Trumpet diameter

[Birel99]

When determining inlet trumpet diameter, would you want the largest diameter alowed by the rules?

At 18K+ RPM, I assume a venturi will be to restrictive to provide any gains?

Regards,

[G-Rock]

The diameter should be the same as the valve area or slightly larger. Length is more important. Short trumpets for higher top end and long trumpets for more low end power.
G

[ISLAMATRON]

So the trumpets will be getting a little longer this offseason

[ackzsel]

So the trumpets will be getting a little longer this offseason

Are they allowed to change that?

[ISLAMATRON]

So the trumpets will be getting a little longer this offseason

Are they allowed to change that?

Yes those and the injectors

[Ciro Pabón]

Sorry for intruding into something is not my field. I also ask for forgiveness for any barbarisms, I've been reading about it for three months, so I have no knowledge but "dangerous ignorance"...

However, I'm anxious to test my new knowledge on the "forum-test", to learn a bit, if someone contributes and corrects this post.

I become involved into the tuning of a motorcycle, so I "discovered" the Helmholtz theory.

The idea is that you can model the piston and the moving mass of air as a spring-mass system, known as a Helmholtz resonator. I'm fairly used to this, as is the base of the asphalt roughness measurement (modelling the suspension in a similar way), but I digress.



The first thing that seem curious to me is that the air behaves like a spring, with a "stiffness" (if I compare the equations to the ones of a suspension) given by:



where a is the speed of sound, A is the area of the piston, is the density of air, V is the volume of displacement of the piston. Thus, the stiffness of air is proportional to the speed of sound! Beautiful.

Now, if you plug the frequency of the system into the basic equations, for a trumpet length L, you arrive to:



Which I believe is the basic equation for trumpet length L. I used it (rather naively, I imagine) to define the tuning I talked about. Notice that the frequency you have to use, is the one of the piston, same goes for the volume V and the area A.

In the end, it's clear from the equation (I hope) that for faster traveling waves, a shorter pipe is needed. I wonder if you could construct a manifold with an "extra path". This extra-path could be controlled by a valve, which could be opened or closed depending on RPM. Has this ever been tried? Is it legal? Would it be useful?

On the other side, I read about "organ pipe theory" (but I did not used it).

What I got from organ pipes is this:

When the cylinder goes down, the pressure in the intake drops, thus an "expansion wave" is produced. This wave travels through the intake up until the junction where all the individual intakes begin. There, the wave "sees" a larger area and it reflects. This reflected wave becomes a compression one. This compression wave travels down the intake, "in reverse". The key is to dimension the intake in such a way that this compression wave arrives to the intake valve right when the valve closes, so this wave will help to "fill" the cylinder. Waves move at the speed of sound, so for you to get this effect you have to be very precise about the length of the trumpet.

At the exhaust, everything seems to happen in reverse: the geometry of the exhaust creates a reflected expansion wave that helps to empty the cylinder.

Am I wrong?

So, to summarize, I agree with G-rock: area should be more or less the same as the valve area (so you do not restrict the flow) and lower RPMs need longer trumpets.

[mep]

Hi Ciro,
Great post you are the best again.

With this equation I tried now to calculate the trumped length.

L= (a²*A)/(4*π²*V*f²)

With the f as the frequency.
In Ciro's equation is the angular frequency used.

I took:
a=343 m/s
A=π*d²/4= 4,9 *10^-4 m² d=25mm rough guess
f=300 1/s
V=3 *10^-4 m³

And I got L=5,4cm
Is pretty short but sounds reasonable.

[donskar]

Ciro wrote:

In the end, it's clear from the equation (I hope) that for faster traveling waves, a shorter pipe is needed. I wonder if you could construct a manifold with an "extra path". This extra-path could be controlled by a valve, which could be opened or closed depending on RPM. Has this ever been tried? Is it legal? Would it be useful?

Not only in use, but quite common. The Toyota Tundra (and many other production vehicles) has a dual path intake manifold. Probably far too complex for F1. LMAO

On the other side, I read about "organ pipe theory" (but I did not used it).

