turbo vs turbos vs boost

[rtviper]

I think I know what lbs of boost mean. What would it matter if you use 1 turbo or 2 when it comes to building boost in a motor. If you have a motor that runs x.x in the 1/4 on 20 lbs of boost with a single turbo setup what would using a twin turbo setup do to accomplishing the same et? Would you be able to run the same times with less boost? This question came to mind while reading claims of performance numbers from a twin turbo setup that seem a lot better then one would get with a single turbo setup on a similar motor combination. The difference seems to be several lbs of boost less to achieve the same performance.

 

[chevyII]

no, 20lbs of boost with a stock turbo and a larger one is different. The turbos are rated as CFM. Boost is just pressure above the engine vacuum. Twins vs single is another story. Do a search on the twins alot of info.

 

[jhanson1750]

Has to do with volume of air moved at a given (psi) pressure. Twins can typically be smaller each which spools quicker.

 

[rtviper]

Well I guess I dont understand what boost is. I thought it was a measure of excess above what the motor will take vs what is being forced into it. I thought the only way to change that is to make changes to the motor so more of the intake charge will enter the motor before the excess builds up to 20 lbs again. In this situation the motor will now make more power with the same 20lbs of boost showing on the guage. You are saying if you add a bigger turbo to the same motor you will make more power at the same boost even though you havnt maxed out the smaller turbo?

 

[rtviper]

1750 I understand they will spool quicker I was just surprised they could make a lot more power with less boost on the same motor.

 

[Jerryl]

I am no expert but my understanding on the subject is in the combination of boost, CFM and gr/sec. Gr/Sec is one of the most important ones.
20 PSI boost at 300F is not the same as 20 PSI at 70F.
Neither is 700 CFM at 300F vs 70F ..........

 

[Ormand]

"Boost" is simply the ratio of turbo compressor discharge pressure to inlet pressure. If it is pulling in air at atmospheric pressure (14.7psia) and discharging at 29.7psia, then there is 15 pounds of "boost". The way to make more boost is to use a bigger compressor, or to turn it faster, or both. The "efficiency" of the turbo affects the power you get at a given boost. Since the compressor uses exhaust pressure to turn, a less efficient compressor requires more exhaust pressure, and the higher back pressure on the engine cuts the power output. A less efficient compressor will also heat the air more, and hotter is not as dense, and thus makes less power. Running two small turbos to get 20 psi boost can put the turbos in a more efficient range than running a big turbo to get the same flow/pressure, and the smaller turbos have less inertia, so they will spin up quicker.
Boost is NOT the "excess" of what the motor will take. Boost is just a way to get more air (pounds of air) through an engine of a given size. You compress the air, with a turbo, or a blower, and BOOST the power. The engine doesn't change, but because it is taking in more pounds/hour of air, it can burn more pounds/hour of fuel, and make more power. The temperature of the air is a key factor, which is why the intercooled cars make more power than the "hot airs"- cooler air is denser air, and more pounds per hour.