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Discussion Starter #1
Just got done with the first prototype of my 50mm throttle bodies, im kinda saticfied with the result sind i had minimal with tools. I used at hand held file, a drill and a steel cutting sawblade.
Its a mix up of sport classic and hypersport throttle bodies and som home made parts.
The bike started right up and ran great during idle with out blower motor or idle rpm thingy. And responded well to gas blips but it will go on the dyna some day januari.
The pics are a bit blurry but they are taken with a Nokia in bad light.

Nex run in the dyno will be with theese and termignoni spaghetti headers.







 

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Okay, I'm going to show my ignorance here :eek: so please be kind ...... is there a benefit in increasing the size of the throttle bodies if you do not increase the intake valves at the same time?

NN
 

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Theoretical improvement in top end with the right exhaust, but you're right that it would also take larger valves and porting to get the full available benefit.
 

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i think its great . i just purchased some myself. thinking outside the box what i like to do . major i think you will be proven wrong . when the cylinder clears and intake ocurrs it should improve the charge for better combustion. its the same as opening the airbox or putting stacks on the throttle boddies
 

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Theoretical improvement in top end with the right exhaust, but you're right that it would also take larger valves and porting to get the full available benefit.
I also think that the fairly short intake stacks would favor the top end as well. Of course, you'd have to add fuel to get more power as well, not just more air.
 

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i think its great . i just purchased some myself. thinking outside the box what i like to do . major i think you will be proven wrong . when the cylinder clears and intake ocurrs it should improve the charge for better combustion. its the same as opening the airbox or putting stacks on the throttle boddies
Yes, but there is a limit to the benefits to opening up the intake. Without more fuel, no more power is going to be made. I think the biggest advantage of this arrangement is that the intake is now more straight than with the air box. Eliminating bends in the intake allows the resonate tuning to function far more effectively. The pressure waves in the intake do not flow around bends very well. This has been shown to be true in horn loudspeaker design, for example, where the expansion has to be folded to make it fit into the living room. If ordinary bends are used, then the pressure waves from the driver get lost. If the waves are thought of more as light beams, you can see why this would be the case. They get lost in the bends. So, the best solution for folding up a horn, or an intake tract, is to use reflectors instead of bends. I hope this makes some sense. The best of course is a straight path.
 

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Discussion Starter #10
First of all this is just a experiment since i like to learn and teory is one thing and trying is another so i like to try.
I bought the 916 throttle bodies on ebay for nothing 70€ and then some skrews and bolt 10€.
The reason for me trying this out is that i have gone through the whole engine cams, porting, valves, bigbore, high comp etc. And the engine is killer but i think there is some more to gain. One big plug in the system is the exhaust that is 36mm inner dia which is to small for an tuned engine, i have now gone up to 45mm. Recommended is same size as exhaust valve plate and for tuned engine a couple of mm bigger.
The intakes are 50 mm at the top but 46,5 mm at the bottom where it meets the engine, that isnt taht much bigger then stock so i gues ther wont be a loss in torque.
I will run the bike in the dyno and adjust the ecu off course.
I allso thought about the bigger intake and small valves, seems like there should be some kind of restriction there and the bigger intake will go lost. But there is mor then just size, smaller hole flows faster etc etc. I can tell you that where the engine meets the black sock to the intakes the whole is smaller then valve plate size.

I ran the bike with the stock throttle bodys only, no bell mouth, no black bend that goes up to the airbox and got.

Hp: 98,79 hp
torque: 96, 42 nm

With the black bends and with custom intake bells 52mm high and wide mouth style, no airbox and mapping.

hp: 102, 67 hp
torque 102, 39 nm

There are several things that can happen.

1. I loose torque due to the to big diameter for the engine.
2. They are to short for the cams specs, i will try longer intake bells aswell.
3. Get a high hp number but a bad curve and low torque.
4. By eliminating the bends i get a downdraft kinda intake(good).
5. They work out great and i get an improvment.
6. Prabably ten more things that might happen.
7. I think these will use the 1-2 grade harmonics instead of 3-4 that the normal intakes use wich are stronger and gets 4-5% more boost. (dont know about this one just read and calculated some) (bad)
 

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Your expectations are very realistic. As you appear to be completely aware, how you choose and tune all the other components can yield various results on your list.
 

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Discussion Starter #12
I hope for a gain but if the result isnt what im expecting ill just go back to the stock throttle bodies again.

I have done plenty of reserch on intakes, exhaust and done some calculations and with the new throttle bodies and exhaust it seems to match up quite good in lenght etc.

Cant wait for the dyno.
 

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Can someone help me out here? I'm pretty familiar with acoustics and an engine intake system is very simply an acoustic circuit. However, all the engine tuning references to velocitys stacks t^hat I have looked at have a different terminology and I can't figure out what is going on.

In all the acoustics texts I have read (and I have read all the classics), there is an assumption that a tube less than a quarter wavelength long behaves as a pure mass/resistance--ie. the resonances due to internal reflections are so small as to be effectively ignored. From reading the engine tuning references, though, it seems that tuners are worried about resonances even in pipes shorter than 1/4 wave.

