ZLover4Life

Not true. How do I know this? Because if it were true, the entire fuel system on the 300ZX could not handle any significant boost level, and I (s well as you) know that to be untrue.

This is because fuel pressure increases directly with boost pressure. As it is 6:47am, I don't remember what the vacuum is at idle, but I know it's in the vicinity of -15psi. When the car is at 0 vacuum, that means fuel pressure increases by 15psi as well. That means when you're just accelerating with no boost at all (or, in the case of an NA, accelerating almost at all), you have about 44.1psi + 15psi = 59.1psi of fuel pressure.

If you're statement that out fuel lines couldn't handle more than 62psi were correct, that'd mean that a Twin Turbo 300ZX could not boost more than 2.9psi without bursting a fuel line. (Using onlineconversions.com and knowing my car sits at close to -500mmHg, it's telling me that's about -10psi, which seems to low to me... but even so... that would mean an accelerating Z32 sees 54.1psi of fuel pressure. By your statement, a Twin Turbo could not boost more than 12psi without bursting the fuel lines.)

Guess what... I've done 26psi and never bursted a fuel line.

Those lines handle much more than 62psi. They handle at least 85psi, in my experience. 125psi is not impossible, realistically.

CanyonCarver

Most standard high pressure fuel injection hose is rated to 125 psi

snwbrderphat540

sorry, but you sure about that, are you sure its not 15 inHG. thats what vacumme is usually measured in

ZLover4Life

In parenthesis, I stated that I know my car sits close to -500mmHg at idle (this is from my boost controller's readings), and went on to discuss that, at that conversion, his statement suggests that the car couldn't boost more than 12psi. No matter if I was wrong about 15psi of vacuum or 15inHg, I know I'm right that my car idles at -500mmHg, and that his statement about bursting fuel lines is wrong.

snwbrderphat540

oh yeah i can see that and the point was made clear and almost hard to follow lol, just pointing out the psi/inHg difference as to that could confuse someone trying to follow what you explained, but the 500mmHg kinda clears that up i guess huh.... fine..... nevermind.....

soooo where is this guy that started the thread just like to hear some feed back from him now.

ZLover4Life

Well it was 6 in the morning when I wrote that post... lol

btw, I know vacuum usually isn't measured in psi (it's strange to speak in terms of negative pounds of anything), but for the purpose of the fuel system and it's direct relationship to the vacuum/boost pressure, it was necessary for me to do so. To say that fuel pressure increases by 1psi for every 51.715 milimeters of mercury or for every 2.036 inches of mercury only confuses people. But saying that fuel pressure increases by 1psi for every 1psi decrease in vacuum/increase in boost is very easy.

Probably embarassed to post because he didn't test the fuel pressure like we told him to and came back with a bogus lie because he was too lazy to do it right. But I could be wrong.

Goofyz

at sea level air pressure is normally about 14.7 psi. So at idle you should get about (negative) -14.7 psi. What we would normally call the full vacuum pressure of a car (natural or boosted) this 14.7psi is also felt on the liquid in the fuel tank but liquid builds pressure under its own weight too. So under normal conditions (non pressurized tank) the fuel line pressure would be a function of the height of the fuel in the tank + the atmospheric pressure hence the time for the tank to drain out is generally the volume, V, divided by the flow rate, Q. However, the flow rate Q is a function of the deltaP across the opening which is in turn dependent upon the height, H, of fuel in the tank. This is solved with integral calculus (I use S for the integration 'squiggle'):


t = S (A / Q) dH

using this you can get how long it takes to empty a tank if you know its dimensions. Your fuel pump adds pressure to keep the drainage at a rate consistent with the value needed to keep the air/fuel mixture correct. Since that value is always in flux due to throttle position car manufactures overdue the pressure at the tank and regulate it down the line. Its much easier to regulate fuel pressure through a line than over a changing volume of fuel in a tank. So, you get a fuel pressure regulator which is controlled by the vacuum pressure of the engine. That gives you the major acceleration control pre-boost levels. Post boost levels are when the fuel pressure regulator is run at its highest opening and then the major fuel regulation is controlled by pulse length regulation of the ECU/O2/MAF sensors loop at the fuel injector.

at least, that is what my dad told me...

snwbrderphat540

yeah....... see im out of AP physics..... soooooooo stop. lol, im not reading anything else that has a formula in it.

ZLover4Life

That whole first paragraph is correct but irrelevant because of the second sentence of the second paragraph. We have fuel pumps that change how the tank empties. Everything in the second paragraph is correct as well, but it's nothing new to the conversation. But there is one statement that bugs me a bit...

This ignores the fact that Fuel Pump Control Unit adjusts the speed of the fuel pump, which also regulates fuel pressure. Yes, injector pulse length is altered, but that's in conjunction with the fuel pump speeding up to provide higher fuel pressures and, thus, more fuel for higher RPM/higher boost.

