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Junior Mint
2,461 Posts
Discussion Starter · #1 ·
Several years ago I had typed a post explaining to a member over at tds how to calculate oil usage. I had saved it in a word document because I needed to think on it for a while.

In this case I was discussing my old inline pump set up that used a 15 degree and a 17 degree pump. At the time I was on BD injectors and I had accurate flow bench numbers for them. Also, somewhere around here I had calculated the oil produced by a Gen3 style pump but that is not at my finger tips at this time. Easy enough to calculate however as a Gen3 runs at a 1.32 OD and the cc/ rev are available online. Also in there I have some assumptions regarding internal injector flow that were just assumptions at the time. But they turned out to be true.

Here is a copy of that post:

"An example of oil requirements for stock BD injectors would go something like this:

My stock Alliant BDs flowed 244cc volume stock and the stock 5 hole nozzles on the flowbench at 2800 icp flowed that 244cc in about 3.4 ms if I recall correctly. This is important info.

The stock HPOP turns at 85% of crank speed.

The 17 degree pumps, pump about 7.2 cc of oil per HPOP revolution so that equals about 6.12 cc per engine revolution.

The 15 degree pumps, pump about 6.8 cc of oil per HPOP revolution so that equals about 5.78 cc per engine revolution.

Keep in mind these #’s assume 100% efficiency and that is never going to happen.

So you have a 15 degree pump on your truck and you want to buy an inline big oil system and you want to know if you have enough oil? (you do, but just barely and here is why)

Oil needed is easily calculated. 6:1 injector is 244 * 6 = 1464 mm^3 PER injector. Of course 4 injectors fire per engine revolution cause it’s a V8 4 stroke.

I figure the lowest RPM anyone is going to ask for full pw is around 2400 RPM (just guessing really) so 2400 rpms/ 60 seconds = 40 RPSecond. Which is 25ms (1000/ 40) per crank revolution. So regardless of injector pw we have one injector BEGINNING to fire ever 6.25 ms. (25ms/4=6.25ms) Full pw at the lowest RPM is always going to tax the HPO system the greatest.

Now due to injector constraints (should be the nozzle but sometimes it’s not) we are emptying these injectors in 3.4 ms. The injector accepts oil for this period of time but the rest of the time the pressure regulator is doing all it can to bleed off that excess oil. So that means that the injectors can only accept 54 % of the oil that the HPOP(s) are providing (3.4ms*4 injectors) /25ms which in the case of a 15 degree pump and a 17 degree pump equates to (6.12cc+5.78cc)*.54= 6.43 cc of oil. Now our requirements are of course (1464mm^3*4=5.856 cc of oil to run these injectors.

Of course this assumes 100% efficiency but just looking at stock injectors and the (lack of) performance offered on 1 stock HPOP makes it clear that a 40-50% overallowance is not overkill AT ALL. Furthermore, the higher the desired pressures the more inefficient the system becomes.


What happens when the pw is so long and the RPM are so high that the oil system has to provide oil for multiple injectors at a time? PW gets cut back of course.

What happens when the nozzles and/ or injectors get really large and the pw is further shortened? Oil requirements go up substantially.

What happens if the same large nozzles/ injectors are not dropping HPO psi on a dual pump system when measured at the rails? The only obvious answer is that the injectors are not really getting all the fuel out. Maybe a restriction inside the injector is making the nozzle NOT the restriction (as it should be) and resulting in poor atomization at the NOZZLE where it matters? (i.e. 400cc injectors, 12 thousandths nozzles, high oil pressure AT THE RAILS on a twin HPOP system, no power, high smoke, burned up motor is an example that comes to mind)

IMO, this is the primary reason that Dockboy has always done so well w/ his “little” stock BD injectors. The atomization was there because the nozzle holes were small enough to make sure that the nozzles were the restriction. And I’ve been giggling for years when I saw people say things like DB has stock nozzles w/ .001” holes etc. Those BD nozzle holes are around .00085” and there are only 5 of them! They are small and they work good!

Hope that helps someone."
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