I spelled everything out concisely on account that many others may read through this and become confused.

As for the larger turbo, the same indicated boost pressure with a larger turbo equates to a higher mass airflow rate for two major reasons in the real world. At a given indicated intake manifold pressure:

- The larger compressor is more efficient at pressurizing the system, so less shaft speed energy is wasted as heat.

- The larger turbine is more efficient at depressurizing the system, so less backpressure equates to reduced pumping losses.

Both result in greater VE and/or thermal efficiency, which translates into greater torque, and greater torque with respect to rpm = greater hp.

 

Sounds like a YES to me. It's what I've been saying from the start of this thread. It's actually stuff you probably had a hand in teach me from past threads.

 

As confusing as this discussion became, for all I know everyone is saying the same thing. Sometimes it just gets lost in translation.

 

There is no mystery reason beyond what you stated. The only thing is that the charge density and backpressure is never the same between a larger and smaller turbo at the same pressure given they are pumping into the same engine. There are cases where they can be close like a more restrictive stock engine with a 35r and a stock turbo running 15psi, but generally nobody runs a big honking turbo on a stock setup engine.

I am probably the closest it gets to somebody running a larger turbo on a stock(ish) engine and even then I have larger IC pipes, intake, cams and exhaust. If I were to compare my power between this turbo at 15psi and a stocker, the difference wouldn't be too impressive though. Crank the boost up to 25psi and the gains increase as the engine is able to process more air then the stocker can efficiently produce.

Also keep in mind the engine can process a certain VOLUME of air, so while a stock turbo and a 35r may produce the same volume of air at 25psi, the 35r will contain much more combustable air molecules as the density is greater.

The only thing I would be uncertain of is how much backpressure would play a role versus air density. I'm sure this depends on how much boost you run and how much air your engine can process. So again, the more air your engine can process (built engine) the worse backpressure you would see at the same boost pressure. Now this has got me paranoid about running my dinky .63 turbine when I build my engine hah.

 

tq is to blame not so much hp. when a rod bends and a piston goes thru your block hp is not to blame. its all tq. Thats why you see evos have over 1000hp but the tq number is way down in the 500ft-lbs. 2.0l aren't know for tq. just my 2 cents

 

check out this thread I started a while ago.

https://www.evolutionm.net/forums/en...-safer-dd.html

 

I'm not picking on you, as same has been stated on this forum several times - but horsepower is nothing more than a unit that is based torque*rpm.

1000whp EVOs carry the torque high in the rpm range and those builds are far from what people drive on the street...

 

Yep totally agree. There is a couple things I've learned from this thread. First, is that I need to remind myself that just because a stock turbo can make and hold a low boost pressure like 10 or 14 psi, doesn't mean it is in it's efficiency range. I need to remember, that the motor and setup might be a very good pump, and require so much mass air flow, that the stock turbo has to work extremely hard and out of it's efficiency range to produce that low boost pressure.

The other thing I learned is as shaft speeds increase, back pressure increases. I guess exhaust gasses blowing through a turbine wheel spinning like crazy have a harder time getting through than a slower spinning turbing wheel. I only thought about the creation of back pressure in terms of the size of the turbine housing.

 

Well, what is it that drives the turbine wheel? Think about it.

 

ccrain, you are 100% correct and I feel sorry for you getting bashed by so many people that just follow the crowd and don't understand what's going on.

The whole straw comment...that made me

The 'straw' in this case is our engines. That is a FIXED constant. The straw is not changing size.

Everyone sees a compressor map of a big turbo and they see that at 20 psi it can flow a crapload of air and just assume that's why a big turbo makes more power as the same psi as a small turbo. The fact is that the compressor map is just that...a map for that compressor. You fit your engine to it by calculating the CFM that your engine (irregardless of the turbo) can flow.

