Many small engine manufacturers use exhaust backpressure specifications for emission conformance guidelines. Such specifications are often given to a muffler manufacturer for use in designing an exhaust system.

There does not, however,  appear to be any consistancy between engine manufacturers (or suppliers of mufflers)  as to the procedure used for measuring back pressure. We've seen measurements made in the first chamber of the muffler, in manifolds at locations immedately downstream of the exhaust ports, in the bends of exhaust manifolds, with different probes, in mid stream, at the tube walls, etc.  We've seen drastic differences in measured backpressure depending upon small changes in location of the measurement.  In twin cylinder engines there can be large differences in back pressure readings between cylinders (with similar manifolds).

This  presents the question for general discussion. Can the forum share some good techniques for taking back pressure measurements? Are there accepted practices and advice for making pressure measurements in manifolds? Since measurement technique appears to affect the results so dramatically, can there be any validity in back pressure specifications for assuring that an engine conforms to emission standards?

When I worked for a major diesel engine manufacture in their engine lab, we used two types of probes.  The best probe was inserted into the middle of the exhaust stream with a right angle bend that faced directly into the flow.  This kind reads stagnation pressure directly.  The second type was just a round hole flush with the exhaust wall.  With this type, you have to know the flow rate, temperature and cross sectional area so you can calculate the stagnation pressure.  It is generally less accurate but is easier to install and and less disruptive to the flow.  

In any case, for best accuracy you want to be in a straight section at least a couple of diameters long.  Since these were big engines in test cells, it was pretty easy to get repeatable measurements of backpressure.  I expect it may be a lot tougher if you are trying to measure a chainsaw mufflers backpressure.

As clarification, most engines under 10 hp (including chainsaw engines (about 1 hpScrew pumps) will have mufflers that bolt directly to the external engine port (no manifold).Back-pressure measurements are made in the first cavity of the muffler and measurement location is generally loosely defined.  There has been little discussion whether the measurement should be made near the cavity walls, in the middle of the cavity, or what influence the flow has on the measurement.

Engines over 12 hp generally have mufflers with pipe manifolds. Such manifolds are typically 1.125 inch diameter tubing, rougly 1  foot long with one or two bends in them. The first bend almost always starts wihin an inch of the engine exhaust port. When the measurements are made in the manifold, the effect of flow is continually a subject of discussion as is the effect of tube bends, and method for measuring pressure.

In the past, small engine manufactures often allowed the end user (for example a lawnmower manufacturer) to retrofit the engine with their own muffler/ manifold design. With tighter emission controls, engine  companies needed to tighten their requirements.  One general specification is to limit maximum backpressure at WOT conditions.  Unfortunately, there is extremely poor correlation between pressure measurments made by various parties.  

I've measured backpressure in boats on a fair sample of medium and high speed Diesels from multiple manufacturers.  

I haven't run across a requirement for measuring or calculating stagnation pressure.  Engine manufacturers seem to universally assume that the backpressure will be measured either at the ports they provide (few do), or at the exit plane of the turbo exhaust flange(s).  Some document or imply temporary insertion of a custom thin measurement flange, which they don't specify, supply, or sell.

Marine exhaust system manufacturers traditionally provide a radial pyrometer port in their turbo flange, and it's common practice to measure and report backpressure there, at the bore wall, without correction.

In a typical yacht, there is no room for a straight pipe of any length anywhere near the engine, and often there isn't room for a turbo flange that's thick enough to accept a pyrometer probe, so you will find systems with ports downstream, usually somewhere in a small radius elbow.  Since the largest single pressure drop is usually in the transition from the turbo flange bore to the first elbow, and the flow distribution within an elbow is a mess, such systems always catalyze contention about backpressure measurements, unless they are deliberately oversized.

Engines with multiple exhaust outlets typically give different backpressure readings at each outlet.  Wildly different for staged turbos that are shut off.

One big contributor to uncertainty is the dynamics of the measurement.  In a boat, the only backpressure measurement that anyone really pays attention to is at WOT.  Absent a dead calm sea, and sometimes even then, the boat is pounding up and down, so the gauges and the technicians reading them are being tossed around.  If the boat's exhaust is underwater, the backpressure also changes as the boat rolls laterally, and that happens with every steering correction.  All of that means that the gage needles move around, a lot.  I have seen people record max/min values and interpolate, report just the max or just the min, or try to estimate where the needle spends most of its time.  I personally have found it helpful to insert a valve that can be throttled to damp the gage movement.  That gives a stable, if slow responding, value.  I have also seen people use Magnehelics with all of the unused ports plugged, which gives a wonderfully low, stable, erroneous reading.

Mike Halloran
Pembroke Pines, FL, USA