Exhaust and intake resonances

Competition engines and ancillaries - general discussion
Robert
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Exhaust and intake resonances

Post by Robert » Sun Mar 17, 2013 8:46 am

Hello Guy,

This is a question about matching well the resonances of an engine system (intake and exhaust).
From your experience, is it better to shift the intake and exhaust resonances in order to spread the humps on the rev band or is it better to have the same revs ?
So is it better to have more humps with given amplitudes or less humps with bigger amplitude ?

Hoping I'm clear enough. Thank you !
Kind regards

Robert
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Tue Mar 19, 2013 4:09 pm

Hello,

I cannot imagine there is no answer about this question...
Waiting for the specialists, will be patient. :)

Robert
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timinator
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Re: Exhaust and intake resonances

Post by timinator » Tue Mar 19, 2013 5:20 pm

Hi Robert, Speaking of general applications for gas engines is not very satisfying. As a rule the engine is most efficient for a range of about 1500 rpm. The best answer to your question would be to pick where you want that 1500 rpm and tune to have no humps(?) in that region. Having more gears in your transmission makes tuning easier. I look for the highest average horsepower in the rpm range that racing rules or customer budget limits the specific engine.

TomLouwrier
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Re: Exhaust and intake resonances

Post by TomLouwrier » Tue Mar 19, 2013 9:43 pm

Bonsoir Robert,

Interesting question but complex matter behind it.

Tuning 'both sides' of the engine together with the cams (both lift curve and timing) usually give you a somewhat peaky characteristic, you get those humps and dips in the torque curve. The idea of using inlet and exhaust resonances to even out those peaks would seem like a logical thing to do. However, in doing so you may shoot yourself in the foot.
Think about this: are the dips really losses or are the humps actually your gains?

I feel there is a very large possibility of actually ending up with the worst of both worlds. You may get the resonances cancelling each other out and have a flat curve, without any of those humps. This is because the ram effect is lost. So you end up with just the dips. It may even be less than that because the waves are now actually fighting each other.
The resulting performance will thus be lower than the average of the bumpy curve. You have no way of using the performance from the humps by placing them in a useful part of the rev range. And that, actually, is the true meaning of "tuning".

Mind you: don't take me for an expert here. This is all gut feeling. I have absolutely no figures to back this thought up. Resonance theory, harmonics etc are not my thing. Maybe next year :-)

regards
Tom
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Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Wed Mar 20, 2013 1:23 pm

Interesting thread, although I have thought hard about a useful reply - very sorry was unable to write at all till today Rob.

Some worthwhile observations by others.

Maybe I can get you to figure this out with a bit of GC 'Q&A', might be more interesting than me just giving a straightforward reply.

Let's start with a hint - see turbo torque curve below. Why are there no 'humps'?

GC
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turbo torque.GIF
turbo torque.GIF (11.32 KiB) Viewed 2535 times
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Wed Mar 20, 2013 2:33 pm

Hi Guy,

It is a pleasure to read you.
I see a BIG hump at 3500 !

I try an answer: To fill the cylinder we need 2 phenomenon’s, the flow and the acoustic (superimpose onto the first). Blower certainly brings more flow without need for acoustic ?
In fact my concern was how to increase filling with the help of well tune acoustic part of the engine.

I am right ?

Kind regards,

Robert
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Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Wed Mar 20, 2013 2:51 pm

Yes the big rise in it of course the turbo going to high boost. Then the torque curve shows a complete absence of peaks/troughs in the output...

Yes, there are no wave effects on-boost on the turbocharged engine. The cylinder - which is in depression durin the intake phase sees 'overpressure' right outside the valve.

Unlike the atmo unit whose outgoing negative waves (of any frequency) only see (useful) overpressure at the end of the rampipe...

Even for a 'given' engine (ie: one with a particular spec of CR,cams, cam timing, exhaust and intake setup etc) With so many variations in cylinder pressure (pos or neg), piston speed, temperature - it is no wonder we see variations in the torque curve all over the place. And this is quite normal.

And don't forget that there may well be far more peaks and troughs in the atmo curve than we realise, since measurements tend to be taken in the transient state (ie: accelerating up to max speed) at relatively big intervals, say 500 or 1000 rpm. I have done bench-dyno step-tests with 200 rpm intervals and it's funny how 'unsmooth' a good torque curve really is. Of course in the real-world we just drive thru those points and don't notice the minor variations.

The cylinder fills because it is sucking in - and this filling can be aided by sheer momentum of incoming charge (which is partly why the torque curve evens-out at and over max torque), beneficial inlet tract pressure waves and better cylinder purging on the overlap...

G
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Wed Mar 20, 2013 3:15 pm

From my very very small experience I want to share here for analysis my “results”.
This is the curve of my engine (130TC, original cams, preparation of the head following the best I could do the GC recommendations, valve grinding by GC, guide from GC, CR 10.7:1)
I calculated the resonances of the intake with 2 different methods and the resonances of the exhaust. I can observe (looking carefully the dyno curve) that the humps and the dips on the curve are quite near of the calculated.
The drawing synthetizes both types (calculated and observed) and contains the dimension for acoustic approach.
I am not at all a motorist but coming from electronics I believe in waves. I think also that an engine is like a music instrument (we speak about horn, trumpet …) and like in it the waves in the “pipes” can be tuned for the best (or the worst!).
So in addition to the ram effect I believe that the acoustic wave can add to the filling. So to pressure, the torque, the power.

On the drawing I mentioned also the bandwidth of the torque > 90% of the max torque (2500 rpm) and the bandwith of the torque > 95% of the max torque (1500 rpm)

I am humbly waiting your comments.

