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Old 09-10-2014, 02:22 PM   #1
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Default Why does my MAP decline slightly over WOT pull?

On this street pull, the log shows that MAP falls off slightly as RPMs approach 7K.

Is this a bad map sensor or a leak in the hose? I used a 1/8" hose which is actually a bit smaller something like 7/64ths.

How badly will this affect tuning?

There is alot of noise in the AFR, I was trying a different ground which is worse than what I was using when I tuned VE.

I am grounded to the engine above the TB, which I heard was the ECU ground. Is there a better ground, or should I be using shielded wire?

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Old 09-10-2014, 02:29 PM   #2
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Where is your wastegate gettings it's boost signal? If it's right off the compressor housing, that's probably your problem. Try moving it to your cold side pipe, as close to the throttle body as possible.
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Old 09-10-2014, 02:33 PM   #3
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Originally Posted by curly View Post
Where is your wastegate gettings it's boost signal? If it's right off the compressor housing, that's probably your problem. Try moving it to your cold side pipe, as close to the throttle body as possible.
This is a naturally aspirated NB with bolt ons.

I tapped into the cruise line close to the firewall hole so that the line would be as short as possible.
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Old 09-10-2014, 02:43 PM   #4
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How much does yours drop? I just looked at a log on my car and it had a gradual drop from 100.2kpa to 97.4kpa over a 2000-6700rpm 4th gear pull. I'm assuming it's normal and just a result of restriction in the intake. Turbo should definitely fix it though.
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Old 09-10-2014, 02:45 PM   #5
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Originally Posted by pdexta View Post
How much does yours drop? I just looked at a log on my car and it had a gradual drop from 100.2kpa to 97.4kpa over a 2000-6700rpm 4th gear pull. I'm assuming it's normal and just a result of restriction in the intake. Turbo should definitely fix it though.
I dunno, looks like it drops about 3-5% at 7K. I suppose I can just take this into account in my VE table.
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Old 09-10-2014, 02:49 PM   #6
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Here's my log, looks similar:

Attached Thumbnails
Why does my MAP decline slightly over WOT pull?-kpa.jpg  
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Old 09-10-2014, 02:52 PM   #7
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Quote:
Originally Posted by pdexta View Post
I'm assuming it's normal and just a result of restriction in the intake.
^ This.

Everything in your intake tract is a restriction. The air filter, the throttle body, the inlet piping itself, all restrictions.

The pressure drop across a restriction is proportional to the volume flow per unit time through said restriction. The more air you try to pull through the intake system, the more pressure drop across it.

Try this experiment: Get a common drinking straw. Put it between your lips, and inhale through it very slowly. Feel much resistance? Nope- very little restriction. Now, inhale through it very rapidly. A lot harder, eh? And your MAP (Mouth Absolute Pressure) drops considerably as well. If you were to install a hose-barb fitting through the side of your cheek, you'd be able to instrument this and see basically the same effect as in your log.

That's all you're seeing here. It means that everything is working normally, and your engine is obeying the laws of physics.
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Old 09-10-2014, 02:54 PM   #8
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Old 09-13-2014, 08:24 PM   #9
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Very timely question! I too had noticed that my logs show 95-97kPa during sustained WOT conditions longer than a couple of seconds (non-FI/NA VVT engine) and had wondered if I had a leading vacuum line to the MS3 box.

Thanks for the information.
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