the myth of pop pressure and pump timing

[articfarmram]

Ok guys, my truck is giving me a hard time now and need some input.. here it goes, I was night guarding a ship this week and got this great idea to remove my stock injectors and pop test them and clean them while I was sitting in the engen floor of the trawler making shure this old pile of junk didnt blow up on us threw the night.. any how I removed them and pop tested them the spraypattern was flawess but the poped around 210- 220 bar stock is 260bar I punched out washers that where 0,05mm and put them in each injector till the 260 goal was reached. micro messured the total of all washers and went to a dieaselshop and bought new washers that fitted eached injector disasembled all of them and put them in a ultrasonic microsolve bath. once clean I sprayed all the parts with injector pop testing dieasel fuel "REALLY clean and filltered dieasel" reasembled tourqed and installed them back in place.. when i started the motor it idles aroun 200 rpm lower and it runs a bit wierd all cylinders are fireing... ok thats a set back.. my theory is that when i bought the truck it had really low idle.. I bumped it up and never thought of it again til now, is it a chance that the inj. pump is slipped a few degrees and "as the pop pressure drops they come in thousnds of a millisec sooner than stock" since the injectors where poping so low and if the pump is slipped it held in hand and kind of kept it in balance.. now the injectors are back to stock the slipped time is more noticeable.... If anybody has any input please by all means chip in Id like some opinion thaks for reading this

[God prefers Diesels]

Sound possible since higher pops retard the timing. So a lower pop would advance it. So you could've.

[hartracing390fe]

lower opening pressure would imatate advanced timing. reset your timing with the good injectors and then set your idle, should be fine.

[v8440]

When I read the title I thought someone had a reason why they thought pop pressure and timing are actually unrelated. All this does is seem to support that.

[Jagger]

I have a theory. If the lift pump is weak or the overflow valve is weak, the amount of fuel that gets into the plunger will be less since more pressure would mean more fuel getting in before the port closes. At idle the pressure is only 17-23psi and the port is barely opening since all you need is enough fuel to idle. This means that at idle, pressure differences would change the amount of fuel getting into the plunger. Now if the pop pressure is low, more fuel is going to be shot into the engine. I think this is true because if the pop pressure was say, 0psi, it would shove all the fuel into the engine after it got past the delivery valve, but if the pop pressure was much higher, it would only shoot in the fuel that was at the higher pressure. Therefore, by increasing pop off pressure, it decreases the amount of fuel, making it idle 200RPM lower and having to readjust your idle so that the amount of fuel reaching 260bar = the amount that previously reached 215bar. It would do this by obviously pushing the rack forward some more to allow for more fuel to go in so more of it will be at a higher pressure. If there is less fuel getting in, then it reaches pop off pressure and quickly drops below it and runs out the return line. I guess that means it could have nothing to do with fuel pressure from the lift pump or overflow valve and could just solely be the nature of the pop pressures changing.

Just my theory anyways, I don't know for sure.

[v8440]

My understanding is pop pressures affect timing also, because it takes longer for the pump to produce higher pop pressures. That right there IS retarded timing. If it takes longer to reach pop pressure, injection occurs later.

As a somewhat separate issue, there's the idea that fuel system restrictions can retard timing. You touched on that in the first part of your post. If the restriction is such that the engine needs more fuel than the system can deliver, cavitation occurs inside the barrels. Cavitation produces air bubbles that must be compressed before pop pressures can be reached-retarding timing. This is why trucks that have fuel system restrictions or dying lift pumps will sputter and blow white/blue smoke when they're being asked to make more power than the fuel system will allow. It's the same symptoms as slipped timing because the same thing is happening with the timing, just for different reasons.

[Jagger]

I read a 300 page thing on timing and I can probably sum it up in a paragraph. Yes the timing is retarded if the pump is left unscathed from the previous idle speed. I don't believe this to cause a 200RPM drop though. At idle speed the engine should be more retarded anyways since the engine has static timing which means they have to set it for low and high rpm operation, so 13.5* is probably the average of it. At low RPM the fuel does not need a lot of time to get to the point of producing power, so you would retard timing at low RPM. At higher RPM the fuel needs more time so you advance it so that it is producing power right at TDC or slightly after. That is why all these truck pullers are running 20-30* of timing when they are doing 5000RPM.

Now as for the thing I read, atomization of the fuel (which is better at higher pop pressures) has an effect on timing as well. When the fuel shoots into the combustion chamber, the fuel expands like hell. If you put your hand in front of an air hose, the air is freezing coming out. The fuel is the same way, the more it atomizes, the more surface area of fuel there is absorbing the heat and this retards timing because it absorbs it and cools the area intensely, taking longer for it to combust. As pop pressure decreases, timing advances.

It should still be more efficient with the retarded timing at low RPM (idle speed) so should not drop in RPM, the only thing there is to factor is the amount of fuel at 260bar. I really think the low pop pressure shot in more fuel since more of it was at 215bar. Now that his pressure is up, he is trying to use the same amount of fuel to reach 260 bar but less of it is able to so less fuel is shot in, less fuel in the combustion chamber = less RPM.

The only thing that is throwing this off is the fact that fuel is pretty well incompressible. Therefore it would reach whatever pressure it wants to get out of the barrel, without compromising its volume since it isn't a gas. If there is cavitation in it, then it would follow all the laws of compressibility and my theory would work. When I put a clear hose on mine, there is a constant draw of tiny bubbles that were going through it. My engine runs perfect though, like his might have, but the air can be compressed and cause the drop in engine RPM since a higher pop pressure would mean more time is spent compressing that air so the injection event would end with little fuel reaching 260 bar.

Where is the half a cent emoticon when I need it.

[Art]

Originally Posted by articfarmram

.. now the injectors are back to stock the slipped time is more noticeable....

This makes it sound to me like the additional torque required from the increase in pop pressure simply caused the pump's gear to slip on the shaft.

[SteelCityDiesel11]

so, if im getting what youre putting out there...then theoretically; if im running original stock injectors with lets say 130,xxxmi on em and i throw a set of new/aftermarket sticks in, theoretically i should reset my timing? because my pop pressure will not be the same on my new injs as the ones im pulling out. unless the differences between the two pops will not make a significant impact on inj timing?

[v8440]

It depends on how severe the difference is. If the timing has not slipped, simply restoring pop pressure to stock levels won't cause undue performance loss. However, if timing is stock, advancing it a few degrees just about always gives a little more power and a little more mileage. You'll usually see about 1-2 mpg overall improvement.

[SteelCityDiesel11]

i understand that. but...does the difference in pop between a worn injector and a new/aftermarket inj cause a difference in timing notable enough to warrant a reset? i understand that the difference can vary, but how much can the pop drop on a worn stick before it becomes tooo much? -just trying to get an idea because i just set timing w/ 125xxxmi inj and im looking to throw some ddp's in or something and if i 'should' get timing RE frickin done then i guess i will haa

[articfarmram]

thanks for all the input, great stuff and well written posts.. I think my theory is correct wont really know until I get out there and set the timing, Im going to take the injector out again and have them pop tested in one of those fancy digital pop testers maby the tester I used was wrong and I set them all to high. Im going to begin with resetting the time back to stock and try it out then probably going to set it to 14,5-15