Big injectors with small pulse width
 

So it seems like the trend is continuing to go larger on injector size while keeping pulse width low. Just today I saw fleece dyno’d a truck at 700hp on 250% over injectors, a stock cp3, and a fleece cheetah turbo. The truck ran clean as a whistle and it really got me thinking.

From what what I understand this makes things easier on the motor while still allowing for great power potential but is there a point where the injectors are “too big” and pulse width can’t be lowered anymore? How large can one go on injector size before cylinder wash down starts becoming a problem if the truck is street driven and sees some idle time? At what point does idle cleanliness start to become affected? Thanks for any info.

That's on the extreme end of the spectrum. Surprised that a stock cp3 would hit 700hp. Around the 100-150% range you will start getting into some haze accelerating with traffic that might not be very apparent in the daytime but you'll see it at night when someone's headlights are behind you. The trade off is that when you start increasing pulse width the rail pressure drops and you'll start seeing more smoke, more heat, and lower power. It is harder on the pump as well when it's at max volume and pressure can't keep up. You won't wash down a cylinder unless the tuning is terrible.

My truck has 150% overs (they might be bigger than that because I was dropping rail on a 2200us tune) and that is the biggest I would want to run on a daily driver that needs to pass a visual emissions test.

But if you were to step up to a set of 250% overs and drop your pulse width lower yet what is to be expected? You may actually find that it could hold rail pressure and still make the same power (maybe more?). I’ve talked with members of the Firepunk team over the phone before and I was told that it’s not necessarily the size of the injector that drains the rail, it’s how long the injector stays open. I’m not sure however if the pulse width could be lowered enough to clean up the idle haze from larger injectors. A tune could most likely be created to pass visual emissions testing, the motor just has to rev and not produce a large amount of smoke. It wouldn’t be a tune you’d want to drive around much on though.

I am a fan of using large injectors and reducing the pulse width. My 05 runs less then stock duration and makes 1100hp.

Not all large injectors react the same way. Idle haze is more of a product of injector and nozzle design, than nozzle flow. Look at the flow sheets and see how even the flow is at low flows. Even flows that are lower then what you plan on running. This is a good indicator on what will happen at idle flows.

The lowest duration that the ecm will command is 160us. If in your tuning you command less then 160us, the ecm is skip that injection event. With large injectors you may have problems getting low enough duration for it to idle without skipping events (rough idle). You need to not only reduce the main event, but you will need to reduce ( or eliminate) the pilot and post events.

As a general rule. Longer injection events with smaller injectors is better for fuel economy, and short injection events with larger injectors is better for producing power, especially at higher rpms.

Paul

I opted for a 75% over nozzle from DDP to try to hit 800-850hp as clean as possible since I’m in the heart of the environmental consciousness. I can’t say whether or not I’ve hit that number but I can say I’m very happy I went with that small of an injector (which isn’t really that small); the drivability of the setup is awesome imo

Great info Paul, thanks. So by running a larger injector with a shorter injection event you are increasing your rate of injection, does this allow you to run less timing and therefore make things easier on the engine? I’m currently running a set of exergy 100% overs but I have a set of industrial injection’s stage 1 cobras that I’m contemplating putting in incase I want to step things up in the future. My head is off and getting redone so now is the time to decide.

It's not so much about making it easier on the engine, as it allow you to concentrate where the peak pressure occur in the combustion cycle. Generally we want the peak pressure to occur in the area of 7 to 13 degrees after top dead center. This value will vary depending on a lot of mechanical and thermodynamic variables. For simplicity sake, if we say that we want the combustion to go out in that range, then as rpm increases the start of the injection must start earlier. The earlier the combustion starts then the more reverse pressure (pressure while the piston is still moving up), will happen on the piston. Having a shorter duration and still deliver the required amount of fuel will reduce the amount of reverse pressure that's applied. The higher the rpm, the more this comes into play.

For mileage though a nice steady push works better then a hard sledge hammer like blow. This is competition diesel though. Who cares about mileage! I can race the whole weekend on a 1/4 tank of fuel. Pretty good mileage in my book! :-)

Paul

I have not drag raced the truck or put it on a dyno but I am hoping the 150% overs will be enough for my goals. I am hoping to be around 900.

Those injectors should get you there fairly easily in the right setup.

Did industrial ever get you a spec sheet on your cobra or is that highly confidential nda material :poke:

They are still being gone through and nozzles changed over to the late spray pattern. I had half a mind to ask Tyler for a spec sheet but I want one for my injectors and not just a standard sheet for any set of cobras. After what Paul said I want to take a close look at the flow numbers on the lower end of the spectrum as well as up top.

I have no idea if I’m allowed to know the exact numbers and even if I am I’m not sure I can share them...

I have a hard time understanding if you own something and the manufacture thinks there so special they can’t tell you exactly what it is. Maybe that’s just an excuse for covering up junk imo.

Secret squirrel stuff :doh:

I agree, but it turns out the injectors come with a flow sheet showing exactly what they’ll do. Industrial’s line of cobra injectors have shown they can make all the power you’d ever want and do so reliably, I wouldn’t call them junk.