How far away from the port can i put the injectors?

[olie05]

I want to go with an ITB setup, but I'm thinking about drivability. The throttles i'm going with have the injector in them, but for convenience I'd like to have them further away from the head (less angle on the runners). So there is going to be a limit here:

 

Too close too the head = bad runner angles, better driveability

 

Far away from the head = great runner angles (almost straight) but loss in driveability. (more homogenous fuel air mixture)

 

I'm trying to avoid adding a second set of injectors close to the head.

 

Seeing the accurate injection setup it looks like it places the injectors about 5" from the head (maybe more with a cannon manifold) and i'm wondering how streetable this is, at idle and low speeds, for example.

[thehelix112]

Having the injectors as far away as possible increases fuel atomization and hence improves combustion/power etc.

 

F1 engines have the injector exit a couple of inches from the END of the runner IIRC.

 

Also an article on autospeed specifically about this somewhere.

 

Dave

[Tony D]

There is no 'drivability' issues from having your injectors in the throats of the air horns! Key is to have the injectors impinge on the wall at about a 5 degree angle just before the throttle plate. That is what Ford Testing showed.

 

Of course, backfire issues keep them from doing this on most current Port-Injected engines. Anybody remember the HO 5-Liters tendency to immolate because of pooled fuel due to an ECU issue? They used two standard Bosch 45# injectors on their TBI setups...

 

But I digress. Your drivability will be fine with the injectors further away. What the injectors do close to the intake valve is improve low-speed emissions through the vaporization of the petrol when it hits the back of the valve.

 

If you go to the TWM Induction site, you will see now they have standalone supplementary rails that allow fueling from the ITB Throat/Airhorn area now for most of their setups. It makes more power, but most OEMs will keep injection between the closed throttle plate and the intake valve for that backfire safety factor. Practically speaking with a backfire and the injector out in the air horn you can have a bad situation happen if you don't immediately open the throttles to 'suck the fire out'---which is kind of counter intuitive for most drivers. Of course if you are in a properly designed metallic airbox, you can greatly mitigate the backflash issue. This all assumes N/A. On turbos airflow may dictate that you keep each injector somewhere in each respective runner simply to keep fuel from being blown off course. Many JDM apps had injectors mounted in the firewall side of the plenum shooting down the runners for the L-Engine---but I'm thinking they had a diffuser at the entrance of the plenum to keep velocity air from blowing straight across the fuel streams. HKS is good for making something that looks like it blows straight into a plenum, but in reality goes within a concealed chamber and is passed out into the main plenum through a complex diffuser setup.

[RTz]

Below is a nice article comparing opposite injector locations...

 

http://www.usrallyteam.com/content/t...ng_article.pdf

[RB26powered74zcar]

Below is a nice article comparing opposite injector locations...

 

http://www.usrallyteam.com/content/t...ng_article.pdf

 

 

Thanks for the link, and I'd really like to read it, but >> 404 Error Found

[X64v]

Found it.

 

http://www.usrallyteam.com/content/tech/injector_staging_article.pdf

 

The ellipsis was getting in the way of the url.

 

EDIT: Wow, that's an excellent article. Not only did it cover the advantages/disadvantages of stand off injectors, it touched on Braap's throttle body sizing arguments, rontyler's plenum air distribution work, and Drax240z's dyno tuning thread as well.

[olie05]

Believe it or not, I read that article before posting this question. The article revolves around an engine with "high" and "low" injectors. In my case I am going to be running "medium" injectors... where the injectors are neither right behind the intake valve nor right behind the throttle valve, but rather right in front of the throttle valve.

 

After reading Tony's post, I have been convinced that i will have no problem running the injectors right in front of the throttle.

 

edit... wording was wrong....

[Tony D]

Just remember for that kind of setup to work, the throttle plates must be cracked open slightly just like on an early carburettor setup to allow air/fuel mix to make it to the intake tract...

 

Having them behind the butterflies allows idle air to be controlled by a single point screw...which is easier as then you only have to synch the multiple butterflies for mechanical phasing, as airflow should follow identically oriented throttle blades. Though aside from synching the idle airflow, an IAC can still be used for additional air for cold startup and A/C idle-up if needed.

 

On an aside though, if you have something like a Mikuini Manifold where the airflow is almost straight down after the throttle plates, many of the ITB setups with injectors after the throttle plates will be shooing 'down' the manifold instead of at the opposite wall at a 45 degree angle. Combine this with the high-velocity air coming across the throttle plates at partial throttle openings and you can see that right behind the plates can, depending on the manifold, not necessarily be all that bad.