Explain boost/psi to me!

Jforce05

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I've been around cars and diesels my whole life and I feel like a dumbass asking but the only stupid question is the one you didnt ask. I always thought more psi= more horsepower.

But then you see guys with a turbo @10psi with WAY highet rwhp numbers than twin screws @ 20psi. I realize supporting mods mainly being the fuel system plays a huge role.

How exactly does it work?
 

R.D.P.

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Same everything else a 20lb twinscrew setup will top a 10lb turbo. Now 16lb on the turbo and you are probably passing the twinscrew in HP. Why? Parasitic loss from turning the blower and with a PD blower - heat. The only thing better than compressed air going into the combustion chamber is cold compressed air, well and nitrous lol....
 

Tezz500

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I've been around cars and diesels my whole life and I feel like a dumbass asking but the only stupid question is the one you didnt ask. I always thought more psi= more horsepower.

But then you see guys with a turbo @10psi with WAY highet rwhp numbers than twin screws @ 20psi. I realize supporting mods mainly being the fuel system plays a huge role.

How exactly does it work?

lots of good videos can example pressure and A/F ratios... its way too much to type imo..

http://www.turbos.bwauto.com/products/turbochargerprinciples.aspx

Basic Basic... a few reasons (given all else is the same) a lower pressure boosted engine may have moreHP due to the fact that the VOLUME of air being pushed into the Combustion chamber is actually MORE... though the pressure is less... (think cooler air charge denser AIR. more air less pressure)
 

Jforce05

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Basic Basic... a few reasons (given all else is the same) a lower pressure boosted engine may have moreHP due to the fact that the VOLUME of air being pushed into the Combustion chamber is actually MORE... though the pressure is less... (think cooler air charge denser AIR. more air less pressure)

Thats exactly where I was getting confused
 

SonicDTR

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Boost is actually a measure of restriction. If you increase the flow through an engine(heads/cams/exhaust) then measured boost pressure will drop, but power will increase.
 

Tezz500

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Boost is actually a measure of restriction. If you increase the flow through an engine(heads/cams/exhaust) then measured boost pressure will drop, but power will increase.

this also. an engine is basically an air pump. the more you can shove into it (air meaning Oxygen needed for combustion) the more power its going to make. dont forget you got to remove it once you get it in there. but were just focusing on BOOST and intake volume etc for this lesson.
 
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SteedaX47

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Different blowers/turbos have efficiency ranges, as they can only flow so much air. take a stock m112 from a terminator. its almost right at its efficiency peak from the factory at 8psi on the dohc 4.6. any more than 14psi and youre really only blowing hot air.

Think of a garden hose. you can make a small diameter hose flow just as much or more than a larger hose if you apply enough pressure to the smaller one right? but eventually, youll hit the maximum flow for that hose and wont really be able to force more water through it, whereas on the larger hose, the same pressure can flow more water. make sense?

so... lets take a larger supercharger, say a whipple 2.9, it has a MUCH higher efficiency range and will make more power than the eaton at the same 14psi since it will be pushing cooler air into the manifold. but 14psi on a 2.9 isn't quite in its efficiency range yet so its really not doing the job it could do. so lets bump up the boost to... 18psi. now its about in the operating range it should be and will make great power while probably having cooler (or close to) air going into the intake than the eaton at only 14psi.

same thing can be said about the 3.4 vs the 2.9 along with turbos vs superchargers.
 
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R.D.P.

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Different blowers/turbos have efficiency ranges, as they can only flow so much air. take a stock m112 from a terminator. its almost right at its efficiency peak from the factory at 8psi on the dohc 4.6. any more than 14psi and youre really only blowing hot air.

Think of a garden hose. you can make a small diameter hose flow just as much or more than a larger hose if you apply enough pressure to the smaller one right? but eventually, youll hit the maximum flow for that hose and wont really be able to force more water through it, whereas on the larger hose, the same pressure can flow more water. make sense?

so... lets take a larger supercharger, say a whipple 2.9, it has a MUCH higher efficiency range and will make more power than the eaton at the same 14psi since it will be pushing cooler air into the manifold. but 14psi on a 2.9 isn't quite in its efficiency range yet so its really not doing the job it could do. so lets bump up the boost to... 18psi. now its about in the operating range it should be and will make great power while probably having cooler (or close to) air going into the intake than the eaton at only 14psi.

same thing can be said about the 3.4 vs the 2.9.

This is all true, but OP is mainly talking turbo vs supercharger - parasitic loss is I believe 10-15% depending on the setup. So that means if I could get some other power source to spin my Whipple other than the crank, like say the engine exhaust - boom, +60whp on the same boost level.
 

Jforce05

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So all in all its mostly an efficiency thing? You can spin the eaton to 15psi but since you are overworking the supercharger it creates hot air being forced into the intake? While a whipple at 15psi is working way less to create the 15psi so the cooler air intKe temp is in fact the reason for the added horsepower?
 

SteedaX47

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This is all true, but OP is mainly talking turbo vs supercharger - parasitic loss is I believe 10-15% depending on the setup. So that means if I could get some other power source to spin my Whipple other than the crank, like say the engine exhaust - boom, +60whp on the same boost level.

Lol youre right, I totally skipped over the foundation of his post, my bad lol.

So all in all its mostly an efficiency thing? You can spin the eaton to 15psi but since you are overworking the supercharger it creates hot air being forced into the intake? While a whipple at 15psi is working way less to create the 15psi so the cooler air intKe temp is in fact the reason for the added horsepower?

the cooler air charge isn't all of it that's a big part of it. back to the hose thing, the whipple has a higher airflow capacity. small hose=eaton, big hose=whipple. same pressure through the larger hose will move more water/air

when I say efficiency range, I mean more of how the air flows through the case of the blower. or even through the housing of the turbo and around the impellers inside. they work better/worse at different rpm
 
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SonicDTR

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Lol youre right, I totally skipped over the foundation of his post, my bad lol.



the cooler air charge isn't all of it that's a big part of it. back to the hose thing, the whipple has a higher airflow capacity. small hose=eaton, big hose=whipple. same pressure through the larger hose will move more water/air

when I say efficiency range, I mean more of how the air flows through the case of the blower. or even through the housing of the turbo and around the impellers inside. they work better/worse at different rpm

You run into this same concept on turbocharger sizing. A larger turbo might wind up making the same(or less) peak PSI boost, but flow way more volume, and therefore more power. You can spin a little turbo waaaay past its efficiency range, or get a bigger turbo and spin it a bit less and make the same power easier.
 

2001sleeper

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The ELI5 version:
The objective is oxygen in the cylinder bore.
A larger cylinder note will allow more oxygen (air) in, but for those that lack displacement forcing additional oxygen in and generating higher cylinder pressure will allow more fuel to be combusted to drive your Pistons down the bore faster.
Hence the people that say no replacement for displacement.

Turbo or supercharger is just a means to compress the air making a denser stream being forced into the engine. However, compressing air is not free as it takes work to force the molecules to get closer together. This work then generates heat and this the battle between compressing air and removing the heat generated.
 

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