slim_boy_fat

I am getting the 'essential' sage 1 if you like upgrade. Accuboost, chips, shims and FPR.

One problem I have the screw on FPR rates at 2.5bar. If i fitted one from a 968 its apparenty 3.8bar, would that casue problems. Surely the fule pump can only supply fuel at a certain rate and the chips will only alow the injecors to fire in the correct amount of fuel?

What do people think?

Its going to be an expensive option to try and get the 3.0bar screw fit FPR.



p.s. All opinions welcome, especiall from those who know abit about it subject.

Sam Lin

Danno's (GURU) chip kit comes with everything you refer to, including a new 3-bar FPR.

Sam

Gary Gaukler

I concur with Sam; but if for some reason you want to buy separate items, Speed-6 sells 3-bar FPRs. www.speed-6.com

HTH,

Gary.

slim_boy_fat

For those who cant read, my car has the screw on FPR, NOT the push fit, hence the need to find a screw on FPR!! Try and keep up guys.

Sam Lin

I did read that, I just assumed you knew you could cut the screw fitting off and use the barbed fitting.

Sam

Danno

I'm working up an adaptor-fitting to install the clamp-on 3-bar FPR on '90-91 951s that use the screw-on FPR.

". Surely the fule pump can only supply fuel at a certain rate and..."

I woudn't go above 3-bar because you're decreasing the flow-volume of your fuel-pump when you increase pressure. Going from 2.5-bar to 3.0-bar will have already dropped the max flow-rate from 370-375rwhp to about 350-360rwhp.

"... and the chips will only alow the injecors to fire in the correct amount of fuel? "

Only if they are programmed to scale back the duty-cycle across the board to account for the higher fuel-pressure. Going up to 3.0-bar on the FPR gives 9% more fuel-flow. By decreasing the duty-cycle in all the fuel-cells by 9%, you end up at the same fuel-delivery as before, but at a lower duty-cycle. So instead of hitting 100% duty-cycle on the stock injectors at around 240-250rwhp, they can deliver that same amount of fuel at 91% duty-cycle (still too high). This allows you to squeeze 260-270rwhp out of them at 100%. I don't recommend that anyway; you should stay around 80% max due to overheating the injectors and driver-transistor in the DME when running above that.

slim_boy_fat

Want to keep it stock, thanks.

slim_boy_fat

Well its your chips that i am using

If thats the case then why not just stay with the 2.5bar FPR.

What would be the problem with
A. Just using standard FPR 3.0bar
B. Using a 968 3.8bar FPR

Does the FPR keep the fuel pressure at a maximum rate? So what does the pump push out? What is it rated at? I am sure i read somewhere that the pump is 3bar max.
All the regulator does is prevent pressures increasing above a set value. i.e. 3.0bar

Danno

"Well its your chips that i am using
If thats the case then why not just stay with the 2.5bar FPR."

I used a 3.0-bar FPR because it will allow a little more fuel-delivery with stock injectors. Some of the TurboS cars with our latest chip-kits @ 18psi are in the 270-280rwhp range. This is above the 100% flow-delivery of the stock at 2.5-bar.

"What would be the problem with
A. Just using standard FPR 3.0bar"

Our chip-kits DO come with a standard 3.0-bar FPR. However, I haven't been able to find a Bosch 3.0-bar unit with the thread-on fuel-fitting as used on the '90-91 cars. That's why I'm making a short hose-adaptor people who have that model-year.

[b][i]""What would be the problem with
B. Using a 968 3.8bar FPR

Does the FPR keep the fuel pressure at a maximum rate? So what does the pump push out? What is it rated at? I am sure i read somewhere that the pump is 3bar max.
All the regulator does is prevent pressures increasing above a set value. i.e. 3.0bar"

No, the FPR doesn't keep fuel-pressure at the maximum-rate, but is based upon manifold-pressure. Both the stock 2.5-bar FPR in the 951 and the 3.8-bar unit in the S2/968 work on a rising-pressure mechanism pegged to manifold-pressure. So on a 951, vacuum/boost is fed into the FPR from the intake-manifold and pushes on a diaphragm inside the FPR. At idle, the vacuum will pull down the fuel-pressure to about 2.0-bar. Under boost of say.... 18psi, this will result in 3.8-bar of fuel-pressure.

The pump can put out 75-85psi or 6-bar. However, the flow-rates goes down with increasing pressure. At 2.5-bar, the stock pump puts out about 1700cc/min. At 3.0-bar, this drops to 1500cc/min. At 3.8-bar this is only 1200cc/min.

Using 3.8-bar on a 968 is fine because it will never see boost. Due to the rising-pressure nature of the regulator, the additional pressure generated by boost will cause pressure to be too high and the fuel-pump won't be able to deliver sufficient fuel-volume at that pressure. At 18psi with a 3.0-bar FPR, the fuel-pump will only deliver enough fuel-volume for about 340-360rwhp. If you use 3.8-bar FPR, then at 18psi, the fuel-pump will only send enough fuel for 310-320rwhp.

This is painfully obvious when dyno-testing and programming chips/EFI systems. At some point on the RPM band, the air-fuel mixture curve gets leaner and leaner on a 30-degree angle upwards. No amount of increasing fuel in the data-maps will increase the fuel-delivery. Using larger injectors (with appropriate scaling of fuel-maps) will end up with the same break-point where the mixture heads upwards towards lean, but at a lower duty-cycle on the injectors. The fuel-pump is just maxed out.

