Helmholtz resonance intake manifold vibration

[TurboRA28]

Hello, i'm totally out of my league here, but can the helmholtz resonance cause what might feel like a vibration in an inlet manifold?

I'm asking because i've made up a new inlet manifold for the 3sgte, but since fitting it around the 4000rpm mark it gets a nasty resonance/vibration in the inlet manifold.

The old manifold didn't suffer this problem.

So i'm thinking ive upset some inlet tuning . I've tried reading about helmholtz but just not sure if it applies? Or more importantly, any quick fixes?

Thanks!
Joel

[MWP]

Youll probably find it has nothing to do with airflow through the intake.
I think itll be caused by normal engine vibrations which do resonate with the intake at certain RPMs.
Itll be different to the old intake as its different size, weight, mounting, etc will tune it for different frequencies.

Not much you can do really other than braceing it to the engine better.
Mybe run a couple of braces from the underside of the manifold down to the engine mount?
Not a nice way to do it, but itll really firm up the intake and stop the problem... itll also help to reduce fatigue on those alloy welds in the future.

[TurboRA28]

Thanks for that mate.
Damn I need a TIG! Next manifold is going to be mild steel I think

[Celica RA45]

3sge have a harmonic problem from 2000 to 3000 rpm ,even if you change the rod ratio it helps alittle bit ,but taking 2kilos off the crank helps also just put up with it like i do ,cant see out of rear vision mirror till past 3000 rpm

[MWP]

Damn I need a TIG! Next manifold is going to be mild steel I think

I like the idea of carbon fibre myself... just epoxy bits to it

Seriously... it looks like carbon isnt as difficult to use as people (well i did) first think.
Lots of very good info here if you havent seen it before:
http://www.racingcomposites.net/unm

[30psi 4agte]

Turbo ra:

I know exactly what you on about here. I have had this prob for years now on my 4age. I have a custom plenum and it has a nasty vibration !!!!!!!!!!!!!!

So bad that if it isnt braced and i rev the motor it will vibrate the throttle boddy that bad that it will keep the engine running at 4000 rpm all by its self.

I ended up putting a brace from the under side of the plenumn to the engine it worked for a while but then it tore through the plenum!!

I pulled the manifold once again and took it down to get welded and some heavy duty bracing done on the plenum.

I ran 2 adjustable rods with rose joints at each end and so bad is this vibration that the rose joints fucked out within 2 weeks. ------ I now have made SOLID adjustable rods

I now run 2 braces from the underside of the plenumn to the engine. Havent had it crack yet but never say never.

Just so you guys know the motor is not out of balance and has actually been balanced! The only thing i can put it down to is engine harmonics or reversion in the manifold.

[kingmick]

its what is called resonance. i just mentioned it in the thread abount the flat spot in the 4ag.
ive had hats crack becase of it. part of the problem when people have half an idea making things is that the things they make for an engine, dont take into account how the engine works.
with out looking at the setup in person etc it could well be a balance problem. make sure you look at you balancer.
have you changed the clutch and flywheel?

[brett_celicacoupe]

maybe you could also weld some webbing into the inside edge of each of the runners. this should eliminate any flex in the runner tubes themselves. then have a rubber bushed brace from the cam cover or head.

[TurboRA28]

Thanks all for this info.. Yeah I don't think it is an engine balance problem, it was all recently balanced by a very reputable engine builder.

Interesting to know others have had this problem too!

Does things like the thickness of the metal used for the runners/plenum have an effect on it? Or more the shape/sizing, etc?

30psi, did the bracing get rid of the vibration? or just stop it breaking?

Kingmick, what kind of things should I be researching more to understand this and design something that doesn't get this resonance? As the factory Toyota gear doesn't suffer problems like this so you must be able to reproduce that somehow. I cant run the factory plenum as it would poke out of the bonnet

Brett, I like the webbing idea! I noticed the factory gear has a lot of webbing over it.

Thanks
Joel

[TurboRA28]

MWP, how does carbon stand up to boost?

[MWP]

MWP, how does carbon stand up to boost?

Very well if its thick enough... just like any other material, alu, steel, etc.

[Gold28]

G'day Joel,

If what Celica RA45 says is true and the 3s has harmonic problems, then the problem may not be too hard to fix.

You can't change the harmonics of the engine, they are a simple multiplication of the RPM of the masses (crank, cams etc) and can't be changed.

What you can do though is to change the mass and stiffness of the components so that their natural frequency doesn't lie close to a harmonic of the engine. Think of your plenum (complete with TB etc) as a tuneing fork. It will want to vibrate at certain frequencies. If you make it stiffer (eg bracing) it will vibrate at a higher frequency and if you make it heavier it will vibrate at a lower frequency. What you don't want is for it to vibrate at your engine harmonics.

A simple example would be to get a ruler and clamp it to a bench. flick the ruler and watch it vibrate. Tape a mass to the end and it will vibrate slower. Clamp it so that the ruler overhang is shorter (stiffer) and it will vibrate faster.

Unfortunately it will have numerous modes of vibration, each with it's own critical exciting frequency. The first mode may be flapping up and down, the second may be torsional mode and the third may be bending or one of a number of other modes or a harmonic (factor) of the first or second mode.

As for fixing it, you really need to know what the mode is. Does it do it when you are free revving the engine or only under load? if it does it free reving then just take a good look at what it's doing and try and figure out the direction it is vibrating. Then brace it accordingly. If for example the critical mode is a twisting mode, with the plenum rocking forward and aft, then a brace in the middle is going to do nothing, the plenum may even just excite a vibration in the brace.

Adding, subtracting and redistributing weight is a good way to do it but once again you really need to know what the modes are.

I would even hazard a guess at it being a fore/aft rocking mode you have, in which case you may be better off putting a brace at the throttle body.

Wow I can bat on.

[TurboRA28]

Hey Anthony, mate as usual that is great information.

To answer a few of the questions there, it only does it free revving, not under load.

Removing or adding a brace directly in the centre of the plenum down to the block seems to have very little, if any, effect on it.

So maybe the brace around the throttle body will assist?

Would it be possible for me to say strap weights to it and that would change how it behaves? More weight will make it vibrate at different frequency.

[brett_celicacoupe]

the way it is now, the manifold is most likely behaving similar to a metal ruler if you bolted that to the tappet cover pointing straight up. with the engine running, the ruler would bending left to right in the same way your manifold would be, but at a different frequency. so the way to stop it vibrating left to right would be to put a brace from the top of the ruler to something on the left or right of it (in your case, from the top of the plenum to the head/tappet cover).


id say you might want to be warey of making a solid brace on your plenum. if you do use one, the manifold will still flex left to right but the stresses near the top mount will be far greater. so any weaknesses along the runners (ie welds ) will have a higher chance of failing.

using a rubber bush in the brace should allow the runners and plenum to move side to side 'as one' . the rubber will also allow 'steady' take up of each individual sideways movement.

in this graph, blue is mild steel, pink is aluminium and yellow is cast iron




you can see from this graph that aluminium has a lesser elasticity range than mild steel. at the point where the aluminium plateus (from its first major linear increase) is where it has been stretched to a point where it wont return to it's original shape (elasticity range). your aim is to keep to the aluminum from stretching/twisting/bending outside of this elasticity range. as mentioned before though, this elasticity range is much less than that of mild steel

anyway, thought that may be kinda relavent information. the graph came from one of my pracs


cya


brett

[kingmick]

very good post! + rep