Ok check my math (injector and pump flow)

So I was doing math this morning figuring the flow of 300cc injectors at 3500rpm.
Injector flow as far as I know is based on 1000 shots.

So 300x8 is 2400cc, but since its a 4 stroke motor, at 1000rpm flow is half that at 1200cc.

At 3500rpm it would be 1200cc x 3.5 = 4200cc per minute.

4200cc per minute is 1.1gpm.

1.1gpm is 66gph.

My Walbro, according to this graph, at 62psi is around 50gph.



Looks like I need to do the dual SD pump setup.

Anyone know the flow specs on a stock pump?

Big Bore said:

So I was doing math this morning figuring the flow of 300cc injectors at 3500rpm.
Injector flow as far as I know is based on 1000 shots.

So 300x8 is 2400cc, but since its a 4 stroke motor, at 1000rpm flow is half that at 1200cc.

At 3500rpm it would be 1200cc x 3.5 = 4200cc per minute.

4200cc per minute is 1.1gpm.

1.1gpm is 66gph.

My Walbro, according to this graph, at 62psi is around 50gph.

Looks like I need to do the dual SD pump setup.

Anyone know the flow specs on a stock pump?

Click to expand...

Math check...

3500 rpm
1750 "shots" or injection events per cylinder per minute
8 cylinders
1750 "shots" or injection events times 8 cylinders equals=
14000 "shots" or injection events for the motor over one minute

Since 7.3 injector capacity is rated as number of CCs flown over 1000 shots, we need to divide the number of shots (in this case 14000) by 1000. This equals 14.

14 times 300cc equals 4200cc per minute.

4200cc=1.1 gpm=66 gph

Looks good by me.

I don't know of a published spec for the stocker pump, but I believe the common thought is 30 gph. My truck had a p74221 carter on it as the stocker...it took mine just under 2 minutes to pump a gallon (as pumped out of hose connected directly to the outlet of the pump).

I'm running dual pumps with 250/200s and works great, as of now I don't have an in cab fuel pressure gauge.

But on the plus side if you lose a pump you can still limp it home.

Dieselboy. said:

I'm running dual pumps with 250/200s and works great,

Click to expand...

250's @ 3500rpm flow 55.44 gph. If 2 SD pumps flow 30gph each/60gph total, thats more than enough for your needs. Unfortunately it's about 10+gph less than what 300's flow at 3500 rpm, and I may push the rpm's to 4000.

However I do have a spare Walbro, and in theory two of those in parallel flow 100gph, which is way more than I need even at 4000rpm. Walbro's are only $110 ea for a replacement and I get the redundancy just like the SD pumps in parallel.

BTW thanks V-ref for the math check.

Big Bore said:

BTW thanks V-ref for the math check.

Click to expand...

You bet on the math check...brave of you to post math publicly.:biggun:

For a quick/easy/rough rule of thumb....with assumptions being 3750 rpm maximum and the tuning/injectors are able to dump 100%of the injectors potential volume in one event......It looks like the math could allow a fella to take 1/4th his injector size in CCs, and that would match up to a GPH spec for his system, that would factor in a small margin.

Example-

160cc=40 gph
200cc=50 gph
250cc=62.5 gph
300cc=75 gph
350cc=87.5 gph
400cc=100 gph

And so on....lots of other factors involved, but maybe a rough rule of thumb to get a ballpark idea, and then go forward into the details if one so desired.

OP-Looks like you've already got the details covered!:rockon:

V-Ref said:

You bet on the math check...brave of you to post math publicly.:biggun:

Click to expand...

I almost didn't factor in the 4 stroke.

Remember - those numbers are only good if you are emptying the injectors in your scenario.
For instance - here is a 238cc injector and a 455cc injector that have the EXACT same fuel pump requirements





For further comparison - here is a "standard" 238cc hybrid with 80% tips:



The "standard" 238cc hybrid and the "fast" 238cc hybrid have very different fuel pump requirements at any given scenario.

Also as RPMs increase - the likely hood that you are emptying your injectors decreases because the allowable time for an effective injection event decreases. At higher RPMs everything is faster EXCEPT the actual injection rate of your injectors which is for all intents and purposes static no matter what RPM for any given ICP number.

I just wanted to throw some confusion on the fire this fine Monday morning.

What is the window at 4000rpm?

Jason beat me to it...


Your injection duration must be considered as you will not be emptying but the tiniest injector at 4000rpm with current injection rates unless you just like drowning an engine in early/late fuel and melting the manifolds off.

Big Bore said:

What is the window at 4000rpm?

Click to expand...

At 4000RPMs you have 24 crankshaft degrees of rotation every millisecond - the FDCS signal that controls the IDM to fire the injectors becomes unstable if it is operating above say a 70% duty cycle (for a safe number) - that puts the maximum window at about 63* of crankshaft rotation - or about 2.62ms of PW. Some tuning strategies - including my own - put the target injection window much tighter than the 63* maximum.

I was going to include some nice math here to show you fuel rate for the pump required based on the available injector refill time and actual numbers - but.... my math isn't working out on paper and I don't have time at the moment to figure out what I screwed up.

Good info. God this is tough to keep up with lol

As for flow the walbro about equals dual SD from what I have tested. Both will handle 200% nozzle injectors running typcial PW. But here is the reason duals are better even beyond the redundancy. A single pump has to use more volts an amp thru its pump than a each of the SD. That means that much more eroding of the brushes etc. Two smaller pumps are running lower power each but together equal the single should have less wear and last longer. I think this also is clearly shown with the track record of these pumps and setups. We know the bosch SD have a great track record and the reason is they are not asked to do both high volume and pressure just high pressure with moderate at best volume (25-28 gph)

Good read. So an A1000 will feed 250/200s adequitly?Jegs says the A1000 flow 70lbs/hr?

No that is way way low. A gal of #2 diesel is 7.15lbs @ 72 degree. That would not even be 10 gals per hour. You are missing another 0 at the end of that. 700lbph or 100 gph

That is more than enough for 400% nozzles running max available PW. The issue once again is the brushes and commutator. At that pressure and volume the pumps do not last. You will also notice that the A1000 is specifically not rated for diesel. Their Marine version that is more costly is. Apparently it has special coatings etc that prevent the accelerated wear diesel causes of the armature bushings, brushes and commutator. I have been looking and review all the various pumps. Other than the fuelab that is the only that might work. In fact given the way marine motors are run my bet is it would be good. Another thing I noticed is the regulators have the same ratings. The normal aeromotive bypass reg we all typcial use are not rated for diesel only the marine version is. Something to consider.