Let's See You Answer This One...genius'

[Munkyb0y]

R33, stock except for 3 inch turbo back mandrel exhaust, pod filter.

I found that i had a compliance 2.5" cat, so i figured it's restricting the system, and changed it to an xforce 3" highflow.

A quick rundown on my boost gauges. they're both splitfire. Gauge A runs off the factory nipple on the plenum and is mounted on my steering column. Gauge B runs off the hose going to the bov(was there when i got it), and sits in my glovebox for double checking stuff like this.

Before the new cat, Gauge B showed about 0.5psi above Gauge A. So boost was showing as 8.5psi on A, and 9psi on B.

After the new cat they show 9psi on A, and 8psi on B.

If they both went up or both went down, it'd make sense. but they've gone in opposite directions. wtf?

Firstly, someone re-assure me that there is no way the new cat is causing more restriction than the compliance cat (which was not gutted or damaged). ie. it was a sound move, and of benefit to the system, even though i couldve gone with a more expensive option like a 5inch big body catco, but i didnt.

Secondly...what is with the boost readings?

I've confused myself, and you, no doubt. If you've read thru this, congratulations, you're more patient than i am.

[R31Nismoid]

ambient temps both the same?

you werent taking one reading from a cold night

and the other form a hot day or something similar?

either way, if both gauges have taken two different readings contrary to each other... makes me wonder about how accurate they are

[Munkyb0y]

the readings were at the same time, so same temp.

had a passenger reading the one in the glove box.

[Devils_Advocate21]

boost guages are not acurate at all, also length of vacuum hoses may make some difference aswell, and guages are only a guide not a real representation of what the hell is going on, if you wanted results dyno it then see how acurate the guages are, compared to dyno result of boost, plus dont forget the error that may be seen from reading at an angle, same as if passenger looks at your speedo from an angle it will look different to what it really is, same as if you go by your speedo guage then your stupid as 99.9% of them are in accurate.

My opinion on this

Not only saying that, how do you know there isnt the slightest leak in one of the vacuum to guage hoses??? :S

Edited November 21, 2006 by Devils_Advocate21

[Munkyb0y]

i understand that gauges arent accurate etc. and leaks can cuase bad readings.

but...the gauges were reading about 0.5psi apart, and now they're reading 1 psi apart in the opposite direction.

u see?

also, is it even possible that a highflow cat can cause more restriction than the standard cat?

even if it is the cheap xforce option i went with?

[R31Nismoid]

the readings were at the same time, so same temp.

had a passenger reading the one in the glove box.

So... took inital reading... fitted cat, went for a drive immediately after fitment and took another?

Thats weird.

The gauges should be accurate, that is if your not running 11ty billion meters of hose...

was your old cat stuffed? collapsed inside?

[Munkyb0y]

So... took inital reading... fitted cat, went for a drive immediately after fitment and took another?

Thats weird.

The gauges should be accurate, that is if your not running 11ty billion meters of hose...

was your old cat stuffed? collapsed inside?

no, i didnt explain that properly, or you misundertood

the readings before and after the cat were not taken on the same day.

what i meant was, the readings showing the difference between the gauges is taken at the same time. ie, i'm not going out, reading one gauge, then coming back in, swapping etc, then out for another reading.

but i'm still baffled as to how one gauge showed higher by 0.5psi, and now it's showing lower by 1psi.

if the gauge on my steering column is correct, then changing the cat netted me extra flow which resulted in ~0.5psi gain (i'm happy with that).

If the other gauge is correct, then changing the cat actually cost me flow, losing about 1psi (not so happy).

the standard cat looked to be fine inside. all mesh intact, no loose rattling, etc.

[Sydneykid]

Gauge A runs off the factory nipple on the plenum

Gauge B runs off the hose going to the bov

Where does the hose going to the BOV come from?

If it comes from the plenum, they usually do, then my guess is the BOV is leaking 1.5 psi.

cheers

[Munkyb0y]

Gauge A runs off the factory nipple on the plenum

Gauge B runs off the hose going to the bov

Where does the hose going to the BOV come from?

If it comes from the plenum, they usually do, then my guess is the BOV is leaking 1.5 psi.

cheers

hmmmmm, interesting. yes it's coming from the plenum.

why now though? why not before the cat? is it maybe an extra 1psi that did it?

it's a gfb hybrid bov (full plumback), and the spring is set to quite soft. would tightening the spring fix a leak?

if not, how do u fix a bov leak?

[R33S2]

Have you tried swapping the vacuum lines to see if you get the same readings?

[Munkyb0y]

Have you tried swapping the vacuum lines to see if you get the same readings?

yeah i was going to replace the line to the gauge that reads low, and see if it was just that.

but i'd also like to know how to check/fix a bov leak.

will tightening the spring on the bov do anything for a leak?

[Munkyb0y]

so how do you check for, and fix a bov leak ? (gfb hybrid bov)

if there was a leak somewhere else, it would affect both gauges equally, correct ?

[Sydneykid]

so how do you check for, and fix a bov leak ? (gfb hybrid bov)

if there was a leak somewhere else, it would affect both gauges equally, correct ?

Check is easy, disconnect the vacuum hose from the BOV and block it off. Then compare the gauges. If the guages now read the same, then you know you have a leak. So check the hoses and the clamps between the BOV and the plenum, a 1.5 psi leak is easy to have and hard to find. If you can't find any leaks then it's the BOV, try tightening it.

cheers

[mad082]

did you have the same person reading gauge b both times. and were they looking at it at the same angle. or maybe the person just rounded up, or the needle had flickered.

go for a drive and look at gauge a and have someone look at gauge b. then compare readings. then swap places and repeat. then you will be able to tell if it is how the other person is reading the gauge.

[Munkyb0y]

yeah, i thought of that. and i will get someone to video cam the gauge so i can check it myself afterwards. lol.

but all this heat in syd right now. not the best time to be pushing the skyline. might wait for a cooler day.

i might just change the hose from the plenum to the bov, and then see what the reading says.

what diameter is that hose anyway?

[Munkyb0y]

i put a new silicon hose on from the plenum to the bov, with a new T-piece just before the bov, running to gauge B. all very secure, i cant imagine there's any leaks in the line.

gauge B is still reading 1 psi lower. tonight is a cold night, so i got 10psi on A, and 9psi on B.

the bov itself is bolted up well.

what bothers me is that there might be a pressure leak somewhere, hence the readings.

will adjusting the spring on the bov, to a harder setting, make a difference?

any others ideas of what to try, or where to look?

[J_Red33]

So did you actually try swapping the gauge connections over to see if the gauges still read the same when they are swapped?? What happened when you did?

[Munkyb0y]

no, i havent swapped the gauges over.

i guess i should try that. the reason i havent is that the drivers side gauge is a bitch to remove, and the hoses in the engine bay arent long enough to swap over. i might extend the hoses and try that next.

[Munkyb0y]

yeah damn it, wouldnt you know it, it was the gauge playing up.

swapped gauges around and the bov line showed 9psi, while my factory nipple showed 8psi (with gauge B).

so gauge B was giving a false reading.

i shouldve tried this first, but i didnt imagine the gauge would all of a sudden give a false reading, after reading consistently for so long.

anyways, let that be a lesson to me.

good news, there's no leak

thanks for the help