I know that our engines are supposed to "handle" 10psi at max. But then Im going to assume thats coming out of a S16G turbo. The cfm of a 14b turbo is I think around 405, and a big16g should push somewhere near 575cfm. So then, 10psi really isnt equal between the two turbos. The stock 1G 14b is set at 10psi. Does anyone know what this equates to in equivalent boost from a 16g? Because assuming I run 6psi from a 14b, its going to create much less power then 6psi from a s16g, correct? I was going to run 6psi, but maybe I can keep it at 10. Is 10psi safe to run with a 14b? Thanks.

No matter what, 10 psi is 10 psi, I would never push that much on our engine. The boost gauge checks how much psi your pushing thats in your intake manifold. And I would guess that you lose at least .5 psi

So then the cfm doesnt matter at all? I imagined that at x amount of boost the turbo is putting in y amount of cfm. So if the amount of air changes, the amount of boost should change too. Wouldnt more air be entering the engine if the cfm is higher?

The CFM does matter. But boost is boost..your just moving more VOLUME of air...
A t25 @11psi will certianly not make the same power as a SS16G @ 11psi...a t25 is like 350 cfm where a SS16G is 480cfm. So the 16g is definitally going to make more power...does that make sense? Either way your car will not handle 10lbs without some mods done to it...either a tti racing head gasket (if you want to keep stock pistons)or a set of forged pisons ect ect ....

Thanks for the answers :) But I still dont really understand. Maybe im just slow.

So then if the CFM doesnt really matter, then technically I can get a 20G turbo and set it to run at 6psi and get massive power gains? That doesnt sound right to me. Assuming that the problem that would cause engine damage would be caused by having too much air in the engine, wouldnt the volume of air entering the cylinders actually matter?

Lets hypothetically assume that some turbo out there makes 400cfm at 5psi. Now lets also assume that another turbo makes 200cfm at 5psi. Assume that the cfm of the turbo doubles every time the psi doubles. Now lets turn up the boost. You run the 400cfm turbo at 10psi, and the 200cfm turbo at 10psi. In reality, youre pushing in 800cfm vs. 400cfm into the engine. Wouldnt the 400cfm turbo be much more likely to cause damage then the 200cfm?

>wouldnt the volume of air entering the cylinders actually matter?

It's pretty much the main reason why 5 psi is 5 psi. An engine only demands so much air. For arguments sake, let's say our engines demand 170 CFM @ 7000 rpm. Running 5 psi of boost compresses the air so it is more dense, but it doesn't change what the engine demands. The same volume of air is demanded by the engine, but it has more mass because it is compressed 5psi above atmosphere - effectively turning 170 CFM into 230 CFM. So as long as a turbo can give you 230 CFM @ 5psi - you should be able to run it at 5psi. For 10psi boost, the 170 CFM turns into 285 CFM. So as long as the turbo can give you 285 CFM @ 10psi - you should be able to run it at 10psi.

Bigger turbos can be more efficient compared to smaller turbos, putting out cooler air. Cooler air is better.

<EDIT>
>In reality, youre pushing in 800cfm vs. 400cfm into the engine.
This is not reality
</EDIT>


95 Eclipse RS : 5 speed
Jeep TB writeup - http://www.dimensia.com:81/jimbo/JeepTBfor2gnt.html

Psi is not psi on separate sized turbos. CFM ratings for each turbo dictate how much air can flow out of them. A smaller turbo, will flow less CFM per each pound of boost, over a larger turbo with more CFM's at each pound of boost. How do you think big ass turbos make such great power? They can flow a lot more air per each pound of boost over a smaller turbo. IMO, 10 psi on a 14b should be your limit, which by taking a guess, would equate to the S16g's airflow of around 7-8 psi. I dont know a whole lot about the 14b, but thats just a guess. To make this simple: Take a hose, 1/4" in diameter, and put 50 psi of water pressure through it. YOu get x amount of flow through that hose. Now take lets say a garden hose, with 50 psi of water pressure through it, and it will obviously flow LOTS more water even though its at the same pressure. Pressure and flow are related, but not the same..

