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Discussion Starter #1
I have a feeling I’m gonna get flamed for this newb question, but I honestly don’t know the answer.

I’m using the autometer 4303 boost/vacuum gauge. At idle, reading off manifold vacuum it shows a negative value, which means there is vacuum, when I get into boost it obviously reads a boost pressure. It was dyno tuned at 14psi using a Greddy profecB spec 2. I’m not sure how he verified it was 14psi, but when I do some test hits, I can’t seem to get the gauge to read over 10psi.

My question is, is boost showing on the gauge anything higher than the manifold vacuum at idle, so 10psi of boost is actually 15psi when idle is 5hg of vacuum? Am I simply reading the gauge wrong?
 

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I have a feeling I’m gonna get flamed for this newb question, but I honestly don’t know the answer.

I’m using the autometer 4303 boost/vacuum gauge. At idle, reading off manifold vacuum it shows a negative value, which means there is vacuum, when I get into boost it obviously reads a boost pressure. It was dyno tuned at 14psi using a Greddy profecB spec 2. I’m not sure how he verified it was 14psi, but when I do some test hits, I can’t seem to get the gauge to read over 10psi.

My question is, is boost showing on the gauge anything higher than the manifold vacuum at idle, so 10psi of boost is actually 15psi when idle is 5hg of vacuum? Am I simply reading the gauge wrong?
10psi is simply 10psi, this is the amount of boost (positive air pressure) that the engine is seeing. Not additive to the vacuum reading.
 

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The only reason it might be different is from different reference points. 15psi at the compressor outlet might only be 10psi at the manifold due to plumbing configurations/restrictions.

...Or you have a boost leak.
 

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Discussion Starter #4
Okay, thanks for clarifying.

One big issue is that EVERYTHING is pulled or referenced from one manifold vacuum source on the Edelbrock intake. Wastegates, boost controller, bov’s, fuel pressure regulator, and gauge are all tied into the same. I’m assuming where one is pulling, one is pushing etc. there’s just a lot of air to be moving around. I’ll check the vacuum lines, most are stainless hard lines, but some are rubber.

I may have just not have been reading it properly, the car gets quite rowdy and I tend to lift before 6500rpm. Hard to keep my eyes on everything at once, especially when I can’t record that value in datalog to play back later.
 

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Okay, thanks for clarifying.

One big issue is that EVERYTHING is pulled or referenced from one manifold vacuum source on the Edelbrock intake. Wastegates, boost controller, bov’s, fuel pressure regulator, and gauge are all tied into the same. I’m assuming where one is pulling, one is pushing etc. there’s just a lot of air to be moving around. I’ll check the vacuum lines, most are stainless hard lines, but some are rubber.

I may have just not have been reading it properly, the car gets quite rowdy and I tend to lift before 6500rpm. Hard to keep my eyes on everything at once, especially when I can’t record that value in datalog to play back later.

Gauges have some dampening or latency in the movements. Otherwise they jump and bounce around.

Your only p[roblem with Tee'ing a bunch of stuff is the fill volume and fill rate of the things you T. If you had 50 things that all ran small lines and had no domes, it would fill fast and follow the boost. If you have just one thing with a big dome volume to fill, it can delay pressure change significantly.

It adds latency or "response delay" to the pressure because the volume has to fill (or drain) and that requires flow.

For example, a 2" diameter dome diaphragm that moves 1 inch has 3 cu in of air. You have to fill or drain that volume to fully change pressure. Usually the hoses all are meaningless compared to domes.

Personally, I'd run large fill volume things like a BOV and fuel regulator off their own manifold tap independent of any data logging or MAP pressure monitors. I'd also use a check valves on things that only need vacuum, like the vacuum T that runs power brakes and heater controls, or on any PCV system. I would not push boost back onto the vacuum tree or anywhere else it is not needed.
 

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Discussion Starter #6
Sounds like a plan, I’m not running brakes etc of that line, just everything related to boost and boost reference. There is a larger port in one of the runners 3/8” I believe, I may be able to split up some of the vacuum load. If given 2 ports, and needing to run boost reference to the things I listed (wastegates, fuel pressure regulator, bovs, gauge, boost controller) what would you run together on a dedicated circuit? I hated the thought of using that port because of its large diameter, but I guess necking it down to -4 line wouldn’t be a terrible idea, just to split the load. It was tuned like it is currently setup, so I know it currently works, just never liked the thought of so many tees.
 

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Discussion Starter #7
A caveat off of that question is, both ports are showing manifold vacuum after throttle blade, so there should be no “real” difference, correct? Running the fuel pressure regulator and bovs on a dedicated line like suggested would seem to clean things up a bit.
 

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Before going through all this I would contact your tuner and see how he measured the boost. Did he measure maximum boost in which it can spike to 14 then settle down to 10. See if he logged it so you can see what the boost graph looks like.

ks
 

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Discussion Starter #10
That’s exactly what I don’t know, and he can’t remember, it was over a year ago. It was done using the profec b controller, so you don’t actually set a boost level, it’s a percentage. I’m assuming he did it by gauge, because that’s the only way I can think it can be accurately measured. Also, I don’t believe it spiked due to the overboost pressure system built in to the controller. It was tuned twice, once at 14lbs and at 8, which was off the wastegates springs. He said it was very conservative for the setup and we simply ran out of time that day. I’ve just been running 14lbs since.
 

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Discussion Starter #11
I’ve got the dyno sheet, it shows the max boost around 3500 at 13.8lbs and pulled all the way to 6500. Like I said, I don’t know of any other way it can be measured other than manifold pressure, unless it was somehow through the mass air metering?
 

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It's possible the WG's are opening. I would check the lines and verify that they are sealing.

ks
 

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That’s a big rabbit hole to go down. You have a few items I’d try.
I would try and get away from T’s and use dedicated sources for fuel pressure, BOV & WG.
Once you separate the feed source the next rabbit hole is reference point. You have: 1) compressor cover, 2) pre intercooler, 3) post intercooler, 4) pre throttle body, 5) post throttle body.
What you decide to source from where, the internet will never agree with. I have my $0.02 but your comfort may be different?
 
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