Tuned Pipe Wave Velocity

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Timbo Whalen

Well-Known Member
Supporting Member
Joined
Oct 25, 2015
Messages
1,632
Hi-

Has anyone done any research on the wave velocity (fps) and the exhaust gas temperature (EGT), or concluded such

within a reasonable hypothesis, with regard to the high nitromethane %'s we run in these engines?...

The formula for determining the tuned length (TL) of our pipes utilizes a given engine's peak rpm in the equation.

That seems to create the problem of the engine not getting 'on the pipe' in a loaded state.

It seems that using a figure of 75% of peak rpm allows the pipe to hit at approx. 3/4 throttle, and so on.

Be interesting to hear any thoughts that others have, rather than 'it just works best with the pipe set at 11.5" from

center of cylinder to where the diffuser and baffle cones intersect.'... pipes vary in design and it seems prudent

to establish the mean point of reflection within the baffle cone and measure to the piston face at the exh. port.

with regard to tuned length (TL)...

Thoughts, anyone?...

Thanks for any input-

Tim

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Tim there is more to it than just what you are showing in the examples.

The pipe will pull a vacuum in the case all the way back to the carb if it is flowing right.

Intake timing will also effect how a pipe comes on and also port timing and design.

Some eng with the same pipe set at the same length with the same prop will be on pipe as soon as thy hit the water.  

When others will struggle to get on pipe.

So why dose this happen???????? 

Is it  the pipes fault??????

Just some thing to chew on........................................
 
Peak rpm, fuel, exhaust temps are just a few of the points we need to consider when designing a pipe.

As much as we want to achieve peak power at a given rpm, our engines have to lug the prop off the beach, and run through the engines power band (get on the pipe) to hit peak rpm.  Hence a pipe designed for peak power at max rpm will not be the fastest pipe or even a good pipe as you need the engine to be able to accelerate from lower rpm’s to get to peak rpm, i.e. it’s all a trade off.

The vacuum David discuses is the purpose of the diverging cones of the pipe, that section of the pipe creates a low pressure to help draw the fresh air fuel charge into the cylinder.  The diverging cone or cones are what create a high pressure wave that super charges the air/fuel mixture into the cylinders bore just prior to the piston closing off the exhaust port.

As far as getting on the pipe, I think there is even more to it than the power band, the engines dynamics come into play.  I know when I raced shifter karts if I took a took a wrong which resulted in me short shifting and getting the engine out of it’s powerband it seemed to take me several revs through the powerband and shifts to get the engines dynamics back in sync.  I don’t know if that had to do with the carburation becoming out of sync, the crankcase having some extra fuel, fuel starvation, the pipe having less hot gasses or what, but I could definitely tell the process of getting an engine on the pipe has more going on than just pegging the throttle and waiting for the engine to wind out.

Also as David mentions engine mods to timing affect how the pipe works.  So the question is, do you do you match engine mods to match the pipe, or do you mod the engine and design and test handfuls of pipes to match the engine mods???  Then you throw in air temperature, humidity and pressure and add in water conditions for the day.
 
Remember unless you are making your own pipe your hands are tied. 

Cretin mods to the eng will work good on most all pipes. 

I have the mind set that if you let it flow the pipe will work better. 

How hard the cone has to work to draw the case down will affect how much is loaded in it at a cretin RPM.

Intake valve timing will also affect how much you can draw in at a cretin time in the RPM range.

You can only pack back in what you suck out. 

If there is nothing to pack you get nothing.

Have bin seeing some of the affects here in some new engs I have moded .

Changing intake valves  timing will have a big effect on how much you will pack in the cylinder.

Some engs that flow very good will not be able to burn all that fuel and clear out the eng as I cal it.

To much of a good thing is a bad thing some times.

You have to add more plugs to burn it all and clear the eng out.
 
The pipes are made of the wrong material to achieve both power and RPM.  The aluminum dissipates heat so you then need to run shorter lengths to get the RPM and thus lose low end.  This is why the wrapping of pipes has been catching on more.  

I don't have any scientific evidence to back my claim, just results from personal testing. So I'm concluding that under higher temps the pressure wave travels faster.
 
Tommy tape is your friend.  

As the eng loads up at High RPM the pipe will hold more heat and shorten the tune length.

