Sunday, January 10, 2010

SVO Greentop 42# testing at 3 and 4bar

This is nothing new; I've written about it before HERE. However, I don't know why this is still such a ripe source for arguments, thus I figured it's time to revisit.

A friend of mine got some SVO Greentop 42s, and send them to Deatschwerks for some cleanup and flow testing. He wanted the injectors tested not at some arbitrary rated pressure that the injectors will never see in his application, he actually wanted to see what they would flow at the intended rail pressure. He called up Deatschwerks, talked to whoever did the service, and asked about the additional testing at the GM standard of 58psi. They said no problem, and sent him not one but two flow sheets: one at 43.5psi and one at 58psi. This sort of data should once for all settle all SVO flow discussions, as it's done on the same bench, with the same injectors, by the same technician, hopefully with the same methodology. I love the smell of science in the morning!

Quick disclaimer is in order: I usually avoid any sort of endorsements on principle, as I feel commercialism ruins science, but I am going to make an exception here. I've talked in the past with David Deatsch and he's been very interested in what our little tuning community needs. So if you really want to get to the bottom of 'why my trims are off on one bank' sort of problems, give them a call.

Back to tuning...
So Daniel forwards me his flow sheets, I enter his data into Excel, and let's take a look at what we see:

or for people who like graphs:

So we have 8 injectors, they all flow a little differently.  The red bars are the flow values tested at 3bar (43.5psi).  The purple bars are the flow values tested at 4bar (58psi).  The green values are a theoretical flow at 4bar calculated using the Bernoulli's formula.

As you can see, there's a discrepancy between the theoretical and the measured flow values at 4bar.  The interesting part is not that they are different, but that they are different in a consistent manner.  There's a whole area of mathematics that deals with this exact phenomenon, called 'Residual Analysis.'  I've been told to cut it out with the math, so here's the super short version that's relevant to this post.  Residual Analysis tells us that if there's a distinct pattern in our 'theory vs reality' then the underlying assumptions or models are wrong.

So there are few possible explanations here:
1.  The injectors actually take on different flow characteristics at 4bar vs 3bar.  Maybe they're more suitable to higher pressures, as they all flow more than they should.
2.  There's a fairly consistent error in measuring pressure or flow.

So I decided to quickly explore option 2, as I wanted to see at what pressure the injectors would flow as much fuel as the flowed values would suggest.

So using some numerical trickery, I asked Excel "what fuel pressure would I have to apply to the injectors I have data for at 3bar, to get the flow seen on the 4bar flow sheet?"

And what Excel came up with was 411kPa, or 4.11bar, or 59.7psi.  This is not just for one injectors, this is across all of them.  Here's the results in graphical form:

As you can see, the theoretical and the real bars are much closer to each other this time.  Some are over, some are under, but they're all close.

So this leaves us scratching our heads:  is the flow bench indicating less pressure than there really was at the time of testing?  Wouldn't that show up at the 3bar flowing too?  Why doesn't the theory and the real results line up closer?  What if it's the flow numbers that are skewed not the pressure?  What other 'hidden variables' are we dealing with here?

Despite all this, I'd much rather use these flow numbers, than just assuming 42lb/hr @3bar converted to 4bar ignoring all the assumptions we've made in that process.  Especially if you combined it with accounting for fuel pressures at given manifold vacuum, as I've demonstrated before.

And to come full circle to establishing the flowed pressure of SVO Greentops:  they all flowed between 43.0 and 43.5 lb/hr at 3bar (42.5psi).  Not 39.15psi, not 40psi, not whatever else people claim.  Granted, that's also not perfectly aligned with the official 42lb/hr flow, but that probably can be attributed to freshly cleaned injectors.  I wouldn't be surprised if the OEM's underrated their injector flows to make up for the average car user than never uses fuel injector cleaner.

Hope this helps,

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