lil' help with boost

Discussion regarding the software diagnostic solution

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lil' help with boost

Post by RuS4ty » Feb Wed 14, 2007 6:56 pm

I after logging some pulls the other day with vag come I have some questions... and instead of writing them up again ill just copy and paste this message I sent to a guy who knows all about vagging. I guess I just don't understand what hes saying with finding the deviation between the "two". What other one do I compare it with?

please excuse my laziness with the c&p-ing.

Like Marc says, you need to adjust it. He gave you the correct formula.

To find out if there is a problem you need to compare requested boost pressure to actual boost pressure. It is the deviation between these two values that would indicate problems. If there is only minimal deviation, then that means your chip is only asking for 14 psi.

I know GIAC chips peak to 16 or 17 psi, but only hold at 14 or 15 psi.

Originally Posted by RuS4ty
Hey Marc referred me to you for a question about vag-com.

I was logging my boost and torque with it the other day and my boost peaked out at 1940 mbar, which I believe is about 28 psi which is high. Marc says that I'm supposed to adjust for the ambient pressure with the formula (mBar-1000)/1000*14.50377.... is this true? It would then bring me down to about 14psi which is low. When logging I used the advanced measuring blocks and "boost pressure - (actual)", so does that not automatically adjust for the pressure?
The torque peaked at about 350Nm on some pulls.

also, mods are just a chip(apr).
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Post by ValveFloat » Feb Thu 15, 2007 11:40 am

There must be a measuring block for requested, or commanded boost. That would be the boost level that the ECU is telling the WG controller is appropriate for the current condition.

If you find that block, and compare it to the actual boost, which I assume is the MAP sensor output, you would at least know if the turbo is producing the boost that the ECU is asking for.
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Post by Mikewire » Feb Thu 15, 2007 11:51 am

I don't think vag-com calcs absolute pressure and adjusts because it measures in mBar - hence the formula, but I could be wrong. If I remember correctly I don't think you have a gauge, so there is nothing to compare your readings from vag-com to.

You can measure acutal vs. requested boost - is that what you are measuring? Or are you just reading mBar and Tq?

Are you getting any fault codes when you do a scan? This may tell you if you have a problem with any of the control valves.

Sounds like the guy didn't understand your question. You were asking "Will vag-com adjust for ambient pressure?" And he just skipped over your question and told you to compare values.

If you really peaked @ 14 psi with an APR then it sounds like something isn't right, because from what I understand and with APR you should be peaking higher. You may have a vacuum leak or a loose hose? I have limited experience with turbo setups, so maybe someone else can chime in with their experience.

Check this site out for a bunch of turbo calculators: ... sary.shtml

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Post by Ramrod » Feb Thu 15, 2007 12:18 pm

If both are indicated in mbar, you shouldn't need to convert. Just compare, but I'd convert to psi anyway . Log Block 115. Past that you'll have to ask the experts.

Here's a link to DIY pressure testing if you end up going that route. ... tester.pdf
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