you designed and built the intake to make use of it.
If you cant make use of the first harmonic wave there is allways the second and third wave.

 

Here you can use this mathematical model.

http://www.grapeaperacing.com/GrapeA...ionsystems.pdf

We used it for our FSAE team. This model was fairly accurate, 150-250 rpms from predicted outcome. We tested 4 different lengths with good results. Although, we were running through a restrictor, so your results could vary. We used 2nd and 3rd harmonics as the 1st harmonic required extremely long lengths. On our dyno graphs, you could clearly see the torque peaks near the predicted harmonics.

I have a spreadsheet that will calculate where your harmonics will be based on engine inputs.

 

Thats a good read there. Some of the Helmoltz theory stuff was a little confusing but I think I understand it.

Can you email me the spread sheet you used?? 95ttoplt1 at fbody dot com