Sean Hall

FUEL PRESSURE/FLOW CONSIDERATION:
As fuel pressure increases, maximum flow the fuel pump will support, decreases. In other words, total horsepower a given fuel pump will support decreases, as fuel pressure increases.

IMPLICATIONS:
Cars, or chips, using a 3bar(~44PSI) fuel pressure regulator will support roughly 50HP less than a system using an adjustable fuel pressure regulator set to 34PSI. Refer to page 95 and 96 of Maximum Boost, for reference.

CONCLUSIONS:
1) Set fuel pressure to the lowest pressure recommended for your injectors(At idle). Thus, you will have the most fuel pump/HP headroom.
2) Make sure you have an adequate fuel pump. At 3bar, or 44PSI, the most a stock 951 fuel pump will support is roughly 340WHP; at 34PSI, its roughly 390WHP.
Disclaimer: The above conclusion is based on my experience and is to be used as a point of reference. Additionally, Bosch does not make its' fuel pump datasheets readily available. So I had to draw these conclusions based on my own experience. Your mileage can and will vary.

Keep in mind, if you run out of fuel flow, the sympton will be a lean condition under full power. If you're on the edge of your fuel pump's capability, you probably won't see a lean condition until after the fuel pump has been under 100% load for a period of time.

I hope this is helpful for you guys.

Sean

Sean Hall

Someone asked me to post my story; how I came to the above conclusion. So, pour yourself a cup of coffee. Or, in my case, maybe a whiskey is in order... Here it is:

I had just rebuilt my 2.5L motor - rings, rod bearings, etc. During the rebuild, I changed the camshaft to an Elgin 330T Cam. Given the different profile of the cam, my APE chips needed to go too - way too rich in the mid range. So I contacted Danno to asked him if he'd help me dial in my car's fuel curve. He agreed, and I got a set of his chips. After I tweaked the fuel curve on my ARCII, the car ran great with the new chips. The Fuel curve never fluctuated from where I wanted it. In fact, I ran 22 pounds boost with no problems. The car just ran great. I really liked the chips. Way better then the APE chips.

The setup wasn't perfect though. I asked Danno to tweak a few settings on the fuel curve and he suggested we get the fuel curve dialed in with the ARC2 set to zero across the board. I agreed. And since I had 55# injectors he also recommended I set the DME's FQS switch to the 55# injector setting and incease the fuel pressure to 3bar, so I did. (Seems harmless enough, Don't it! )

After sending many chips back and forth and spending half a day with Danno tuning my car, we finally got the fuel curve dialed in with the ARCII set to zero across the board. But during all this, another problem crept in... the a/f ratio was going lean under full power after several test runs. We tried everything. We couldn't figure it out, and we were running out of time, so we left it as it was to solve another day. I took the car home and tried everything I could to solve the problem.... And after several weeks, still no answers.... somedays it would be 'okay' and others 'not okay'. Really weird.

Then came the Santa Barbara "fun run". All the cars got onto the first open road and I opened 'er up: passing 911s and other 'Llama-cars' and after a few "Who's Your Daddy" sprints, my car began to go lean and began to ping. So I turned down the boost - still pinging. I turned it down some more, to 18psi - still pinging. So I babied the car the rest of the event but the damage had been done. By the time I got home, it was blowing oil all over the engine compartment. When I tore it apart, I had two broken compression rings: #2 and #4.

So, there's the story of how Sean Hall learned all about fuel pressure.

Only if I had known how much fuel pressure affects the fuel pumps capability!... Oh well. As they say, hind sight is 20/20. And in this case, hindsight was about five grand $$$.

Sean

Danno

"2) Make sure you have an adequate fuel pump. At 3bar, or 44PSI, the most a stock 951 fuel pump will support is roughly 340WHP; at 34PSI, its roughly 390WHP. "

That's about right. I think you got the last '34psi' mixed up with something else? Like '54psi' perhaps? I have never seen a 400rwhp 951 that hasn't had an aftermarket high-volume (not high-pressure) pump upgrade installed.

There's no getting around upgrading injectors and fuel-pump if you're aiming for more than 350rwhp. That's because there's an inverse-relationship between pressure vs. flow-volume. As you increase pressure, the total-volume delivered by the pump goes down. To get the highest total flow-volume possible, it's best to upgrade to larger injectors and keep the fuel-pressure on the low end of the range. Then program your chips or EFI system to reduce the duty-cycle in relation to the increase in injector size. Sean in your case with 55# injectors, I recommended 18psi. More boost would need 65# injectors to keep fuel-pressure low.

