Exhaust: how close is too close?

We planned to use an existing crossover-exhaust that was previously fitted on another RV-4. It suits us well, because both crossover pipes are in front of the oil sump and keeps us the space behind the oil sump free of exhaust pipes. The todays Vetterman crossover exghaust has one pipe in front and one behind the oil sump.

As you can see in the pictures below the exhaust pipes have low clearance in some places.
- Pic 1: approx. 0.12 Inch between each other and between a pipe and the oil sump
- Pic 2: approx. 0.16 Inch between the pipe and the oil fitting (used for inverted oil system)



For those having experience with similar installations: is this too close? Especially the clearance between the pipe and the fitting? There is no space to place a heat shield in between.

Thanks for your advice

Andreas, there is no absolute "yes or no" answer. The pipe will heat the oil fitting and sump, but only measurement will tell how much.

Just a data acquisition thought

That angle is a tough spot. I have one for the same purpose but mine is 1/2" away from the ex pipe. The concern is mostly that there is no oil flow and we don't want coking within the fitting. A data suggestion - just thinking out loud. Leave off the connector hose for a data flight. Then to measure the temp, consider adding a stub so you can stick the thermocouple probe (1/8") into the center of the elbow. That way it will be surrounded by oil (appropriately stagnant). If the temp is not frying the oil then OK. Then put the full oil return back on after the data run. Likely heat transfer back to the sump through the body of the fitting will be greater than the oil, but the stabilized temperature is the question as it relates to the oil and hose attached near by.

The downside of being too hot would be coking and plugging at the fitting, or getting the hose too hot at the fitting end.

Please post the results if you do this to quantify the distance question for the masses.

Hi Andreas, what you have is a very old "Tolle" crossover exhaust. It was likely built back in the late 80's.
Affording to what Larry Vetterman has told me, it was common practice back in the day to put a dent in the pipe, or mill the sump back slightly to make the exhaust fit better. The refusal by Larry to do either of these was what got him started in the rv exhaust business years ago.
While you shouldn't have an issue with exhaust heat due to proximity to the sump or the fitting (my opinion) the Tolle exhaust give up considerable performance do do its pipe connection orientation, i.e. 3 goes to 4, 2 goes to 1. This is what allowed tolle to put both pipes in front of sump.
The reason one pair of pipes on the Vetterman crossover goes behind the sump is because of tight clearance on the 4, and it is a more efficient flow design, i.e. 4-3/1-2. Hope this helps. Clint

Can you wrap the exhaust pipes (fiberglass exhaust wrap) and then also fire sleeve the oil line and elbow?

One of the fittings on the inverted system has very little flow it; is a stand pipe going above the oil level in the sump with drain back oil from the separator.
The other sump fitting has continuous oil flow while right side up; it is the oil pick up for all operations, except while inverted. This oil pick up fitting would be self cooling with little worry while inverted because inverted durations are comparably very short.

I'd also worry about rubbing/rattling from engine vibration.

Understand the physics

Understand the physics first. Without having to solve the mathematics or model the physical system you can gain insight into how the system will respond by understating what is going on and what parts of the physics are linear verse the parts that are non-linear. There are many non-linear things taking place here and the human brain in not adaptive to non-linear phenomena.

The maximum heat flux W/m^2 at which radiation may be transmitted is given by the Sefan-Boltzmann law q''=?T^4 where ? is a constant and T is the absolute temperature to the fourth power. The energy to the surroundings is q=eA?(T1^4-T2^4) where e is the emissivity, A is the surface area, T1 and T2 are the different surface temperatures. Another concept is the view factor, it is geometry specific and difficult to determine, but looking at simplistic orientations from a point source radiation it will generally result in an equation of the type of Area/(Area+4*L^2) where area is the receiver and L is the distance between surfaces. The point being that it is non-linear and falls off with quickly with distance. Surface reflectivity and emissivity all also have an effect on energy transmission.


I would suspect that you may have an issue.
Solutions? Increase distance, use an radiation shield with an insulator if possible. A radiation shield could be made from stainless steel and some hose clamps (look at the heat exchanger design concept for the cabin air.) This is a very similar design issue that is seen on cars near the catalytic converter. The temperature on the catalytic converter can reach 2000F surface temperature and be in close proximity to the floor. Automobile manufactures use a heat shield with a reflective surface with an insulator on the back side. As the temperature of the reflective surface goes up, so does it's ability to radiate heat away, the insulator increase the temperature of the reflective surface. I would model the solution after the automotive design. A good piece of asbestos sure could come in handy here . Short of some asbestos, go to the salvage yard and see if you can obtain some material for a heat shield.
You can also test your paint and see at what temperatures it starts to degrade. IF it starts to yellow, that will also be a good indication of how hot it is. Not sure of what the material is under the paint, but if it is aluminum, you may lose the heat treated properties before the paint yellows.

Thank you for all your inputs. For instance, I am not in the operation phase yet where I could take measurements. But it's a good thought.

Clint: Thank you, I was always wondering what type of exhaust this is. And now, I finally understood the difference between this one (Tolle) and the Vetterman crossover. Changing the exhaust type would mean changing the entire layout in the lower engine compartement. We have a Rotec TBI on an O-360-A1A. And not having a pipe behind the oil sump came in handy because of the available space for controls and fuel lines without having exhaust pipes too close. A 4-pipe is unfortunately a "no go" in Switzerland, unless we reunite 2 pipes and add a Liese muffler.

Yep, more distance would be the best choice (theoretically & practically), but will involve quite some changes.
The idea from the catalytic converters of cars seems interesting. I will check this out.

By the way, we already use integral sleeved oil hoses. Might add additional shielding, where possible. But needs to be primarely radiation shielding.

The concerned fitting is the oil return fitting in the picture:
http://musclebiplane.org/image3/invert4.jpg