This is all a useless tangent though (sorry to say). This thread can be summed up simply as:

Z is getting too much fuel.
Owner claims 125psi of fuel pressure, but might be full of sh*t.
Stock fuel hose can handle 125psi of fuel pressure.

CanyonCarver

Manifold vacuum in an internal combustion engine, at idle, at seal level should be ~18-22 in-hg. This is od school tech. You can tell alot about the condition of your motor by monitoring the varying vacuum pressures thru your rpm range.

Here is an interesting read for those that are interested. --->>> Vacuum Measurement <<<---

How is it that this thread got onto this subject?

ZLover4Life

When it was said that the fuel lines couldn't handle more than 62psi, I explained that it was a ridiculous statement because fuel pressure varies directly with manifold pressure, and therefore the lines must withstand significantly more than 62psi.

CanyonCarver

oh yeah.

iirc, some of the tt.net guys are seeing fuel pressure of 80+ psi when boosting.

ZLover4Life

I have no doubts of this, but I'm sure they're also running smaller injectors and increasing the fuel pressure with an adjustable FPR to compensate for less injector flow. 80+psi of fuel pressure with a stock FPR would presuppose about 30psi of boost, and not many people run that much. lol

tc300zx

i gess the aftermarket fpr with guage in it might be broken im getting anoter fuel pressure guage, its got new high pressure lines on it and it was leaking a little bit ill re test it...

ZLover4Life

In my completely honest opinion, you shouldn't be putting any FPR (or fuel system component) in your car unless it came in a box marked "Nissan" or "Nismo." But yeah, get an inline fuel pressure gauge (they have them at AutoZone) and T into the fuel line... let us know the readings.

91zxtt

For what it's worth.....my boost/vacuum gauage reads 18-20 psi at idle.

snwbrderphat540

inHg. COUGH* couhg* ehem.... wow sorry boot that little tickle in my throat

ZLover4Life

No, that's in PSI (didn't we already go through this?). And that's actually a healthy vacuum reading in PSI.

18psi of vacuum is 36.64inHg, 20psi is 40.72inHg. So his -20 to -18 psi vacuum reading is -40.72 to -36.64 inchces of mercury.

And Eric, for future reference, you'll never see an inch reading anywhere on our Z cars. They're metric... so the readings would be in mmHg if they were not in PSI.

snwbrderphat540

my bad, i was thinking aftermarket boost guage, i forgot the Z32's boost guage actually has NUMBERS on it stock and not just little pictures like most stock boost guages, i officially apologize then. guess its been a while since ive even seen teh inside of a z32 let alone a TT one....

ZLover4Life

Bingo.

Aftermarket gauges have different ranges for the tick-marks when they convert from one measurement (PSI) to another (inHg), like this...



Since the gauge on the TT is equally spaced on both sides and numbered equally, it's basically reading negative psi. As we've discussed, that's not a good way to read vacuum, but, as I also said, when discussing the direct relationship between boost/vacuum and fuel pressure, knowing the vacuum in psi makes it easier to see.

CanyonCarver

All I know is that vacuum readings at idle will be ~18-22 in-hg. You can convert between in-hg & psi, and the metric system all that you want, You won't have 40 in-hg vacuum tho.

Goofyz

when they do the comparison of those 4 vacuums late night on TV with the blue water in the columns that last one gets way more than 40 inch of Blue water... which as everyone knows is way prettier than mercury and when it freezes sometimes falls from airplanes which makes it airborne...try that with mercury.

ZLover4Life

I stand corrected... your agreement with Eric made me wonder if I might be wrong, so I dug up Ash's 6 P's post and, you're right, -22 to -18 inHg (not psi) is a proper vacuum reading at idle. This is approximately -10psi, which means at idle the car should measure about 35psi of fuel pressure (the difference between vacuum/boost and fuel pressure must be about 45psi... I remembered 45 was the number, just forgot that it was the difference between the two). That's what I get for not touching a fuel system in 3 years...

And Eric, the stock TT boost gauge does read in psi only, but only goes to -14 and never reaches that... so it does read about -10psi.

For anyone who wants to read...
http://www.twinturbo.net/net/viewmsg...&msg_id=922678

snwbrderphat540

yeah while i cant doubt your knowledge on your own stock boost guage, when you weresaying those average numbers for vacumme, i was like.... uhhhh, i swear 15-20 in HG was standard not that in PSI.... but w/e that still doesnt do anything for this guys fuel problem nor my ignorance to the TT guage reading in PSI. lol...

91zxtt

What's funny is up until just now, I had never noticed the in-hg on the left side of my boost gauge....I just saw the psi on the right side and figured that was what it was being measured in. I'm lame... ..thanks Eric for pointing that out. That makes me feel much better.