intake manifold ram effect 1

[kimbo2]

I am designing a custom intake manifold on a rotary engine. I want to take advantage of a ram effect at around 4500rpm and hopefully no negative effect between 3000-7500 rpm. I have dyno tuned many cars and seen some crappy up and down torque curves and want to avoid this. Can anyone help me with a formula to calculate the behavior of a set-up?. Is it a helmholtz resonator or is it just the time for reflection of a sound wave up and down a single runner only? at the moment I am going to use a 2.6 litre plenum and 12" long 1 1/4" diameter runners to the 4 intake ports. Any ideas Thank you in advance.

 

[GregLocock]

You are right, the easy way to tune it isn't a Helmholtz resonator, it is a half wave pipe.

In a traditional air intake, starting from the engine, you have the length of the individual runners, then you have the plenum, then you have the pipe form the plenum to the filter. Each of these lengths can be important, tho the runner length dominates usually. The cross sectional are of the plenum needs to be at least 4 times the area of the runner (10 really), to get proper decoupling. The pressure of the wave inverts when it hits the plenum.

As the intake valve closes a slug of air hits the back of it. This creates a compressive pulse which is bounced up the runner. It hits the plenum, inverts, and comes back down the runner. Ideally this then hits the intake valve as it opens. You can use multiple paths (ie bouncing the pulse off a closed valve) which will reduce the desired runner length, but probably reduces the optimum efficiency.



So you need to know the time interval that you want between pulses at the valve, which is easy to estimate from the speed and the cam timing. Typically it'll be 400-500 degrees of crank on a normal four stroke. You have to allow a bit extra from the 'perfect' timing to allow an effective opening, say 30 degrees.

Speed of sound is 330 m/s at room temperature, but your charge temperature is probably 50 degrees C or so (ours is anyway) and it varies with the square root of absolute temperature.

Typical length for a second harmonic pipe at 3500 rpm is 80 cm.

Cheers

Greg Locock

 

[GregLocock]

Check out SAE paper 2001 01 1211 if you want to design a Helmholtz resonator to do the same job. I don't pretend to understand how this works - I use Helmholtz resonators to make things quiet, not more powerful.
Cheers

Greg Locock

 

[kimbo2]

Greg, do you know wher I can find this paper on the web?
Thanks for your help!

 

[GregLocock]

You have to buy SAE papers. Or photocopy them.

Cheers

Greg Locock

 

[FireLover]

For a paper that receint, you can download it off of the internet... For a fee. I think for SAE members, it is $10 a paper. Go to

http://WWW.SAE.ORG

and select the bookstore. Then look for technical papers and search the number given. SAE papers are a great source of information.

 

Hi
1) Please describe me some about "VARIABLE LENGTH MANIFOLD" and "VARIABLE AREA MANIFOLD"

2) So I want some about "MANIFOLD RESONANCE"

3) Then tell me about "RELATION BETWEEN (MANIFOLD NATURAL FREQENCY) and (SPEED ENGINE)"

4) And my last question is about "HOW TO CALCULATE MANIFOLD NATURAL FREQUENCY IN ENGINE"

If you have any formula in 4 up questions please send me all.

Best Regards.

 

Hi
1) Please describe me some about "VARIABLE LENGTH MANIFOLD" and "VARIABLE AREA MANIFOLD"

2) So I want some about "MANIFOLD RESONANCE"

3) Then tell me about "RELATION BETWEEN (MANIFOLD NATURAL FREQENCY) and (SPEED ENGINE)"

4) And my last question is about "HOW TO CALCULATE MANIFOLD NATURAL FREQUENCY IN ENGINE"

If you have any formula in 4 up questions please send me all.

Best Regards.

 

[GregLocock]

Take a look at

http://www.grapeaperacing.com

That explains the basics, at least for the length tuning and effect depending on speed.

I haven't heard of variable cross section before - other than in the special case of inside the carb. Cheers

Greg Locock

 

hello. has anyone checked the math on the grape ape racing site? I tried some of the equations for runner length on the tuned manifold page, and I can't get the same answers as shown. thank you