sup_dawgTT Posted June 3, 2009 Share Posted June 3, 2009 hey guys first time posting in the UK forums but iv been on the supraforums community awhile. just a question please, iv just hooked up by boos gauge following the MKIV.com guide and is only reading vaccum. it reads 20 vaccum on idle, but then under load it reads around 11 vaccum and under full boost it goes to 0. any ideas why? iv hooked it up to the vaccum pressure sensor line using a T piece. thank you Ash Quote Link to comment Share on other sites More sharing options...
Digsy Posted June 3, 2009 Share Posted June 3, 2009 Sorry for the dumb question, but your car is a twin turbo isn't it? Quote Link to comment Share on other sites More sharing options...
Wez Posted June 3, 2009 Share Posted June 3, 2009 sorry for the dumb question, but your car is a twin turbo isn't it? Quote Link to comment Share on other sites More sharing options...
kaahari Posted June 3, 2009 Share Posted June 3, 2009 If your car works ok, maby broken boost gauge? You should see positive numbers from ~1800rpm and up under load. Quote Link to comment Share on other sites More sharing options...
JustGav Posted June 3, 2009 Share Posted June 3, 2009 Sorry for the dumb question, but your car is a twin turbo isn't it? Hahahaha, you know, that was my first thought as well.... shouldn't laugh. Quote Link to comment Share on other sites More sharing options...
mawby Posted June 3, 2009 Share Posted June 3, 2009 Sorry for the dumb question, but your car is a twin turbo isn't it?I would expect vacuum to increase with load if it didn't have a turbo. Quote Link to comment Share on other sites More sharing options...
Ric Posted June 3, 2009 Share Posted June 3, 2009 you're not just revving it at standstill are you? that's not under load. get the car moving then you will see + pressure Quote Link to comment Share on other sites More sharing options...
Marmoot Posted June 3, 2009 Share Posted June 3, 2009 Hey Sup_Dawg. Whereabouts in NZ? I'm in Auckland. Should meet up sometime. IMHO for stock it'd be hard to even get into boost on 1st gear. Stick it on 2nd or 3rd and go up a hill somewhere. Otherwise, perhaps a boost leak? Quote Link to comment Share on other sites More sharing options...
Digsy Posted June 3, 2009 Share Posted June 3, 2009 I would expect vacuum to increase with load if it didn't have a turbo. Nope. Manifold depression will be high at idle and will gradually approach atmospheric pressure as the throttle opens. Quote Link to comment Share on other sites More sharing options...
sup_dawgTT Posted June 3, 2009 Author Share Posted June 3, 2009 hey guys, yup its on stock twins lol i should have mentioned that. i did sit and rev it i saw vacuum increase which was a sign that the gauge was registering something so i thought ok everything running fine lets take it out for a boost. upon driving the gauge does move up to 10 vacuum, and stays there under load of the first turbo, upon planting it it moves to 0 or 1 positive pressure. i have no leaks but ill change the vacuum hose tomorrow and try again. should i just hook it up from the FPR line instead? since thats how the GReddy manual installs it? Quote Link to comment Share on other sites More sharing options...
jacko Posted June 3, 2009 Share Posted June 3, 2009 Are you sure you have taken the vac line from the right pipe? Quote Link to comment Share on other sites More sharing options...
sup_dawgTT Posted June 3, 2009 Author Share Posted June 3, 2009 im positive since i followed the MKIV.com instructions which was take the line from the pressure sensor. but now im thinking maybe take the FPR line instead? im not sure about the gauge but it reads fine. its reading negative vacuum pressure upon warming up, but it wont move over to the right hand side and show positive boost pressure. any help please? how would i do a boost leak test? Quote Link to comment Share on other sites More sharing options...
Digsy Posted June 3, 2009 Share Posted June 3, 2009 OK Are you sure there isn't a non-return valve between the intake manifold and your t-piece? Can you trace the vacuum hose back to the manifold? Quote Link to comment Share on other sites More sharing options...
sup_dawgTT Posted June 3, 2009 Author Share Posted June 3, 2009 i used the T piece i got in the box which featured a two way connection and the third part had a brass or some sort of metal fitting inside i think which is a one way valve? but anyway it has a metal fitting which said to connect the boost gauge line into and connect the boost pressure line into the two ends that run straight across. Quote Link to comment Share on other sites More sharing options...
Marmoot Posted June 4, 2009 Share Posted June 4, 2009 I remember I had that before. Turned out the boost gauge had gone to the crapper. Supdawg, you don't happen to have one of those $20 TradeMe/eBay boost gauge do you? Quote Link to comment Share on other sites More sharing options...
