Exhaust and intake resonances

Competition engines and ancillaries - general discussion
Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Mon Mar 25, 2013 11:16 am

Thanks Tim,

all good with me.

The assumptions may however be valid only in that case - if indeed they are correct.

G
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Re: Exhaust and intake resonances

Post by timinator » Tue Mar 26, 2013 6:56 am

Hi Robert, The wave tuning that I think you would be more interested in is that coming from the exhaust side of the engine. Properly designed exhaust systems do create sizable power gains.There are some very good programs available for calculating exhaust header systems. Then you can combine the intake and exhaust waves to best advantage. Where the combination of intake/exhaust wave tuning is most valuable is when the piston is at tdc and not yet exerting any draw on the incoming air/fuel column. The pressure differential created by the intake/exhaust wave effect starts the movement of the air/fuel column into the cylinder starts earlier. The added energy imparted causes higher velocity allowing more filling later in the cycle.

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Re: Exhaust and intake resonances

Post by Guy Croft » Tue Mar 26, 2013 12:43 pm

Tim,

you are doing well but there is a lot more to it than that. Did you buy my 2010 book?

I dwelt at length on this very subject.

G
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Re: Exhaust and intake resonances

Post by timinator » Tue Mar 26, 2013 2:55 pm

Yes GC there is much more than that, and I do not wish to write a book on this subject one short post at a time. I was hoping to direct Robert to your book without just saying go buy GC's book. Now however, I have said it anyway. Sorry for the commercial.

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Re: Exhaust and intake resonances

Post by Guy Croft » Tue Mar 26, 2013 3:11 pm

no worries Tim, as I often say we are all friends here, it just happens I own the site as well as write on it so take the 'odd liberty'..!

Keep on writing here as you can, because the subject is so involved and yet so much a part of the subject of race engine prep I myself am torn betw leaving the thread alone and joining in..!



G
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Robert
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Re: Exhaust and intake resonances

Post by Robert » Tue Mar 26, 2013 7:49 pm

Hello,

Concerning the books, I have both. The first one is full of highlights of many colours. It helped me a lot to prepare the head and the engine. Thank GCT (former GCRE).

I take the occasion you give me to give a feedback to Guy concerning the second one. In fact it is difficult (for me) to read it because the writing style is certainly very rich but difficult for a non-English. It is too much "literary" for my technical English. It is not a criticism but just an opinion. The first one was more accessible. Nevertheless I find very useful data inside.

Sorry to digress... I could not imagine the initial topic take us so far!

Robert
GC_131

Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Wed Mar 27, 2013 11:59 am

mmm, Rob, the first book was written in the 3rd person style, the second in the first-person, deliberately.

The second was written in the same style I would use if I was teaching someone at my workshop.

G
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Re: Exhaust and intake resonances

Post by 1NRO » Wed Mar 27, 2013 8:54 pm

Interesting topic, one I have some interest in.

Can I ask a question to those that might fancy offering an answer.

If we can assume a race dedicated engine and an ability to predict with accuracy (haha) the tuned lengths of both intake and exhaust runners what portion of the used rpm would be worth targeting for them to be in sync with each other?

I can appreciate there's not really a correct answer but I'd be interested to hear what people would choose to do if they could.

TIA

Nik
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Re: Exhaust and intake resonances

Post by timinator » Thu Mar 28, 2013 4:24 pm

Hi Nik,

If we can assume a race dedicated engine and an ability to predict with accuracy (haha) the tuned lengths of both intake and exhaust runners what portion of the used rpm would be worth targeting for them to be in sync with each other? [quote][/quote]

There are programs that predict intake and exhaust runner length with great accuracy.

The exhaust system can be made to be quite good for about a 4000rpm spread. The "How To Build Max-Performance Chevy Small Blocks On A Budget" by David Vizard makes this claim on page 124. Although I have other sources that make the same claim, this was easiest for me to find.

The program I use for exhaust tuning gives lengths for best torque or best horsepower. So making a target to have it in sync with intake runner length is fairly straight forward. How you use this information on the track is up to you.

Also keep in mind that where to use maximum power is sometimes a problem. When is power off the corner more important than terminal speed on the straights? When is a cam change required to achieve this instead of advancing/retarding the cam?

The biggest varible is the driver. Some are just better than others. How do you tune an engine for a driver that just spins the tires to much? Or can't keep off the rev limiter?

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Re: Exhaust and intake resonances

Post by Guy Croft » Thu Mar 28, 2013 5:02 pm

Put up some links Tim, could you please?

I have numerous simulation results from Virtual 4 Stroke and would interested in comparisons.

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Re: Exhaust and intake resonances

Post by timinator » Thu Mar 28, 2013 8:00 pm

Wow GC you have Optimum Power Tech's Virtual 4 Stroke! That is like $30,000 worth of programming. I can't believe that you need anything from me. Even Prof. Blair and Associates 4stHead program is only(?)$10,000, which is way more than I am going to spend.

I use Engine Pro by RAS which is an analysis program so I can get a quick idea if the collection of parts people want to put together in an engine are compatible. I gives recommendations for improvements in addition to producing estimated tq and hp curves. It also gives best intake length and volume, header length, collector size, cam timing, etc...