As you slow down and cool off the pipe it acts longer.
 
I don't believe the loss in power is due to the shorter pipe needed for cooler exhaust gasses.  Our engines convert the heat of combustion into mechanical motion.  The more heat we can produce, through the fuel we burn and how hot we can run, the more power we make.  Big hot fire, big power.  If we over cool the engine, or if the pipe cools the exhaust gasses off, then the heat is not being used to produce power and we're wasting the mechanical energy required to pump the air and fuel through the engine.  Insulating the pipe, or running a longer or smaller diameter stinger is a means of retaining more heat in the engine and making more power.

But as with all good things, too much of a good thing is a bad thing.  If we run our engines too hot, we get detonation and we melt pistons.

Even though it is hard to visualize, the tuned pipe is effectively the same device as a turbo supercharger, but with no moving parts.  The reason turbos are placed as close as possible to the exhaust ports and the headers and turbos often wrapped in heat tape is so that the exhaust gasses running through the turbine are as hot as possible as that gives the maximum power to spin the turbine, and subsequently the compressor.
 
Tommy tape is your friend.  

As the eng loads up at High RPM the pipe will hold more heat and shorten the tune length.

As you slow down and cool off the pipe it acts longer.
Yes but you can retain more heat with a different material. 

Heat sinks are made of aluminum and the purpose is to dissipate heat quickly.  We want to hold that heat and use it to our advantage so why would we want aluminum?  Most likely for cost and ease of production.
 
I don't believe the loss in power is due to the shorter pipe needed for cooler exhaust gasses.  Our engines convert the heat of combustion into mechanical motion.  The more heat we can produce, through the fuel we burn and how hot we can run, the more power we make.  Big hot fire, big power.  If we over cool the engine, or if the pipe cools the exhaust gasses off, then the heat is not being used to produce power and we're wasting the mechanical energy required to pump the air and fuel through the engine.  Insulating the pipe, or running a longer or smaller diameter stinger is a means of retaining more heat in the engine and making more power.

But as with all good things, too much of a good thing is a bad thing.  If we run our engines too hot, we get detonation and we melt pistons.

Even though it is hard to visualize, the tuned pipe is effectively the same device as a turbo supercharger, but with no moving parts.  The reason turbos are placed as close as possible to the exhaust ports and the headers and turbos often wrapped in heat tape is so that the exhaust gasses running through the turbine are as hot as possible as that gives the maximum power to spin the turbine, and subsequently the compressor.
The shorter pipe is for a faster return of the pressure wave due to the heat loss from pipe.  If you hold the heat you can run a longer pipe for launch and still achieve the high RPM.
 
Tim there is more to it than just what you are showing in the examples.

The pipe will pull a vacuum in the case all the way back to the carb if it is flowing right.

Intake timing will also effect how a pipe comes on and also port timing and design.

Some eng with the same pipe set at the same length with the same prop will be on pipe as soon as thy hit the water.  

When others will struggle to get on pipe.

So why dose this happen???????? 

Is it  the pipes fault??????

Just some thing to chew on........................................
Dave, well aware of all that, and it's fine if designing a pipe from the ground up.

There must be a relatively mean constant regarding wave velocity, as also a

baseline temperature for EGT.

In today's world, the timing numbers for any given displacement are all quite similar.

Blow down duration with a tuned pipe is, perhaps, most important.

Wouldn't think it would be the pipe's fault, as it's an inanimate object.

Thanks for input, Dave.
 
Peak rpm, fuel, exhaust temps are just a few of the points we need to consider when designing a pipe.

As much as we want to achieve peak power at a given rpm, our engines have to lug the prop off the beach, and run through the engines power band (get on the pipe) to hit peak rpm.  Hence a pipe designed for peak power at max rpm will not be the fastest pipe or even a good pipe as you need the engine to be able to accelerate from lower rpm’s to get to peak rpm, i.e. it’s all a trade off.

The vacuum David discuses is the purpose of the diverging cones of the pipe, that section of the pipe creates a low pressure to help draw the fresh air fuel charge into the cylinder.  The diverging cone or cones are what create a high pressure wave that super charges the air/fuel mixture into the cylinders bore just prior to the piston closing off the exhaust port.

