I have heard people say, for example, they are running 10 Psi on a larger turbo... a T-66 perhaps and get tons more power. Now, what I'm wondering is, if I run 10 psi on a stock TII turbo, or on a huge upgraded turbo, why would I get more power out of the bigger turbo?? The intake manifold would have the same interior volume for the air and the same inlet diameter; so how does that work?

Thanks

Aaron

 

Little turbo is out of its efficiency range (spinning too fast) at higher PSI and heats up the air charge- some of that additional psi is simply from the air expanding from the heat. Larger turbo is not beating up the air; cooler air charge=more air molecules per PSI of air in system= more HP!

 

You're equating boost with airflow volume.&nbsp They are not the same.&nbsp The bigger turbo WILL flow more air volume, period.&nbsp Air flow volume is mesured in "cfm" or cubic feet per minute.



-Ted

 

OK, but isn't the engine and intake tract a set displacement. Once you fill it FASTER with the higher cfm the extra air is simply pressurized FASTER, but if you keep it to the same psi on high and low CFM turbo where is the extra air going?

 

You're thinking static.&nbsp The intake air flow is a dynamic moving system.&nbsp Yes, pressure and intake tract volume is constant, but this only applies to a static system.&nbsp You're missing the all imporant dimesion of velocity.&nbsp CFM is the measurement of the volume of "gas" moving at a specific velocity.



-Ted

 

Ya, I still don't get it
same rpm, same port duration, same PSI of air trying to force its way in the same amount of time. Only thing I could see is if there was a pressure drop in the intake runner as the port opened w/ low CFM and no pressure drop w/ High CFM. Wouldn't these pressure variances in the runners show in the overall PSI or is it like reversion that is dampned by the length of the intake runners?

 

This is the ONE most important thing you need to grasp.

Incresed Volumetric efficiency ! The effect of running a better flowing exhaust manifold, much larger turbine wheel, bigger rear a/r housing, FMIC, along with larger piping etc ALL improve the engine VE. Instead of having around 65% or 70% VE you can have up to 85% VE or more with stock porting @ max bhp rpm, also add to this that the rpm at wich max power is made is generaly lifted as well is what makes for up to a 30% increase in engine power even when comparing low boost settings like 10psi.

Not only this, you can realize much better fuel economy and much better throttle resonce with the increased VE as well, as you will make good power with little boost and be able to run leaner fuel mixtures compared to the "chocked" std or high flowed std turbo as it will need to run much more boost and with it a richer mixture to get the same power.

My "street ported" TII engine is a classic case of this. At 14.5psi It makes 500bhp @ 8000rpm due to it's much better VE, yet when driven "normally" will do over 500km on 50lt of fuel.

So in a nut shell, bigger turbo's + associated mods (intake, IC, header etc) = incresed VE and incresed peak rpm which = much more power for same boost levels.

Cause remeber when you go to look at what compressor to match to an engine, you need to know the VE, and to know this you need to know the effect of a lot of factors, It's a bit like which came first the chicken of the egg?... You will only know the VE when you have a firm grasp of what effect larger IC's, ports, exhaust manifold design, turbine specs, IC efficiency, etc etc have. Then you can match you're compressor section (Turbo eg 60-1) for the pressure ratio's you plan to run.

It gets quite complex

 

That's exactly the way I understand it. Obviously a properly sized compressor is going to be more thermally efficient at 10psi than a stock turbo, but the main gains are from un-plugging the cork in the exhaust so to speak . The stock turbo is more like a supercharger, in that it's restrictive exhaust housing allows for increased exhaust velocity/spool-up, but bottlenecks the engine and acts like a parasite on the top end.

 

Gee RICE RACING, so much for being simple...



-Ted

 

The flow is important and the cfm at the psi dictates that, if 2 different turbo's one small, and one large are both building 10 psi of boost at 300 rpm, being that the engine is a positive displacement device, both turbo's are moving pretty much the same volume of air, what comes into play though, is how dense that volume is, how tightly packed the o2 content is, the cfm may be the same, but if the small turbo is heating the air excessively, there is less overal o2 chargine in a given volume, there is more 02 in a cubic foot of air at 10 psi at 100 degrees than there is at 200.. Density recovery, thats why intercooling works...
Another part of the gain of larger turbo is being able to hold that 10 psi throughout the rev range, a smaller turbo overspeeds, it cannot deliver the volume being eaten up by the engine to hold the boost, and thus you drop off boost.
The exhaust side of things is the last part of it, and why I am not a fan of Hybrid turbo's, the larger exhaust housing decreases back pressure on the motor resulting as RR said, greater VE, with a hybrid you can pack more air in, but you are increasing the backpressure on the motor by increasing the charge going in, while the boost gains are there, the penalty of backpressure removes not all, but a fair chunk of the power gains. Max

 

Looks simple enough to me! The stock turbo severely chokes the exhaust. You can't get more air in without first getting the old stuff out. By having a much freer-flowing exhaust, you can get more air in, but you also need a compressor size that can move the air efficiently, especially in your target boost range, otherwise it just beats the air.

