intake manifold ram effect
intake manifold ram effect
(OP)
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.





RE: intake manifold ram effect
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
RE: intake manifold ram effect
Cheers
Greg Locock
RE: intake manifold ram effect
Thanks for your help!
RE: intake manifold ram effect
Cheers
Greg Locock
RE: intake manifold ram effect
RE: intake manifold ram effect
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.
RE: intake manifold ram effect
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.
RE: intake manifold ram effect
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
RE: intake manifold ram effect