As far as getting on the pipe, I think there is even more to it than the power band, the engines dynamics come into play.  I know when I raced shifter karts if I took a took a wrong which resulted in me short shifting and getting the engine out of it’s powerband it seemed to take me several revs through the powerband and shifts to get the engines dynamics back in sync.  I don’t know if that had to do with the carburation becoming out of sync, the crankcase having some extra fuel, fuel starvation, the pipe having less hot gasses or what, but I could definitely tell the process of getting an engine on the pipe has more going on than just pegging the throttle and waiting for the engine to wind out.

Also as David mentions engine mods to timing affect how the pipe works.  So the question is, do you do you match engine mods to match the pipe, or do you mod the engine and design and test handfuls of pipes to match the engine mods???  Then you throw in air temperature, humidity and pressure and add in water conditions for the day.
Very much understood...however, the topic centered around a baseline tuned length (TL)

for an OEM pipe, as provided by the engine manufacturer.

Thanks.
 
The shorter pipe is for a faster return of the pressure wave due to the heat loss from pipe.  If you hold the heat you can run a longer pipe for launch and still achieve the high RPM.
Basically... at present, we're experimenting with ceramic coatings on/in pipe, header, squish/bowl area, and piston crown.

At this point, the thermal stability is much more consistent once the ideal needle is found.

We're playing with bleed resistor ID now to control inner pipe ambiance and pressure.
 
Basically... at present, we're experimenting with ceramic coatings on/in pipe, header, squish/bowl area, and piston crown.

At this point, the thermal stability is much more consistent once the ideal needle is found.

We're playing with bleed resistor ID now to control inner pipe ambiance and pressure.
Interesting.  How much weight is added form the ceramic coating?

I've had good luck with Carbon Fiber.  Doesn't seem to absorb the heat.  Really have to watch the cooling when using the Carbon pipes though.
 
Last edited by a moderator:
I don't believe the loss in power is due to the shorter pipe needed for cooler exhaust gasses.  Our engines convert the heat of combustion into mechanical motion.  The more heat we can produce, through the fuel we burn and how hot we can run, the more power we make.  Big hot fire, big power.  If we over cool the engine, or if the pipe cools the exhaust gasses off, then the heat is not being used to produce power and we're wasting the mechanical energy required to pump the air and fuel through the engine.  Insulating the pipe, or running a longer or smaller diameter stinger is a means of retaining more heat in the engine and making more power.

But as with all good things, too much of a good thing is a bad thing.  If we run our engines too hot, we get detonation and we melt pistons.

Even though it is hard to visualize, the tuned pipe is effectively the same device as a turbo supercharger, but with no moving parts.  The reason turbos are placed as close as possible to the exhaust ports and the headers and turbos often wrapped in heat tape is so that the exhaust gasses running through the turbine are as hot as possible as that gives the maximum power to spin the turbine, and subsequently the compressor.
Yep... did someone say detonation?... an ounce or two of KL-600 in a gallon of fuel will take care of that.

Will even allow a bit tighter squish (or higher CR) and a hotter plug. All good things- heat is power.

Hotter gasses expand exponentially better. Clausius' 1st Law and Boyle's Law validate.
 
 All the programs work off the same old formulas. Thy all add up the same way just different input parameters is all.

Thy all just predict top power at a cretin RPM.

Did some logging of EGT with my eagle tree set up a few years ago when I was all wound up in it. also had a few pipes made different ways.

Castor oil has more effect on EGT's than the tape will ever have........

Still have the pipes and run the one on my VAC1.05 eng.

If you are deep into it and can do lots of back to back testing you will see the differences. But with out hours of testing it is all just jiber jaber.

Best to try different pipes and land on one you like. Then mod the eng for flow. 

How you drive the boat will have more of a factor on what pipe you like. One pipe will not work as good for one persons driving set up.

How it comes off the corner or how it powers threw the corners or how it tops out at the end of the strait.

I had two different pipes made with two different rear cone set ups. both where 2 stage rear cones. thy both work totally different.

One come on hard in the corners and peters out at the end of the straits and one is steady in the corner and spools up a the end of the straits.

Two different ways to drive the boat or different hulls.

monos need that punch in the corners and don't like to dive in to the corners spooled up.

Hydros like to dive in HARD and maintain in the corners then unload and launch.

So 2 different pipes.
 
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