xu9j4 with itbs, injector location

[joris_andriesse 8]

Hi all,

I did quite some searching on the topic, couldn't find the answer, so hope the subject hasn't been covered to often before.

I purchased a set of Satchell itb's to run on my xu9j4 and with these you have 3 options for your injectors. Namely, using 8 injectors, using 4 injectors in the OEM location, using 4 injectors in the bodies.

 

I didn't have the idea for using 8 injectors, so it will become either 4 OEM location or 4 in the bodies. What would be the best loation?

Some additional info.
The mi is slightly worked. Lightend and balanced crankshaft and flywheel, flowed head, PT stage 1 outlet camshaft, PT stage 2 inlet camshaft, CR on 10.5-11 to 1, will be running on VEMS stand alone (installed and tuned by DP-engineering). No sollid lifters/uprated springs or whatever, so I recon that max rpm will be 7-7,5k.
The car will mostly be used on the road, so i'm not after max hp at max rpm, but a nice driveable car for some 'slow' sunday afternoon cruising

[welshpug 1,657]

for a road car I would use the oem location, as that will work better for low rpm part throttle area, though going 8 injector is very easy, you just put the feed to the front rail and loop the return off that rail to the original one

[joris_andriesse 8]

Thanks.
Placing 8 doesn't seem a lot of work in installing, but I would expect that that gives quite a hassle to sort that out while configuring the standalone management. But that's just based on "I think"

[Tom Fenton 1,541]

Depending on how the ECU handles it, it can be really very easy. Certainly with an Emerald ECU it’s a couple of tick boxes and tell it at which RPM to start using the outlets and what percentage of total fuel to use them for.

[joris_andriesse 8]

10 hours ago, welshpug said:

for a road car I would use the oem location, as that will work better for low rpm part throttle area, 

And why is that, how does that work?

#justtryingtounderstand

[SRDT 175]

oem location is made for smooth operation and efficient combustion, you can see that injectors are more or less aimed at the valves.

Injectors on the bodies are aimed at the manifold walls, that mean you will have a lot more wall wetting:

https://www.highpowermedia.com/Archive/wall-wetting-the-tau-factor

[joris_andriesse 8]

Thanks!

[Thijs_Rallye 87 4 Cars]

On 12/25/2021 at 1:23 AM, joris_andriesse said:

And why is that, how does that work?

#justtryingtounderstand

Fuel tends to stick and remain in fluid form in the relatively cold ports. In the OEM position the fuel is aimed at the intake valve where you will have less fuel "stuck" to the port, which decreases HC emissions. 

 

One of the reasons to use the injectors further away from the intake valve is for increasing the time for the fuel to atomize and mix with the air, which in general is more applicable for high rpm (8,5k+) / high horsepower engines. And even those often use staged injection where the injectors that are furthest away are switched on at higher RPM where the air flow speed is relatively high.

 

It all comes down to valve open time and the amount of air / fuel that you can get into the cylinder in that time. Back in the day the fuel flow from big injectors was hard to control at low pulse widths, which is where staged injection comes into play. 

 

With modern big injectors and ECU control strategies that low pulse width behaviour has been improved quite a bit where it now often is possible to have a decent idle with those big injectors.