Compressor Size Question.

[budget_crx]

I have a friend that is trying to convince me that with all other variables the same (chargepiping, intercooler) a smaller turbo would be the "faster" turbo. (because bigger turbos lag) He is claiming that 10psi is 10 psi, no matter what the size of the compressor, basically: a T4@10psi would have no gain over a T25@10psi. He is also claiming that Compressor size does NOT effect the CFM of a system, so the T4 and the T25 would have the same CFM if all other variables were constant, even though the T4 has a much larger inlet and wheel. Can anybody prove this to be true/false so this arguement can end?

thank you

 

[bison]

Tell your friend he is a moron and has no idea what he is talking about. There are several factors involved in choosing the correct turbocharger for a given application. He is especially wrong about the 10 psi is 10 psi. The useful compressor efficiency for the intended application is determined by the mass flow needed at a given shaft speed for a given engine size. The exhaust housing type and size plays more of a role on the spool rate for a given turbine/compressor with all else constant. Not the size of the compressor. The engines air use in cfm is constant when the intake is pressurized or under vacuum. The airflow on the turbo inlet is not. The cfm will increase with shaft speed on the inlet side. There are considerations such as the surge limit when choosing a turbo combo. Ball bearing cartridges change the possibilities a lot these days too making spool up with larger turbines and a/r's much faster. Ive only touched on a small amount of the physics involved because im too tired to type. Others will be chiming in and hammering your friend. If what he was saying is true then i could put any turbo on the engine in my sig and as long as it hit the same boost pressure it would run the same. Wrong, wrong, wrong. A too small turbo would be blowing a lot of hot air just like your friend. Tell him to start doing some research before blurting out nonsense.

 

[budget_crx]

a direct qoute:


"bigger turbos have more potential to create bigger hp gains due to being able to move more cfms. but if you have the same chargepiping with a small turbo as with a bigger turbo, you will move the same amount of cfm with the same amount of boost. if you were to up the size of the chargepiping when going to a bigger turbo, you will make more hp."

 

[budget_crx]

The engines air use in cfm is constant when the intake is pressurized or under vacuum. The airflow on the turbo inlet is not. The cfm will increase with shaft speed on the inlet side. .

so a larger inlet will yield more airflow, thus increasing the cfm capability over a smaller inlet, correct?

 

[ijames]

a direct qoute:


"bigger turbos have more potential to create bigger hp gains due to being able to move more cfms. but if you have the same chargepiping with a small turbo as with a bigger turbo, you will move the same amount of cfm with the same amount of boost. if you were to up the size of the chargepiping when going to a bigger turbo, you will make more hp."

This would be true if the charge piping were so horribly undersized that its restriction dominates the system. In any real system the piping is sized so that its restriction is minimal.

Your friend needs to learn to read turbo compressor maps so he can see the different efficiencies for different wheels at some constant pressure ratio and cfm, and then he would understand that 10 psi is not 10 psi because the temperature is not the same so the mass of air is not the same so the hp potential is not the same. Go to Turbo Regal Web Site and read the tech articles by John Estill on turbos.

 

[bison]

so a larger inlet will yield more airflow, thus increasing the cfm capability over a smaller inlet, correct?

I should have said cfm flow into an engine is pretty much constant at a given rpm regardless of vacuum or pressure in my other post.

At higher shaft speeds it will allow more airflow as long as its on the right side of the surge limit on the compressor map. The pressure drop on the inlet side is not as critical as having the proper size compressor wheel for an application. A restriction will cause a slightly lower efficiency on the inlet. The time in my sig was done with a stock TR GM MAFS with one screen. It is a fairly large restriction and i bet i only choked off 20-25hp in my instance. If an equal restriction was after the compressor wheel it would choke off a lot more causing a big pressure drop hurting performance at higher boost levels. Not only does it cause the drop, more wheel speed will be needed to get the intake manifold pressure up to the desired level adding more heat to the air. You pretty much looking at 12 degrees per 1 psi of boost minimum. So for a 4 psi pressure drop your adding close to 50 degrees extra heat to the air before it goes through the intercooler. The stock TR intercooler neck is a great example of this.