I finally decided to go the larger injector route. I decided on 550's but not Bosch or any of the others mentioned on the forum. Also, I learned a few things about injectors that I did not know before. Specifically, how much more fuel they can inject at higher fuel pressures. Injectors are rated at 3 bar so our std 450's probably flow close to 600 at the std 58 psi that our cars run at and about 640 at 66 psi several guys have settled on after a host of mods. The 550's flow around 700 at 58 psi and over 750 at 66 psi. What I have decided on is a flow matched set of BlackOps 550's that have a proprietary operation that provides a more atomized stream of fuel to the cylinders and have stainless steel parts. They can easily provide all the fuel that I might ever need at a duty cycle around 70%.

 

What fuel pump setup are you running?

 

The SL55 with the AEM fuel pressure regulator and free flow filter from Rob.

 

Flow is balanced against the boost pressure and the boost RETARDS the flow rate by reducing the pressure DIFFERENTIAL. At high enough boost the fuel would flow backwards which is why I have my pressure regulator balanced against the manifold pressure, WoodyBeen trying to get this out for all to see and review. Enjoy.

 

Hi,

running injectors @ 85% is a ideal target.

You need to retune for bigger and especially different injectors but you probabely know this. You might be save @ wot but nothing else.

It may be better to check and modify the stock injectors to your desire. No guessimating here.

Fuel Injector Modification

Greetings from Germany

Steven

 

1. Woody's on the money - injectors are measured at 3barg in and 0barg out (atmospheric), if you apply 1bar back pressure flow will be reduced.

2. "In general" flow increases proportionally to the square root of the pressure applied....So... doubling the pressure increases the flow by approx. sqrt(2) = 140%.


Applying these 2 facts....
for a stock 450cc (@3bar) injector running off the in filter 3.9bar FPR and against a WOT manifold pressure of 1bar (14.7psig) the injector would see a total drop of 3.9-1 = 2.9bar - lets round that to 3bar......so its going to be able to deliver 450cc/min.
Now the design should have about 30% fat in it to account for a max duty cycle of 80% and some wear and tear/blockages etc. So you'd be aiming for 315cc/min.....In stock condition.

The final equation to add is "how much more fuel do I need if I up boost from 1bar (stock) to 1.5bar (stacked).
Additional air = [1atm+1.5]/[1atm+1] = +25%
So you should need to provide at least 25% more fuel - probably a little more for cooling, but then there's a little less air due to intake restrictions and inefficiencies.
+25% is hopefully going to +25% more hp...;>)
315cc +25% = ~400cc/min (rounded up)

So a 450cc injector "could" provide this - but you've significantly cut into your 30% head room. "Feel lucky punk!"
If you apply the 30% head room you'd need 520cc/min.

To get 520cc/min out of a 450cc (3bar) injector you need 520^2/450^2 x 3bar = 4bar
Because your mani prs is 1.5bar - you need to add that to your 4bar because that's the drop across the injector = 5.5bar from the FPR.....
......or you could just change the injectors out for 550cc (3bar) injectors and then hope your pump and FPR can deliver the fuel....I'll save that for the next post.

 

Part 2.
The stock Fuel Pump and in filter FPR needs to service 6 injectors delivering 450cc/min = 162,000cc/hr = 162L/hr at the stock fuel pressure regulators setting of 3.9bar.
Now this includes the 30% head room mentioned in the first post. So really its 113L/hr.

Applying the +25% more fuel requirement for a stacked 1.5bar manifold pressure makes 113Lphr into 140Lphr.

Now looking at the fuel pump curves for the Stock pump, the WalbroGSL392 and the SL55 below.
A NEW stock pump can deliver 165Lphr at the stock FPR pressure of 3.9bar. So it should be able to service the stacked 1.5bar (22psi) condition if you simply change the injectors. However if it's worn and running 10% off spec, say at 145L/hr you going borderline and an upgrade to the Walbro would be recommended.

You can see the SL55 is way off scale and all that excess fuel is simply getting recirculated back through the filter to the tank.


Now if you were thinking you could keep your old injectors and just change the FPR and pump....the stock pump will only do 115L/hr at the 5.5Bar FPR setting - but you need 140!....no dice.
In that case you'd cover it with the Walbro again.
Yes - this is telling us that an FPR of 5.5bar off a new Walbro (or SL55) pump should deliver enough fuel to feed stock injectors fuel to support 22psi boost.

L8tr....I just wanted to check the fuel line losses aren't too significant and it comes out at about 0.2psi for 3/8th" ID steel tubing at 140Lphr (0.6GPM). Which is getting significant and worth keeping in mind.