>>>To make this simple: Take a hose, 1/4" in diameter, and put 50 psi of water pressure through it. YOu get x amount of flow through that hose. Now take lets say a garden hose, with 50 psi of water pressure through it, and it will obviously flow LOTS more water even though its at the same pressure. Pressure and flow are related, but not the same..<<<

Nice analogy :thumbsup I started to try and explain it but it didn't make sense. I'm glad you thought of something easy.

Just because a turbo is rated at 500 cfm @ 2.0 PR does not mean that the turbo flows that amount of air in our engine at 15 psi boost pressure. The engine mass air flow is determined by the displacement, the RPM, the volumetric efficiency, and the air density (or plenum air pressure and temperature). At a given RPM and at the same plenum air pressure and temperature, the same amount of air flows regardless of which turbo is used.

Bigger turbos put out cooler air - so you can get more power out of them.


95 Eclipse RS : 5 speed
Jeep TB writeup - http://www.dimensia.com:81/jimbo/JeepTBfor2gnt.html

Alright! Great discussion going on here :)


>Just because a turbo is rated at 500 cfm @ 2.0 PR does not
>mean that the turbo flows that amount of air in our engine at
>15 psi boost pressure.

Well, it may not be exactly that much air, but if you think about it, thats what a turbo is doing. Its "forcing" all the air it can for each pound of boost. Think of psi as the "work" to getting the air into the motor. We are forcing more air than the motor, VE, and displacement can normally take on. Thats where the power comes from. When we force more air and fuel into the motor, we get more power.

The engine mass air flow is determined
>by the displacement, the RPM, the volumetric efficiency, and
>the air density (or plenum air pressure and temperature). At a
>given RPM and at the same plenum air pressure and temperature,
>the same amount of air flows regardless of which turbo is
>used.

But we must remember, we are forcing the air/fuel mix into the motor with boost pressure. VE, and displacement has a bit to do with it, but we are squeezing the mix into the engine. A bigger turbo can "squeeze" more air into the motor, because of a larger compressor wheel, that can collect and push more air over a smaller turbo.
>
>Bigger turbos put out cooler air - so you can get more power
>out of them.

Actually, it almost is the opposite. With larger turbos, comes more heat from compression of the air. Think about a air compressor for like air tools. Feel the outlet of the compressor. I'll bet its pretty darn hot. Why? Because its compressing lots of air. Compressing air particles creates heat in the process. Pure physics there. Yes, colder air equals more power, but not that much. A T25 turbo at 10 psi will get blown away from lets say a T3/T4 hybrid turbo at 10 psi. It just flows more air. Take a look at each of those turbo's compressor and turbine wheels, and you will see the big difference in their sizes. A larger turbo flows more air, plain and simple. :)
>

The very very short answer:

Running a 14b at 10psi is "ok" on our engine.

Running a 16g at 9psi is "ok" on our engine.

Running a T3 at 8psi is "ok" on our engine.

And yes, CFM matters. Don't believe me? Hook a 3cfm air compressor up to your intake at a whopping 50 psi. Hint: You won't blow your motor.

>>Bigger turbos put out cooler air - so you can get more power
>>out of them.
>
> Actually, it almost is the opposite. With larger turbos,
>comes more heat from compression of the air.

Ummm.. no. The heat will ALWAYS be less. Consider your laws of thermodynamics. I can't really explain it (so ask corbin, or better yet, google) but the simple fact remains - the greater the mass of air that flows through the compression mechanism, the greater the distribution of heat through that given air mass. Thusly, overall, a lower output temperature at compression.

For intersting reading regarding the theory of thermodynamics, check this out: http://chemcases.com/fuels/fuels-c.htm

However, as far as 10psi from a 14b (or a T25, or whatever) i think it comes down to intake pressure vs compression ratio. I think 10psi of intake pressure regardless of temperature is too much on 9.6:1 static compression with your average 2GNT'ers tuning ability. yeah, you'll break something. Can it be done? Sure.. Can your average Joe do it? I dunno.. maybe. Maybe not. Do you want to risk it on your daily driver?

