By Eric Brisbon 

All diesel injectors have a specified opening pressure which must be set.  This is normally accomplished through a series of shims and tested on a pop tester that allows you to verify the correct opening pressure and visualize the spray and sealing characteristics of your injector.  It also normally makes a noise when you test it.  Technicians generally refer to this noise as chatter (or bark). 

The noise (which is arguably unspellable phonetically) is a result of the fuel exiting the nozzle faster than the delivery from the pop stand. Stated differently, the efflux of fuel from it exceeds the rate of delivery to it (thank you Burman and Deluca).  Either way, the result is an audible chatter which happens each time the opening pressure is achieved, and the needle opens and closes in rapid succession.  Most seasoned fuel injection veterans have come to expect this phenomenon.  So what happens if it doesn’t?  Usually, if you are the tip manufacturer you get a call telling you that your nozzles are no good and a bunch of questions on how to return the product.  However, there can be more to this story. 

Popping an injector also checks the spray characteristics and sealing as mentioned.  Should you have a rip in the hose (no good atomization), leaks after closing (poor valve sealing), or you have a rapid pressure decay (excessive stem leakage), the customer is probably correct, that the tip is not right.  However, if all three criteria are good but the injector doesn’t chatter, do I have a bad tip? 

A nozzle does not chatter on the engine with the possible exception of starting RPM. They open at the right pressure and close at the end of the injection event(s). All other things being equal, why do I care if they chatter on a pop stand?  Perhaps I don’t.  Personal experience says that tips with very high sack volume and larger orifice sizes (as in high performance tips) do not chatter consistently, if at all.  Sometimes they chatter and then go away after repeated tests.  Perhaps the fuel velocity and flow from a tip with large sacs and orifice areas impede the relative rates of discharge (higher volumes and area means lower pressure) against the rate of delivery thus reduce or eliminate chatter? 

Also, to be considered is the way the needle seats in the body.  By design there is a slight differential angle between the valve and body to allow for very slight deformation to compensate for irregularities in surface texture or concentricity.  A heal seat is preferable and will normally result in a good chatter with poorer results the closer we are to a toe seat.  Of course, all of this is moot should seat concentricity be poor. 

Most of us consider a solid chatter to be the sign of a good, high quality tip as we have come to expect that.  Just be mindful that should you be working with modified or higher flow part numbers be sure all the other salient aspects are in check as you may be throwing out the baby with the bathwater.