I think people are not aware of, or perhaps maybe not paying much attention to a fundamental property of fuel pumps, which is that they deliver the most fuel at low pressure, and fuel delivery decreases as fuel pressure increases.

Using data for Walbro fuel pumps provided by Mardigrasmax's website, I put together this graph, which displays fuel delivery for Walbro's 190 lph, 255 lph and 255 lph high pressure pumps as a function of fuel pressure. In addition, a line representing fuel delivery required as a function of engine power is included. Engine power at the crank is represented by the upper horizontal axis.

The graph is very revealing in that it shows that for an engine making 350 hp (that would be about 290 hp at the wheels for a manual transmission and about 265 hp at the wheels for an automatic) the fuel required is about 30 gallons per hour. If you look at the fuel delivery for a Walbro 255 lph pump (not the high pressure pump), fuel delivery starts to go below 30 gph at a fuel pressure of about 75 psi. With the high pressure pump the fuel delivery goes below 30 gph at about 95 psi.

This means that if you are using an 8:1 fmu to drive your 240 cc/min injectors at above 75 psi in order to supply enough fuel to make 350 hp at the crank, you better be using a high pressure pump or you will be dangerously lean.

This may be the reason for the recent rash of blown engines.

Edit: The fuel pressure vs engine power line is based on 240 cc/min injectors. Larger injectors will require less fuel pressure at a given engine power.

 

I thought this was common knowledge for anyone that's running boost?? I'm running 190lph Walbro with my direct port nitrous. I know I will never bump my fuel pressure above the OEM spec. It flows way more than enough at 43 psi to support up to 450 hp

 

very useful info. thx a lot...
Does this only apply to the in tank pump? what about the inline pump that comes with the stillen kit? Do I need a high pressure in tank pump if I am using an addtional inline pump from stillen?

this might explained the high EGT I have recently. to correct this problem, do I need to leak more boost at FMU or use a 6:1 in order to drop the fuel pressure a little?


now I am a bit confused, if you see fuel pressure before the injectors, that means more fuel are pushing into the engine. How does high fuel pressure affect amount of fuel delivered? If the amount of fuel delivered is bettet at low fuel pressure, why do we need FMU at first place?

 

another good link: http://www.hondata.com/techfuelpump.html

it has more graph on fuel pressure vs fuel delivery.

 

another one from Walbro




from this article:
http://www.vfaq.com/pump-Walbros.html

 

I don't have any data for the Vortech inline pump. It would be interesting to see how it performs as pressure increases.

The fuel pump delivers more fuel at lower pressures, but the fuel delivery into the engine is insufficient at the stock fuel pressure of 43 psi because the injectors aren't big enough to provide sufficient flow at 43 psi when boosting. So one way to increase the fuel delivery is to increase the fuel pressure by using a boost-referenced fmu.

Fuel delivery increase through an injector is proportional to the square root of the pressure increase. So if you need 40% more fuel you need to double the fuel pressure. At 86 psi the injectors can provide sufficient flow, but now the fuel delivered by the pump decreases from 60 gph to a little less than 20 gph (for the 255 lph Walbro pump). Twenty gph is sufficient for only about 240 hp, so you end up lean at high rpm if your blower is pumping enough air to support 260 hp.

 

thx for the quick response, stephen. I understand that we need fuel pressure in order to delivery more fuel. However, how does fuel delivery decrease as fuel pressure increase? I always thought that if I see fuel pressure before the injectors, that means there's fuel to the engine. I never thought of fuel delivery. So if there's a sweet spot of fuel pressure....that means I dont need that much of fuel pressure for my injector size. I have seen >110psi on my gauge at redline, 2nd gear.


I have two pumps instead of one....I wonder if this situation applies to my case. is there any way that I can test the fuel delivery?

now this got me worried about my high EGT, where I hit 815 C at peak from 75mph to 110mph. I checked the plugs last week, they all seem fine. the electrode is gray and the insulator is light brownish. I always thought my high EGT is a result of my restrictive stock cat-back.

thx for your explanation and sorry for any inconvenience.

 

Steve -

I'm a tad confused. For guys with 370cc injectors (like us), I'm thinking that we'll need in the ballpark of 45-50psi of fuel pressure, but for discussions sake lets say we max out at 60psi of FP.

That means that at 60psi the 255lph HP pump is flowing slightly over 50gph....50gph could support over 500hp based on your chart.

I thought the limit of the 370's was well under that. Do we hit a "duty cycle wall" first?

 

Well, you have a good point there. If you are seeing 110 psi on your fuel gauge then that is what the injectors are seeing and the fuel delivery is based on 110 psi. Apparently your inline pump can deliver all the injectors can take at 110 psi. If the pump couldn't deliver then you would never get to 110 psi in the first place.

815C may be a little high, but it's not too high as long as you're not hearing any detonation (although even light detonation that can't be heard is damaging in the long run), and more importantly your plugs don't show any signs of detonation.

In your situation I'm more worried about the 110 psi fuel pressure and the danger of injector lock up. You really ought to consider going the 370 cc/min route.

 

What that means is that the fuel pump can supply well over what the injectors can deliver at that pressure, so you're safe.

The power that injectors can support is rated at a standard fuel pressure, i.e. 43 psi. JWT says that 370 cc/min injectors can support 420 hp at 43 psi, and this corresponds with what the RC Engineering tech website says if you assume a BSFC of .5 . If you increase the fuel pressure, then the 370's can support higher power.

So you think you're going to need more than stock fuel pressure? Are you going to the really small pulley sizes?

 

No. I had just thought I read that 370's could support Xhp, and was wondering why that number seems so much lower than I got from your graph.

Now it makes sense. Thanks!