Pipemax by MaxRaceSoftware is the header design software that I have. It gives much more technical information on header criteria.

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Re: Exhaust and intake resonances

Post by tricky » Thu Mar 28, 2013 11:16 pm

A race engine designed for that use will probably spend a lot of time in a fairly narrow range, so I would say it depends on the gearing and wheel speeds as to what ranges are useful. In that situation, I am inclined to add my instinct and say that the inlet and exhaust would be best slightly offset, within that range but with the peak exhaust resonance above, that is later in rpm's as as to be the stronger action. I think (purely from reaserching) the exhaust tuning has a more pronounced effect over inlet waves. That could well be down to the higher temps I guess.

Having put a ton of effort into making exhaust and inlet manifolds to (try) and make use of these effects on a 16v, my ideas have been along the lines of aiming the inlet peak pulse high into the revs at aprox 7000 rpm (1000 short of expected redline). Exhaust tuning is more aimed at aprox 5000 rpm to help bring the torque up. Bit of an experiment, but this is for a road car albeit an extreme one !

One more point to make/open for debate :-). From the reading I have done and help from others I can see that taking advantage of a certain 'length' of runner at one rpm point can have a similar aid at a lower point in the rev range, not withstanding the fact that it will be to a lessened effect based on a different number of wave reflections.

Richard
Twice as many valves

Guy Croft
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Re: Exhaust and intake resonances

Post by Guy Croft » Fri Mar 29, 2013 1:05 pm

I wish Tim!

No I make no claim to have that software but I worked intensively over a 4 year period with someone who did have access to it, we were able to share sim/dyno/rolling-road/flowbench data and amass a great deal of invaluable information.

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Re: Exhaust and intake resonances

Post by timinator » Wed Apr 03, 2013 4:34 am

tricky wrote:A race engine designed for that use will probably spend a lot of time in a fairly narrow range, so I would say it depends on the gearing and wheel speeds as to what ranges are useful. In that situation, I am inclined to add my instinct and say that the inlet and exhaust would be best slightly offset, within that range but with the peak exhaust resonance above, that is later in rpm's as as to be the stronger action. I think (purely from reaserching) the exhaust tuning has a more pronounced effect over inlet waves. That could well be down to the higher temps I guess.


The higher temperature only changes the speed which the waves travel in the exhaust. The exhaust tuning has three benefits it adds to the production of power. The tuned wave created by the properly sized header system, when it reaches the end of the system, sends a negative wave back to the chamber. This negative wave extracts the last of the exhaust gas out of the chamber so the entire mixture at ignition is combustable. Because of valve overlap at this time the negative wave produces a lower pressure than atmo which starts the incoming intake air/fuel with more energy. The negative wave continues up the intake tract until it either runs into a plenum or the end of the ram pipe where it is reflected back down the intake runner as a positive wave. This positive wave adds energy to the incoming air/fuel.
As GC stated producing an intake wave has so many variables planning on producing them at a slight offset if at all would be difficult.

This is my understanding of exhaust tuning. I would appreciate being told if this is incorrect or incomplete.

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Re: Exhaust and intake resonances

Post by Guy Croft » Wed Apr 03, 2013 4:01 pm

extract Chap 9 my 2010 book


"Exhaust pressure basics - and effects:
Exhaust valve opening can be anywhere from BDC (pressure-charged engine) to half-way down the expansion stroke. The cylinder state at EVO depends on firing pressure and varies between engines of different Vs and Vr and depending on exhaust cam timing. The gas pressure can be well over 2bar. One might expect this would cause flow straight to the port at opening, considering that port pressure should be low on an n/a unit (near atmospheric) and, although potentially higher on a turbo unit (1 bar gauge or more - it goes up and down with boost), still well below cylinder pressure. If the pressure ratio (Rp) across the valve is more than 2:1 the flow will choke initially but as the valve lifts higher gas will move into the port at a rate proportional to the valve curtain area. Blowdown has commenced. If the valve rate is fast enough and sufficient residual pressure exists in the cylinder, a primary positive pressure wave as high as 7bar gauge can develop. (Note: Already the danger of too much exhaust lift with a state of high back-pressure should be obvious gas is not going to vent properly. That causes major cylinder heat and contamination problems and will upset Vr and potentially injection even too. If you have high back pressure - you either have high boost with commensurate high cylinder pressure at EVO and your engine runs fine, or it doesn't because the turbine and housing is badly matched).

Gas flows because of differing pressure states between regions. The pressure wave emanating at EVO does not leave a depression behind it as often stated though it is true that it will carry gas particles some distance down the exhaust system. It creates a positive pressure state in the pipework at first and the effect of it motion is no more than that of an ocean wave passing under floating object. The wave will move the object some short distance and that object will then come to rest until the next wave. Gas particles will move in proportion to piston speed only when there is no wave action at all present, which is never. Or maybe when the exhaust valve is shut, and then some. And, unlike the inlet tract where the secondary 'pulse' gives the most benefit (wave travels up and down the tract from valve to rampipe twice after valve opening, trying to use the primary wave would make the tract way too long) on the exhaust side it is the primary wave we're most interested in. And where that really comes into its own is when happens to when it is incident on (sees) the pipe end and change of volume at the 1st collector.

