Tuning Megasquirt - Oversized injectors

Im in the process of tuning a 89 Fox mustang, 333ci stroker, AFR185's. MS2PNP standalone. at the moment Im running 24lb injectors and Im probably going to get the car dyno'd and have a bit more tuning done on the dyno in the spring. Before then Im looking to get the correct sized injectors for what this engine should output.

Car should easily make right in the 350rwhp range, maybe a bit more. In the future around 400rwhp would be ideal so I'd like to have some growing room as well. Now I've calculated it out and 36lb injectors would be fine for a car making 350, ideally I'd like 39-42lb but they are not availible to me at a reasonable price due to being in northern canada. border and shipping kills

Local speed shop here has nothing between 36lb and 47lb injectors. Now Im leaning towards getting the 47's.

Anyone with experience tuning megasquirt know if going to too large an injector cause any driveability issues or tuning problems?

thanks

Rushcanuck said:

Anyone with experience tuning megasquirt know if going to too large an injector cause any driveability issues or tuning problems?

thanks

I run sixty pounders in my ls street truck with microsquirt. No idle or response issues all summer. The base tune I had to work off of did not use 60lbs injectors so I rescaled the fuel map, but you can also just change it in req fuel settings and it will get you close.

32lb injectors will flow 50HP per injector... I know because I made 200HP on a neon using 4 .injector flow is a function of fuel pressure....that 200HP I made was with a nominal PSI of 53psi...the higher you turn up fuel pressure the more the injector will flow. With a proper fuel injection system, pressures of up to 70psi are perfectly safe(I myself have run as much as 65psi before) THat being said, there are other things to take into consideration...boost as an example...a 1:1 fuel pressure regulator(the most common type) raises fuel pressure 1psi for every lb of boost(to counteract the increased pressure inside the intake tract). A RRFPR(rising rate fuel pressure regulator)is an old way of tuning turbo cars and raised the fuel pressure between 5-10psi for each lb of boost....this is a factor because that car running 53psi...if it were working under 20lbs of boost...all of a sudden that fuel pressure would be 73psi using a 1:1 FPR or 153-253psi(never run pressures this high) with a RRFPR.

The way all this applies to your application is simple...you want to make 400HP without boost on 24lbers? Simply raise the fuel pressure to about 60psi or so, you will have enough safety margin in your injector pulsewidth for it to be fine.

Assuming that you are using stock foxbody fuel pressure(39psi) you can use 24lb injectors turning up the pressure to 65psi(make sure all your fuel injection line is new and that you are using fuel injection hose clamps)

Those 36lb injectors sold by your local speed shop would flow like 46lbers @65psi....assuming they were rated at 36lbs at 39psi to start with.

https://www.deatschwerks.com/fuel-calculators/flow-vs-pressure-calculator

A useful link for you that will tell you what size your current injectors will flow like if you turn up the fuel pressure to X

One last thing to keep in mind: Injectors are rated at a given pressure....some 24lb injectors may be rated at 39psi...others at 49psi...still others at 53psi...make sure you know what you have.

To address your other question: Idle quality...the more the injector flows, the worse the fuel atomization...large injectors drip more than spray...the bigger the injector, the more noticeably your idle quality WILL suffer. Keep in mind that this type of thing is subjective and ignition timing has a lot to do with it...but as an example...I personally like my cars to idle at 1000RPM...some people hate that and want their cars idling at 650-750rpm...I think this type of idle is fine, they do not.

36# injectors will get you over 400hp

The 24#'ers are good for 300hp

I ran 120# injectors in my old 2.3 4 banger. You should be fine. Jcamper

Isn't 400 hp right about where SBF blocks start to crack in half?  

47 lb/hr injectors won't present any real tuning problems here. Even an MS1 would probably have been OK on that, and MS2 has about 100 times more pulse width precision.

I agree, 47lbs isnt enough to worry about idle quality....you have to get up to about 650cc(62lbs/hr) before you would even start to notice

In reply to Wicked93gs :

lots of info here, thank you. I currently dont have an adjustable fuel pressure regulator on the car. had acouple failures previously and getting replacements is a bigger pain, so I ditched it for a factory 39psi regulator that I can afford to carry a replacement.

The car doesnt and never will have boost, Im already nearing the limits of a stock ford HO block and dont need much more for what I'm doing with the car.

Currently I am running ford racing 24lb injectors, they are calibrated at 39psi. The injectors Ive been looking at are also calibrated at 39psi

Now how much will having a marginally larger injector really have an effect on idle quality? honestly speaking the difference between 39 and 47lb injectors are not that much larger

Rule of thumb is 3x the nominal injector size for gasoline at 100kpa is the low load tunable limit. Example 30lb/hr needed on the motor, you can get away with about 1000cc/min without major low load problems. Injectors with better low load data are easier to deal with, especially on ms3 where you can have deadtime and low pw curves.

In reply to Rushcanuck :

I suggest always getting an aftermarket 1:1 FPR....it is very easy to adjust things if you wanted to say...run E85 as an example....and they give you a lot of adjustment in injector flow. I myself have used the el-cheapo Ebay FPRs like this:

These stupid cheap regulators have never failed me(they are very simple devices, so this is no surprise) and I have used them to great effect on a half dozen cars...at $25 a pop(they seem to keep dropping in price) you can always afford a backup just in case, plus they have a built in gauge, which is nice since it allows you easily check function and verify current fuel pressure(which also can be an early warning of a failing fuel pump)...I used to use the expensive Aeromotive regulators until I realized these did the trick just as well for about 20% of the price.

In reply to Wicked93gs :

I didn't realize how cheap they have gotten. You are right it's worth having a spare. Mine is still going but doesn't have a lot of miles on it.  

I assume megasquirt works a good bit like the Haltech in my RX-7.  Fuel pressure is just one factor influencing the amount of fuel metered by an injector.  The primary factor would be the "ON" time for the injector, specified in your fuel map, which should be very adjustable.  If I was to err, it would be on the side of "too big" rather than "too small."  Too big, and you might loose some resolution at idle and just above.  Too small, and you could exceed the duty cycle of the injector at WOT and run lean.

Re: idle, a lot depends on which injector you use. Newer ones like EV-14 can flow a ton a fuel and still atomize very well at low pulsewidths. They actually idle better than the stock 1.6L injectors that were in my Miata. 

There are real idle issues that come with huge injectors, but 47s aren't even close to that threshhold.