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Different boost readings (Is this normal??)


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Now that the truck is broken in, I have been getting on it testing everything out before I go to ...




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Old 03-25-2012, 07:25 PM
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Different boost readings (Is this normal??)

Now that the truck is broken in, I have been getting on it testing everything out before I go to the dyno. My pulley combo on a stock engine should produce around 13 lbs of boost with Novi2000. My engine is not stock. My heads are fully ported and intake has been cleaned up, x-metal tb, forged pistons/rods, bassani headers, no cats, true dual 3" exhaust, custom cam for blower, etc.

With the boost gauge hooked into manifold vacuum (along with brakes, and everything else that needs vacuum or boost) I am getting a max of 5lbs of boost. This doesn't seem high enough. I next get a reading at the pipe from the blower to the intercooler and this shows no vacuum and constant boost pressure of between 2-5lbs, even while just driving normal. When on the gas hard I got max boost of 10lbs. So to see if it is my intercooler causing the problem I get a reading from the pipe that goes between the intercooler and the TB and it goes to a max of 7.5lbs of boost.

My intercooler is costing me 2.5lbs of boost?? Does that sound right?? And then I am loosing another 2.5lbs once into the intake??

Is this normal or what the heck is going on???

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Old 03-25-2012, 07:38 PM
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I know several guys have had lower than expected boost levels with ported heads. That is normal.
Also, switching to t-bolt style clamps will help eliminate additional boost loss through your system.
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Old 03-25-2012, 09:22 PM
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Quote:
Originally Posted by mysrt10 View Post
I know several guys have had lower than expected boost levels with ported heads. That is normal.
Also, switching to t-bolt style clamps will help eliminate additional boost loss through your system.
All clamps are t-bolt. I expected the port work to make the boost lower but are the differences in the boost readings normal? Like an intercooler costs 2.5lbs of boost and then loose another 2.5lbs going into the engine? Something just doesn't seem right.

Where does everyone else hook their boost gauge up at?
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Old 03-25-2012, 10:30 PM
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Sounds to me like you have a leak somewhere, I have mine plumbed into the intake just behind the throttle body.
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Old 03-25-2012, 11:34 PM
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Could pressure be leaking from the IAC valve? I have a filter on this instead of being plumbed into the intake tubing. Does the IAC close under full throttle?
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Old 03-26-2012, 12:19 AM
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Try plugging it off to see. You will have to screw in your idle screw a fair ways to get the truck to idle if you plug it off.
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Old 03-26-2012, 12:33 PM
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Quote:
Originally Posted by Mighty1 View Post
Could pressure be leaking from the IAC valve? I have a filter on this instead of being plumbed into the intake tubing. Does the IAC close under full throttle?
Yes an intercooler can lose a couple pounds of pressure but introduce a cooler charge therefore preventing detonation.

Yes....the IAC is closed on wot and anytime you have the pedal down, once you lift, IAC takes over idle and decel.
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Old 03-26-2012, 03:00 PM
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Originally Posted by VENOMOUS1 View Post
Yes an intercooler can lose a couple pounds of pressure but introduce a cooler charge therefore preventing detonation.

Yes....the IAC is closed on wot and anytime you have the pedal down, once you lift, IAC takes over idle and decel.
Thanks for that info Was getting worried about what the IAC did under full throttle.

So then the drop in pressure from the intercooler is within limits at 2.5 lbs. So I have 7.5lbs going into the intake but the reading from the boost gauge hooked into the brake booster line (along with meth and extra fuel pump, etc.) at the back of the intake is 5lbs. Would another 2.5lbs drop off from the front to the back of the intakebe normal?
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Old 03-26-2012, 08:41 PM
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Are you conducting these readings under same conditions and under same load? Your manifold positive pressure should be the same after the intercooler as it is the the manifold unless your manifold gaskets are leaking and blowing boost out before your boost guage nipple.
No you shouldn't loose another 2.5 lbs!
Blow off valve/wastegate working right?
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Old 03-26-2012, 08:52 PM
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Boost at the rear of the manifold could very well be 1-2 pounds lower than before the tb butterflies or the fitting right below the tb. The intake plenum size is fairly small for the displacement of the engine coupled with modest intake runner size. Imagine it as a reserve tank of compressed air waiting to be sucked down. At rpm when the engine is in boost you have 10 thirsty cylinders taking gulps of air in pulses. These waves of vacuum pulses will reduce pressure at the rear of the manifold compared to in front of the tb. Also that rear fitting is right near the opening of one of the runners. This distance from the tb and proximity to the runner provides a good vacuum signal to the accessories and further strengthens the likelyhood boost pressure measurement would be lower at this location. .hope this makes sense.
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Last edited by JeffBoyette; 03-26-2012 at 08:55 PM.
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