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Old 12-08-2014, 12:06 PM   #161
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Yeah, we have multiple guys saying that Yellow Tops (425's) are not enough for 240whp, and this thread saying that stock (240) is adequate. I still cannot see how both answers are correct. I'm not even debating 85% vs 100% DC.
It's interesting, isn't it?

I typically tend to side with the formulas & math crew, which would indicate that four 230cc/min injectors are undersized by around 40% (making all the usual assumptions) for ~250 BHP at full-tilt-boogie.

But then you have folks come along and post up dyno sheets saying "nuh uh, we clearly are making this power with those injectors."


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The other variable is the red line. If you stop at 6K, then your 100% is 20mS, but if at 7200, then it is 16.7mS. So you can squeeze more out of smaller injectors if you lower boost (power) at high RPM.
Actually, this is not true.

Power (generally expressed in HP) is all about fuel flow per unit time. If you are flowing so many cc/minute of fuel, then, all else being equal, you can expect to be producing about so much horsepower, regardless of engine RPM.

While it's true that you can sustain higher manifold pressures at lower RPM, and could therefore, in theory, produce more torque at a lower RPM, you forget that HP is developed as a function of torque * RPM. So if torque is increased but RPM is decreased, power will remain the same.



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Old 12-08-2014, 12:20 PM   #162
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Thanks Joe.

Put another way, If I want the same HP, I still have to add the same total fuel per event, therefore, if the injector needs 16.7mS at 7200, it would need the 20mS at 6000 if the boost (air flow per event) is turned up to make that same power. And, I have to be making more torque at that lower RPM (duh )

Has my perspective been re-aligned correctly?
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Old 12-08-2014, 12:32 PM   #163
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Put another way, If I want the same HP, I still have to add the same total fuel per event,
The same total fuel per unit time, not per event.

Let's say that it takes 1,000cc/min of total fuel flow to achieve a certain desired HP.

With four injectors, that's 250cc/min. And that's regardless of how fast the engine is turning.

This is the reason that the common injector-sizing formulae do not require RPM as an input into the calculation. As RPM increases, the maximum available injector duration per cycle (in ms) will decrease, but this is evened out* by the fact that the number of cycles per second increases by the same proportion.



* I am handwaving over dead-time, as on paper that's one factor that does not scale proportionately and thus does factor into the power-vs-RPM debate, however I am presupposing for the sake of this conversation that the injectors in question are already operating at 100% duty cycle anyway, and their dead-time is therefore effectively zero.
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Old 12-08-2014, 01:13 PM   #164
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if your math says you don't have enough fuel, but your AFR gauge shows you do, which do you believe?

they've repeated hitting 230rwhp on stock injectors at least 3 times now. Dyno sniff plots after a cat tend to be half to a full point lean so the plot here showing 12.0:1 at redline seems pretty good to me.

Would I do it myself? probably not.
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Old 12-08-2014, 01:18 PM   #165
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I like static injectors.
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Old 12-08-2014, 01:38 PM   #166
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Is there any evidence that suggests that if an injector actually sits at 100%DC for a few seconds at a time will it lock open? You will only be hitting 100%DC towards the top end of the RPM scale anyways and only during WOT pulls. If you are in fact hitting 100%...I would suggest we'd have to increase the power output until we start seeing lack of fuel being able to maintain a target AFR.

I'm going with no.

We have plenty of pwm solenoids on our cars that already do more times than an injector will in a day.


I'd also argue it's just not safe to go 100%DC simply because there's no "oh ****" room at that point.
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Old 12-08-2014, 01:43 PM   #167
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Is there any evidence that suggests that if an injector actually sits at 100%DC for a few seconds at a time will it lock open?
Has anyone ever suggested that they would?

I've never heard this before, but I cannot imagine why it would be true. There's no obvious mechanical property of the injector that would make it behave any differently after having been open for 10 sec vs. 10 ms, and presupposing that we're talking about high-impedance injectors, no harm should come to it by way of heat, either.

Now, I have heard that if you cross your eyes and make a face at your little sister that it will stick like that.

