I was on the RC Injector (RC Engineering site) and found some calculators to estimate required injector sizing for a given crank HP.

http://www.rceng.com/technical.aspx

Question/Concern:

K24/18gs are capable of 625 crank HP at 1.3 bar. Using the calculators, the required injectors are 61 lb/hr assuming 85% duty cycle & 5bar (73.5psi) fuel pressure. Crank the duty cycle to 100% and 52 lb injectors are required at 5bar pressure. The stock injectors are 410cc (approx 39 lbs) and will need a duty cycle of approx 132% to support 625 crank Hp

Does this right or I am missing something? Are the k24/18 kits supplied with only a 5bar pushing the duty cycle this hard? Seems risky to crank the wick up without enough fuel.

 

I think according to one tuner with a flash on stock K16 you're already out of fuel, so you're right.

 

This is pretty spot on. In reality, from a safety and good tuning prospective.... Flashed cars need injectors.

Thats a very hard sell in this market.

 

I just had a discussion with Todd K and Akram about this matter. 60 pounders are good for 24/18s but when going w/ bigger turbos and all the things that go along w/ them (secondary pump, intake etc.) it is better to go bigger.

Logically, bigger would then be 72 pounders, but the 72's are really 60 pounders with just larger holes punched in them.

Thus we are going w/ full 83 pounders for my new set up. I was concerned about drivability but was assured that w/ the proper tune it will not be an issue.

 

wouldn't it matter if you are running stock fpr or 5bar? Most of the lb mentioned is at 3bar, so if you are running 5 bar fpr, it would be higher.

 

The calculations are based at 5bar (73.5 psi).

 

check out sprayitracing.com the bosch ev14 are direct drop in. 52lb & 72lb are rated at 3bar.

 

Our injector pricing is identical or even slightly better then this including plug adapters...

Emails sale@eplabs.net if you need anything .

 

Obviously, you should consult your tuner, but I think you will have a better running car with smaller injector at a higher pressure than bigger injectors at a lower pressure. Atomization is key, and you need pressure to get that. Just my .02.

 

Warning long post but important subject IMO.

Injector manufacturers rate flow at a specific pressure. Normally, 43.5 psi is chosen as most OEM fuel pressures are in this range.

There are basically three ways to supply more fuel (combination of the three is normally used as there are limitations to each):
1) Increase fuel pressure
2) Increase duty cycle
3) Increase injector size

Increasing pressure will flow additional fuel & the new flow rate can be calculated:
Square root of (New pressure/Old pressure) X Old Flow rate
Example (Square root (73.5psi/43.5psi)) X 58 lbs/hr = 75.4 lbs/hr

Estimated required injector size can also be calculated:

(Estimated Crank HP X BSFC)/(#injectors X duty cycle)
BSFC = Brake Specifc Fuel Consumption = 0.60 to 0.65 for turbocharged applications
duty cycle - RC Injectors recommends 0.80 max

At 625 crank HP - approx 82 lb injectors are required at 43.5 psi. At 5 bar 63 lb injectors are required. Smaller injectors can be used but with increased duty cycle.

Given the calculations, I cannot understand why anyone would risk a $25K motor with high HP & stock injectors? IMO, the kits are packaged with only a 5bar to keep the cost down. For many, costs will include both injectors & install labor.

EPL has some nice injectors (Bosch EV14s) at a really good price. Cheap insurance in IMO.

 

Keep in mind you need to be tuned to run larger injectors. Tony sells the injectors & handles the ECU update.

 

Absolutely agree. This is exactly why I spent a little extra for the bigger 72#ers with my kit. I know many are running fine with just the 5 bar but still....

 

That has always been my philosophy, but I am not going to second guess Mr Protomotive and Mr Motorwerks. I expressed my concerns about stalling and only drive ability due to running at the low end of the duty cycle but apparently the tune will resolve those issues and I will have extra safety.

 

Just a small correction..

BSFC for a 996 turbo is around 0.55 + chassis dynos are optimistic when compared to the way the formula works, so you need to reduce your findings by a good 15-20% when choosing injector sizes and you will still be safe...i.e. a 996TT needs less fuel to produce HP than other turbo engines.

Stock injectors are rated at 3.8 Bar of fuel pressure.. For those having a boost controller it is safer for the tuner to use larger injectors just in case they get out of control..

 

Perfectly stated. I remember tuning my old turbo small blocks and they were happy at 11.0 A/F ratio. These cars like it much higher than that. I still only have 58lb injectors with a 5bar and nearly 600whp. My injector DC is what most would consider high, but not dangerously high. My Meth injections helps keep it in check. I am very happy with the way my car runs.