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Can someone explain turbos and boost to me???

419 Views 5 Replies 6 Participants Last post by  Ormand
I know I should really know this by now but here is something no one has ever really explained to me...

How come you can crank a stock turbo on a car to blow 30 psi and run say 11.90 and then put a larger turbo on and run the same or better ET with much less boost? (assuming you have the same injectors and they are adequate). Isn't 30 psi 30 psi no matter what turbo???
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You are correct, pressure is pressure. But in terms of making power in an engine, it's a function of pressure and volume.

The relationship isn't trivial but a general statement would be that increasing one, and leaving the other would help make more power.

So with the stock turbo, increasing the boost makes more power (to a point)

With a larger turbo, the volume of air (CFM) is increased hence you can make the SAME power but with less boost..... dunno

The theory is actually alot more complex than this but I think I got a close generalization....
I might add ad could be wrong...

In theory, if you have an engine at sea level that has a displacement of say 5.0 litres this is at 14.7 psi air pressure. So, if you "force" another 14.7 psi into the engine (via turbo or supercharger) then you theoretially double the displacement. I am not sure if this is %100 correct though. That is why when you go into the mountains, you car is less powerfull, there isn't enough air pressure or "weight" to push the air in (on a naturally aspirated engine)

So you would have a 10.0 litre engine. Even so, 7 psi can be had on almost any "stock" engine. So now, your honda civic is now a 2.25 litre engine.

A few months ago (2 or so) Hot Rod magazine did a comparison of turbo and supercharging. Even though the turbo setup created something like 800hp on a ford (WITHOUT intercooling) and the super made close to it, it was a comletely different power band..

Done rambling.

this is a nice link that explains it well, I think. http://www.gnttype.org/techarea/turbo/turboflow.html

Note that air density is not always proportional to boost pressure as turbo efficiency affects the charge temperature. A small turbo and a larger turbo may produce the same boost in psi but the larger one, if more efficient, will heat the air during compression less than the smaller one. Therefore there are more molecules of air in its charge. (see my comments under madhat's "boost readings")

<small>[ October 24, 2003, 05:52 PM: Message edited by: Steve Wood ]</small>

Yup, PV/T,that's where it's at.
Some turbos are just more efficient at it at different boost levels than others.
Yes, what Steve said. (as usual) There is a calculator at the Dodge Stealth site that lets you find the density, for a combination of turbo boost plus intercooler. Turbo Calculator
It gives the density of the air, not just the pressure. The engine wants mass flow, the more mass of air, the more power. If the pressure goes up, but the temp goes up, too, then you may get LESS pounds of air, at MORE pressure. Higher boost, lower power. That's the "hot air" disadvantage. The calculator is fun, you can see that the IC is helping even if it is heat soaked, and only cools to, oh, 150 degrees.
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