What I got from organ pipes is this:

When the cylinder goes down, the pressure in the intake drops, thus an "expansion wave" is produced. This wave travels through the intake up until the junction where all the individual intakes begin. There, the wave "sees" a larger area and it reflects. This reflected wave becomes a compression one. This compression wave travels down the intake, "in reverse". The key is to dimension the intake in such a way that this compression wave arrives to the intake valve right when the valve closes, so this wave will help to "fill" the cylinder. Waves move at the speed of sound, so for you to get this effect you have to be very precise about the length of the trumpet.

LJK Setright has a brilliant explanation of how harmonics applies to high performance exhaust systems in The Grand Prix Car 1954-1966‎. I use the term "brilliant" NOT because our more technically astute readers will learn from it, but because failed engineers like me can understand it! (He even explains the theory as it applies to reverse cone exhausts often used on small displacement engines.

[Carlos]

I know I've mentioned this reference book before; but I must again, it's really that good.

Scientific Design of Exhaust and Intake Systems by Philip H Smith

It isn't the final word on the subject. Maybe the first

Best price would be a used copy from an internet bookseller.

Wrong Ciro? No, your right. An intake trumpet is the same size as the valve as is the intake port. IMO

EDIT - That would be true for a single cylinder 2 valve head. Which is my sentimental default, as my first motorcycle was a Ducati 250cc single.

[lkocev]

In the end, it's clear from the equation (I hope) that for faster traveling waves, a shorter pipe is needed. I wonder if you could construct a manifold with an "extra path". This extra-path could be controlled by a valve, which could be opened or closed depending on RPM. Has this ever been tried? Is it legal? Would it be useful?

To answer your question Ciro yes it has been tried, and yes it has been banned. This is useful because it would help make a maximum torque curve throughout the rev range, some production performance automobil's have these type of variable runner length systems. It think so BMW had one a few years ago that was not like what you described as a valve changing runner length, but that it was contiuosly variable, in other words it had a minimum and maximum runner length and anything in between, I'll see if I can find details....

[riff_raff]

Birel99,

I've never seen a rule that limited the inlet "trumpet" diameter. The purpose of the trumpet shaped bell mouth is to provide a non-turbulent and smoothly accelerating airflow into the intake runner. The point of minimum flow area, and thus highest flow velocity, will be at the annular area between the valve and valve seat. So the inlet runner and bellmouth should be shaped to provide high intake charge mass inertias at the valve, to maximize the "ram effect". But to do this efficiently you should also strive for as low an acceleration rate in the intake flow along the length of the runner as possible.

Volumetric efficiency can also be improved significantly at certain operating frequencies (but also hurt at other frequencies) by taking advantage of acoustic reversion effects in the intake runner. To maximize the reversion's acoustic signal, the flow passage should have a very abrupt change in cross-sectional area. However, this is not usually good for flow. The bellmouth shape has been found to be a good compromise.

Regards,
Terry

[timbo]

I think that considering that airbox creates additional pressure when vehicle goes at max speed, trumper length must be greater (since velocity of sound is higher at higher pressure)?
Anyone knows whether this effect is considered when choosing trumpet length?

[ISLAMATRON]

I think that considering that airbox creates additional pressure when vehicle goes at max speed, trumper length must be greater (since velocity of sound is higher at higher pressure)?
Anyone knows whether this effect is considered when choosing trumpet length?

yes it is, for F1 at least.

[Conceptual]

In the end, it's clear from the equation (I hope) that for faster traveling waves, a shorter pipe is needed. I wonder if you could construct a manifold with an "extra path". This extra-path could be controlled by a valve, which could be opened or closed depending on RPM. Has this ever been tried? Is it legal? Would it be useful?

To answer your question Ciro yes it has been tried, and yes it has been banned. This is useful because it would help make a maximum torque curve throughout the rev range, some production performance automobil's have these type of variable runner length systems. It think so BMW had one a few years ago that was not like what you described as a valve changing runner length, but that it was contiuosly variable, in other words it had a minimum and maximum runner length and anything in between, I'll see if I can find details....

That wouldn't be a Trumpet.... That would be a Trombone!

[Henning]

Check out the Mazda 787B that won Le Mans in 1991. It has variable length input trumpets. I don't have a video to hand to post, but it looks very cool to see the trumpets moving up and down.