Scott Posted June 4, 2009 Share Posted June 4, 2009 I would expect vacuum to increase with load if it didn't have a turbo. Nope. Manifold depression will be high at idle and will gradually approach atmospheric pressure as the throttle opens. I'm confused by this, why would it approach atmospheric pressure? If the engine is under load the pistons pull air in hence the sucking noise from the induction kit. That would surely register as a vaccum? As for the problem at hand, i would agree with the others and say fooked boost gauge. Does the stock gauge read positive pressure? Could you try the feed from that? Quote Link to comment Share on other sites More sharing options...
4packet Posted June 4, 2009 Share Posted June 4, 2009 I'm confused by this, why would it approach atmospheric pressure? If the engine is under load the pistons pull air in hence the sucking noise from the induction kit. That would surely register as a vaccum? No, because the throttle is wide open so there is very little resistance against the flow of air and hence very little vacuum. Quote Link to comment Share on other sites More sharing options...
sup_dawgTT Posted June 4, 2009 Author Share Posted June 4, 2009 i have an auto gauge lol so its a bit crap but should work, i might take it back tomorrow to see if its working but im reading positive psi but only around 1-2 psi which is strange under full boost is it not? theres no leaks i checked today and yesterday, o Marmoot flick me a txt [removed by mawby] Quote Link to comment Share on other sites More sharing options...
mawby Posted June 4, 2009 Share Posted June 4, 2009 i have an auto gauge lol so its a bit crap but should work, i might take it back tomorrow to see if its working but im reading positive psi but only around 1-2 psi which is strange under full boost is it not? theres no leaks i checked today and yesterday, o Marmoot flick me a txt [removed by mawby]Please send private information like phone numbers via PM. Quote Link to comment Share on other sites More sharing options...
sup_dawgTT Posted June 4, 2009 Author Share Posted June 4, 2009 sorry mate its just that every time i go to send a PM it logs me out of my account and i only just got logged back in, again sorry. Quote Link to comment Share on other sites More sharing options...
sup_dawgTT Posted June 4, 2009 Author Share Posted June 4, 2009 any idea how i could find out if its the gauge since i dont know anyone near me with a spare one. Quote Link to comment Share on other sites More sharing options...
Scott Posted June 4, 2009 Share Posted June 4, 2009 No, because the throttle is wide open so there is very little resistance against the flow of air and hence very little vacuum. Little vacuum = vacuum though I know the only resistance is from the air filter but it would still be a negative pressure, regardless of how little. Quote Link to comment Share on other sites More sharing options...
Digsy Posted June 4, 2009 Share Posted June 4, 2009 (edited) Little vacuum = vacuum though I know the only resistance is from the air filter but it would still be a negative pressure, regardless of how little. About 2kPa (or 0.3PSI) - hence "approaching atmospheric pressure". The inlet depression at idle will be greater because the pistons are trying to suck 3 litres of air every two revolutions against an almost fully closed throttle. You will still hear a sucking noise from your induction kit because between the air filter and the turbo inlet it is sucking - even when on boost. We are talking about pressure (or vacuum) inside the manifold. When you are at full load the manifold pressure will be positive in a pressure charged engine, or virtually atmospheric on an NA. At idle they wil both register negative manifold pressure. Edited June 4, 2009 by Digsy (see edit history) Quote Link to comment Share on other sites More sharing options...
sup_dawgTT Posted June 4, 2009 Author Share Posted June 4, 2009 so i should be seeing a relatively strong amount of positive pressure once accelerating? im getting 1-2 psi under heavy throttle use where my foot is almost reaching the floor. im not sure why, i hope its the gauge but could it be a bad tune? i havent gotten the car tuned so its stock but i have replaced the turbos? that could be the problem? i do admit the original pair of turbos did feel to pull harder. Quote Link to comment Share on other sites More sharing options...
Scott Posted June 4, 2009 Share Posted June 4, 2009 About 2kPa (or 0.3PSI) - hence "approaching atmospheric pressure". The inlet depression at idle will be greater because the pistons are trying to suck 3 litres of air every two revolutions against an almost fully closed throttle. You will still hear a sucking noise from your induction kit because between the air filter and the turbo inlet it is sucking - even when on boost. We are talking about pressure (or vacuum) inside the manifold. When you are at full load the manifold pressure will be positive in a pressure charged engine, or virtually atmospheric on an NA. At idle they wil both register negative manifold pressure. Surely the figure would depend on how restrictive the filter is? Or is the 0.3 a measurement without the filter in place? Quote Link to comment Share on other sites More sharing options...
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