anyone make their own tuned exhausts ?

[fairchild1934]

what kind of black ninja magic goes into making a tuned exhaust system ? anyone tried it ?
is it any more complicated than just the resonant length of the pipes ?

[tf104]

Clint at Vetterman makes such an awesome exhaust I would not even consider making my own,

[Redneckmech]

Great question, it is a little bit of black magic with few calculations thrown in! While I have not built one, I did my mechanical engineering senior design project on optimization of a single cylinder four stroke 250cc engine and there are a lot of variables to consider. The variables having the largest effect are: Header pipe primary diameter, Header pipe primary overall length, Collector type inlet/outlet diameters, and tailpipe diameter and length. Each bend in the system will have a slight effect on the pressure wave but they can generally be discounted.

With a tuned exhaust system, the goal is to have a negative pressure wave present at the exhaust valve upon opening for the exhaust cycle. This is done by creating a resonance within the exhaust system.
As the exhaust valve opens the high cylinder pressure vents into the exhaust. Just as the valve starts to open it releases a large pressure wave that travels down the exhaust pipe near the speed of sound and as it continues to open, the exhaust gasses begin to flow at just under 1/4 the speed of sound.
As the pressure wave reaches the end of the exhaust pipe it is reflected back towards the cylinder as a negative pressure. If the exhaust is tuned properly and the engine operating at the RPM it was tuned for, will arrive at the cylinder during the valve overlap period and will scavenge the residual exhaust from the cylinder and initiate the flow of intake charge into the cylinder.
The largest factor affecting the negative pressure wave is the primary length so we can set this based on the RPM we want to operate at. For a four cylinder engine we would want to pair cylinders that are 180 degrees apart so we can make most efficient use of the negative pressure wave. For a four cylinder Lycoming we would want to pair cylinders 1-2 and cylinders 3-4 based on the firing order of 1-3-2-4 as those pairs will be 180° out of phase of each other. Now here is where the magic happens!!
Since we have a margin of valve overlap where the intake and exhaust valves are both open we can use the negative pressure wave from #2 cylinder to draw the exhaust out of #4 and visa-versa. Since both intake and exhaust valves have a small overlap, the exhaust being drawn out of the cylinder with the negative pressure wave will draw a clean air-fuel mixture into the cylinder through the intake. If tuned properly, the exhaust system will create a strong enough draw that due to the flow characteristics of the air we can end up with greater than atmospheric pressure within the cylinder since the air cannot stop flowing at the instant that the exhaust valve closes!
The downside is that this will only work perfectly at a given RPM but since GA engines typically are run in a very narrow band we can make much better use of it than an automotive engine. As with everything, exhaust tuning is a series of compromises but we can tune for takeoff power or cruise depending on where we want the greatest efficiency.

The equations for basic tuning are relatively simple:
To determine primary pipe length: P = [(850 x ED) / RPM] - 3 where RPM is engine speed, ED= 180° plus the number of degrees the exhaust valve opens before TDC, P is the primary pipe length if 4-1 manifold or primary pipe length plus secondary pipe length if 4-2-1 type manifold.

Optimal inside diameter of the pipe can be obtained from: ID = sq root [cc / {25 x (P + 3)}] x 2 where CC is cylinder volume in cubic centimeters and P is primary length in inches.

Tailpipe internal diameter is given by: IDT = sq root [(cc x 2) / (P + 3) x 25] x 2

Collector length is given by:CL = [(ID2- ID3) / 2] x CotA where ID2 = diameter of Collector inlet and ID3 = diameter of Collector outlet and CotA = Cotangent of angle of Collector taper



Generally a 4-1 manifold will restrict the power band but give more peak power while a 4-2-1 manifold will give a wider range of operation but roughly 5% less peak power than a 4-1. In a 4-2-1 manifold shorter primaries will increase the top end power but give less mid-range.

Ideally the primarys should come off of the cylinder for about 4" before any bends but this could be difficult depending on your cowling.

If you want a really good book on exhaust systems, look up PH Smith's "Scientific Design of Exhaust & Intake Systems" it is an older book but is still the gold standard for design!

