onfir3

Hey guys, I've got a strange problem. I'm getting some funny readings on my digital gaugem, like from 0.1 to 0.4 at idle, and .8 to 1.1 on the highway at between 4.5-5.7k rpms, and it ocasionally hits 1.3 or 1.4 for a fraction of a secont only to fall back down a second later. I hooked a boost gauge up to the ruber/braided line going into the ecu, and it red 10lbs vac at idle, 20lbs under load.. so thats probably the wrong line, huh :-D Checked the line comming off the wastegate, realized, thats probably not it either.. Anyway, I also checked the wg spring and assembly, its all straight and together. Um, so my question would be 1. What line do I use to measure boost, and 2. what does the vacuum at the ecu mean? Thanks for helping me, the somewhat intellegence impared Audi enthusiast. Cheers!
~NICK

mcorrow

If you are adding a boost gauge, you would want to "T" off the hose going into the ECU (sounds like that's what you did)... 1 atmospheric pressure is 15 pounds (1 bar = 15lbs)... Anything below 1 bar is "vacuum", because you've got less than normal pressure. Anything above 1bar is "boost". each .1 bar is about 1.5 pounds of pressure, so .4bar of boost would be 6 pounds of pressure..... Now, you need to add the 1 atmospheric pressure to that, so 15 pounds plus your 6 pounds is 21 pounds, which is the max that the stock spring and wastegate should get you.. so, if you're seeing 20 pounds on your gauge, that ought to be about correct.. My 5000cstq shows .4 at idle, and i believe that is correct..
Boxer actually had a pretty great write up a few days ago for someone about a similar type of question..

So, it sounds like your system is working properly...

Boxer, am I anywhere near close?

SteveAngry

Audi's stock gauge measures using Absolute pressure. It is also not very fast reacting or accurate.

Stock boost is 1.3-1.4 bar absolute. Anything below 1.0 bar is vacuum on the stock gauge. .4 is what you'll usually see at idle.

Using a good aftermarket gauge with a stock ECU you should see 10 in Hg at idle, pull around 20 in Hg downshifting, See 5-6 lbs of boost.

If you're seeing 20lbs of boost on a aftermarket gauge on a stock ECU you've got problems.

BTW, all the numbers given might be different depending on your elevation.

Steve
Michigan

onfir3

we revved it, and it kept going more and more into vacuum. it went from 10lbs vac to 20lbs vac. I guess, I'll keep trying with that, but where does the boost line originate from. Oh, and thanks for the Physics lesson guys, but I got boyle's law and bournelli's and all that jazz in HS, I gotcha :-) I'm sorry for my ignorance, though, I didn't think the gauge was supposed to read vac? as in it always reads 1.0 bar, since thats ambient pressure(im 150ft +sealevel :-}) so that boost would read 1.0 to 1.4bar. Since 1.0 bar=14.7psi=740 somethings of mercury and KPa.. Anyway! So, if it reads .4ish at idle, and 1.1- 1.4 under load, its good? Um, Oh. Thanks!
~NICK

boxerfan

mcorrow is almost right... he does fine until the addition part. Steven is concise.

Let me guess... you are reving the motor in your driveway! No load = No boost. In fact (again as covered a few days ago) as Katman pointed out... the motor is trying to suck more air through the TB than is allowed... hence you create more vac. You can't test until you put the system under load. Try a 3rd gear pull and see what you get.

onfir3

Erm.. Oh, i suppose if it can't suck air so well thru the Intake, that makes sense `_` hehe, thanks! I shall try this.

qpower

LOL 20lbs on stock ecu would definitly be a problem , fun for a little while though lol lol

onfir3

I get it. I just had to be shown, but applying everything I know, I should have sat and figured that out myself. Thanks. I believe my boost is fine then, I'll check tomo. Oh, Boxerfan, I know ive talked to you about a chip and spring, I am selling the car to my roomate, i told him to go to you, and when he's ready he will. Still wanna get a k24 or something similar. Thanks for the help thus far, couldn't have gotten this far without you.

Twisted5000

10 Hg seems a bit low to me at Idle. I'm getting near 18. Maybe just the elevation change?

mcorrow

I assumed we were talking absolute pressures...hence my addition...