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Brendan · Joined: 14 Jul 2008 Posts: 14

If you have aftermarket fuel control, is there any need to vacuum reference your fuel pressure regulator? Does it matter what type of fuel control you use - MAP vs Alpha-N for example? The reason I ask is, I recently logged my fuel pressure vs. RPM and got the graph below. Green is RPM, red is fuel pressure:

You can see areas at constant, WOT, where fuel pressure varies by over 5 psi. My engine is a NA 4-cyl BMW S14 with ITB's and a notoriously poor MAP signal. My thought is the pressure fluctuations are due to MAP fluctuations / resonances. I would think that removing the vacuum reference would give a more stable, constant fuel pressure, and make tuning fuel more accurate, but I've never tried without the vacuum signal.

Can anyone confirm / deny my suspicions? Should I give it a try?

Thanks in advance.

Brendan

sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

The idea behind raising and lowering the fuel rail pressure in line with the intake manifold is to keep a constant pressure ratio across the fuel injector.

The primary reason we want the injector to operate at one consistent pressure delta is to ensure that 1ms of injector open time has the best chance of always equalling the same amount of fuel flow at all times, it keeps the maths simpler. Secondary reasons include maintaining a consistent spray pattern... and of course in "extremely" high boost situations to make sure fuel flows out of the injector into the manifold rather than air heading into the rail Laughing

For low boost or NA applications with plenty of injector head room and good injector lag times it's easy work around a static rail pressure without any downsides.
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Brendan · Joined: 14 Jul 2008 Posts: 14

Thanks Sam - what do you mean by head room, just having an acceptable (not maxed out) duty cycle? What would you consider good lag times?

I have to confess that I don't know lag/dead times for my current injectors, but am going to test to find out, and may be replacing the injectors regardless once I look at duty cycle (Am in the middle of a rebuild and re-evaluating my fuel system)

RootesRacer · Joined: 04 Apr 2005 Posts: 460 Location: Arvada, Colorado

Brendan · Joined: 14 Jul 2008 Posts: 14

sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

Hi Brendan sorry to confuse, the reason I mentioned head room was because I had qualified it as a statement true for both NA AND "low" boost applications, think 8psi Eaton chargers etc with a referenced linear response reg the rail pressure would jack up to the tune of 8psi when on full load, so if you remove that feature you'll lose some flow potential from the injectors as intake pressure increases and the fuel rail doesn't follow, hence you'll need enough duty cycle head room left to simply open them for longer to compensate.

Roots is correct in saying that without a reference line to the reg in an NA application the live baro on the day becomes the pressure which the fuel pressure reg see's.... but as the intake manifold maximum potential pressure is also baro it's of no consequence so it can be ignored.

My comment on lag times again is perhaps not particularly pertinent to you specifically but what I was getting at is that some of the larger flow rate diaphragm style fuel injectors suffer from quite a large increase in lag time (dead time) with increasing pressure ratio, so in some applications having the rail pressure drop in line with intake pressure at for instance "idle" it can be the difference between dropping the injector into say the bottom 1% of it's duty range or not, which can in turn be the difference between a smooth idle and a rough one.
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Pantera EFI · Joined: 12 Feb 2005 Posts: 1266 Location: So. California

FIRST, you need to inspect fuel injector location.

When the injector is in the manifold, below the butterfly you will have a Delta Pressure change when the engine is "throttled".

When the injector is ABOVE the Throttle plate you will have a lower Delta Pressure change if any.

This effect could CHANGE when the engine is boosted.

Lance

Brendan · Joined: 14 Jul 2008 Posts: 14

Bryan90GL-X · Joined: 04 Jun 2011 Posts: 2

Brendan,
Ive been playing with this for a few years myself. I have a 2L Mazda engine with ITBs running alpha-n. I understand all of the theory behind the fpr and how it is SUPPOSED to work but from just experience alone Ive had better luck without the vac reference. When I did have it hooked up, I always found myself having to tune around it, instead of with it. My idle tps-based fuel numbers had to be MUCH higher than even the next higher tps value. Also caused issues with accel enrichment. Thats just my personal experience, I have heard it done the other way with no problems. Good luck!

PS. For a cleaner map signal, are you using a good sized vac canister? Also make sure your TBs are balanced, regardless if you use map at all, your engine will thank you for it!

sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

Brendan · Joined: 14 Jul 2008 Posts: 14

Settled, I'm going to try it... I'll probably run a fixed/non-adjustable FPR in the 3 - 3.8 Bar range.

Bryan90GL-X · Joined: 04 Jun 2011 Posts: 2

Hey Brendan, how'd you make out with this?

Brendan · Joined: 14 Jul 2008 Posts: 14

This is a project for this winter for me - I am dealing with a spun bearing due to detonation right now. I noticed the fuel pressure fluctuations I was getting in my logs which is what led me down this path...

sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

Brendan · Joined: 14 Jul 2008 Posts: 14

BMW S14 Engine. Bearings are a known weakness with this engine for whatever reason. The theory is that the force from the detonation transmits through the rod, deforming the big end of the rod and making it easier for the bearing to spin.

The new engine has uprated rods, better oil cooling and baffling, and I'm doing my homework to make sure I don't screw something up again from the tuning and reliability side...

sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

Absolutely, there's no question at all that producing loads of cylinder pressure when the piston and rod are so straight that they don't yet have mechanical advantage over the crank will certainly decrease big end bearing life.

It's just not something which gets discussed in normal tuning circles very much, but I tune a lot of endurance racing engines so it's something I think about a great deal.
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Brendan · Joined: 14 Jul 2008 Posts: 14

Bumping up an old thread I started last summer as I have an Injector Dead Time question.

I'm now running ID725's @ 3 BAR. Pretty easy to enter the provided dead time data into my ECU, but I have to enter the data for one fixed fuel pressure and a range of voltages.

If I were to leave the vacuum reference disconnected - this means there is now a non-constant / varying injector delta pressure - wouldn't this throw off the dead time values? For example, with my vacuum reference left attached, my fuel pressure would vary by ~10psi based on the graphs I posted earlier.

Is this anything to worry about? Looking at the injector specs, it looks like the dead time would only vary ~.05msec for a 10 psi range.

Thanks in advance.

nfn15037 · Joined: 22 Dec 2005 Posts: 243 Location: Boston, MA

Probably less to worry about than the variable pressure ratio across the injectors you were dealing with before!

stevieturbo · Joined: 29 Aug 2006 Posts: 845 Location: Northern Ireland

I wouldnt worry about it. Variations will be very small, and if you're battery voltage isnt around 14v then you have other issues to worry about anyway.

For years many ecu's ran fine before these dead times were used.
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Pantera EFI · Joined: 12 Feb 2005 Posts: 1266 Location: So. California

I would also "not worry" where the reference location was positioned (manifold/free air).

I would WORRY if a fuel leak could cause a fire, it WILL, one reason OEM's go to manifold.

When one understands the above stated calibration standards:

1. Injector fuel outlet location
2. BARO correction slope NPC/PPC
3. Supercharged/non-supercharged

Then the engine will operate fine.

Lance