As you said, there are two main reasons: lower VE and reduced temps...that's it. Plain and simple. CFM is going to be the same between the smaller turbo and the bigger turbo at the same boost pressure when connected to the same engine (at a constant VE..in actuality the bigger turbo's hotside will increase VE). It's the VE and temp difference that changes the mass airflow, with the bigger turbo having a higher mass airflow than the smaller one.

Everyone get's hung up on comparing the stock turbo at it's choke limit, like 23 psi at redline, where it's spitting out flames basically. The CFMs the same....it's just a very, very hot CFM with a very low VE, which results in a lower mass airflow for the smaller turbo. The bigger turbo still has a relatively cool charge, along with a bigger hotside (bigger VE).

A good test would be to run a stock turbo or 50 trim or something small and a huge turbo like a 35r, 40R, etc, at 6psi and see how much difference in power there would be (keeping timnig, AFR, etc constant). My bet would be it would be pretty small...actually I can probably figure it out exactly.

The fact of the matter is that airflow (CFM) through an engine is dictated by the following equation:

Airflow (CFM) = PR[RPM*V.E.*Cid/3456]
PR=Pressure ratio=(boost in psi+atmos(psi))/atmos(psi)
RPM = RPM of engine
V.E. = volumetric efficiency at RPM being measured
Cid=cubic inch displacement= 122 for our 2.0L engines

To get pounds of air: n (lbs/min) = P (psia) x V (CFM) x 29 / (10.73[ft3ˇpsiˇ °R-1ˇlb-mol-1] x T)
P=atmospheric pressure

In the CFM equation, the only thing that changes CFM for the bigger turbo is VE. For the corresponding mass airflow, the only thing that changes for the bigger turbo is reduced T (due to increased compressor efficiency).

I would have to look at the compressor maps of two turbos, but I bet there is even a boost level where the smaller turbo would produce more power than the bigger turbo, due to it being more efficient at that PR(keeping RPM, timing, AFR) the same.

I don't know why so many people don't grasp this concept...but to have 20 people coming in here laughing at you and bashing you with stupid comments is just stupid.

JohnBradley is the only one who tried to explain things. But, I think he isn't even taking into the account that the engine is a fixed constant...the CFM stays the same, with respect to VE. It's mass airflow that is changing for the reasons mentioned.

Edit: There are other equations that are involved as well, such as IC efficiency, which is compounded and makes the hotter outlet temps of the smaller turbo make even less power, but this is as simple as I can state it for easy understanding.

 

And, btw, to answer the OP's question:

No, boost doesn't blow a motor. The pressures of combustion are what...in the 1000s of psi? Boost is negligible in that. It's the total cylinder pressure and the ability of the internals to withstand it that blows the motor.

That's why detonation and preignition are bad. It's the spike in cylinder pressure that does the damage.

 

those posts should be sticky eric!
everytime I argue those points I get shot down too. Basically on bigger turbos it's just less heat and bigger turbine side making the power.

 

And a few miscellanous other things, such as bigger intake pipes with less vacuum, etc. Not to mention that the lower temps and higher VE allows for a more aggressive timing and fuel curve. Also, people usually just don't upgrade the turbo...they upgrade the cams, head, etc, which increases VE even more.

All of these add up to allow a bigger turbo to make more power at the same boost level. But it's just not because of the typical response that I see a million times that a 'bigger turbo flows more air, comparing straws and what not'.

I just did some calculations, and simply from a change in 60-80% compressor efficiency and a change of +10% VE, I calculated roughly +60 whp for a stock turbo and 35r sized turbo at the same 23 psi of boost at 7500RPM (this is assuming everything else is the same...intake pipe, IC, cams, o2 housing, exhaust, etc, etc). Add to that 3 more degrees of timing, and now you're looking at +100 HP.

 

Post 50?

Ted said it and Aaron said it, there are some terms that seem to being used interchangeably that might not normally be interchanged. Aaron was raised around boosted domestics so he tends to think in those terms I've noticed.

 

Made my day!

And learned some things from this post as well. Thank you.