Jump to content
SAU Community

Recommended Posts

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.

Link to comment
https://www.sau.com.au/forums/topic/144130-lets-see-you-answer-this-onegenius/
Share on other sites

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

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 by Devils_Advocate21

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?

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?

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 :D

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.

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.

:D cheers :)

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.

:D 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?

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?

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.

:nyaanyaa: cheers :happy:

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.

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?

  • 2 weeks later...

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?

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.

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 :D

thanks for the help :D

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • Update 3: Hi all It's been a while. Quite a lot of things happened in the meantime, among other things the car is (almost) back together and ready to be started again. Things that I fixed or changed: Full turbo removal, fitting back the OEM turbo oil hardlines. Had to do quite a bit of research and parts shopping to get every last piece that I need and make it work with the GT2860 turbos, but it does work and is not hard to do. Proves that the previous owner(s) just did not want to. While I was there I set the preload for the wastegates to 0,9bar to hopefully make it easier for the tuner to hit the 370hp I need for the legal inspections that will follow later on. Boost can always go up if necessary. Fitted a AN10 line from the catch can to the intake hose to make the catchcan and hopefully the cam covers a slight vacuum to have less restrictive oil returns from the head and not have mud build up as harshly in the lines and catch can. Removed the entire front interior just shy of the dashboard itself to clean up some of the absolutely horrendous wiring, (hopefully) fix the bumpy tacho and put in LED bulbs while I was there. Also put in bulbs where there was none before, like the airbag one. I also used that chance to remove the LED rpm gauge on the steering column, which was also wired in absolute horror show fashion. Moved the 4in1 Prosport gauge from sitting in front of the OEM oil pressure gauge to the center console vents, I used a 3D printed vent piece to hold that gauge there. The HKB steering wheel boss was likely on incorrectly as I sometimes noticed the indicator reset being uneven for left vs. right. In the meantime also installed an airbag delete resistor, as one should. Installed Cube Speed premium short shifter. Feels pretty nice, hope it'll work great too when I actually get to drive. Also put on a fancy Dragon Ball shift knob, cause why not. My buddy was kind enough to weld the rust hole in the back, it was basically rusted through in the lowermost corner of the passenger side trunk area where the wheel arch, trunk panel and rear quarter all meet. Obviously there is still a lot of crustiness in various areas but as long as it's not rusted out I'll just treat and isolate the corrosion and pretend it's not there. Also had to put down a new ground wire for the rear subframe as the original one was BARELY there. Probably a bit controversial depending on who you ask about this... but I ended up just covering the crack in the side of the engine block, the one above the oil feed, with JB Weld. I used a generous amount and roughed up the whole area with a Dremel before, so I hope this will hold the coolant where it should be for the foreseeable future. Did a cam cover gasket job as the half moons were a bit leaky, and there too one could see the people who worked on this car before me were absolute tools. The same half moons were probably used like 3 times without even cleaning the old RTV off. Dremeled out the inside of the flange where the turbine housing mates onto the exhaust manifolds so the diameter matches, as the OEM exhaust manifolds are even narrower than the turbine housings as we all know. Even if this doesn't do much, I had them out anyways, so can't harm. Ideally one would port-match both the turbo and the manifold to the gasket size but I really didn't feel up to disassembling the turbine housings. Wrapped turbo outlet dumps in heat wrap band. Will do the frontpipe again as well as now the oil leak which promted me to tear apart half the engine in the first place is hopefully fixed. Fitted an ATI super damper to get rid of the worn old harmonic balancer. Surely one of the easiest and most worth to do mods. But torquing that ARP bolt to spec was a bitch without being able to lock the flywheel. Did some minor adjustments in the ECU tables to change some things I didn't like, like the launch control that was ALWAYS active. Treated rusty spots and surface corrosion on places I could get to and on many spots under the car, not pretty or ideal but good enough for now. Removed the N1 rear spats and the carbon surrounding for the tailpipe to put them back on with new adhesive as the old one was lifting in many spots, not pretty. Took out the passenger rear lamp housing... what do you know. Amateur work screwed me again here as they were glued in hard and removing it took a lot of force, so I broke one of the housing bolts off. And when removing the adhesive from the chassis the paint came right off too. Thankfully all the damaged area won't be visible later, but whoever did the very limited bodywork on this car needs to have their limbs chopped off piece by piece.   Quite a list if I do say so myself, but a lot of time was spent just discovering new shit that is wrong with the car and finding a solution or parts to fix it. My last problem that I now have the headache of dealing with is that the exhaust studs on the turbo outlets are M10x1.25 threaded, but the previous owner already put on regular M10 nuts so the threads are... weird. I only found this out the hard way. So now I will just try if I can in any way fit the front pipe regardless, if not I'll have to redo the studs with the turbos installed. Lesson learned for the future: Redo ALL studs you put your hands on, especially if they are old and the previous owners were inept maniacs. Thanks for reading if you did, will update when the engine runs again. Hope nothing breaks or leaks and I can do a test drive.
    • No those pads are DBA too  but they have colors too. I look at the and imo the green "street" are the best.
    • I’m not sure what happened I told them about sonic tunes free OTS tune and the next the I know .. I was booted..   To funny 
    • Yea - I mean I've seen my fuel pump which is decades old and uh, while I'm not saying this with real knowledge... but I sure get the ick at using anything in the fuel system that produced the state of that pump. Many years ago I went through multiple pumps (and strainers) before I dropped the tank to clean it out with extreme violence. I'm talking the car would do maybe 50km before coming to a halt, which resulted in me cleaning out the filter with some brake cleaner and going on my way. None of my stuff ever looked like what came out of your fuel tank. I don't think I'd be happy with it unless every single component was replaced (or at least checked/cleaned/confirmed to be clean here).
×
×
  • Create New...