Injector rating for a target power output
 

Is there a formulae for working out the necessary rating of injectors for a target output?

I am planning to use either a 300rwhp or 400rwhp engine for my wife's '85 shopping cart, and want to work out the necessary injectors. Reading various specs of forum members engines, it seems 2700cc total for 300rwhp and 3600cc for 400hp is typical. However, I can not find anything beyond this derived rule of thumb. The build will be conventional, 12AT with series 5 intake and turbo with FMIC, FMOC, dowelling etc. running at 12psi.

Hey Paul,

Take a look here: http://www.megamanual.com/v22manual/minj.htm It discusses injector sizing and has a simple calculator for you. The main thing you need to know is the engine BSFC. This number tells you how much power your engine can make for a given amount of fuel. I don't know the numer for the rotary, but I think it has been posted on here before. For the calculations, you want to use fylwheel HP, of course. Once you have the size of the injectors, go a bit larger to give some overhead. When you have the fueling in pounds/hr, multiply by 10.5 I believe to get cc/min.

If you have an ECU with staged injection, you can play around with the sizing difference between the primaries and secondaries. I believe 2/3 of total flow given by secondaries is a pretty good number (although some 7s run 50% in stock form). Smaller injectors will have more control, so it is good to have a bit smaller sized on the primary for good fuel control at idle.

Basically you want to be sure that you will always have enough fuel for an 85% duty cycle on the injectors. Some people just stick the largest injectors they can get a hold of, but you will lose control that way. A properly sized setup is the way to go.

Kent

Paul I had that same set up quite a few years ago and I just used 13bT injectors for the primaries and 12aT injectors for the secondaries. The rest of the fuel system was the standard pump filling the surge tank, a bosch 975 main pump and a malpassi rising rate fuel pressure reg.

Hope that helps.

12AT. I think I will go down the same track for my wife's car, but may look at various options for physically adding the two injectors. My own 12AT is still with two 880cc injectors with a slightly improved stock TO3 to give around 225rwhp. To build above that limitation is a jump stage, 4 injectors with a TO4 starting around the lower 300rwhp level. I am using a Mapassi at present but noticed Revhed is critical of them on his 300rwhp 13BT.

GSL-SE. Thanks for a very useful generic table in your reference.

Doing what all good forum members should do, go search, I found the following on the Ausrotary site posted some years ago by a Melbourne engine builder:-

"For the record these are a rule of thumb for engines I have built successfully over the years :
• 2 X 13BT and 2 X 12AT with good fuel volume and pressure will go to around 260 RWKW ,
• 4 X 12AT injectors will go to around 310 RWKW ,
• 2 x 12AT and 2 X 1000 cc Rochestors , 340 RWKW ,
• 2x 12AT or 1000 cc Rochestors and 2 x 1700cc indy blues = more HP than your 13BT will ever make."

Multiplying by 1.3 is useful information for US comparison

Paul

12AT injectors are getting old, harder to find and are quite pricey. You can get new Siemens 900cc injectors for a good price and combined with stock 13BT primaries should be ideal.

Yea,that sounds about right.

I run 550 primaries and 800 secondaries and its PLENTY.I dare say Im too rich up top,which is probably hurting my peak HP,but its better to play it safe and protect the engine.
Most guys running 1200-1600cc secondary injectors are just being cautious.Thats a hell of a lot of fuel,they usually will never run those injectors anywhere near 85% duty,unless they are going for big numbers.300-400 wheel HP is moderate output for a 2 rotor,although its nothing to skoff at either.

what engine management are you going to be running?

if your on a budget, go with t2 primaries (550) and 12at for secondaries(720).. that will limit you right around 300 hp(safely)..

if your going to run a programable ecu and have some extra money then i would do a s4 secondary fuel rail with a/n fittings(instead of the stock restrictive banjo type) and a set of 1200 cc injectors so there is pleanty of room to add more fuel if need be in the future :)

680RWHP12A- I use a Microtech ecu, the only question is whether to use fuel/ignition or keep to the fuel/12AT dizzy set-up as on mine. Budget is not an issue at this level of fun. My wife complained my 220rwhp roadster lacked kick so thought of building her a fast shopping cart as a spare time project. Then she can leave her Outback turbo for serious shopping.

The lack of kick is due to the smoothness the 12AT turbo comes in low down. My earliest turbo cars had all the driveability of a BP.

I run 850 Pri and 1600 sec. IIRC that should be good up to 600rwhp or so...

This is the philosopy I am using based on the reference given gsl-se;

"Injectors that have too large a flow rating will make it difficult to tune the engine at idle and cruise. Injectors that have too small a flow rating can starve the engine of fuel at full power, and seriously damage your engine. To determine how big your injector's flow rating should be, multiply estimated horsepower (HP) of your engine by the brake specific fuel consumption (BSFC)* and divide by the number of injectors and the desired duty cycle and you will get a rough estimate of injector size:

InjectorSize = (HorsePower * BSFC) / (#Injectors * DutyCycle)"

I will be using a BSFC of .55 and 85% duty cycle. First impression is to go with Revhed and Steve's 550/800 combination now I understand the theory behind the practise. It's great when the two line up!

I thought that I would add this. I did not look at SE-addicts calculator but I am sure that it is similar to this one. And for those that are too lazy to take out a calculator here is another option... :)

http://rx7.com/cgi-local/3rdgencalc.cgi

For 300rwhp I would run 550/850. The span is not too far apart so that you don't have to worry about stepping problems. And the Primaries are small enough that you should get better MPG's