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Old 12-08-2014, 01:45 PM   #168
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I have personally had PE850's lock open on a subaru while tuning before. Car flooded and died. Would not restart. We noticed fuel pouring out of the header after a couple attempts. I learned a valuable lesson that day.

I'm pretty sure the guy still has them. I can ship them to you for inspection.
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Old 12-08-2014, 01:45 PM   #169
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I've read it more than a few times that "it's bad to go 100%DC on an injector because it will stick open/fail".

here and elsewhere on the truthnet.


edit: see ---^
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Old 12-08-2014, 01:47 PM   #170
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Is there any evidence that suggests that if an injector actually sits at 100%DC for a few seconds at a time will it lock open? You will only be hitting 100%DC towards the top end of the RPM scale anyways and only during WOT pulls. If you are in fact hitting 100%...I would suggest we'd have to increase the power output until we start seeing lack of fuel being able to maintain a target AFR.

I'm going with no.

We have plenty of pwm solenoids on our cars that already do more times than an injector will in a day.


I'd also argue it's just not safe to go 100%DC simply because there's no "oh ****" room at that point.
I probably didn't use "static" properly. I agree with you. If it were a problem to run at 100% DC, the MX6 would have had problems long ago.

I just don't like running injectors at 100% full open, period. For yes, the reason you say, no headroom. And also because any measure of control goes right out the window.
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Old 12-08-2014, 01:57 PM   #171
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I have personally had PE850's lock open on a subaru while tuning before. Car flooded and died. Would not restart. We noticed fuel pouring out of the header after a couple attempts. I learned a valuable lesson that day.
Well, that's interesting. I presume that the injectors had been operating at 100% DC immediately prior to the failure, and that root cause analysis indicated same as the proximate cause of the failure?

I learned something today.

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Old 12-08-2014, 02:52 PM   #172
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I'd also argue it's just not safe to go 100%DC simply because there's no "oh ****" room at that point.
It's not that there's no "oh ****" room - there's no room of any kind. In fact, there's less than no room. The dyno sheet being discussed here shows 12:1 AFRs (assuming the readings are post-cat) with 100%DC injectors at 1000ft of pressure elevation in 94*F weather. SAE correction on that sheet is 8%, which means the car actually only has fuel for 213whp if the pulls are done at sea level and 77*F ambient temps (SAE standard J1349). What happens when the owner of that car takes it down to sea level and merges onto the freeway on a 50*F morning?
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Old 12-08-2014, 02:53 PM   #173
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hopes he put his IC behind the radiator.
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Old 12-08-2014, 03:47 PM   #174
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hopes he put his IC behind the radiator.
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Old 12-08-2014, 08:33 PM   #175
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he becomes a statistic and gets to hear a new margarita joke
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Old 12-08-2014, 08:41 PM   #176
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It's not that there's no "oh ****" room - there's no room of any kind. In fact, there's less than no room. The dyno sheet being discussed here shows 12:1 AFRs (assuming the readings are post-cat) with 100%DC injectors at 1000ft of pressure elevation in 94*F weather. SAE correction on that sheet is 8%, which means the car actually only has fuel for 213whp if the pulls are done at sea level and 77*F ambient temps (SAE standard J1349). What happens when the owner of that car takes it down to sea level and merges onto the freeway on a 50*F morning?
He lives here at sea level and we've had those temps recently. Jerry, how's it been?
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Old 12-08-2014, 08:47 PM   #177
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hopes he put his IC behind the radiator.


dei.jpg

There's always this.
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Old 12-09-2014, 10:23 AM   #178
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He lives here at sea level and we've had those temps recently. Jerry, how's it been?

He says he hasn't been doing full throttle redline pulls.
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Old 12-09-2014, 10:31 AM   #179
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Don't get me wrong, but I have bounced off the rev limiter several times, which in the four years of ownership before turbo I've never hit the rev limiter

Sounds like here, people are expecting my car to grenade. So far, so good. No pinging, nothing out of the ordinary day that I can tell.
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Old 12-09-2014, 10:50 AM   #180
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My revlimiter is my shift light.
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