fcs13

Quick question guys, my boost guage is reading vacuum instead of boost. I have (I think) everything hooked up right. Is there a particular place to hook the vacuum line to on the intake manifold? Also, I know for a fact my boost controller is not tuned yet as we finally have the car starting correctly now. Long story short, the wire harness was screwed, but now its fixed. So now are the stages of getting it on the road and one of my mechanics questions was why is it reading vacuum instead of boost. Is it infact because my wastegate has not opened due to my B/C not being tuned or what? info please! thanks,



-B.



(sorry for the newb question, but I gotta learn somehow)

Leetheslacker

do you have it hooked to the TB?



some of the vacuum ports on the tb will only show vacuum and not boost.



the best place to use is to tee it into the line from the lim to the boost sensor.

1988RedT2

When are you observing vacuum readings? At idle?



A turbo motor will show vacuum readings at closed throttle and low-load conditions such as cruise. If your mechanic doesn't know this, you should get a new mechanic.

RX7 13B 4 UR AZZ

I second 1988RedT2 all cars N/A or F/I will read vac at idle

fcs13

Ah, okay see I am coming from non-turbo cars. though I have been around them for quite some time now I still havent really played with the boost guages or supporting goodies. The car was sitting at idle so thats prolly why I was confused with his question. Anyway, thanks for the replies, very helpful.



-B.

ColinRX7

To help you understand vacuum and pressure charge, check this out:



A blow off valve's function is to dump pressure in the intake path when you let off the throttle. A vacuum line runs to the blow off valve, so when you snap the throttle shut, the engine begins sucking air past the plates rather than being fed air, which creates vacuum. Vacuum, than can be used to pull open a valve and dump the boost pressure in the intake path. With the throttle plates closed, there is a great reduction in flow, which greatly increases pressure in the intake path, as the turbocharger is designed to force "x" psi of air into the motor without any flow restriction. If there is a restriction, the air slams into the throttle plates and wants to turn around.



Compressor surge is something that happens when the intake path has a charge of air that can't be purged fast enough (or sometimes not at all). Although your blow off valve actuates, it can't flow the entire charge out the valve, and the result is pressure reversing at the throttle plates, and going back to the compressor (harming the compressor), and then going back towards the throttle plates, until the intake charge has been successfully purged.



What else happens when the pressure charge hits the turbine wheel of the compressor? The blades want to stop turning. Not good for performance. Purging the boost helps keep the turbine speed at a high level of RPM so you require less time to reach peak boost RPM.



Those guys with that sound that goes "CHOOOOO CHO cho cho" are not cool.



A defective blow off valve, improperly sized blowoff valve, or a bad connection to the blow off valve (since it relies on vacuum to open) can damage your compressor.



That being said, I would plumb the boost gauge to the vacuum line of the blow off valve. Why? Your boost gauge has become a diagnostic tool. You will know when your motor is producing vacuum when you let off the throttle. If you detect no vacuum on your gauge, you have a problem with the car. This means vacuum isn't getting to your blow off valve, which is bad. The vacuum line has become disconnected or clogged (for whatever reason), and you're seeing the result at the most integral part of the system. This will allow you to say "Well my car is acting funny, but at least I know there's no problem with the vacuum at the blow off valve. Perhaps the valve is sticking?" (if such a problem should arise). Plumbing it in this manner will also allow you to play around with different vacuum fittings on the car to find out which fitting has the fastest vacuum response, which will actuate your blow off valve promply enough to continue current turbine RPM the best.















And to go off topic a bit, what do you open-vent guys think of this:



The blow off valve is open at idle, correct? Because the vacuum is holding it open. And since this blow off valve is in front of the throttle plates, the AFM and/or air filter is further away from the actual throttle plates. The blow off valve (being open) has less restriction to atmosphere, the intake charge being used by the BACV (bypass air control valve, for idle intake air) is bypassing the throttle plates, and bringing air in from where? The blow off valve! Why? It's just easier for it to pull through the valve since it's open! Dirt can get in, but most importantly, this air is not being noticed by the AFM, resulting in a lean engine idle condition! Or another scenario, how about this: Generated rejected heat by the motor is turning the exhaust side turbine slightly. This in effect causes the compressor blades to turn slightly, pulling air through the AFM, and the air only venting to atmosphere. The AFM is telling the engine "x" charge of air is coming in, when it's just bypassing out the open blow off valve at idle. What does that mean? Your engine is running needlessly rich. I'm pretty sure the reason the blow off change is vented back to the compressor inlet from factory is to prevent this "scavenged air" from becoming atmospheric air charge and causing all kinds of tuning and idle problems, while potentially allowing dirt to enter the charge. But hey maybe I'm just new, and you guys all knew that already.



You could also just ditch the AFM with an updated EFI system, which won't use one, allowing you to run an open vented blow off valve without any idle or tuning issues (since intake air charge is calculated based on vacuum and temperature).