Resonance tuning


#1

Intake Resonance tuning.

Recently I had posted something here and someone got upset and suggested it was an “opinion” and “wrong”. If you want to question the following you are welcome. However, just saying “you posted something wrong and need to retract it” is pitiful and won’t be responded to (or may even be deleted). Please, add to the discussion with constructive criticism and further the learning here for all.

I am quite interested in the topic of resonance tuning. Here is a fairly good youtube explanation of a few principles. I thought it was a good one. If you have something better please share.

I have been reading up about exhaust resonance tuning. Merge collectors, four into one vs four-two-one, exhaust angles needing to be less than 15deg so as not to harm flow and so forth are things I’ve known about for years, and have a tiny bit of first hand dyno experience through the development one friend does on his race car. Tonight I was reading a bit by a ex F1 engineer about headers not needing to be equal length so much as “equal volume” for computational analysis and then only improved through real world testing. Of course cylinder temperatures diff and hence exhaust speed in runners will never be equal and the radius of bends has an effect too. When I find a simple explanation, preferably visual I will share.


#2

I understand what he is meaning by the bounce back of air as it completely makes sense but cant see it working properly to the theory.
I get that there would be unused energy but in all honestly with how the valves open so quickly I have my doubt’s as to if it would be completely possible to have the tune perfect where the air would be on the bounce back going towards the valve.
I think it is a good theory but one you could never make perfect.
I also don’t think the air would do a complete travel to the bounce back location when the valve closes on acceleration as the air coming in would have stronger force or velocity and it would almost first create a fight between the 2 air streams momentarily until the stronger force wins which and probably by that time the valve has opens and all air is sucked through into the chamber as all of it is still under pressure.
On deceleration then yes it would as the air coming in to the port would have possibly the same or weaker velocity than the bounce back air so it could possibly do a bounce back or even help to change the air direction of what comes in after the initial meeting and then all sucking through the valve again but weaker than b4.
I think the only way to stop the loss of air pressure velocity or whatever it is called would be to have a valve system for the valve so as to when the valve closes the wasted air can divert somewhere else and not cause a pressure build up or fight with incoming air in the chamber.
Why I tend to think this way as when looking at a turbo setup with a supercharger that has a blow off valve. One that does not have a blow off valve does get the gobble noise of the air coming back out but only when the butterfly closes for a quick deceleration. I dont see any reason for this same principal not to be in the chamber also but believe it doesn’t quite happen due to the quickness of the valve opening and closing.


#3

There are pulses in the air. Think sound waves. The sound does work as pressure waves. Whey are rampipes or bell mouths sometimes called “trumpets”? Have to do with sound. Sound causes vibration of the particles of the medium through which it travels.

Think about two strokes. The perfect tune is often associated with a savage power band. The inlet and exhaust ports can have a lot of overlap. But if you block the exhaust port with something other than the piston then fuel would stay in the cylinder. The shape of a two stroke exhaust is such that the waves will, at a particular rpm" reflect back and stop fuel coming out the exhaust.


#4

I didn’t even think of sound coming in to the equation. That makes everything different. I actually thought that the trumpet’s and bell mouth’s were just called that because of the shape’s hahahaha.


#5

Just think, you can “hear” an engine come on “song” when making power/torque.


#6

I understand it like this: you have a pipe and that pipe has a certain natural frequency, which is related to length, like a musical instrument. When air moves through the pipe a standing wave is created inside the pipe. On an engine most of the time everything is mush inside the pipe, but at certain RPM the frequency of the valves opening/closing matches the natural frequency of the pipe, so the standing wave reaches the valve when its open. This creates basically a turbocharging effect, pushing more are into the cylinder than normal. Your right when you say its not perfect, because is only works at specific rpm, but on the other hand it is perfect in the regard that it is an exact science that functions in theory and practice. The science behind it can be difficult to understand, but I think people who have studied music or fluid dynamics would understand the ins and outs very well.


#7

The frequency is of course the distance between the pulsing waves (the peaks thereof), so at particular length the energy leaving hits the atmosphere (inlet or exhaust). At particular revs the frequency might match the tract it travels through such that the wave helps with extra filling (inlet) or scavenging (exhuast), or particularly like on two strokes keeps the inlet charge from going out the exhaust. I recently read something (went back to find it and can’t - it’s one of two things on the web I regret losing [the other had to do with forward facing inlets, including size and position) written by an retired F1 engineer who explained that they would also take a pipe’s volume into consideration and that came into play when bends were greater than something like 17deg since working on centre line length was giving them okay computer simulations that were not working in practice. When they accounted for volume the maths came closer to getting predictable real world results for them.

There are some good SAE papers on this stuff that show research done on 600cc bike engines.


#8

Having recently designed a set of curved trumpets I had wondered if the centreline method was correct. There is a big difference in distance between the outside and inside of the curve, particularly on tight turns.

Another thing to mention is the effect of valve timing - different cams cause an engine to have a different tuned rpm even with the same tract length.


#9

Yes the pulse has to “not” hit a closed valve. Most noticeable on two strokes where “powerband” comes in has much to do with the exhaust timed pulses stopping fuel leave the cyl. Same with an exhaust pulse on a four stroke having an effect on valve overlap. The I guess there is pulsing that helps extra cyl filling and cyl scavenging.

Number of cyl also has an effect. Tuned 4cyl exhaust with a proper collector or flat plane V8s work well for pulse tuning. Not so good on 3cyl and 6cyl.