Negative pressure? What all does this mean and do?

Spiff44

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Hello fellow enthusiasts..

Old timer here.. been doing Honda engine swaps since the early 87's... but recently bought myself a new 2019 SI. Its my first turbo application. (have to say.. its no replacement for a mean B16)

I thought I had a pretty good understanding of how turbo's work until I got this car and started watching the boost gauge as I drive. I was very surprised with how often its under negative pressures.. When cruising in 6th at about 3000RPM (80-85), it pretty much stays at -5 pressure. This blew my mind!

I never heard about it before and searching around there is little information on "negative boost". So I am here looking for more information from someone smarter than me. To me, it doesn't make sense.. the only time I might expect something like that would be under sudden throttle back or similar. But under constant modulation? Just seems weird. (then again.. turbos and superchargers sound impossible if I didn't already know that they work....here, have some FREE power!)

Can anyone shed some light? Seems like I have to go to about half throttle to start pushing positive boost.. so what effect is the negative pressure doing/causing in the meantime? Apparently its good for MPG..

Thanks
Spiff
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gtman

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I wouldn't worry about those negative "pressure" numbers. When cruising, it's just telling you the turbo isn't being used. No boost in other words. You'll be using boost when accelerating or going up a hill under load, etc...

You definitely wouldn't want to be boosting all the time because that wouldn't be great for longevity or mpg's.
 

amirza786

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That is completely normal. It will only go into positive when in boost. Normal driving, depending on load to the engine will stay in negative and head to positive pressure when needed
 
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Spiff44

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Thanks.. yeah,.. I figured it was normal... but how? If you're under constant throttle.. it just doesn't make any sense that its creating negative pressures. My theory would be that its probably the turbo overhead.. until the extra restriction of the turbo is overcome in the exhaust path, that drag is manifesting as negative pressures... sounds good to me.
 

amirza786

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Thanks.. yeah,.. I figured it was normal... but how? If you're under constant throttle.. it just doesn't make any sense that its creating negative pressures. My theory would be that its probably the turbo overhead.. until the extra restriction of the turbo is overcome in the exhaust path, that drag is manifesting as negative pressures... sounds good to me.
The turbo has an electronic waste gate that opens and closes as needed to release the excess boost, otherwise you would have an over boost condition
 


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Thanks.. yeah,.. I figured it was normal... but how? If you're under constant throttle.. it just doesn't make any sense that its creating negative pressures. My theory would be that its probably the turbo overhead.. until the extra restriction of the turbo is overcome in the exhaust path, that drag is manifesting as negative pressures... sounds good to me.
The pressure is measured in the intake manifold before the air enters the engine. On a normal, naturally aspirated engine the pressure is always negative, because the engine sucks air into the cylinders from the intake manifold.

On a turbo car, it works the same way. When the engine is running, it's sucking air from the intake manifold, causing a negative pressure. The difference is that under a load demand, the turbo can push more air into the manifold causing the pressure to cross over 0 to be a positive number, which is "under boost".
 

gtman

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OP, you're overthinking this. Those negative numbers simply represent no boost. If you want to monitor parameters, things like fuel trims, MAP (boost), IAT's, knock control, etc. are much more important.
 

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Thanks.. yeah,.. I figured it was normal... but how? If you're under constant throttle.. it just doesn't make any sense that its creating negative pressures. My theory would be that its probably the turbo overhead.. until the extra restriction of the turbo is overcome in the exhaust path, that drag is manifesting as negative pressures... sounds good to me.
A turbo engine that's not spooling and an NA engine are basically the same - operating in vacuum. The engine is sucking in air in order to burn fuel. Sucking in, just like a vacuum cleaner. No boost = vacuum.
 
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Spiff44

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The pressure is measured in the intake manifold before the air enters the engine. On a normal, naturally aspirated engine the pressure is always negative, because the engine sucks air into the cylinders from the intake manifold.

On a turbo car, it works the same way. When the engine is running, it's sucking air from the intake manifold, causing a negative pressure. The difference is that under a load demand, the turbo can push more air into the manifold causing the pressure to cross over 0 to be a positive number, which is "under boost".
Thanks for this post and the others.. Okay, yeah.. this all does make more sense. I guess I was expecting a more linear effect from the throttle.
Now I kinda wanna see what boost from a supercharger would look like. That would have to be more linear, right?
 


JT Si

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Thanks for this post and the others.. Okay, yeah.. this all does make more sense. I guess I was expecting a more linear effect from the throttle.
Now I kinda wanna see what boost from a supercharger would look like. That would have to be more linear, right?
I'm not sure if a supercharger also has something similar to a wastegate that allows no boost conditions. A quick google makes it look like traditionally, superchargers do not meaning you would be correct.

On a turbo, it's ideal to have a negative pressure when cruising as it means less air -> less fuel -> higher fuel economy.
 

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Thanks for this post and the others.. Okay, yeah.. this all does make more sense. I guess I was expecting a more linear effect from the throttle.
Now I kinda wanna see what boost from a supercharger would look like. That would have to be more linear, right?
supercharger usually has a bypass valve or else you'd be boosting at idle and cruising situations.
 
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Spiff44

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Learned some new things! I didn't realize that the waste gate was more than just an over boost protection, but that its helping to modulate through the entire power band.
 

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Thanks for this post and the others.. Okay, yeah.. this all does make more sense. I guess I was expecting a more linear effect from the throttle.
Now I kinda wanna see what boost from a supercharger would look like. That would have to be more linear, right?
Yes because a supercharger is driven by belt off of the engine crankshaft. That means as the engine runs through the RPMs, the boost increases linearly like you stated with the rise in RPM. There is no boost lag so response is better in a supercharged engine rather than a turbocharged engine.
 
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Spiff44

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Yes because a supercharger is driven by belt off of the engine crankshaft. That means as the engine runs through the RPMs, the boost increases linearly like you stated with the rise in RPM. There is no boost lag so response is better in a supercharged engine rather than a turbocharged engine.
But I bet there is still a brief period of negative pressure as that initial drag of the impeller gets overcame.
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