If manifold pressure was all that mattered, all you need to do is heat up the air, because air expands as you heat it. But that's not what matters, what matters is maximizing the MASS of air that gets in the combustion chamber - you do that with manifold pressure, minimal manifold temperature (how good is your I/C / how efficient is your compressor?), and getting as much of the exhaust out of the chamber as possible. (Porting isn't entered into it yet!)

 

My understanding after having read all these posts is that efficiently getting a large quantity of air in is just as important as efficiently getting it out, and that all this is equally as important as cooling the intake charge as efficiently as possible as well. Does that sound right?

 

Rx7_Turbo2(Graham), Its also very important that you paint the volute of the compressor blue, if its a different colour than your shift boot, you will lose power....Max

 

Hahaha I will have to add that to my "to do" list before I install the new turbo

 

OK, larger A/R and exhaust flow as Rice pointed out helped me get it. You do raise the VE this way. I know how uncorking the rotaries exh. can help as I had NA before my TII. Stock turbo is restrictive on a free flow exh TII; look how much exhaust energy you have to divert through the wastegate to keep boost down. Thanks guys!

 

One point that might make things easier is that the choke point is the TURBO, NOT the engine.&nbsp If you think about the turbo being the most restrictive on the entire system, then it makes it easier to understand.



-Ted

 

I think I have a headache now!

 

ok so what about having a small turbine and housing with a huge compressor and cover....

obviously it'll build up backpressure hurting VE...

but what if i'm using a MASSIVEwastegate (maybe even one per runner) with well designed piping? wouldn't it be possible for this to flow the necessary gas volume and hence have quick spool ups and minimal back pressure at top end?

 

Exhaust flow will eventually be choked but the small turbine.&nbsp This it doesn't matter what your compressor section can handle.

Sure, up until the point the turbine section physically chokes due to too much exhaust flow...


-Ted

 

ok if i understand correctly the W/G must keep a certain amount of backpressure in the turbine in order to keep the turbo spinning. But anymore backpressure than this minimum requirement will hurt performance...

What if my external W/G setup is large enough and designed well enough that it will keep only the minimum back pressure required to keep the turbo spinning and no more?

wouldn't this mean i could have quick spool ups and no loss of top end?

 

No, because you're totally getting too much into the exhaust side of things.&nbsp The turbo produces boost due to engine load and differential between turbine exhaust flow and compressor intake flow.&nbsp If you flow just enough gases to spin the turbo, you get very little differential and thus very little boost.

Here's what your system will do...
You stomp on it.
Exhaust flow increase.
Turbo starts to spin, creating boost.
Wastegate opens, releasing most of the exhaust energy.
Turbo slows down.
Boost goes down.
Your overly efficient wastegate ends up wasting a lot of the exhaust energy that makes a turbocharger such an efficient power component.


-Ted

 

Only if it's broken! The wasegate should only open enough to bypass enough exhaust gases to maintain a set boost level. It doesn't (or shouldn't) just pop right open.
Assuming the wastegate is big enough, as exhaust flow from the engine increases the wastegate sets up an equalibrium between exhaust gases passing through the turbine and bypassed exhaust gases to keep boost constant. That's how I understand it anyway.

 

NZConvertable....I agree 100%!!!!!!!!!

So back to my original question...

If the small housing (say .80AR) and 2 big external gates can flow the same CFM as a bigger housing (say 1.15AR)and one external gate then why would there be a loss in top end power??

Or is it that the difference in CFM between a 0.80AR housing and a 1.15 housing bigger than what 1 big external gates will flow...if ya get what i mean??

cheers

 

you have to look at it this way, a if a turbo is making like 60 hp, it is taking about 30 hp away from the engine. therefore you're only netting 30 hp more.

right?

 

How is a turbo taking horsepower away from the engine?