So like that thread that Sean Hall posted about a month or two ago, it's best not to try an increase fuel-delivery by dramatic increases in pressure. This can only work with small stock injectors to squeeze a little more out of them. It's best to just get larger injectors and pick the correct settings on your chips.

slim_boy_fat

Thanks for that, its through necessity that i am thinking about using the 3.8bar regulator not bucause i want to. As you say trying to get hold of one to fit 90-91 cars is not easy.

If i had to choose then would i be better just keeping the 2.5bar one or get the 3.8 or not bother fitting the chips, accuboost and shims untill i can get the 3.0bar FPR.

Thanks

Slim.

Danno

Please send me an email, we'll talk more about this offline. THanks -danno

slim_boy_fat

Done..

Highlander944

This is interesting.

Now I'm a confused let me talk it through to see if I understand correctly...

The fuel pump moves the fuel to the engine through the fuel pressure regulator. Based on the description given, this regulator will increase the pressure so that, when the injectors pulse, the increased fuel pressure allows more fuel to enter into the engine. Is this correct thinking?

Thinking about the injectors themselves. What is the difference between the stock (what about 43#), 55#, 65#, and 75# injectors? Does it require a stronger 'gate' to start and stop the fuel flow at the higher pressure? Is this what a bigger injector does? So on chips like the Guru chips, the injector pulse width is adjusted for the different injectors?

Based on what was said earlier as you increase the fuel pressure you decrease the volume of fuel which will be delivered to the engine. Given a fixed engine efficiency it would seem that one could generate a graph which would predict a maximum rwhp given a particular fuel delivery systems. The intersection between how much fuel can be squeezed through a given size injector and how much fuel the fuel pump can provide at a given head pressure would seem to be 'maximum'. Is this correct? So the trick would be getting enough air into the engine to burn this maximum amount of fuel at the stokeyometric (I really can't spell) ratio.

So what then is the maximum injector size that a stock fuel pump can use? I'd love to plot this out and see it in excel.

Am I thinking clearly on this matter and could someone give me some numbers...

Peter.

Fen

My understanding:

The fuel pump delivers fuel to the engine. Based on Danno's figures it can build 6bar of pressure (after that presuambly it stops as it can't pump any more fuel into that pressure).

The regulator controls how much of this 6 bar can be seen at the fuel rail/injectors. Its a pressure restrictor then, and it allows 2.5 bar, 3.8 bar or whatever max to the injectors. It can't bring a lower supply up to 2.5 or 3.8 or whatever it is set to.

The injector numbers relate (again based on Danno's info elsewhere) the amount of fuel that will pass through the injector if it is open for a period of time (lb/hour?) with 3 bar pressure pushing that fuel through it.


What Danno is saying is that the standard pump can maintain 6bar maximum pressure, but if you introduced a leak then it wouldn't have to be very big for the pump not to be able to maintain that pressure. As the pressure reduces there is less effort in maintaining it = less effort to move fuel into that pressure = more fuel moves in for the same effort. If you follow that then what it means is that the standard pump can maintain 3.8 bar with a leak of 1,200cc/min, at 2.5 bar it can cope with a leak of 1,800cc/min. Consider the injectors as leaks.

Danno's explanation reads a little like the pump will put out 1,800cc/min with a 2.5 bar regulator but only 1,200 with a 3.8, but in reality it would still supply 1,800cc/min with a 3.8 bar regulator if that much were required, just that it would never get more than 2.5bar pressure into the rail. the problem comes when the chips run the duty cycle of the injector expecting a certain pressure to be there and the pump is not able to provide that pressure - if the injector is open for 1/2 second then less fuel will pass through with 2.5bar pushing it than 3.8 bar. A second problem is that if you force feed the cylinder (with a turbo) then there is pressure in there and it takes more pressure behind the fuel to get the same volume into a cylinder the greater the pressure already in there, so if the pump can't deliver what the ECU ecpects in terms of pressure the shortfall in actual fuel delivered could be significant.

Highlander944

I understand what your saying but....

If the fuel pump is rated like other pump I've ever seen then as the head pressure increases the volumetric flow will decrease. Thus if you required all the fuel that 75# injectors could provide, you'd have to move to a higher pressure pump AND higher pressure hoses.

My thinking as this relates to fuel injection...

Mission: Get more fuel and air into the combustion chamber so that there is bigger boom. == More RWHP.

The amount of time to get the fuel into the engine will be fixed by the cycle of the piston. So, if I wanted to get more fuel into the engine how would I accomplish this? I believe I would increase the pulse width of the injectors. Once at maximum width, I would have to increase the fuel pressure.

Thus, I believe the pressure is needed to deliver more fuel within a given window.

How is this increased pressure derived? It would seem that the fuel pump provides this pressure. The fuel pump is constantly pushing against this higher pressure, thus the impeller or whatever is spinning is encountering higher resistance and the volumetric flow is reduced.

What I'm trying to say is that the fuel pump must be matched for both a given pressure AND a given volumetric flow. The window in which the injectors may pulse should be well known. Given this, you can plot the volumetric requirements at a given pressure for a given injector. Do this for multiple pound ratings. This would be a set of curves that define the volume of fuel at a given pressure required to generate maximum horsepower. Next you would draw the stock pump curve. Any injector volume curve that goes above the pump curve would result in lean combustion.

I would really like to plot these out. Heck as I think about it, I bet you would adjust the pulse width for different size injectors so that they do not overshoot the volumetric capacity of the stock fuel pump. Am I close Danno? You could tweak the base curves using this pulse width to ensure the engine will not run lean.

So does anyone know the maximum window of time allowed for the fuel injector pulse? Perhaps the injector manufacturers have volumetric ratings on their injectors, that would seem a reasonable specification. Also, on these four cylinder engines, is each cylinder firing equally spaced? And is 1 rpm one 'sequence' of all four cylinders firing?