:driving: Turbo Boost Gauge readings

[nvno1]

Good Day All,

I was reseaching and not able to get a true understanding with Boost gauge reading.

I have a aftermarket boost gauge and wanted to know what it should read vs what i am getting to help with some troubleshooting. At what Rpms should the Hg be in and when does it go to possitive numbers. Meaning when does the needle on the gauge leave the -Hgs and goes into the measurable boost numbers?

I know it seems like i'm, however i cant find anything concrete on this matter.

Thanks in advance,
SJR.

[dudemaaan]

Hg " is vacuum you will see this at idle and off throttle. Anywhere from 14-20"hg, depending on your porting. Positive pressure will start as soon as you step on the gas and the turbo begins spooling. For stock twins you should see 10psi by like 2k rpms with a small dip to 8 psi at 4k rpms and then right back to 10 psi. Single turbos will just depend on their size. Some gauges read in bar, in which case you will have to convert my psi measurements.

[nvno1]

Originally Posted by dudemaaan

Hg " is vacuum you will see this at idle and off throttle. Anywhere from 14-20"hg, depending on your porting. Positive pressure will start as soon as you step on the gas and the turbo begins spooling. For stock twins you should see 10psi by like 2k rpms with a small dip to 8 psi at 4k rpms and then right back to 10 psi. Single turbos will just depend on their size. Some gauges read in bar, in which case you will have to convert my psi measurements.

WOW thanks for the prompt response. Well the car is a 87 TII,stock motor, RB downpipe and walbro fuel pump. The gauge reads in PSI, my gauge doesnt go that high. Well she is not on the road yet(no driveshaft right now).

[Phoenix7]

you won't build boost in neutral....with stock injectors you should not see more than 10 psi but, like I said, you won't build boost without load.

[nvno1]

Originally Posted by Phoenix7

you won't build boost in neutral....with stock injectors you should not see more than 10 psi but, like I said, you won't build boost without load.

great, however what should the Hg be at 750, 1000, 1300?

[RETed]

Vacuum reading don't really matter except for maybe at idle.
When driving, when you see vacuum on the gauge, it just means the turbo isn't working to produce boost.
Trying to see your vacuum numbers while driving is a royal waste of time.


-Ted

[NoDOHC]

I think a stock ECU rotary should run about -15 to -18 "Hg at Idle. As you gently rev the engine in neutral, this will dip a little about 1200 rpm (-18 to -21), but remain relatively consistant after that.

With an aftermarket ECU I run about -15 " Hg when cruising on the highway and about 21 " Hg at idle (Timing advanced over stock).

[dudemaaan]

Ignore what i said about 10 8 10 boost, thats for 3rd gen rx7's with sequential turbos. Didn't think about it being a 2nd gen. I wouldn't worry too much about vacuum provided you have good compression numbers. But stock ports usually pull over 16". My large street port pulls around 15" at 1000 rpm.

[nvno1]

Originally Posted by RETed

Vacuum reading don't really matter except for maybe at idle.
When driving, when you see vacuum on the gauge, it just means the turbo isn't working to produce boost.
Trying to see your vacuum numbers while driving is a royal waste of time.


-Ted

Wow Ted, i feel so honored you replied on my post. I'm not driving the car right now, my main reason for asking about the vaccum is the idle is not set and i wanted to know if at 1000rpms, if the reading is right? I have checked for vac leaks and nothing has surfaced, however i dont have another way of verifing there is no leak. So i figured the vac reading on my boost gauge would fall as a back up.

[nvno1]

Originally Posted by dudemaaan

Ignore what i said about 10 8 10 boost, thats for 3rd gen rx7's with sequential turbos. Didn't think about it being a 2nd gen. I wouldn't worry too much about vacuum provided you have good compression numbers. But stock ports usually pull over 16". My large street port pulls around 15" at 1000 rpm.

, Yeh i was still looking around and nothing seems very concrete on the matter on hand. I was just trying to see if there was a leak(still havent found any) and having low vac reading would state a vac leak right if the compression, fuel pressure and ignition is good right?

[vex]

Originally Posted by nvno1

Wow Ted, i feel so honored you replied on my post. I'm not driving the car right now, my main reason for asking about the vaccum is the idle is not set and i wanted to know if at 1000rpms, if the reading is right? I have checked for vac leaks and nothing has surfaced, however i dont have another way of verifing there is no leak. So i figured the vac reading on my boost gauge would fall as a back up.

Ah, the mysterious missing vac leak, where you know you have one, but just can't find it.

Try to get a smoke machine. That should show you exactly where your vac leak is. I am in the process of doing this, but have a few things to do first before I do (mainly earn money).

as for boost and vac readings. Are you sure your gauge is grounded properly? Not having a proper ground will give you very messed up readings. If you have, expect to have anywhere from 14-20+inHg in vac. I'm currently pulling in around 14inHg on a fresh rebuild (though compression is absolutely fine). Good luck though.

[nvno1]

Originally Posted by vex

Ah, the mysterious missing vac leak, where you know you have one, but just can't find it.

Try to get a smoke machine. That should show you exactly where your vac leak is. I am in the process of doing this, but have a few things to do first before I do (mainly earn money).

as for boost and vac readings. Are you sure your gauge is grounded properly? Not having a proper ground will give you very messed up readings. If you have, expect to have anywhere from 14-20+inHg in vac. I'm currently pulling in around 14inHg on a fresh rebuild (though compression is absolutely fine). Good luck though.

Hey thanks for the info. The gnd is good but my Vac readings are not that high(if memory serves me right). With the smoke machine the smoke will be inserted to a opening in the intake somewhere and pressurized through the system with the vac leak showing up by means of escaping smoke?

[vex]

Originally Posted by nvno1

Hey thanks for the info. The gnd is good but my Vac readings are not that high(if memory serves me right). With the smoke machine the smoke will be inserted to a opening in the intake somewhere and pressurized through the system with the vac leak showing up by means of escaping smoke?

Not quite. With the engine running (and you're reading low on vac) start the smoke machine. Point the stream over the engine and look for locations where the smoke enters the engine where it shouldn't be. Gaskets, Hoses, connections, etc are good places to look.

[Flooder]

Originally Posted by Phoenix7

you won't build boost in neutral....with stock injectors you should not see more than 10 psi but, like I said, you won't build boost without load.

Sure you will, just not very much. You can still make your BOV vent when in neutral.

[nvno1]

Originally Posted by vex

Not quite. With the engine running (and you're reading low on vac) start the smoke machine. Point the stream over the engine and look for locations where the smoke enters the engine where it shouldn't be. Gaskets, Hoses, connections, etc are good places to look.

Iknew that. not really..so like i was messing with the car somemore and my Hg reading are a lot lower than what i thought it was like -5@1100rpm and at 3-4000rpms the reading is about -14hg. diffy a vac leak somewhere. my oil pressure is reading good, temp is cool, when i press in my tps the rpms climb. i got it on video but my wk pc doesnt have a dvd player, i get my mgr to load i and send it to me email so i can post it.