Once you begin to max out the flow of the standard turbo you will also begin to reach the max flow of the standard injectors. Once 100% injector duty cycle is reached this means that each injector is constantly open, and for smaller injectors this will not be enough fuel for a powerful engine. The stock S13 SR20DET injectors are rated at 370cc of fuel per minute each, and are normally rated to about 170rwkw. I have surpassed this figure now, but the injectors are well and truly maxed out, running at 13:1 air fuel ratio, which is getting quite lean.
S15 SR20DETs have 480cc injectors standard, but i figured these would not be enough. Therefore, I was after 550cc or higher, as I had seen many cars making up to 250 rwkw with a fuel regulator before. I didn’t want to use a reg, as I prefer to just use the correct sized injector.
SR20 Injector Requirements
SR20s run an usual style of injector, in the form of being side feed and high impedance. Most injectors have a feed from the fuel rail on the tops, so fuel flows through the middle of them and out of the tip. Side feed, however, sit immersed inside the fuel rail, so that fuel is flowing all around them. They take fuel from around themselves and direct it to the tip.
The impedance is simply an electrical parameter to do with the current required to fire them, with most injectors rated at low impedance. SR20 injectors however are high impedance.
Other engines to share this style of injector are the RB25DET, the 1JZGTE, 2JZGTE and the VG30DETT.
HKS side feed 740cc suit SR20
Fuel enters from silver section surrounded by black half way down injector.
HKS top fed 600cc suit RB20/26
Fuel enters from top fittings and travels through the centre to nozzle at bottom.
I managed to get my hand on what I was told to be a set of straight fit 600cc HKS injectors. They were sent with a spare fuel rail and had the remains of some plugs on top.
Here is a close up view of the injectors. They are marked Denso (which make HKS and SARD injectors), but not much else in the way of helpfulness.
The Truth: Mazda 13B REW Secondary Injectors (FD RX7)
My research told me that HKS make 550cc and 740cc sizes for SR20 replacement injectors, not 600cc as stated. The only set that resembled mine were the 740cc seen in the table higher in the page. Then I was reading a magazine and stumbled across a picture of my exact injectors in an advertisement. The ad was for SARD injectors, so they appeared to SARD 550cc SR20 replacements injectors. So I thought they were until one of them would not fire after they were tried on the car. I sold them quickly for $300 and the new owner identified them as the secondary 850cc injectors from a twin turbo 13B REW Mazda rotary engine.
Only coming in pairs on the 13b, someone had obviously scavenged a set from two engines to make up four injectors in total.
I then found further information on this excellent page:
These red 850cc or purple 550cc injectors would make a great budget upgrade with some hunting around. But I had already sold the set so I needed to do something else now.
High Flowing Factory Injectors
What seemed a quick solution was to have the factory injectors high flowed. This was suggested and organised by Unigroup for $120 per injector. They came back at ~650cc which turned out to be plenty. Without an aftermarket reg on factory fuel pressure the injectors are at ~70% with a rich, safe tune. The advantages of these are they require no changes to the wiring or fuel rail, and look totally standard from the outside.
In case I ever need to swap back I picked up a spare fuel rail, reg and factory injectors for $80.