Looking at the numbers from the link above:

Second Pulse Length: =108000/RPM
Third Pulse Length: =97000/RPM
Forth Pulse Length: =74000/RPM
Fifth Pulse Length: =54000/RPM

The Second Pulse works out to an 1/8th wave resonance at an air temp of 150 degrees Fahrenheit, the Fifth Pulse works out to a 1/16th resonance. The Third and Fourth Pulses must be some odd combination of even lower order reflections that are close in proximity.

Now, the acoustics of an intake tract together with a plenum (air box) are totally different. The resonance here is not due to internal reflections in the tract, but rather is a mass/spring (helmholtz) resonance where the air in the intake tract is the mass and the air in the plenum is the spring. Here we are relying on the fact that the tract is short enough that internal reflection resonances can be effectively ignored.

The idea of using a tuned intake system is to reduce the acoustical impedance of the system at the rpm of the resonance. The tradeoff is that you are increasing the impedance elsewhere. However, because the resistance of the system increases with velocity and so the system is limited at the high end, usually you are willing to tradeoff higher impedance at low rpms for lower impedance at high rpms.

Now, I haven't done detailed calculations, but it seems to me that you can get a much, much lower acoustic impedance at that resonance frequency from a helmholtz resonance (air box and intake tract) than you can with the resonances of the intake tract only. This seems to be backed up by the fact that racing teams all use air boxes. So apart from aesthetics, why would you not want to use an airbox? And don't sweat the Pulses--get the Helmholtz resonance right.

Now, just want to repeat--I am familiar with acoustics, not engine tuning so these statements are meant as provocative questions, not deep insights into engine tuning.

regards, John
 

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Discussion Starter #16
Can someone help me out here? I'm pretty familiar with acoustics and an engine intake system is very simply an acoustic circuit. However, all the engine tuning references to velocitys stacks t^hat I have looked at have a different terminology and I can't figure out what is going on.

In all the acoustics texts I have read (and I have read all the classics), there is an assumption that a tube less than a quarter wavelength long behaves as a pure mass/resistance--ie. the resonances due to internal reflections are so small as to be effectively ignored. From reading the engine tuning references, though, it seems that tuners are worried about resonances even in pipes shorter than 1/4 wave.

Looking at the numbers from the link above:

Second Pulse Length: =108000/RPM
Third Pulse Length: =97000/RPM
Forth Pulse Length: =74000/RPM
Fifth Pulse Length: =54000/RPM

The Second Pulse works out to an 1/8th wave resonance at an air temp of 150 degrees Fahrenheit, the Fifth Pulse works out to a 1/16th resonance. The Third and Fourth Pulses must be some odd combination of even lower order reflections that are close in proximity.

Now, the acoustics of an intake tract together with a plenum (air box) are totally different. The resonance here is not due to internal reflections in the tract, but rather is a mass/spring (helmholtz) resonance where the air in the intake tract is the mass and the air in the plenum is the spring. Here we are relying on the fact that the tract is short enough that internal reflection resonances can be effectively ignored.

The idea of using a tuned intake system is to reduce the acoustical impedance of the system at the rpm of the resonance. The tradeoff is that you are increasing the impedance elsewhere. However, because the resistance of the system increases with velocity and so the system is limited at the high end, usually you are willing to tradeoff higher impedance at low rpms for lower impedance at high rpms.

Now, I haven't done detailed calculations, but it seems to me that you can get a much, much lower acoustic impedance at that resonance frequency from a helmholtz resonance (air box and intake tract) than you can with the resonances of the intake tract only. This seems to be backed up by the fact that racing teams all use air boxes. So apart from aesthetics, why would you not want to use an airbox? And don't sweat the Pulses--get the Helmholtz resonance right.

Now, just want to repeat--I am familiar with acoustics, not engine tuning so these statements are meant as provocative questions, not deep insights into engine tuning.

regards, John
I know you can calculate and read, i have done plenty and it all comes down to who har written the text and formula for calculating intales and exhaust. There is a BIG diffrence in using John b Heywood and another guys formula.

I base all my results on the dyno session i did, ran the bike for a almost a day with airbox, no airbox, with wide mouth bells, short, long and noting at all just a throttle body.

And if you got a good number with example 50mm wide mouth bells and then put the airbox on it dropped 2-3 hp and same for torque. it allways showed a better curve all over without airbox.
Then i should allso point out that the airbox im running is using a flat panelfilter for more air intake. The stock one is so bad even with dp lid etc.

I really do belive that a good big enoght airbox is the best way to go or else the racebikes wouldnt have used them. But fitting an airbox in our frame that has enoght volume can prabaly be hard, maby if you got a good ram air system.... jada jada jada it would be great but that is much much work to get the right airbox.
I will run the bike with "pods" first and if it gets me good numbers and i decide to keep the throttle bodies i will make an simple airbox and see what happens.
 

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Can someone help me out here? I'm pretty familiar with acoustics and an engine intake system is very simply an acoustic circuit. However, all the engine tuning references to velocitys stacks that I have looked at have a different terminology and I can't figure out what is going on.

