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Ideal Injector Size For 3.6 S/charged

We are running a 964 motor that has been supercharged (Cartronics) and it is reported that the flywheel power is in the region of 500 hp. We run the standard DME with a DASTEK piggyback system (D8).

When the car is on the dyno at DASTEK it is found that even at the leanest setting on their piggyback the mixture is way to rich.

They have suggested that we look at alternate injectors because at the bottom end they cannot come close to getting an ideal air fuel mixture. (This of course will influence the crispness with which the motor accelerates among other problems).

As a reference:

Standard 964 INJECTOR - Output/Minute 2.4 cc at 3.8 BAR yielding 4.9cc DUTY CYCLE (1000 pulses / minute)

#0280150737 INJECTTOR - Output/Minute 4.9cc at 3.0 BAR yielding 9.2cc (1000 pulses/min) (AUDI QUATTRO RALLY)

The Audi injector is the one that is currently in the car. It is a Saturated Injector at 15.7 Ohm. (The German technician added a JA! after the 15.7 Ohm - so we in jest also call it a 15.7 Ohm, JA! injector. LOL)

We are petrified of running lean under boost so we do not want to revert to the standard 964 injector. Please help. What injector would you suggest we try?

Johan

www.almost.co.za
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Old 01-29-2007, 10:45 PM
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At a guess, some good 55#/hr injectors? The Siemens ones, I believe, drop right into the BOSCH fuel rails.

Are you using some sort of aftermarket FMU / boost sensitive rising rate fuel pressure regulator with your setup, to increase the fuel pressure under load/boost?

Protomotive have custom Motronic maps for aftermarket forced induction setups, which works in conjunction with their dual rate fuel pressure regulators and high flow fuel injectors. They typically specialise in turbocharging, but I don't see why it wouldn't work with a SC setup?
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Last edited by WydRyd; 01-29-2007 at 11:31 PM..
Old 01-29-2007, 11:26 PM
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WydRyd - Are you using some sort of aftermarket FMU / boost sensitive rising rate fuel pressure regulator with your setup, to increase the fuel pressure under load/boost?

Answer: No, the fuel pressure (as measured after the final pump) is fixed at 3.2 bar.

Regards

Johan
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Old 01-30-2007, 01:24 AM
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Having a variable fuel pressure will help you by not having to have too large an injector. This will help the idle due to a better spray pattern.

Ruf used two injectors per cylinder so that only one was needed to spray at low rpm and idling.
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Old 01-30-2007, 06:26 AM
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With the 55 lb injectors the car should idle fine and they should be able to supply enough fuel to net 500 HP. the Unichip is a plus for tuning. Install a wide band o2 so you know whats going on at all times
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Old 01-30-2007, 06:05 PM
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Holly cow 500 HP with a super charger. Do you have pictures of your engine. I am starting to plan a turbo engine conversion but if I could get 500 hp by adding a supercharger maybe I should rethink my plans.

Was it purely a bolt on system or did you have to change the compression ratio?

Sorry to jump off your topic I am just intrigued by your set up. As to the ideal injector size isnt it more of a function of what size injector to run based on the fuel pressure that is being delivered to the injector?

I am not sure if it is more ideal to have a shorter duty cycle by running the larger capacity injector so I am really just throwing out things that I would be asking myself if in your position.

If all of the above does not make sense what I am trying to say is I am not sure if there is a sweet spot for fuel pressure/injector size/duty cycle.
Old 02-03-2007, 10:31 AM
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Is there a formula ... kind of like the formula for choosing veturi size for a carburator there must be something similar for injector size based on pressure displacment and boost?
Old 02-03-2007, 11:09 AM
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Quote:
Is there a formula ...
Yes there is

Right here

you use BSFC (brake specific fuel consumption)

turbo/supercharged engines use more fuel
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Old 02-03-2007, 11:48 AM
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Quote:
Originally posted by Larry Ratcliff
I am not sure if it is more ideal to have a shorter duty cycle by running the larger capacity injector so I am really just throwing out things that I would be asking myself if in your position.
Hi Larry,
Seems most don't like to go above 85% duty cycle. However, Protomotive go much higher than that. The problem is as I said earlier that too big of an injector at idle (therefore at a low duty cycle) will cause a poor spray pattern. This is why Ruf used two injectors per cylinder with only one being used for low rpm and idle.
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg

Last edited by NathanUK; 02-03-2007 at 02:34 PM..
Old 02-03-2007, 02:31 PM
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Here's some good tech info: Link

Something around 55# should work just fine. Siemens #55 are excellent injectors too. You can find them Here relatively inexpensive.

It's generally accepted that running over 80% duty cycle on most injectors for long periods of time will wear them out very quickly. This can cause abnormal failures like failing stuck open (can flood the cylinders) for example.
Old 02-03-2007, 04:00 PM
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Quote:
Originally posted by Porschefile

It's generally accepted that running over 80% duty cycle on most injectors for long periods of time will wear them out very quickly. This can cause abnormal failures like failing stuck open (can flood the cylinders) for example. [/B]
Dead on, and it can also cause them to stick closed(either completely or partially). Problem most people face pushing duty cycles is combining that with insane fuel pressure using rising rate regulators with absurd ratios(like 12:1) Pushing those limits is both potentially dangerous to the motor, but also to the car and yourself(fire/explosion).

85%DC is the fail safe. Most good tuners wont push 80% without stepping up one size and then tuning down, as long as fuel atomization isn't compromised at low speeds/no boost..

For street tuning ability, i've used components referred to as EIC(extra injector controller) or SIC(supplemental injector controller). VERY similar to what RUF used on the CTR as stated above. It allows stock drivability and fuel consumption with the stock injectors, but as boost kicks in, the EIC/SIC automatically references your trigger signal/injector signal and adds fuel accordingly through a second set of injectors based on a programmable table you input via PC.

for those who want more power, but can't quite fork the cash for a complete standalone it's a good option to look into. Still need to control timing; and good programmable boost sensing DIS can handle that for the most part. The EIC I love using the most is by 034 Motorsports.

weblink here: http://www.034motorsport.com/product_info.php?cPath=22&products_id=71

Probably old news to most. Good news to some
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Old 02-03-2007, 10:56 PM
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