Discussion: Why do so most FI "kits" come with piggy back injectors?

[DigitalFusion]

I'm curious as to why so many of the FI kits out there utilize add-on extra injectors and proprietary piggy back controllers for them instead of replacing the injectors completely and then controlling them? What are your thoughts as to why this is so common?

 

[Yamadog]

Well I know on the viper we do it because the ECU cannot be programmed to pull fuel at lower throttle and runs way too rich until wot the extra controller only kicks in piggy back injector at 3/4 to full throttle.

 

[mbarryracing]

Its all a cost factor... they are relatively inexpensive effective solutions to keep the price of kits somewhat affordable.

Most stock ECU's with NA calibrations / maps are not accessible be modified to accommodate being boosted, or for different injectors, etc.
Some OEM ECU's can be reprogrammed, but not easily. There is a rather significant initial investment cost associated with the purchase of the specialty hardware and software for the builder / tuner to be able to reflash the ECU's, then the development cost to create a new specific maps... reason why reflashes alone cost so much.
Aftermarket programmable ECU's are in an expensive cost realm all by themselves.

All that cost would get passed onto the customer.

Another is adjustability and simplicity. Piggyback controllers provide some level of adjustability to the customer.

 

[canuck5]

My experience was this over the years on the nytros stock injectors would run out of fuel at 12-14psi a few turbo builders tried the bigger green injectors but as stated you basically then had super rich stumbley bottom and more fuel for your boost. The answer was secondary injectors as stated they then use boost rpm and elevation to tell secondary injectors to spray and stock ecu does the rest. It does seem like the newest transition tho is going to ecu reflashes Sidewinder for example.


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[DigitalFusion]

Thats what I am having a hard time understanding. I keep finding things that say larger injectors were too hard to tune because they were too rich on the low end. Why it was so hard to get larger injectors to run? Bikeman does it all the time with PCVs. They even sent me a sample map of going from 330s to 550s on a RZR.

Push Turbo Nytro kits now come with 550s to replace the stock 330s, and a power commander.




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[Mtnviper]

Below is a copy/paste out of the Mega-Squirt manual. While it pertains to Mega-Squirt ECU's, the principles are the same for any ECU/EFI system and it helps explain fuel percentage as it relates to pulse width time.



Idle Pulse Width

You have to select your injectors based on the maximum horsepower your engine can produce to prevent the engine from running lean at wide open throttle. But why not just pick the biggest ones you can find?

The answer has to do with idle and cruise pulse widths. If you use very large injectors, your idle pulse widths get very short. This can drastically reduce the mixture ratio control that you have during idle and cruise situations, and lead to very poor driveability and seemingly strange tuning behaviour.

To illustrate, suppose you have established that your engine produces the lowest MAP reading at an idle pulse widths of 1.2 milliseconds, and your opening time is 1.0 milliseconds (considered the 'standard opening time'). Recall that MegaSquirt can only change fuel by 0.1 milliseconds at a time.

And also recall that MegaSquirt assumes NO fuel is injected during opening (which is close to true, since the injectors remain closed until the coils charge, then they snap open at the end of the opening time). Now if the net effect of the enrichments change by 2%, the pulse widths don't change at all. Even if they change by 49%, nothing changes. But once they change by 50%, the pulse widths suddenly changes to 1.3 seconds.

So the next leaner possibility is 1.1 seconds, and the next richer is 1.3 seconds. However 1.3 milliseconds is not 1.3/1.2x100% = 8.3% richer, instead it is (1.3-1.0)/(1.2-1.0) = 50% richer! The mixture becomes very, very rich, and the engine runs poorly.

To confuse your tuning efforts further, it may be that you are already near a threshold, so that a small change in one parameter makes a very big change in the air/fuel ratio in one direction, but no difference at all in the other direction!

But doesn't the EGO correct? Actually, it can't. Even if you set the step size to 1%, nothing happens until the 50% (i.e. 1.3 milliseconds) threshold is reached. That is, the step size only takes effect once the 0.1 threshold of PW is reached. And if the number of ignition events between steps is large, the engine may stumble and die before it recovers and leans out. So in fact you may be better to set the O2 step high (50%), and the number of ignition events low (say 2) so that the average over just a few injections is correct. It is a band-aid approach, however, and likely to induce ignition related problems.

Obviously the converse is true if the engine goes lean. It has to go at least (1.1-1.0/(1.2-1.0)=50% lean before anything happens. If does go lean, it may back fire and die before it gets a chance to become richer.

You might think you can get around this by decreasing the injector opening time (to get a larger "adjustable time") and increasing the VE (or req_fuel), but that doesn't work because the 'ideal' injection time is still 1.2 seconds, and the permissible step is still 0.1msec, regardless of the way you add the components of the pulse width up.

And making matters worse is the fact that many high-performance engines will want even lower pulse widths at cruise than at idle, compounding the tuning problems and introducing more driveability issues. A system with a very short pulse widths like this will be difficult to tune. It will appear not to respond at all to enrichments over a certain range of a parameter (say IAT), then suddenly it will seem to change so drastically that you seem to require an entire new set tuning values.

Now even if your engines idles perfectly at a very low pulse widths, changing load, speed, and other variable (EGO, IAT, etc.) will demand slightly different air/fuel ratios. However, none of them are likely to need exactly the +50% you have to choose from!

This is why several aftermarket ECU manufacturers recommend an idle pulse widths of not lower than 1.7 milliseconds. If yours is lower than this, you need to address it before you will be able to tune your engine for all operating conditions. Ultimately the best solutions are appropriately sized injectors or staged injectors.

 

[DigitalFusion]

Dangit, meant to hit more options and accidentally clicked submit instead.



I wish I could find some detailed information like that for the PCV. Altho I have to admit that I havent had much down time lately to do any deep searching.

Now if the net effect of the enrichments change by 2%, the pulse widths don't change at all. Even if they change by 49%, nothing changes. But once they change by 50%, the pulse widths suddenly changes to 1.3 seconds.

I guess that really sums up the information I am after. What is the cut off for fuel changes that starts playing havoc with tuning? If a person goes from the 324 or 330cc injectors in the Nytro/Viper/Phazer (seriously, what size are they? I have seen both values) to 550cc injectors, they would need to cut about 40% across the board to start with to get back to "stock" fuel levels.

 

[Mtnviper]

Dangit, meant to hit more options and accidentally clicked submit instead.



I wish I could find some detailed information like that for the PCV. Altho I have to admit that I havent had much down time lately to do any deep searching.



I guess that really sums up the information I am after. What is the cut off for fuel changes that starts playing havoc with tuning? If a person goes from the 324 or 330cc injectors in the Nytro/Viper/Phazer (seriously, what size are they? I have seen both values) to 550cc injectors, they would need to cut about 40% across the board to start with to get back to "stock" fuel levels.

Yeah, ultimately it may be one of those situations where you just have to get it running to see if the ECU/PCV are able to compensate for the injector size difference.

A good wide band O2 will help a lot with the tuning. You may already know this, but you have to be careful with wide bands as any kind of a miss-fire will give you goofy readings. If it's a rich or lean related miss-fire they can read the opposite of what it really is. I've chased my tail a time or two because of that!
It might not be a bad idea to install an EGT gauge as well. Some times they can help with tuning A/F mixtures since the readings tend to be more steady. Installing the EGT probe closer to the exhaust port tends to be more accurate, however with a turbo you'll want to install the probe just down stream of the turbo. This is because if the probe breaks off, it won't go through your turbo and damage it!