In all the acoustics texts I have read (and I have read all the classics), there is an assumption that a tube less than a quarter wavelength long behaves as a pure mass/resistance--ie. the resonances due to internal reflections are so small as to be effectively ignored. From reading the engine tuning references, though, it seems that tuners are worried about resonances even in pipes shorter than 1/4 wave.

Looking at the numbers from the link above:

Second Pulse Length: =108000/RPM
Third Pulse Length: =97000/RPM
Forth Pulse Length: =74000/RPM
Fifth Pulse Length: =54000/RPM

The Second Pulse works out to an 1/8th wave resonance at an air temp of 150 degrees Fahrenheit, the Fifth Pulse works out to a 1/16th resonance. The Third and Fourth Pulses must be some odd combination of even lower order reflections that are close in proximity.

Now, the acoustics of an intake tract together with a plenum (air box) are totally different. The resonance here is not due to internal reflections in the tract, but rather is a mass/spring (helmholtz) resonance where the air in the intake tract is the mass and the air in the plenum is the spring. Here we are relying on the fact that the tract is short enough that internal reflection resonances can be effectively ignored.

The idea of using a tuned intake system is to reduce the acoustical impedance of the system at the rpm of the resonance. The tradeoff is that you are increasing the impedance elsewhere. However, because the resistance of the system increases with velocity and so the system is limited at the high end, usually you are willing to tradeoff higher impedance at low rpms for lower impedance at high rpms.

Now, I haven't done detailed calculations, but it seems to me that you can get a much, much lower acoustic impedance at that resonance frequency from a helmholtz resonance (air box and intake tract) than you can with the resonances of the intake tract only. This seems to be backed up by the fact that racing teams all use air boxes. So apart from aesthetics, why would you not want to use an airbox? And don't sweat the Pulses--get the Helmholtz resonance right.

Now, just want to repeat--I am familiar with acoustics, not engine tuning so these statements are meant as provocative questions, not deep insights into engine tuning.

regards, John
I am familiar with acoustics also. I designed my own bass horn. It is a 70 Hz straight horn with hyp-ex expansion.

Velocity stacks are Helmholtz resonators. They are not for matching impedance. The intention is to increase the amount of air entering the intake valve by setting the length such that a resonance occurs at a desired frequency. I have been told that NASCAR engines are designed to match the third harmonic. At resonance, the reflected pressure wave down the runner to the intake valve will hit the valve just as it opens, increasing the pressure at the valve and thereby adding more air. The bell mouth on the stack is merely to smooth the air flow into the stack. They do not use horn theory to design stacks, though I think that would be an interesting idea.

Racers often use a megaphone on the exhaust. This is a conical horn. I do not know their intention with that, but it appears to work. I am curious what would happen if the megaphone was replaced by a hyp-ex horn or something like a tractrix.

Helmholtz resonance occurs in pipes also, not just boxes, from what I understand. If you look at the back side of a loudspeaker with a vented bass driver, you'll see what amounts to a velocity stack in the hole.

For a 4 cycle engine, you divide the desired rpm by 4 to get the intake frequency. So, if say the desired engine speed is 10,000 rpm, then the intake needs to be tuned to 2,500 Hz, or a harmonic thereof. The harmonics will be weaker, yes, but to put a full size intake tract on the engine is usually not practical. Also, just as a straight horn loudspeaker is better than a folded one, an intake runner that is straight is better than one with elbows in it. The pressure waves will be much stronger with a straight path.

The downside to using a Helmholtz resonator is that it has a narrow peak. This is handled by some manufacturers by using a variable length intake runner. They have a motor to move part of the runner to make it longer or shorter.
 

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I may be wrong about the intake runner being a Helmholtz resonator. This is just what I was told by the man who owns that web site. Here's an article on velocity stacks, from a link in Wikipedia:
http://www.profblairandassociates.com/pdfs/Bellmouth.zip
I have not read this yet.

I glanced at it and they appear to be only concerned with the steady state inflow characteristics and not the resonant frequency of velocity stacks.

I look at the reciprocating intake valve as being equivalent to a loudspeaker driver. So, the intake runner is just a tube of constant diameter with a flare at the end connected to a driver. It should have some kind of resonant frequency that can be used to advantage to get more air into the engine.
 

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Discussion Starter #19
This is exactly what i mean:D

You can talk and discuss this for houers days and years and still come up with nada. Everybody has his own recepy.

What is said in text doesnt always traner over to reality.

trail and error mixed with some light math and reading is my way.
 

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I really do belive that a good big enoght airbox is the best way to go or else the racebikes wouldnt have used them. But fitting an airbox in our frame that has enoght volume can prabaly be hard, maby if you got a good ram air system.... jada jada jada it would be great but that is much much work to get the right airbox.
I will run the bike with "pods" first and if it gets me good numbers and i decide to keep the throttle bodies i will make an simple airbox and see what happens.
If you wanted to use an airbox, then you would use a single velocity stack attached to the inlet to the box, and then have the two runners to the engine attached to the box. This is an example for a 4 cylinder engine:
 

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