turbo on a 6 cylinder . . .

Your not suppose to think in PSI terms.

as 10 psi from a T25 is going to be WAY different than 10psi from a T70.

 

i do believe that you can increase the gas mileage with a turbo. that has to be the objective when you choose the turbo size. if you choose a small turbo that will build boost under the low exhaust flow of cruising then you will be capturing energy that is in the exhaust. this will feed that energy back into the motor making it easier to get a good air charge into the combustion chamber. fords new ecoboost line of motors works this way. it also uses a variable vain turbo which allows it to work efficiently at the low exhaust flow and high exhaust flow situations, kind of the best of both worlds.

 

Maverick75,

Sorry, what I meant to say psi absolute pressure in the intake manifold, although I believe the formula to get that is psig + psia = psi absolute.
Yes, a bigger turbo will produce more boost than a small turbo, but boost inside the intake is all the same, 10psi is 10psi.....................kind of funny how physics works that way.
Bryant is correct about getting good mileage, as with my TurboCoupe, as long as I kept my foot out of I could pull over 25mpg in 5th gear.

 

While that is true I don't think your seeing my point. At 10psi on a smaller turbo its flowing less CFM than on a bigger turbo.

Like for instance if a T25 at 10PSI is flowing 150CFM that's good for 100 HP

But a T70 at that same PSI will be flowing way more, like say 450CFM which is good for 300 HP.

That's why you can generalize things with PSI, because turbos flow differently depending on a lot of variables.

While the stock fuel system can support 8psi on a small turbo, it for sure isn't going to support 8 psi on a Huge turbo.

Same goes for bottom end, you can't say oh its safe for 15PSI.
Because two different size turbos are not going to make the same amount of HP at the same psi level. It may be able to handle a small T3 size turbo all day at 30psi, but toss a big T4 turbo on it and it might not handle 10psi before it blows.


The only time you can generalize boost is when your talking about identical engines with identical turbos. Which I think you were doing in your post, but it was hard to understand because this thread went from talking about 6 cylinders to turbo coupe 4 cylinders

 

Maverick75,

I am not a turbo expert and have only fooled around with one in my 88 Turbo Coupe. I understand different, ie., small turbo's flow less CFM that a larger turbo, just like a 650CFM carb will flow less than a 1050CFM carb will.................and yes, on the same motor the 650 may make more HP because the motor is not setup to handle the 1050cfm...........and if the motor has a different cam, intake, heads...............ect., it will produce much more with the 1050cfm.
On my 2.3l turbo (the only exhaust gas driven turbo I have ever fooled with) when the boost gauge read 30psi, it was measuring intake manifold pressure, which is the true pressure inside the intake manifold............if I had gone to a bigger turbo I could have produced more hp because I would be shoving more A/F into the cylinder at the same psi, so I understand that the larger turbo can flow more, which means you can increase the A/F that you dump into the combustion chamber, which will make more HP at the same psi...............and that if you put the T25 on one motor, and then put the T70 on the same motor it may or may not make more hp and could have a chance of buring a piston, breaking a ring land or worse, because you now have much more fuel and air...............150cfm vs 450cfm........although I doubt the T70 would actually make 150hp more...............So, my original statment that talked about psi...............boost is only measured in one of three ways; Bar, mmHg, and psi.......................and the only psi you are really worried about is the psi inside the intake manifold which will soon see the top of the piston and other internal parts.....................IMHO

 

Remember the engine can only ingest so much air at a given boost level. That bigger turbo can support 10 PSI @ 450 CFM. That means on an engine that ingests 450 CFM it will still efficiently provide 10 PSI. A higher CFM turbo will not make the motor ingest any more than a low CFM turbo at the same boost. Except for the fact that the bigger turbo might heat the air less at that boost level and there fore more oxygen will be in the charge going into the cylinder. But on the other hand the bigger turbo will inevitably need more exhaust velocity to make the same boost so it will not make the 10 PSI till a higher RPM is reached. Sizing your turbo CFM to your engine CFM correctly based on the driving you do is key. Don't lie to yourself when you choose and you'll be happier


The little T25 will give awesome throttle response and quick spooling to max boost. Very low in the RPM band. After that the higher the RPM the more of a restriction the small quick spooling exhaust side becomes. The bigger turbo will do the same thing but higher in the RPM band.