[Diy_efi] injector placement question

[Joe Lutz]

Dene, that is actually a very good question, one which I tried to find the 
answer to myself without a lot of luck.  When designing the intake system 
for our FormulaSAE car, I was presented with this same dilemma.  After much 
searching and talking to professionals, including my power plants professor 
who used to work in the Honda F1 engines R&D, I came to the conclusion that 
there is no set formula for injector placement.  Most advice I received was 
based on what has been learned from trial and error and empirical test data. 
  There are a few rules of thumbs though.  Generally as you move the 
injectors farther away from the ports, you loose a little in throttle 
response and low rpm efficiency due to increased wall wetting because the 
airflow is slower.   However farther away from the ports is good for higher 
rpm if the runner is fairly straight.  Reason is the air is moving faster 
therefore if you inject earlier, the fuel has more time to atomize before 
entering the cylinder.  This is part of the reason that F1 engines used 
staged injection with two injectors at different locations for each 
cylinder.  Assuming that you are running port injectors I would put them 
downstream of the throttle.   Are the ports siamesed?  If not, run 6 
injectors, one in each runner.  45 degrees to the runner centerline or a 
little less seems to be a good injector angle.  I would not run 1 injector 
for two ports unless you can time the injector pulses so that each injector 
fires according to the intake events for each of the two cylinders.  Reason 
is if you fire the injector once per rev and then after the injector fires 
the intake stroke for one of the two paired cylinders begins then 120 
degrees later the second cylinder intakes before the injector is fired 
again, guess which cylinder got all the fuel and which one starves.

Joe Lutz


From: Waterline <waterline at ozemail.com.au>
>Reply-To: List for general do-it-yourself EFI talk <diy_efi at diy-efi.org>
>To: diy_efi at diy-efi.org
>Subject: [Diy_efi] injector placement question
>Date: Tue, 01 Jul 2003 09:44:20 +0930
>
>
>
>.....new to this so feel free to laugh at the stupid questions.
>
>I'm planning on injecting a straight 6 cylinder motor (3.3 litre
>202cui).  At the moment the engine has triple 45mm SU carbies on a
>manifold with 150mm long straight runners and it then goes into a head
>which has three inlet ports which split off to two seperate ports inside
>the head.
>
>I am going to make three throttle bodies to fit this manifold but I'm
>wondering about the best positioning for the injectors.  I could build
>the throttle bodies with a place for the injectors just behind the
>throttle plates or I could weld a lug onto the top of each runner on the
>manifold and mount the injectors closer to the ports.  Given that this
>thing will be good for only about 270hp at 6500 what is the best way to
>go for my application?
>
>Also because each injector is effectively feeding two cylinders are
>there any special considerations I need to make there.  I'm thinking
>pulse timing etc.
>
>Do all the common aftermarket computers cope with the type of setup I'm
>suggesting?  I was thinking Haltech or Wolf3d at this stage.
>
>Thanks in advance.
>
>Dene.
>
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