How accurate is injector DC gauge in MT?

disc · Post by **disc** » Tue Jan 24, 2006 11:33 am

As above?
How much that depend on inj. opening time and other constants.

I want to know exactly how much reserve I have on my injectors.
Application is turbocharged and I think its vital.

Bernard Fife · Post by **Bernard Fife** » Tue Jan 24, 2006 12:00 pm

disc,

The duty cycle in MegaTune doesn't depend on the opening time or other constants at all.

It is simply the ratio of the applied pulse width to the available time, for the amount of fuel you are injecting. If the injector opening time is wrong, then the VE will also be 'wrong' to compensate, but the pulse width will have to remain the same to get the correct amount of fuel (and AFR), and the duty cycle stays the same.

Lance.

disc · Post by **disc** » Tue Jan 24, 2006 12:51 pm

Lance,

Thank you. Then is there a way to measure/display real injector DC? I'm not sure how much reserve they have, but I plan more boost in very near future (~2 Bar, just waiting for my 4 BAR sensor) and must have some sort of "insurance". As many calculators I have found on Internet, az many different results I have

.
I need to know the truth, don't want to blow my engine.

Bernard Fife · Post by **Bernard Fife** » Tue Jan 24, 2006 1:48 pm

Then is there a way to measure/display real injector DC?

disc,

Yes, use MegaTune to measure the duty cycle on a running engine. The DC gauge is an accurate and reliable indication (for duty cycles under 100%).

However, if you are asking how to estimate duty cycle in advance, this is more difficult exercise. It depends on the actual flow rate (and the actual fuel pressure), actual opening time, the req_fuel and the rpm (which sets the time availble for injection). Knowing all of these accurately is very difficult in practice.

To calculate duty cycle:

DC = PW/T

where PW = pulse width (including opening time)
and T = available time

PW = REQ_FUEL * VE/100 * MAP/100 * E + accel + Injector_open_time
and
T = 120/RPM (for a 4-stroke cycle and 1 injection/cycle)

Recall that VE represents both the VE and the AFR, and this is difficult to estimate accurately in advance. You can assume E and accel = 0.

So you woud have something like:

DC = (REQ_FUEL * VE/100 * MAP/100 + Injector_open_time) * RPM/120

So, using my engine as an example, with a REQ_fuel of 16.7, a VE of 112, and opening time of 1.0 and a MAP of 100, with a max pulse width at 5500 rpm we would get:

DC = 5500 * (.0167 * 112/100 * 100/100 + 0.001) / 120

= 5500 * 0.0197 / 120 = 0.903 = 90.3%

Lance.

Keithg · Post by **Keithg** » Tue Jan 24, 2006 1:52 pm

I was reading and thought I'd chime in. I am also boosted, though not looking for as much boost as you. I am at 220 kpa or so max. 2.0L 16V 4 cyl and am running 37 lb/hr injectors. They are ragged edge at max torque 80%+. OE with 10 psi max boost were 24 lb/hr. My hunch is that to keep a 2.0L 16V fed properly, you will need MS-II to get any decent idle, because we are talking 42 lb/hr or more to ensure that PW <80% which will probably drive idle PW into the uncontrollably small values for MS-I. IMO, YMMV.

As has been pointed out many times around here, AFR (wb02) is very helpful to know that we are 'close'. The other tool that is neded, especially at high boost is EGT. If you have a knock sensor, that is another piece that can be useful. These 3 tools will help to ensure engine longevity when used properly. These, in conjunction with a dyno, you would have everything you would need to tune for max power. My experience is very limited and is only with WB02 and Knock sensor. I have learned that my engine seems to like 10.5:1 at Max Torque to keep knock away. I really need to get an egt to see what is happening especially at higher revs to keep from melting pistons.

In short, pulse width (input quantitave value) is not important as the outputs... These are how the engine talks to you to tell you what it needs.

KeithG

disc · Post by **disc** » Tue Jan 24, 2006 1:54 pm

That's nice!
Very helpfull

Now I must do some math and I'll have the answer.
Thanks again!

dumetre · Post by **dumetre** » Wed Feb 01, 2006 6:12 am

There are plenty of people running MS1 with 800cc +injectors with good results. A 1:1 reg helps a ton if you plan on running a ton of boost.

boost junkie · Post by **boost junkie** » Wed Feb 01, 2006 11:10 am

2.0L w/ 4 780cc low-z injectors. Static pressure 50psi, with the 1:1 action its about 40 at idle. Pulse width is 1.4-1.5ms. Runs great.

keithmac · Post by **keithmac** » Thu Feb 02, 2006 4:25 pm

I runn 500cc/min on a 1.816v and the idle`s fine (set it to stoich, bounces between 14.2 to 15.6 ish under ego correction). I`ve found most calculators to supply injector specs that are too low, for may application anyway.