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Discussion Starter #1
I've been told/read many claims on how much boost a stock fuel system will sustain or that my motor will explode if I try to run more than 16 psi without upgrading my fuel system. Well here's my argument on the whole matter starting and ending with the airflow meter on our cars. The airflow sensor meters all air entering the engine (very accurately). So for the sake of argument lets say your running a T-25 or 14B (take your pick) at 16psi of boost daily. Also for the sake of argument and simplicity lets measure the total airflow going thru the meter from 0 to 100%. 0 being your engine is off and 100% will represent fuel cut. So a stock car running 16psi of boost is at let say 80% of the meters range. If we try running 20psi+ were at 100% and we experience fuel cut. Now lets compare our factory turbo with a 20G. Lets also say that a 20G flows as much air at 12psi as our factory turbo at 16psi. So now were running a 20G on an otherwise stock vehicle at 12psi and the car feels like our factory turbo at 16psi. Now logic would tell us that our bigger turbo at the lower psi is flowing the same amount of air while still running only 80% of the meters range. Now we bump the boost up to 18psi on the 20G and get fuel cut at 100% of the meters (ecu limited) range. Here we decide to back off to just 16psi on the 20G at 90% of the meters range. Alright with all that out of the way we can deduce that if we strap a 20G on a bone stock car in good mechanical condition we could run at least 15psi all day long no issue. Why (as if my long winded example wasn't enough), because the computer based on airflow supplies a proportional amount of fuel all the way up to fuel cut. Not to mention that a larger turbo wouldn't heat the air as much at moderate boost levels. I would also mention as proof of this "argument" that I have been running 18psi daily on my FP T-28 for the last 2 years on a completely stock fuel system and as long as there are no boost leaks the car has ran much better than my old T-25 at any boost level. Now if I try to run over 20psi the computer reminds me that the factory has determined what levels of boost are good for longevity and gives me nice fat fuel cut.
So my response to those of you who still think running more than factory boost is going to melt your engine, is that if you DO melt your engine you had to be trying pretty hard. The 4G63 is no factory honda motor with a turbo strapped to it that doesn't now how to add fuel in proportion to boost. Bottom line, we are perfectly safe within our factory airflow limits right?
 

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Haha, relying on fuel cut is stupid as hell. I ran [email protected] mph on stock 450 injectors and a 20G so i know what your saying, but fuel cut is not as conservative as you like believeing. Testimony to that are the DSMs with a hole in piston Number two. Even 550's aren't enough.
 

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Do you have any kind of monitoring equipment, or are you just saying 18 psi is safe because it hasn't blown up yet and it doesn't hit fuel cut? If you actually have an EGT gauge, datalogger, etc and it all looks OK, then great. If not, get with the program (and good luck with your rebuild).

I don't tell people that they'll "melt their engine" running more than 16 psi on a stock fuel system. I tell people who post those questions, "I'm not gonna give you an answer because you need to learn to MONITOR YOUR ENGINE and THINK FOR YOURSELF." May seem like the asshole answer, but if you have the right gauges, and tune with your fuel system mods in mind, you should be able to figure out what your safe boost level is.

Anyway - about this airflow meter BS - that's great that you're at 80% or 90% of the airflow meter's range (or whatever BS numbers you made up), but it's pretty short-sighted to just say "OK, the factory ECU and MAS can take care of it because it's still in range." Two BIG problems with that:

1: The ECU does not use o2 sensor feedback during WOT. It goes straight to maps at WOT. The ECU has no idea if the amount of fuel it's injecting from those maps is correct (because there is no o2 feedback). Those WOT maps may or may not be correct for your modified car. This is why we all buy gauges and dataloggers to monitor, and DSMLink/AFC/whatever to tune.

