Boost/vacuum theory questions

[Lsstefan]

Hello. I own an Ecoboost EU PP.
I've been trying to familiarize myself with the tuning theory and stuff and I keep running into this confusion. Can you guys please englighten me?
So from what I've read, a NA engine CAN take up to 1 bar(14.7 psi) of boost during WOT right? Ofc in ideal environment, not on the Everest.
- So I have a turbo on top of it that compresses air more than the engine could on it's own. My digital boost gauge shows maximum 1.3 bar(18.8 psi) of boost. Is this boost over the standard pressure? Like getting 2.3 bar of pressure. Or it's the true pressure including the standard pressure? Like my turbo adds 0.3 bar of boost on top of the standard?
- My buddy has a bigger turbo than mine on his Focus ST (432 hp) and he says he drives with 1.2-1.32 bar(his engine can't handle more atm). How can he make 400+ with the same boost as me?
- What is that vacuum? I know what vacuum is, but for intake pressure? Could it be like the engine takes ALL the air in the intake and since there is no more air there it turns into vacuum?
- Why is it that when I WOT I have boost on the gauge, but when I release the throttle and the RPM stays almost the same I get vacuum? The turbine keeps spinning as the exhaust is flowing fast due to RPMs. Could the BOV release the air from the intake so the mix doesn't become lean?

I know answering my own questions seem retarded but I want to see how I'm thinking about the questions.

Offtopic: my brother owns a miata NA. He wants to add a SC to it, small one. Normally any FI should req a IC, but the whipple SC from.the GT doesn't seem to have one. Why?

Thanks for the answers. Kind regards.

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[transitmaster]

I peak around ~24 psi according to my Cobb ap. I'm around ~700 ft above sea level

 

[Quiksnd]

A boost gauge just shows you a pressure differential. If it shows "0 psi" or "0 bar" with the engine off, then that answers your question. It is not taking atmospheric pressure into account.

A more efficient turbo/intercooler can make more power at the same boost psi if it creates less heat. Boost alone doesn't make power. Cooler/denser air makes more power.

Boost/Vaccuum is measured in the intake manifold after the throttle body. At WOT, the throttle body is open and the intake manifold is being pressurized by the turbo. When you release the throttle, the throttle body closes, the spinning engine is still trying to pump air out but it cannot (close your mouth and plug your nose then try to breathe). That creates a Vaccuum in the intake manifold.

Those are pretty simple explanations to get you started. There is actually a lot more to it, but if I'm wrong anywhere, someone feel free to correct me.

 

[TurboGlen]

BOV's open when the throttle is closed to prevent the tubro from spinning too fast without anywhere for the air to go which can damage the turbocharger.They are not intended to prevent lean mixture but can affect the A/F ratio especially if located after the air meter.

As for forced induction an IC it is not required but does have a lot of benifits. Any time you compress air you increase the temperature which reduces the density and also increases the likelihood of detonation. An IC cools down the air so you get a more dense air charge which creates more power and also are less likely to detonate which can case damage to engines.

 

[Lsstefan]

Thanks a lot fellas.
So when I release the throttle and there is no more air being pumped, the engine runs on inertia? No more burning air/fuel?

 

[dgc333]

Your boost gauge with the engine off is measuring atmospheric pressure which is approximately 14.7 psi or 1 bar. The US spec cars the gauge is marked zero at atmospheric pressure and is marked with psi above atmospheric and "Hg vacuum below atmospheric. The numbers on the gauge are 30 to 0 to 30. Not sure what the EU spec cars look like but since Bar is an absolute measurement I suspect the gauge is marked 0, 1, 2, 3 with the gauge sitting at 1 with the engine off.

With the engine idling or at light load cruise there will be less than atmospheric pressure in the manifold so the gauge will be reading vacuum or less than 1 Bar absolute. As the turbo spools up and starts to create positive pressure the gauge will show pressure above atmospheric psi or over 1 bar. NA engines do the same thing accept they never go above atmospheric.

 

[TheWildman]

Thanks a lot fellas.
So when I release the throttle and there is no more air being pumped, the engine runs on inertia? No more burning air/fuel?

Negative. When you take your foot off of the accelerator pedal, the turbo spools down and your car just runs by itself. No boost. (The turbo is still spinning, but not enough to pump any extra air into the engine) The engine is still burning the air fuel mixture to keep itself running.

 

[arghx7]

Hello. I own an Ecoboost EU PP.
I've been trying to familiarize myself with the tuning theory and stuff and I keep running into this confusion. Can you guys please englighten me?
So from what I've read, a NA engine CAN take up to 1 bar(14.7 psi) of boost during WOT right? Ofc in ideal environment, not on the Everest.

When the n/a engine is at WOT, the pressure in the intake manifold is the same as atmospheric pressure. So if atmospheric pressure is .98 bar, pressure in the manifold is .98 bar unless there is some other form of restriction.

- So I have a turbo on top of it that compresses air more than the engine could on it's own. My digital boost gauge shows maximum 1.3 bar(18.8 psi) of boost. Is this boost over the standard pressure?

Yes.

- My buddy has a bigger turbo than mine on his Focus ST (432 hp) and he says he drives with 1.2-1.32 bar(his engine can't handle more atm). How can he make 400+ with the same boost as me?

The actual amount of airflow entering his engine is higher than yours. A turbo has a compressor side which pumps air and a turbine side connected to the exhaust. On the compressor side, bigger = more compressed air flowing into engine. On the turbine side, bigger = less energy wasted pushing exhaust (less backpressure). That's the simplified explanation.

- What is that vacuum? I know what vacuum is, but for intake pressure? Could it be like the engine takes ALL the air in the intake and since there is no more air there it turns into vacuum?

- Why is it that when I WOT I have boost on the gauge, but when I release the throttle and the RPM stays almost the same I get vacuum? The turbine keeps spinning as the exhaust is flowing fast due to RPMs. Could the BOV release the air from the intake so the mix doesn't become lean?

Here is a picture:

It shows vacuum only, for an engine without a turbo. Even if you have a turbo though, the pressure in the manifold drops when the throttle is closed. Even if the turbo is spinning to make boost, if the throttle is closed the intake manifold will have vacuum. That is why the computer can close the throttle to limit engine torque (vehicle speed limit, traction control).

Offtopic: my brother owns a miata NA. He wants to add a SC to it, small one. Normally any FI should req a IC, but the whipple SC from.the GT doesn't seem to have one. Why?

It doesn't have an intercooler to make the price lower, and to save space (do not have to fit the intercooler). They limit the supercharger so that the air doesn't get too hot.

Negative. When you take your foot off of the accelerator pedal, the turbo spools down and your car just runs by itself. No boost. (The turbo is still spinning, but not enough to pump any extra air into the engine) The engine is still burning the air fuel mixture to keep itself running.

If put the engine in neutral, with foot off the gas and accelerator, you will be idling at less than 1000rpm. Then the engine is still burning air and fuel.

Modern engines turn off fuel injection when you decelerate (engine in gear, foot off the gas pedal, engine speed higher than say 2000rpm). If your car comes with the air fuel ratio gauge in the instrument panel you will see the air fuel ratio go very lean if you are say driving down a hill in gear with your foot off the gas. The engine spins because it is connected to the transmission and wheels. This is called overrun. The wheels turn the engine instead of the engine turning the wheels.

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