Someone correct me if I'm wrong please.

Was reading through another (un-named) forum's FAQ, and came across this:

Question: How much turbo boost can I push on my stock motor? This is a very common question that people ask and there is a misconception about it as well. Short answer, there is no certain amount. Long answer, you have to understand how turbochargers work. If you are a person who is asking this question, not to insult you, but you probably don’t know the full amount of how turbochargers work and how they affect your motor. PSI is only a number measuring the amount of pressure (pounds per square inch) but it’s the CFM of the turbocharger that really matters. We’ll use 2 examples… a Garrett GT3571 and a Garrett GT2052: Garrett GT2052 Garrett GT3571 http://www.turbobygarrett.com/turbobygarrett/images/catalog/Turbochargers/gt35_images/GT3571_731413_1_comp_e.gif These are the flow charts for each turbocharger. As you can see, they flow much differently. One is obviously much bigger is size than the other. Now, with this said, 1 PSI of boost from one turbocharger is obviously not going to compare to 1 PSI of boost from the other turbocharger cause one will flow more CFM of air per PSI. So with this said and you see the difference between each turbocharger, you will slowly understand that you’re not going to get the same power output with 10 PSI from 2 different turbochargers. So this is why the answer is, there is no answer. If you want to know how much PSI your motor can handle, it all depends on the trim, type & CFM of the turbocharger. 10 PSI from one turbo could cause you to have 400 HP while the other could cause you to have 200.

Now I've always understood PSI or "boost" is nothing more than a measurement of backpressure. SO... All things being equal in the setup and just changing turbo's you're going to make roughly the same hp/tq at the same pressure level at the same rpm. Now the delivery may change. spool rate and compressor design are going to change that. But the comment of "10 PSI from one turbo could cause you to have 400 HP while the other could cause you to have 200" is wrong if piping etc are equal.

One thing it gets right, is that CFM is the predictor of power, and different compressors push different levels of CFM at different efficiency levels otherwise one turbo would fit all, and we know that's not the case.

Case in point - 1.6 miata with a greddy turbo kit fitted and tuned to run up to 20lbs of "boost" with the turbo that comes with the kit. Now most of us know, at that level, that little bitty thing starts becoming the atomic hairdryer. Now with a switch to a T3/4 turbo, it will come up to that same 20lbs of waste gate setting and stop building pressure as the waste gate tries to keep the turbine from speeding up more. Because of the compressor design it's not building as much heat, but it is moving the same, if not a bit more air. The engine is still seeing ~1.36 bar of pressure, but not heating the charge as much, therefore making more power.

Or am I just over thinking this?

The pressure in the manifold is a measurement of air in vs resistance to flow. If you had two engines, identical in every way, one with a big turbo, and one with a small turbo, and both were creating 5 psi, the amount of horsepower would be identical.

The delivery of said horsepower might be very, very different. My XC90 with the six has two tiny turbos, and the five cylinder models have one larger one. Total boost is roughly the same, the six makes more power due to one extra cylinder. Delivery is completely different. The six has no lag at all, the five has a bit of lag, but then gives you a more energetic push.

Any blower, though, will create more power at a lower boost level as the airflow characteristics of the engine improve. My little B&M 144 blower creates about 8psi. If I put better heads on it, it would actually create lower pressure in the manifold, but produce more horsepower due to better cylinder filling.

It really depends on what the question is...

For the original, original question- "how much boost can my stock engine make"- every time I hear that, I think the question being asked is what is the capability of the stock motor AND at what point does making more boost become pointless? Which implies that the proper sized turbo has been selected.

And you are asking how to properly size a turbo. Which also incorporates the base engine, because at some point, making more boost/forcing more flow becomes pointless- a death spiral where more boost has to be reacted by more spark retard, and you can double the flow and make less power.

Either way, it's important to know what the core question is.

Keep in mind, the bigger turbo will be less of a restriction on the exhaust side of the engine, so building 5 psi in the intake manifold very well might require more air (and correspondingly mean more power).

Thanks guys. Wanted to make sure I wasn't losing my mind.

well i wouldnt go as far to say that you arent loosing your mind, but yes. i agree with you

Well... I call three people I met on this forum my best friends.... I lost it long ago. But at least my grasp of turbocharging isn't that far off the mark.

Boost pressure is just a easily measurable way of determining increased mass flow.

The less efficient a turbo is, the more heat it puts into the air. So less mass flow. And then there's the amount of power required from the turbine to generate a given amount of mass flow at a given pressure ratio.

Turbo math makes my head hurt.

PSI is definitely not PSI, though. 15psi from a tiny turbo is way different than 15psi from a GT42.

Knurled wrote: 15psi from a tiny turbo is way different than 15psi from a GT42.

I disagree. If all else is equal, 15 psi measured at the intake manifold will be exactly the same.

15 psi at 2500 rpm from a tiny turbo is different from 15psi at 7400 rpm with a big turbo, yes.

Yes, but in the case of installing a bigger turbo, all else isn't equal, as you're affecting the exhaust side and might be affecting charge temps too.

Mind you, unless you go way off one end of the scale, we're not talking about huge power differences at a given boost level from using a different turbo, but there will be some detectable difference.

Exactly... all else is NOT equal, the smaller turbo might be less efficient on the compressor, or it may take more power to spin up to the speed that makes that particular pressure ratio at that mass flow, which translates to pumping losses on the exhaust side.