2: If the fuel system can't keep pressure up during full boost/high RPM use ("fuel starvation" - very likely to happen with a stock fuel pump running >16 psi boost), it doesn't matter what injector pulsewidth the ECU is using. Fuel starvation has nothing to do with whether your MAS is in range, or whether the ECU has hit fuel cut yet. The ECU doesn't know, and doesn't care, that you're fuel starved. It will sense knock, pull timing, and that's about all it can do to protect the motor if you stay on it. With a stock fuel system, you WILL run into fuel starvation issues before you get fuel cut from the ECU. Which means there is plenty of room to do damage without ever hitting fuel cut - and you will have NO CLUE what's going on unless you have the right monitoring equipment.

The ECU is good at running a stock DSM, and that's about it. Don't think it'll all be OK because the ECU hasn't shut it down yet... think for yourself.
 

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Discussion Starter #4
If you can get away with running enough boost on a 20G to hit 12s in the quarter mile on your 450cc injectors with any consistency, than its safe to say that the factory air metering was doing its job. Now if you come back a week later running the same boost or slightly less and your motor pops at the track, you can safely assume that something has definitely changed on your car as far as metered airflow. Probably the most threatening of all leaks that would make your motor go BOOM would be between the airflow meter and the turbo. Say for instance you don't have a good seal on the hose going to the intake of the turbos compressor, you will definatly run lean and at higher boost levels (less safety margin) you will probably put hole in one of your pistons. But all these scenarios are only possible/dangerous because your factory meter is no longer getting accurate readings. If we only had airflow readings based on MAP instead of MAF our cars would be much more reliable with serous mods. And why do you think were talking about the trashy O2 sensor, were talking about MAF here. You know that sensor that tells your computer how much fuel to add for the same amount of air. The O2 is junk anyway. What do you mean the fuel system can't keep up with the factory limits of our ECU, that's like saying the factory put in a fuel cut thats way beyond what they thought the fuel system could handle anyway. That's not the warranty hating mitsubishi that we've all come to love is it now. Yes many of our cars are more than 3,4,7, even10 years old and the factory components are in not so great condition, but thats also make my point in that our vehicles are just fine as long as they are within the factory airflow limits. Not counting the situations caused by poor engine condition.
 

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Discussion Starter #6 (Edited)
**Quote 1: The ECU does not use o2 sensor feedback during WOT. It goes straight to maps at WOT. The ECU has no idea if the amount of fuel it's injecting from those maps is correct (because there is no o2 feedback). Those WOT maps may or may not be correct for your modified car. This is why we all buy gauges and dataloggers to monitor, and DSMLink/AFC/whatever to tune. End Quote**


Uh NO!

Yea the car doesn't reference the 02 during WOT because its too slow and inaccurate! That's the MAFs job, how do you think the ECU knows how to interpret the factory maps. The DSMLink/AFC/whatever are use to tune/run the car based on MAF Hz not some magical linear curve that it follows at wide-open throttle. Try disconnecting your MAF and mashing the pedal. And yes I have a datalogger and read low knock counts at 18psi. 20psi = timing seriously starts getting pulled and running much higher I run into fuel cut.
 

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Off-topic: Christ you are thick-headed.

On-topic: I didn't bring up point #1 (open loop o2 at WOT) so you could rant about how slow and inaccurate the stock o2 sensor is (like you're saying something new to me). The point I made is, THERE IS NO FEEDBACK AT WOT... the ECU has no mechanism to make adjustments for your mods, airflow meter or not. The MAS measures pre-turbo air mass. A given pre-turbo air mass does NOT result in the same intake manifold air mass when you are talking about a stock car vs. a 20G car. Sorry, not gonna happen, not even CLOSE. Guess which situation the stock ECU and fuel system are designed for? The STOCK car running STOCK boost. The pre-turbo air mass that results from your mods may or may not be safe for the rest of your setup.. but you're letting the stock ECU decide what's safe? Come on....
What do you mean the fuel system can't keep up with the factory limits of our ECU, that's like saying the factory put in a fuel cut thats way beyond what they thought the fuel system could handle anyway
As for this pearl of wisdom... THE FACTORY DESIGNED THE STOCK FUEL SYSTEM FOR A STOCK CAR. Stock boost is 12 PSI (maximium, most are less!). Not 18 psi, not 18 psi on a 20G... 12 measley freaking psi. Fuel cut is a safeguard that makes the car cut out in case of a failure that leads to overboost. Fuel cut exists so Joe Average Driver in his stock DSM will realize that the car is fucking up and take it in to get fixed ("uh, gee, it's cutting out when the turbo kicks in, what's goin on"). Your friendly competant mechanic will note that the car is overboosting, and diagnose from there. End result is that the motor lives to see another day.

