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Thread: STI injectors @ 4 bar fuel pressure. Anyone have any knowledge??

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2015-04-03 11:54:19
STI injectors @ 4 bar fuel pressure. Anyone have any knowledge??
Hello Class

I have random question for you.Does anyone have any experience with running STI injectors @ 4 bar fuel pressure?? Specifically; has anyone heard of any issues of running STI yellow top injectors @ 4 bar fuel pressure (especially after back to back highway pulls for a baseline)??

I have a set of 800cc recapped yellow top STI injectors, and apart from having to jack up the fuel table numbers in the idle columns (seems most big boy sidefeeds have a hard time idling at low pulsewidths, which is fine), there is an odd issue where after several pulls (dyno/highway), for whatever odd reason the A/F ratio would skyrocket while idling, similar to having the injectors not firing. In boost, everything else is normal.

Also to note: 4 bar fuel pressure equates to about ~940cc of flow per injector.

SR20DET Blacktop (180SX, 98 MY)
BW EFR 6258
2.5" induction piping
Stock IM
3 inch exhaust

P.S @BenFenner, I am going to tag you as I know you currently run yellow tops in your B13.
Last edited by Boostlee on 2015-04-03 at 11-58-05.
2015-04-03 12:25:01
isnt that the typical drilled injectors issues?

- - - Updated - - -

and this should be in the tuning section.
2015-04-03 12:29:49
I was debating which section. If Ben (or any mod) wants to move it, go right ahead!

@Keo: So no, they are NOT redrilled. They are done by Witch Hunter perfomance. They simply "just remove the difusser plates ". There is a writeup on how it's done. I was going to post it when I found it.

EDIT: found it!

Decapping-top-feed-injectors-how-to is essentially the same process.

However, from what I reading once you get into the higher Fuel Pressures, the latency changes some and all sorts of wonky things begin to happen (like the injectors running a bit "hot" for instance).
Last edited by Boostlee on 2015-04-03 at 12-32-04.
2015-04-03 13:05:49
same shit
2015-04-03 13:30:10
If the injectors are not to blame, it sounds like you're running into issues with the injector latency times (or injector dead times).

With a cold engine, a good tune will dump a lot of fuel at idle to compensate for the lack of heat in the head which keeps a lot of the fuel liquid. Enough fuel needs to be injected to get some evaporated to meet your idle needs. This might be 20ms of injector open time (don't take these numbers too seriously). So the ECU is opening the injectors for 20ms. However, the injectors take maybe 5ms to actually open and start flowing fuel. So you spend 5ms of your 20ms pulse width opening the injector. Then you spray fuel for 15 milliseconds, then the ECU stops sending the signal and the injector closes and stops flowing. So you asked for 20ms of fuel and you got 15 ms instead.

Now when the engine heats up you need a lot less fuel at idle. It is very common to run into the situation where the ECU is signaling the injectors for 5ms of fuel or less. The injector takes all of that time opening so it is only fully open for an instant. It then starts to shut again. You might get a tiny bit of fuel if you're lucky, but not nearly enough for what the engine needs.

This gets worse and worse as injector flow rate increases. This tends to be an issue around 750cc. A proper tune and/or advanced ECU can eliminate most or all of this issue.
Last edited by BenFenner on 2015-04-03 at 13-31-57.
2015-04-03 13:54:15
Curious to see how this turns out..

Following that article, are you saying you can take any side feed injector and take the cap from a top feed and it will allow the side feed to flow more?
2015-04-03 14:03:08
^^ pointless Charles, just get the right injectors and be done with it.
2015-04-03 14:14:56
@BenFenner: Thank you for the explanation! My suspicions were somewhat leading to that direction as I had to jack up the fuel in the fuel tables to get anything to register on the wideband once the car reached a certain temp (read: for the injectors to not spend time in the "dead region") or to adjust the injector scaling much lower (again, same effect).

In addition, your second paragraph sums up the issue with big sidefeeds, high static fuel pressure and the need to adjust up on the fuel map numbers for the vehicle to idle correctly.

There is an injector offset (based on battery voltage or known as "latency" or "dead time" in NISMOTRONIC lingo) which I was delving into before posting this, but that is based of battery voltage. However, another thing which comes to mind is the ECT enrichment which can also be changed (although from the MENU is states is purpose is for post cold startup enrichment).

I am going to reduce the fuel pressure down to 3 bar to start and redo a basemap (with several offsets) and go from there.

@Jybfan04 ( Charles: ) It seems some sidefeeds are built with a "diffuser" and individuals are able to to allow the injectors to flow more. However, you lose important aspects, like spray pattern for instance, so unless you are capable to tune around it, just get the correct injectors.

I received these for a very good deal and wanted to satisfy my curiosity with modified sidefeeds and tuning. This is more of an experiement. ID 1000s will be ready to go in at some point.
Last edited by Boostlee on 2015-04-03 at 14-16-28.
2015-04-03 14:21:33
should've gotten the 850cc power enterprise injectors from me a while back.
2015-04-03 16:15:56
I ran stock 520cc sti injectors @ 4 bar without any issues for years.

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