[Bcone1381]

Can you shoot from the hip, apply your data, and roughly evaluate the exhaust photographed below? All three are different photos of essentiall the same exhaust. The engine is a Lycoming O-360 engine. Cylinders 1&2 are tied together with a cross pipe, and 3&4 are connected with a cross pipe. 1.75" diameter primary pipes, 2.0" tail pipes. This is a Vetterman Standard Cross Over exhaust that Clint makes available for us Bearhawk folks with that engine.

Screenshot 2024-07-05 at 1.24.36 PM.png Screenshot 2024-07-05 at 1.31.03 PM.png Screenshot 2024-07-05 at 1.31.19 PM.png

[fairchild1934]

OK redneck--- can we ask Jared to make you reply a permanent stick for future reference ? where else we donna get that kind of detail. :-) Awesome work. That looks like it would be similar math to what runs a pulse jet--- as it uses the negative reflected pressure wave to draw in fuel and air.

I wonder if anyone has thought of a way to slightly adjust the resonance with a throttle type cable so it could be RPM selected in the cockpit ?
Like just have a pipe on the end that slips an inch or two to vary the resonant length (like a slide whistle )
maybe use a small rack and pinion ---- so you could spin the cable and make fine adjustments at the end of the pipe.
as the length got linger - the round trop time would get longer which should equal a lower RPM.......

[Redneckmech]

Precisely! A pulse jet is working on exactly the same principal.

While a variable pipe could make a slight difference, I doubt that it could be noticed by the butt-dyno or on the tach and the complexity and maintenance of a part operating at exhaust temps may not be worthwhile.

[fairchild1934]

PS---- that's some mighty sexy bending and welding he does ! WOW !

[svyolo]

Since I moved into a hangar this past year, getting ready to fly, the local pilots come by and oooh and aaah my kit build. Not the basic airframe, but some of the engine compartment. Maybe the exhaust the most, but also a few other bits. They can't buy a Vetterman exhaust for any price, and pay 10X the price of my Summit Racing remote oil filter housing. There are a few parts I bought I had huge buyers remorse over the price, maybe the tail wires and wheels/brakes the biggest. A few have said they have never seen new engine baffle rubber. One Southwest pilot with a 182 spent more than my exhaust to repair his 182 exhaust.

Clints exhaust is an easy check writing exercise. Absolutely beautiful and fits like a glove. I also just had to dissassemble one side to heli-coil an exhaust stud and weld on a Anti-Splat part and it is stupid easy and quick to take apart and put back together. A+

[Redneckmech]

Can you shoot from the hip, apply your data, and roughly evaluate the exhaust photographed below? All three are different photos of essentiall the same exhaust. The engine is a Lycoming O-360 engine. Cylinders 1&2 are tied together with a cross pipe, and 3&4 are connected with a cross pipe. 1.75" diameter primary pipes, 2.0" tail pipes. This is a Vetterman Standard Cross Over exhaust that Clint makes available for us Bearhawk folks with that engine.

Based on the photos, that appears to be a crossover exhaust rather than a true tuned system. It is also acting as a 4-2 system so the calculations will effectively be for a 2-1 system (half the engine RPM since they do not join together).
All that aside, it likely has some degree of tuning since unequal length primaries can be designed to use a harmonic of the optimal primary length but doing such will always result in less power than a true equal length system. Essentially, since the negative pressure wave is constantly being reflected back and forth within the pipe, the length can be tuned to use either the 1st reflection, 2nd reflection, 3rd reflection etc.. With an unequal length tuned system there will be less of a balance between the longer primary and the shorter primary since they will be using different reflections but there is still a net gain over a completely un-tuned system.
If I am remembering correctly, most automotive headers use the 2nd reflection, though the 1st reflection is stronger, the resulting pipe length will not fit into any vehicle.

A true tuned exhaust is difficult to package under a cowling due to the need for equivalent length primary header pipes of specific length and thus will be heavier due to the extra material needed. A good example of one can be found in the system made for the Glasair sportsman pictured below (photo source: Kitplanes) or in the systems Power-flow manufactures for a variety of aircraft.



I can try to run some rough/guesstimate numbers on it, but it may be a couple of weeks since I just got back from a family reunion and will be riding the Idaho Backcountry Discovery Route with a group of friends starting later this week and have way too much to catch up on in the meantime...