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Old 11-27-2001, 02:27 PM
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rage2
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Did some research this weekend on injectors, and wondering if I'm calculating something wrong.

My stock injectors (Bosch 803's) are rated at 37lb/hr. I'm at 8psi boost and hitting 100% duty cycle on the injectors (any higher boost and I can see it start to get lean).

My goal is to be able to use 20psi on the track, 22psi for short "scramble boost" runs (such as a drag race) and 18psi as a daily level, so I'm targetting enough fuel for 24psi boost just in case.

So if we convert all this to manifold pressure :

37lb/hour injectors, 100% duty @ 1.55bar manifold pressure

72lb/hour injectors, 51.4% duty @ 1.55bar manifold pressure

72lb/hour injectors, @ 88.2% duty @ 2.66bar manifold pressure

Now, my question... The biggest injectors I've seen put on the 944 turbo's are the 52lb/hr Huntley Racing units. And these are on 2.8L cars running 20+ psi boost. How can that sized injector be providing enough fuel at full boost? Am I calculating something wrong? Is there something wrong with my stock injectors that's messing up my calculations?

I even found a Fuel Injector Worksheet/Calculator website and my numbers came up fairly close.

Hope someone can shed some light on this mystery. Thanks!
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Old 11-27-2001, 05:48 PM
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Luke
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I saw a set of 80 pounders so i know that they are out there. Sorry dont remember where
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Old 11-27-2001, 10:14 PM
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David Salama
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I am also trying to figure out what size injectors to buy. My understanding is that you must also adjust your calculation to consider the fuel pressure. Lets assume you have a rising rate fuel pressure regulator which raises the fuel pressure 2 psi for every 1 psi of boost. At 18 psi boost, your fuel pressure would rise 36 psi. Lets also assume you are running 48 psi baseline fuel pressure with your regulator. Lets also assume you have a 52 # injector. Now, follow the formula:

#/hr adjusted = {square root(total fuel pressure / fuel pressure the injector was rated at (usually 43psi))} times the injector rating in #/hr.

So for this given example:

#/hr adjusted = Square root {((36+48) / 43} times 52 #/hr injector

#/hr adjusted = 72 #/hr fuel

Now you can calculate maximum Horsepower for this set up.

Max Hp = #/hr fuel times (# injectors X .8 duty) divided by BSFC or horsepower per # of fuel per hour or .6 for turbo cars

Max Hp = 72 #/hr X (4 x .8)/.6

Max Hp = 384 Hp

Now you should be able to adjust these numbers based on your injector size, psi boost desired, max Hp goal, and fuel pressure.

Like I said, I am new to this but I believe this is how you make the calculations. Hopefully someone with more experience will confirm my numbers.

Good Luck !
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Old 11-28-2001, 08:53 AM
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Turbo Tim's your man...... He (apparantly) can get any rate (within reason?) of injector you need.

speedforceracing
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Old 11-28-2001, 11:29 AM
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Couple of points...

1) If your going to run larger injectors, I would not bother using a rising rate FPR; it'll unecessarily complicate the tuning process. Stick with a 3-bar (S2) regulator.

2) I'm not terribly conviced of the blanket statement on the Fuel Injector Sizing web site

...
* Brake specific should be .45 to .50 for naturally aspirated engines,
.55 TO .60 for supercharged engines,
and .60 to .65 for turbocharged engines.
...

From what I understand, a thermal well managed motor, with proper head and piston design (which a turbo 944 motor has) should be much better than .65 ... I'd go with .55.

3) So using 500 crankHP, 4 injectors @ 80%, .55 BSFC... I get 42.85 (450cc) injectors...

How much power are you intending to make?

4) Remember the trade off's with larger injectors... then tend to idle poorly (can't get the pulse width small enough, especially with the stock computers injector drivers.

I second the recommandation of ask someone with experience... SpeedForce racing is great, so is Vision Motorsport or Jon Milledge. All of them have extensive experience with the 944T motors.

Raagi
'88 951S
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Old 11-28-2001, 02:45 PM
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rage2
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Thanks for your insights everyone. I spoke with a people and found out the reason why they're recommending smaller injectors is because they use a rising rate fuel pressure regulator along with factory computers. Big injectors idle like crap. But since I'm using standalone fuel management, it makes more sense to use large injectors, 3 bar regulator, etc.

I'm looking to make 400hp at the wheels with a nice flat torque curve up to 6000rpm. Check my 951 mod story thread for what's been done to my car.

I'll keep everyone posted as to how the car runs with the monster injectors.
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Old 11-28-2001, 05:43 PM
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David Salama
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Raagi,

Could you explain your calculations a little more. When I plug 500 hp, .55 BSFC, .8 duty, and 70 psi fuel pressure into to the calculation, I come up with 67 # injectors. How do you get 42 # injectors ??

Thanks !
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Old 11-29-2001, 02:48 AM
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Danno
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The biggest injectors I've seen put on the 944 turbo's are the 52lb/hr Huntley Racing units.
Those are Rochester (MSD-2013) injectors that you can get for 1/2 the price that Huntley is charging.

Another note is that ALL FPRs used on the 951 is rising rate. Just that the aftermarket ones allow you to adjust the starting pressure. Even better is the Cartech FPR which allows you to set an idle pressure, then the point when pressure rises and also the rate at which pressure rises.

As for injector flow ratings, you always need to specify the pressure at which you measured this flow. I think Bosch and most people use 40-45psi. It is so common, that a lot of times, people leave out the pressure.

Since you're using the SDS with a quick minimum injector time than the stock Motronic, larger injectors can be used effectively. Also peak-and-hold injectors have better response times. For 400hp, 55lb injectors should be fine, no?
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Old 11-29-2001, 02:47 PM
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rage2
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Really? I didn't know all regulators were rising rate. In my boost correction curve, it seems relatively linear (3 bar fuel pressure regulator).

Anyhow, I used the injector calculator site and came up with 70lb/hr @ 3 bar fuel pressure for 450hp. We'll see if I made the right choice...
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Old 12-13-2001, 04:14 PM
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My motor uses Bosch Motorsports injectors. They are rated at approx. 83lb./[email protected] 3 bar fuel pressure. On high boost I run at approx. 90 psi. fuel pressure. Idle with the vacuum hose disconnected is 75 psi. The fuel pumps are Bosch Motorsports .044. The two pumps are plumbed in parallel. The high pressure and large volume injectors work fine. The fuel mileage, on boost is 4-5 mpg. I didn't think a 4-banger could suck this much fuel. Off boost a rather respectable 16-17.
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