Pressure waves can be of positive or negative type. On reflection at a closed pipe end or change of volume, waves return as 'same sign' or opposite, respectively. And when a negative wave sees open space at a higher pressure, the returning positive wave is of increased magnitude. When the primary wave (the first one propagated) reaches and a change of volume at a collector - the end of the pipe - three things can happen, 1) a proportion of the wave's energy is refracted up adjacent pipes at the first collector, and to a lesser extent the next if there is one, and 2) part of it continues onward and is pretty-well lost, and 3) part of if is reflected back up the pipe to where it came from, ie: toward the cylinder. Phenomenon (1) can be put to work on atmo units with 4-2-1 headers, because a pressure wave will be reflected as same type off a closed valve. Eg: a wave from no. 1's exhaust event can bounce off no. 4's valve, returning in time to give increased pressure around BDC. This is a good thing because it extends and enhances blowdown. Conversely, that reflected wave can cause havoc, especially on units where the primary pipes are very short, eg: on turbo units. Whilst part of the wave incident on the turbine is going to be 'snuffed-out', there may be reflected positive components (turbine entry behaving like a closed pipe) which can still quite easily interfere with another cylinders whose valve is open. Phenomenon (3) is the most significant because the reflected wave lowers the pressure state across the exhaust valve and reduces the pumping loss. That alone can be worth 20bhp on a 180bhp engine because it not only reduces piston work, it gives way better charge density. Moreover it is the only effect you'll capture on a 4-1, unless you make it wildly short in which case all inappropriate will break loose and that I have done. That wave can as easily cross right over the cylinder into the inlet tract, travel right up it to the rampipe and be reflected as a positive. It's not hard to see that under the right conditions it can enhance the inlet secondary. On pressure-charged engines there is no benefit from a negative exhaust wave in the inlet tract because the charge is itself under pressure - all the time on a supercharged engine and certainly in the working powerband of a turbo engine - ramming only works when the returning wave has a column of charge in front of it. In a boost condition the negative wave would simply be reflected.

How easy is it to capture the benefits? It's the opposite of easy. As I said at the front of the book nothing worth having is easy. Try 'easy' and you'll get it all wrong. Take a simple case: a slow-running constant speed single cylinder engine with modest valve timing and an exhaust pipe of say, 3ft length, open to atmosphere. The positive wave at EVO would be reflected at the pipe end as negative. If the pipe length was right, that negative wave would return to the cylinder creating a low pressure zone for the gas to flow to, thus enhancing. It will work provided the valve is still sufficiently open. And, if both valves are open on overlap, that negative wave could equally as well go straight across the cylinder, up in the inlet tract and back down again as a positive, creating a positive pressure ratio across the inlet valve as the cylinder is in depression. Once again, a pressure state encouraging flow, this time charge. But even if the timing is right for all that, remember that wave action dies away over a period, it doesn't just suddenly stop, and even waves with a fraction of the energy of the primary can upset the cycle. Even if the exhaust valve is closed, reverberations can continue in the primary pipes. Now add 3 more cylinders, throw in radical cam timing, join all the pipes together, vary the load on the engine and hold the throttle wide open. Under the right conditions, positive waves can help fill the cylinder and negative ones empty it but predicting that they will work in harmony with cam timing is another matter altogether. Their behaviour is vital to good performance on high speed atmo engines and achieving beneficial states with a good header layout should be considered an integral part of the whole engine package: nothing robs power like a bad system. See what I mean?

Waves travel at the local speed of sound which varies with a temperature condition that is by no means constant along and across the exhaust system. They are attenuated by distance and pipe bore. For best ramming the optimum inlet sound speed is 0.45 Mach (or certainly somewhere between 0.4 and 0.5) which equates to 160m/sec, at 40 deg C and the same optimum is true for the exhaust side: too high and the valves will just choke in their main working range. This is where pipe size begins to look important. On a turbo unit with 800 deg C exhaust gas temperature you could potentially see 656m/sec which is way too high for anything except a superb twin-scroll unit designed for the job..

Even at manageable speeds the waves can pretty well go wherever they like, whenever. They collide and pass through each other, nudge each other out of the way in the most disagreeable terms, either growing or dissipating in the process. Capturing them (in time to be useful) is no easy task and the quest for that Holy Grail, total perfection, depends on an awful lot of 'givens', like bore, valve size, CR, inlet tract length, and those variables open to us without totally rebuilding the motor, including rampipe length and cam timing.

Those things determine the absolute lengths and py/sy ratios on a 4-2-1 or alternatively the length of a 4-1 and the difference between 'perfect' sizes and non-optimised will be a matter of a tenth of an inch, but really and truly you can take a fully optimised header and change the cam timing on the engine it was designed for and find nothing more alarming than that the engine simply has more power/torque 'here' and a bit less 'there' but is nonetheless perfectly suitable to race. If the thing was 'poor' from the outset so will the alterations be also..."

GC
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