Hello All,

I need help on deciding which injector to run. I am currently running an SVO 30lb injector at 42psi. I ran on the dyno this weekend and the car had an A/F ratio of 15-1 across the board (this is WAY lean). I bumped the fuel pressure up to ~46psi and had the same result. I think my 30's are maxed out.Car made only 351rwhp SAE NA but picked up a lot on the 150 wet shot and made 535rwhp SAE. The a/f ratio on the bottle was around 12.9-13-1 across the rpm range. I am trying to decide whether or not to go with 36lb SVO's or 42lb SVo @40psi. Any downsides to going with a bigger injector as far as tuning? I am really leaning towards the 42lbrs as all the calculators I tried came in at ~41-42lb and hour at 80% duty cycle. When putting in the new injectors do I just adjust the injector constant and run the same tune? Any info will be appreciated.

Thanks

Matt

 

No problems tuning with the 42lbers get them in there quick. You will need to adjust the injector constant, and the injector offset vs. battery voltage table for the slower opening SVO injectors.

 

30 lbers should handle 351 rwhp just fine. your 15:1 A/F ratio isnt an injector maxing out issue. Its a WOT FUELING table error.

Dont go wasting money until you fix the tune

I run 42lb injectors on my motor and make over 500 fly wheel HP N/A and my injectors are not even at 80% duty cycle.

 

Just to clarify what everyone is saying here..... you size the injectors for your NA HP, not for your WET N2O HP. 351rw = 440flywheelHP, assuming 20% loss for the TH400 (a good assumption based on my numbers). At 0.50 BSFC (very conservative for NA) and 80% DC, you get a 34.3# injector. Running an SVO at 43.5psi would only require a 32.5#. With an efficient tune and a 0.46 BSFC, you should be able to run with a 32# injector... get a 36# if you are nervous. But don't size the injectors for the 535rwHP on the WET shot.... that isn't how you size them.

You shouldn't be running lean with your current setup.... you'd have to be running 13% shy on fuel to run at 15:1.... and you aren't pushing the 30# SVO's that hard. As advised above, find out why its running lean..... then nchange the injectors.

 

Thanks for the replies guys. These posts are very informative. I was using a 500 fwhp estimate for the injector sizing. I am leaving a lot on the table by running so lean. I am confident it will pull 380-390rwhp once I get this thing fueled right and that comes out to about 490fwhp. I already ordered a set of 36lb injectors and they will be going in on Sat along with a new tune. Hopefully, I can get the a/f ratio to 12.8 and actaully be able to pull more than 6000 rpm's on the dyno. Thanks for the replies again...they are appreciated.

Matt

 

Hey Injuneer, could you explain in a little more detail on how you figure out what size injectors you need.

 

Simple formula, that you can find online at many websites:

(Flywheel HP X Brake Specific Fuel Consumption) / (# of injectors X Duty cycle) = #/Hr per injector

(HP X BSFC)
--------------- = #/Hr per injector
(8 X DC)

Flywheel HP is obvious. If you only have rear wheel HP, divide rwHP by 0.85 for an M6; and 0.82 for an A4 to get approx flywheel HP.

BSFC is a widely variable number, but it is #/Hr of fuel required per HP. The online calculators generally tell you to use 0.50 #/Hr/HP for NA motor and 0.55-0.60 #/Hr/HP for a power adder motor. When I set up my engine and an engine dyno, we got BSFC's in the range of 0.42 to 0.45 for NA, and 0.52-0.53 for a 300-shot on nitrous on top of a 500HP NA setup. I think for a WELL tuned NA motor, you should use 0.46 #/Hr/HP. For a blower or DRY nitrous, use 0.55 #/Hr/HP.

Using "8" is obvious... we have 8 injectors (well... the V6 guys need to use "6").

Duty Cycle is another variable. The online calculators tell you to use an 80% (0.80) duty cycle. That means you should assume that the injector is only open 80% of the time that it takes the engine to complete 2 full revolutions of the crankshaft. As an example, at 6,000rpm it take the engine 0.020 seconds, or 20 milliseconds to complete 2 crank revs. If your injector pulse width was 16mSec, the DC would be 16/20 = 0.80 = 80%

The issue of duty cycle relates to the stability of the injector. If you try and cycle the injector too fast, it becomes unstable, possibly reducing the actual flow, disturbing the spray pattern, overheating the magnetic coil that operates it.... all things that will seriously damage your engine at max power levels. Each type of injector.... stock ball/seat, pintle style Bosch/SVO/FMS, Lucas disk.... has a point where it goes "critical". Not all of them crap out at 80%.... but the "safe" assumption is 80%.

There are people here who claim to be making 450rwHP on stock injectors, so obviously, at least for the fraction of a second they are under max load on a chassis dyno, they will "work".... the question is how long. I recommend that you follow the 80% guideline. Why cut it close. But you can probably go to 85% or even 90% and "get by"..... just do NOT assume anything above 90% DC.

So, and example, lets assume we have an NA engine that makes 500 flywheelHP.....

First, the way the typical online calculator would do it:

(500 X 0.50)
--------------- =
(8 X 0.80)

250 / 6.4 = 39.06#/Hr per injector.

If you want to take what I feel is a more realistic view:

(500 X 0.46)
--------------- =
(8 X 0.85)

230 / 6.8 = 33.82 #/Hr

All of this tells you the #/Hr flow rating required from the injector. But not all injectors are rated at the same fuel pressure. Our stock injectors are rated at 3.0bar (43.5psi). SVO's are rated at 2.7bar (39.15psi)..... if you run an SVO at 43.5psi, it flows more than it is rated for. You can calculate that by taking the square root of the pressure ratio....

(43.5 / 39.15) ^ 0.5 = 1.111 ^ 0.5 = 1.054

So a 36#/Hr SVO will flow 36 X 1.054 = 37.95 #/Hr.

Should cover all the bases.

A real world example.... For my combo... 300-shot (DRY) on top of a 500HP NA motor = 800HP. Injectors are 64# Bosch running at 58psi = 77.9 #/Hr each. Duty cycle is only 70%

Running the injectors at low duty cycle can be a factor to consider in a full sequential injector setup..... you can start to optimize the exact time during the intake stroke that the injector turns off. This is called "end of event" injector timing, and can affect power output. With the MoTeC engine management system, end of event injector timing is one of the programmables.

 

Matt,

I was running my Ford SVO 30s with the 383 dynoing 389rwhp without the nitrous and had no problems. I then added the Zex dry kit and still had no problems until I changed the regulator and forgot to bring up the pressure. Ouch! It was a good run but my last.

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