>>>Bigger turbos put out cooler air - so you can get more
>power
>>>out of them.
>>
>> Actually, it almost is the opposite. With larger turbos,
>>comes more heat from compression of the air.
>
>Ummm.. no. The heat will ALWAYS be less. Consider your laws
>of thermodynamics. I can't really explain it (so ask corbin,
>or better yet, google) but the simple fact remains - the
>greater the mass of air that flows through the compression
>mechanism, the greater the distribution of heat through that
>given air mass. Thusly, overall, a lower output temperature
>at compression.

Right Dino.

Interestingly, this very topic was discussed to some length in the latest Sport Compact Car regarding turbo sizing. Again as stated there, the temperature of the boosted air from larger turbos creating a denser, cooler charge creates more power. Sort of the same concept with intercoolers or a cold winter day.

But 15 psi is the same whether it come from a T25 or a 25g. It's all the same.

Interesting. Then why do we need a larger, more efficient intercooler when we use larger turbos? If what was said is true, then we can use smaller IC's with larger turbos, since the air is cooler. That just doenst make sense to me.. I just cant see a T25 pushing the same amount of air as a S25G or whatever turbo at 15 psi...ok so the air is cooler with a larger turbo, but thats not where the majority of the power is coming from. Is more air. They wouldnt give CFM ratings on turbos if it didnt mean anything. Right? Yes, psi is psi, but there is different amounts of air flow at different pressures. Larger turbos can supply more air per each pound of pressure. Thats the whole point of upgrading to a larger turbo.

You get the bigger, shinier IC because it has to support more FLOW - not necessarily cool more.

What Jason is saying is that yes, 5psi is 5psi as far as mechanical compression goes (the engine having to compress it, and the possible resulting breakage), but its the CFM that makes the power. The more air mass you can cram in, the more fuel you can burn, the more power you cna make. The pressure is unimportant - its the CFM that matters.

>We are forcing more air than the motor, VE, and displacement can normally take on.

I think this has got to be one of the most difficult concepts to really get a hold of. Maybe it's a bit easier to understand with a correct version of the hose analogy.

In the correct version, for simplification, the hose doesn't exist. It's the spicket and the nozzle that are important. The intake is likened to the spicket. The displacement, rpm, and VE dictate how much we can get out of the nozzle - displacement would be the inlet of the nozzle, rpm would be much we sqeeze the trigger, and VE would be how restrictive the nozzle is.

Normally the spicket is set at a constant 14.7 psi. As you sqeeze the trigger(step on the gas), you would get more and more flow out of the nozzle. Even at full spray, you wouldn't get as much as the hose could provide - maybe a little less, because the nozzle is more restrictive than it's inlet(VE). Let's turn up the pressure(add boost) to 24.7psi(10psig boost). Across the entire range of pulling the trigger(revvin it up) - the nozzle will squirt more than it did at 14.7psi. It still won't give you full flow, because the nozzle is more restrictive than it's inlet(ve) - but you'll water the daisies alot faster at the higher pressure(boostin).

It's not a simple subject. There is no simple analogy. This one may not be the best, but the turbo is not the engine. You can't talk about it as if it is the only important componant. Adding a turbo does not increase the volumetric efficiency. If anything it would reduce the VE, due to increased exhaust back pressure. I'd actually like to get people's opinions on how boost affects VE - I've asked before but haven't gotten any responses.

One more thing. There would be absolutely no reason for compressor maps if CFM were constant for a given boost. We could instead use output curves. It would be the same axis(pressure - cfm), but instead of a map - it would just be a curve.


95 Eclipse RS : 5 speed
Jeep TB writeup - http://www.dimensia.com:81/jimbo/JeepTBfor2gnt.html

well like jason said, 5psi is 5psi no matter what makes it. now i gues to try and simplify it down a lot; let's say the # of turns of the compressor wheel from a specific turbo it takes to get to that set psi depends on how much it flows... that's where cfm comes in. you can't tell the diff at low boost, but say for a t25 to build 5psi it'll have to make more revolutions than a s16g would in order to build 5psi. it's the reason why turbo's get maxed out at a certain psi or hp.

Well then if the CFM matters, then what is a safe cfm amount that our engines can handle?