THAT is the reason why we have fuel cut. It's not a limit for you to get right up next to because you *think* it's safe. Fuel cut isn't designed to do anything except notify Joe Average Driver so you get DAMAGE CONTROL in case of overboost. Fuel cut is not there to cover your ass on your modded car. Mitsu doesn't design anything with modified cars in mind... mods cause warranties to be voided, remember? Especially when your "warranty issue" is that you slapped a 20G on and ran it at 18 psi on a stock fuel system, and ended up putting "speed holes" in your pistons. The dealer will gladly fix it... but it'll be on your dime, not the warranty's.

And Blackboost, I absolutely agree... this is circa-1993 Talon Digest material here, not new and exciting info...
 

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Spyder said:
Lets also say that a 20G flows as much air at 12psi as our factory turbo at 16psi.
That makes absolutely no sense whatsoever. If you increase flow you increase boost. This entire thread needs to get a clue.
 

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Re: Re: "Airflow" the mystical force :p

igs said:


That makes absolutely no sense whatsoever. If you increase flow you increase boost. This entire thread needs to get a clue.
Not trying to be picky here, but that's not entirely true. Boost is intake manifold pressure.. in other words, boost is what's *not* getting into the motor. You can have increased overall flow with the same (or lower) boost pressure by opening up the exhaust side of the motor more. Overall flow and boost pressure aren't attached at the hip.

But other than that I agree, this thread sucks.
 

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Re: Re: "Airflow" the mystical force :p

igs said:


That makes absolutely no sense whatsoever. If you increase flow you increase boost. This entire thread needs to get a clue.
In addition to the point he made about exhaust flow, theres also the fact that a larger turbo will tend to boost colder air containing more O2 and need more fuel...although its a small effect in the 12-16 area and sometimes you may need less fuel due to less knock, btu it is an effect or larger turbos...

Sean
 

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Discussion Starter #11
James92TSi you make some very valid points. So the 20G would actually allow more air into the motor even though its registering the same airflow at the MAF as the stock turbo? Sorry guys I'm just trying to get the full picture, not trying to waste your time here.
 

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Quote: "A given pre-turbo air mass does NOT result in the same intake manifold air mass when you are talking about a stock car vs. a 20G car. Sorry, not gonna happen, not even CLOSE."


Sure it does.... what, do you think that the 20G is making "air mass"?
The mass air flow rate going into any turbo equals the mass air flow rate coming out. The turbo is not making air, it just compresses it. Go back to your thermo book.

mat
 

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ummm....

umm...wont it be a lower mass coming out of the outlet, compared to whats going in, because of the incresed temperature of the air, it will be less dense, mass will be different?
 