Robert


Robert
Attachments
Humps & Dips .jpg
Drawing with the different humps and dips
Humps & Dips .jpg (43.92 KiB) Viewed 2471 times
Curves MF3 shrink.JPG
Dyno test
Curves MF3 shrink.JPG (80.58 KiB) Viewed 2470 times
Last edited by Robert on Thu Mar 21, 2013 2:50 pm, edited 3 times in total.
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timinator
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Re: Exhaust and intake resonances

Post by timinator » Wed Mar 20, 2013 4:09 pm

Hi Robert, Just curious what you want to do with this engine? If you run it from 5200 to 6700rpm it looks pretty good. It allows a 7100rpm redline and still is making good power. What type of collector are you using, and is it with open exhaust? Are you looking at any specific problems like wheel spin at 5000rpm?

Robert
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Re: Exhaust and intake resonances

Post by Robert » Wed Mar 20, 2013 5:00 pm

Hi Timinator,
I use it up to the shift light set up at 6250rpm. The ECU cut ignition at 6500. The pistons are cast (105TC OE) not forged.
I precise it is FI and static ignition.
Do you think it can be used more than 6500?

I just want to analyse this case in order to improve it. My header (exhaust) is 4-2-1 (you find the dim in the drawing). The diametre of the prim. pipe is a little to small (34mm) and I would prefer 38mm.

So behind my question there is a project but I always want to know before act... And here I think to be at the right address !!

Robert
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Thu Mar 21, 2013 2:27 pm

I am convinced on resonance existence (the dyno curves show them). If I modify the length of the intake tract I can get a resonance with a lower harmonic (H2) at about 6900rpm.
Since the signal is higher for the lower harmonic (fundamental is not targetable) I wonder how it can improve the torque so the power at this revs?

For that I need to increase the total length of about 40mm and use cam like GC3. Calculation for this case giving hump at H2=6900, H3=4600, H4=3400. The dips being 5500 and 3900.

The attached drawing showing the comparison between both solutions.

So you understand my first question: is it better to shift the intake and exhaust resonances?

Robert
Attachments
Humps & Dips 2.jpg
Humps & Dips 2.jpg (53.08 KiB) Viewed 2454 times
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timinator
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Re: Exhaust and intake resonances

Post by timinator » Thu Mar 21, 2013 6:05 pm

Hi Robert, Very sorry but I am not able to find your exhaust drawing. Could you supply a link please. Having the correct length primary and secondary tubes can be made ineffective by a poorly shaped, sized, or to short collector.
Yes your cast pistons can go to 6500 rpm, having ignition cut at that point would be awkward.
Increasing intake length I believe would be a plus. Keep in mind that each higher resonance adds a lower amount of wave pressure. Also between each beneficial tuned rpm there is an rpm of horsepower loss. Always a compromise to be dealt with.

Robert
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Re: Exhaust and intake resonances

Post by Robert » Thu Mar 21, 2013 7:35 pm

The photo is from Internet I have no snapshot from mine.
The dimensions are in the previous attached picture.

When you say "higher resonance adds a lower amount of wave pressure" do you mean higher harmonic or higher revs ?
I agree concerning the higher harmonic. I learn if it is revs. Thanks.

Robert
Attachments
exhaust-lancia-beta-montecarlo.jpg
CSC Montecarlo exhaust
exhaust-lancia-beta-montecarlo.jpg (9.63 KiB) Viewed 2434 times
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Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Thu Mar 21, 2013 9:12 pm

I don't want to rain on your parade Rob but this is GC Q&A.

Whilst - as I indicated in my 2010 book, this is really great fun, it is not sufficiently robust to design a setup around. The problem is that you cannot simply look at the torque curve and say 'that hump is a harmonic'. What appears to be a 'gain' in the torque curve at one point may simply be an adverse wave effect either side of that point.

You will get 1st and 2nd harmonics typically around 3200 and 6400 and you might as well ignore the others. Whether they turn out to be any use is a whole other matter. Does a beneficial wave arrive at the valve at the right time? What is the right time? When - exactly - and how - does a negative wave propagate from the inlet valve? What about waves reflected from the exhaust collectors travelling right across the cylinder and up the inlet tract?

You simply cannot predict this with maths.

I have dozens of dyno curves - from the last 10 years alone - and I cannot remark simply from the torque curves where the harmonics are, in fact if you do a bit of math on the units the humps in the torque curve are nowhere near where they be expected to be and there are far more humps than harmonics. And simple computations take no account of the most influential component in the entire engine - the camshaft.

This stuff is fearsomely complex and the late Professor Blair's Virtual 4T software was fundamentally all about waves and their effects - inlet, exhaust and combinations. I do know that it took him twenty years and that much of what was used for the computing iterations (where the system is looking for optimum combinations) was empirically derived and necessarily so.

The manifold on the CSC is not a complete disaster but unless they have made significant improvement since the last time I cut one up - the silencer is - open it up and have a look inside. If you want one that works get my design from the MonteHospital.


GC
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Fri Mar 22, 2013 2:15 pm

Thank you Guy for your post.
Sorry I don't understand your phrase "I don't want to rain on your parade Rob but this is GC Q&A" (you know I'm French and I don't master English totally, I would say partially)

The calculations I made are based on tract length and camshaft timing. The relations used come from 2 different books and are very different but give the same results.
I said I observed the humps very near of the calculated ones but I cannot say I'm 100% sure of the coincidence and if the approach can be used so directly.
It is why I ask question in order to validate or not the approach.

My silencer is not the CSC one but the inner perforated tube has a larger dia than the original.

Best regards,

Robert
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