How do you know what size injectors to run?

[mukumu kurubushi]

Hi guys,

I have a target power figure i\'m working towards and been looking into what size secondary injectors i\'ll need. Being my first rotary, i\'ll admit i know only some basics of engine tuning, though im more than willing to learn.

I\'ve compared what other people are running against what power figure they are achieving. Ive also used the fc3s fuel calculator which ive read seems to make assumptions about your set up, so not too sure about what those are and how accurate it is.

I\'m thinking there must be a basic method of actually calculating what fuel delivery rate you need for what power level that newbies like me can use to work out what size injectors we need.

Would anyone be able to explain? This kind of information seems to be hard to find (or maybe im not looking in the right places) and would come in handy for someone like me, rather than someone simply telling me what injectors to get and then i\'m none the wiser.

Thanks!

[G@NDY]

Well according to that, my setup will support 259.1 fwhp. Shame as im hoping for 500+

Spoke to Brett at rxmotors today and he said that I should be okey with standard 550cc primarys and 1000cc secondarys, plus im using an 044 externally mounted pump and swirl pot. And thats on a half bridge with T78.

What Ecu are you running?

[L18M]

i think i\'ve gone bit over kill with 1680cc secondaries.. but all my mods are future-proof for when i get bored of my cheese tub n upgrade it :p

[Prof]

I had a massive post written out then the bloody forum died
 
Anyways - I\'ve found a good link here -
 
http://www.rceng.com/technical.aspx
 
For rotary engines - the BSFC (Brake Specific Fuel Capacity) is about 0.6
 
If you take the stock T2 engine with stock ECU, the primaries fire first, upto about 80%, then fall back to 40% when the secondaries stage. The secondaries will go to about 85%, the primaries stay at 40% (subject to coolant temps - cooler = more fuel)
 
Using their forumula - the stock 200 BHP engine:
 
200x0.6 = 120 lb/hr
 
putting in the figures from the injectors :
 
550 x 2 x 0.4 + 550 x 2 x 0.85 = 440 + 935 = 1375/10.5 = 130 lb / hr
 
The reason for the extra fuel is because the figures assume stoich or close to stoich, and the rotary likes to run rich (for boost, and also during idle etc.) It does explain why chipped ecu\'s and powerFC will make more power with the stock injectors. Much above 300 BHP and you need to go bigger

[m1tch]

Im running stock injectors, stock turbo and a chipped ECU and I will only get about 300 out of it, don\'t forget that if the injectors are too big, the duty cycle can be changed and they will work fine lol Im going to be running stock 550 primaries (keeps low down drivability) and then the 1680 secondaries for when extra fuel is needed. This setup is a bit eccessive, but I won\'t need to change my injectors every again lol

I would say 550 primaries and 800 or 1000 secondaries will suffice if you do NEED more fueling than that you would need to talk to a specilist as you will be running a crazy huge turbo and a HUGE PP lol

[dthebuk]

You can make 300 from stock injectors? How much boost!!!???!

[m1tch]

You can make 300 from stock injectors? How much boost!!!???!

0.9 bar

[Prof]

You can make 300 from stock injectors? How much boost!!!???!

12 psi (on a T04B hyrbid turbo) and that\'s at 90% duty - was actually 318 bhp and AFR\'s were 12

[dthebuk]

You need an uprated fuel pump for this then?
 
My car is running 0.9 bar on stock fueling and cooling and i assumed i was getting about 240 bhp?

[dthebuk]

ps, sorry for the thread hijack

[Prof]

You need an uprated fuel pump for this then?
 
My car is running 0.9 bar on stock fueling and cooling and i assumed i was getting about 240 bhp?

It\'s advisable - although the stock fuel pump is supposed to be good to 300+, but it will be getting old and tired and the wiring to it is shocking !
 
Rewire it - 1300cc is a good reference for that, and/or get a different pump (I run a walbro)

[dthebuk]

It\'s advisable - although the stock fuel pump is supposed to be good to 300+, but it will be getting old and tired and the wiring to it is shocking !
 
Rewire it - 1300cc is a good reference for that, and/or get a different pump (I run a walbro)

currently running a rewired pump i believe. I just thought i\'d be no where near 300 with a stock fuel set up. Time to upgrade the cooling me thinks.
 
awesome

[mukumu kurubushi]

Fantastic, this is exactly what i was looking for, thanks Prof.

My target HP at the fly is say 350ish. So from my calculations (been waaayy too long since ive done any maths!!) theoretically on a stock ECU I\'d need over 1000cc secondaries to achieve that.

I know thats obviously not going to happen due to various things i dont understand yet like pulsewidths and the limitations of a stock ECU. So, lets say I\'ve fitted my PowerFC on stock engine ports, and my primaries and secondaries are both at 85% duty.

My target fuel rate is 350 * 0.6 = 210 lb/hr

My primaries will give me (550 * 0.85 * 2) / 10.5 = 89 lb/hr

My secondaries will need to make up the remaining 210 - 89 = 121 lb/hr

so, (z * 0.85 * 2) / 10.5 = 121

cancelling out, z = ~750cc

So lets make it a nice round figure i\'ll need 800cc secondaries.

Does that sound about right? I would have thought 800s would have more headroom, from looking at other peoples setups. Also, I\'m making an assumption about all injectors running at 85% duty. I\'m not sure how realistic that is for a tuner?

I feel like i\'ve learned something! :wiggle

I had a massive post written out then the bloody forum died
 
Anyways - I\'ve found a good link here -
 
http://www.rceng.com/technical.aspx
 
For rotary engines - the BSFC (Brake Specific Fuel Capacity) is about 0.6
 
If you take the stock T2 engine with stock ECU, the primaries fire first, upto about 80%, then fall back to 40% when the secondaries stage. The secondaries will go to about 85%, the primaries stay at 40% (subject to coolant temps - cooler = more fuel)
 
Using their forumula - the stock 200 BHP engine:
 
200x0.6 = 120 lb/hr
 
putting in the figures from the injectors :
 
550 x 2 x 0.4 + 550 x 2 x 0.85 = 440 + 935 = 1375/10.5 = 130 lb / hr
 
The reason for the extra fuel is because the figures assume stoich or close to stoich, and the rotary likes to run rich (for boost, and also during idle etc.) It does explain why chipped ecu\'s and powerFC will make more power with the stock injectors. Much above 300 BHP and you need to go bigger

[Prof]

I reckon 85% is ok for the injectors - given that you\'re not running them at this rate for any length of time in real life :driving
 
Some prefer 80% as a max.
 
I\'ve picked 800\'s for my secondaries and I\'m aiming for 300 at the wheels - so about 360 ish at the fly - so your figures seem to match my own findings :Thumbs-up

[TiddlesRX7]

Let us know what you do go for in the end. Seems like Im gonna be similar in the long run, as are a few of us.