Here's a sample chart of pressure vs. volume:



Notice that as you increase fuel-pressure from 35 to 45psi, the total volume delivered decreases by about 25%. Then you have to take into account the rising-pressure FPR used in our cars. At 20psi, you've got 65psi of pressure and only 33% of the flow-capacity as before. Here's a link to another pressure vs. flow chart:
Magnum Pumps
Walbro pumps - flow vs. pressure/current drain

Mike B

What is the correct fuel pressure you should have with 55# injectors and say a K27/8. If I read Sean's example he advocates lower than 3 bar.

Sean...Danno???

TonyG

Danno is right on. Pay attention to what he said.

FWIW, I run the stock 2.5 bar pressure regulator and 72lbs Siemens injectors.

Waterguy

The correct fuel pressure is whatever your chip/engine management system is tuned for. Danno has tuned the Guru chips for a 3 bar fuel pressure, because that higher pressure allows a 9% greater flow from the stock injectors. This higher pressure is unneccesary if you go with larger injectors, such as Tony has.

However, as Sean points out and Danno & Tony agree, a stock (or any) fuel pump will have decreased flow at higher pressures. I believe that the Guru website recommends upgrading the fuel pump if you go larger than 55# injectors. Danno has recommended above a larger fuel pump and 65# injectors if your target is more than 350 rwhp. Do you expect to make more than 350 rwhp with your K27/8?

I suspect there could be other factors, such as a 15+ year old fuel pump, old wiring, frequency of fuel filter replacements, etc. It would be foolish to apply the original pump specifications to our cars without allowing for a factor of safety due to age.

Sean Hall

Danno,
Nothings mixed up. Remember, its an *inverse* relationship.
In fact, increasing fuel pressure by 10 PSI reduces reduces fuel flow by roughly 5 gallons. 5 gallons equates to 50 horsepower. Thus, 44PSI will *reduce* a fuel pumps capability by 50HP, when compared to 34PSI.

You don't have to take into account the rising rate FPR when factoring out the *difference* in fuel flow. In both cases the difference, in PSI, will be the same.

Ie one car:
34 + 40psi(rising rate) ----> difference 10PSI

3bar car:
44 +40psi(rising rate) ----> difference 10PSI


Here's a chart from Maximum Boost, for your reference(page 95). The gallons/horsepower relationship, is on the next page.



Im confused by your last sentence:
"At 20psi, you've got 65psi of pressure and only 33% of the flow-capacity as before"


Sean

Perry 951

Mike -

What chips are you running? FYI, I am set at 38psi idle, #55's, 27/8.

Mike B

Thanks Brian.

I am thinking ahead. Presently I have the stock chips and injectors but ordered a set of Siemens 55# injectors from Racetronics that will probably arrive today. I also ordered Danno's K27 chips and hope they will be here today too. Couple that with the Lindsey Fuel rail buy and I am done with under the hood!

Perry 951

Let me know how the chips work out. I have a set of custom Technodyne chips and although I like them, they have some rich areas that I cannot tune out.

I played with my fuel pressure for a few days, anywhere from 20psi to 50psi static, 38psi seems to be the magical number. On boost, that means I am seeing a safe 53psi.

Mike B

I certainly will Brian..Thanks for the info.

Bengt Sweden

And then there is the claim of a better spray pattern with a little extra pressure. Does anyone know for sure if this is true?

Bengt

Bill

Don't forget that there are other choke points, like the lines and the filter. The smaller the fuel line, the higher the pressure is required to flow a given volume. And how well does the OEM filter perform?

Alan C.

Let's see. Weldon pump, fuel filters, dial-a-flow and regulator coupled with -8 lines along with 83 lb. injectors. That combo lead to a blown wallet and a very healthy head gasket and ring set
Alan C.

Danno

"And then there is the claim of a better spray pattern with a little extra pressure. Does anyone know for sure if this is true? "

I forgot the exact paper, but do a search on the SAE site for "high pressure fuel injectors". A little extra pressure on stock & aftermarket injectors don't do much to enhance the spray pattern. Might be better on aftermarket injectors because they don't need that plastic end-cap to develop the proper cone-shaped spray. The real enhancement is going with specialty-matched 100psi fuel-pumps, that can actually deliver the proper volume-flow at that pressure, and injectors with powerful solenoids desgined for that pressure. The results are better much atomization of the fuel when used with LARGE injectors like 75-lb/hr and above under idle and high duty-cycle conditions.