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matmaster said:
Sure it does.... what, do you think that the 20G is making "air mass"?
The mass air flow rate going into any turbo equals the mass air flow rate coming out. The turbo is not making air, it just compresses it.
Read what I said again. Yes the mass of the air (a quantitative measurement) is the same pre-turbo and post-turbo. However, the AIR MASS arriving at the intake manifold will have different characteristics (temperature, pressure, density) with different turbos, pipes, ICs, etc. that will require in tuning even though the turbo is ingesting the same mass (quantitative) of air at the MAS. I don't think you understand difference between the terms "mass" and "air mass" as I used them, so I'll explain:

My quote was: "A given pre-turbo air mass does NOT result in the same intake manifold air mass when you are talking about a stock car vs. a 20G car. Sorry, not gonna happen, not even CLOSE. "

"Air mass" in that quote represents of the body of air being ingested and all of its properties. I am not talking about the mass (quantitative measurement - grams) of the air. If I were referring to mass (quantitative), I would say the "the mass of the air," or "amount of air" rather than use the term "air mass." As I understand it, that is the conventional way of using these terms to avoid confusion.. but apparently it's confused more people than have understood it.
Go back to your thermo book.

mat
Insert witty snobbish retort with rolleyes smiley and my name in lowercase letters here.
 

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Checked back for a reply and caught a couple typos in my last post. I'm past the 10min edit deadline... and my post doesn't make sense without these changes... heh

1st paragraph, 3rd sentence, add "changes": "...will require [changes] in tuning..."

3rd paragraph, 1st sentence, delete "of": "...represents [of] the body..."

:D
 

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ok sounds like there are pletny of smart people here to help me. Hope im not jacking a thread if iam tell me but don' be an ass. I have a 90 gst, stock except air filter, no exhaust from the DP back. Boost is at 10 psi, nothing else upgraded. I did a log and my car is advancing timing good enough now by 6k rpm's i hit a couple of counts of knock and timing starts to retard, i see that my airflow doesn't increase beyon 1607, i know that's as much as the logger will show(says so in pocketlogger.com) also my injector D/C are at 94% or so from 6k to 7k and i see that as soon as my mas reads 1607 HZ the injector P/W starts to gradually decline that's why im guessing im getting the knock. Could it be that im maxing out the stock fuel system at 10psi, the 02 volts i don't look as much but they were from .86-.88 pretty good i would say. Any input will be greatly appreciated
 

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whitegst97 said:
ok sounds like there are pletny of smart people here to help me. Hope im not jacking a thread if iam tell me but don' be an ass. I have a 90 gst, stock except air filter, no exhaust from the DP back. Boost is at 10 psi, nothing else upgraded. I did a log and my car is advancing timing good enough now by 6k rpm's i hit a couple of counts of knock and timing starts to retard, i see that my airflow doesn't increase beyon 1607, i know that's as much as the logger will show(says so in pocketlogger.com) also my injector D/C are at 94% or so from 6k to 7k and i see that as soon as my mas reads 1607 HZ the injector P/W starts to gradually decline that's why im guessing im getting the knock. Could it be that im maxing out the stock fuel system at 10psi, the 02 volts i don't look as much but they were from .86-.88 pretty good i would say. Any input will be greatly appreciated
Check for boost leaks. It seems odd that you would be aproaching 100% DC at 10psi.
 

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yeah, 1607 hz is the max the MAS can read, I believe, it's below 28-30 lbs/min (which is where you get fuel cut) I'm surprised you're not getting fuel cut at like 6K. Also, are you using the stock boost gauge? If you are that things is total garbage and unreliable, grab a boost gauge and see what type of boost you're running, I know that you said you have no exhaust from the DP back, does that include the cat? Becasue, I know my T-25 creeps with my DP/Catback combo, that is until the wastegate flapper blows open at 5000 and the boost drops down to like 13 psi


oh, and the whole 20g vs t-25 argument: You might be getting the same amount of "boost" but the air is much denser, technically allowing you to run leaner(with the same amount of fuel) without knock. Thats why people use intercoolers and an intercooler can make such a huge difference. A T-25 at 16-19 psi is putting out 300 degree air, which by the gas laws, is a whole fucking lot less dense than 110 degree air, at the same pressure. "Boost" is only what the fuel pressure regulator see's to increase fuel pressure, to the engine it doesn't matter too much, it's the "Metered" air, as in "Metered Air Flow" Sensor, not "Mass Air Flow" sensor
 
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