Ive been doing a little research and thinking about the common topic of "a bigger turbo at the same psi will make more peak hp". This still makes no sense to me. First of all, I think it would help the most to see a car on a dyno, first with a t3, then with a t3/t4, doing runs at 7psi and note the difference in power. IMO, with identical IAT and psi, the horsepower should not vary at all.

Why? The basic equation from which MAP-based systems function from is PV = nRT. where n is the number of moles and R is some constant (look it up), P = pressure, V = volume, T = temperature. So, an increase in pressure will result in a decrease in volume and a proportional increase in temperature.

Cfm, on the other hand, rates the airflow for a given period of time. Heres how I see it: the volume available between the compressor outlet and the intake valves is always constant. So, psi represents the pressure of that air at a constant volume. Increasing the cfm flowed by the turbo MUST increase the psi. The only way it wouldnt is if the engine is consuming air at a rate faster than the turbo can produce it.

Of course, these are my thoughts and may be incorrect, I know the math is correct, but perhaps the applications have a fault. Feel free to correct me or provide input.

 

WOW, you got it right! Most people don't get it. The only reason power would increase is with an increase in compressor efficiency (lower air temps) or a bigger exhaust wheel/housing which would decrease exhaust backpressure and increase the ve of the motor.

 

Alot of ppl think bigger turbos flow more CFM at the same PSI, but like cobra2326 so accurately put it, more CFM forced into a given engine will always increase PSI.

 

The formula we all use for calculating cfm to plot compressor maps also reflects what you are stating.

(cid x rpm x .5 x VE%)/1728 = CFM without boost
CFM (no boost) x PR = CFM under boost

You can see that to calculate CFM under boost we do not need the size, trim, A/R, etc of the turbo.

Like Tony1 said, the only reason we see more power at the same psi on a bigger turbo is the efficiency of the turbo (intake temps), so obviously the intercooler plays a big role here as well.

 

I think the bigger cause is the larger exhaust/wheel housing with less backpressure/restriction.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Muckman &raquo;</TD></TR><TR><TD CLASS="quote">I think the bigger cause is the larger exhaust/wheel housing with less backpressure/restriction.</TD></TR></TABLE>

I think the compressor side would play a larger role. Otherwise people wouldn't bother with t3/t4 style (ie: upgrading cold side), they'd just upgrade the hot side.

That balance of course is tricky between power/lag.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Muckman &raquo;</TD></TR><TR><TD CLASS="quote">I think the bigger cause is the larger exhaust/wheel housing with less backpressure/restriction.</TD></TR></TABLE>

You, my friend, "get it"

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Muckman &raquo;</TD></TR><TR><TD CLASS="quote">I think the bigger cause is the larger exhaust/wheel housing with less backpressure/restriction.</TD></TR></TABLE>

Absolutely. This is why I always say that everyone should have at least a 3" exhaust, regardless of the size of the turbo. Everyone that says "2.5 is big enough for your setup" doesn't have a clue.

 

Argh, I feel like I'm getting part of it....

For arguments sake... standard t3. People will see gains at the same psi just by upgrading the compressor (think t4 etc..). Isn't that different than just thinking that the "larger exhaust/wheel housing with less backpressure/restriction." will be the cause of the change?

Is it me... or are we all sort of talking about the same thing... I'm just talking about the other part of the puzzle?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by splitime &raquo;</TD></TR><TR><TD CLASS="quote">Argh, I feel like I'm getting part of it....

For arguments sake... standard t3. People will see gains at the same psi just by upgrading the compressor (think t4 etc..). Isn't that different than just thinking that the "larger exhaust/wheel housing with less backpressure/restriction." will be the cause of the change?

Is it me... or are we all sort of talking about the same thing... I'm just talking about the other part of the puzzle?</TD></TR></TABLE>

Lets hold everything constant except the compressor side of the turbo (same exhaust, etc.). The only reason you are going to see more power from the larger compressor at the same psi, is the efficiency of the turbo. The larger turbo is more likely to be in a higher efficiency range than the smaller one assuming it's not too big for the setup. This means the larger turbo will have lower intake temps. Lower intake temps = more torque/horsepower.

 

Well, yes, there is going to be a small change in compressor efficiency, but very rarely to people go from a t3 to a t3/t4 w/o going bigger on the exhaust wheel or housing T3 only signifies the inlet flange size. Usually what happens is you end up needing a bigger compressor with a high reving small displacement engine, hence t3/t4's being used. If you need more airflow than a small t3 compressor can provide, then yes, changing only the compressor will change power quite a bit. All depends on how much air you're moving. Generally though, assuming you're not "off the map" with either compressor, most of the gains will come from turbine sizing.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RyanCivic2000 &raquo;</TD></TR><TR><TD CLASS="quote">Lets hold everything constant except the compressor side of the turbo (same exhaust, etc.). The only reason you are going to see more power from the larger compressor at the same psi, is the efficiency of the turbo. The larger turbo is more likely to be in a higher efficiency range than the smaller one assuming it's not too big for the setup. This means the larger turbo will have lower intake temps. Lower intake temps = more torque/horsepower.</TD></TR></TABLE>

K, when rulling out the cold side the above stuff is working in my head. Makes sense from a physics standpoint that efficiency (wheel design, back pressure, manifold flow etcc) will factor in across the board.

Woo, I'm back on board

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">Well, yes, there is going to be a small change in compressor efficiency, but very rarely to people go from a t3 to a t3/t4 w/o going bigger on the exhaust wheel or housing T3 only signifies the inlet flange size. Usually what happens is you end up needing a bigger compressor with a high reving small displacement engine, hence t3/t4's being used. If you need more airflow than a small t3 compressor can provide, then yes, changing only the compressor will change power quite a bit. All depends on how much air you're moving. Generally though, assuming you're not "off the map" with either compressor, most of the gains will come from turbine sizing. </TD></TR></TABLE>

Nod, I know personally I'm starting out with a decent sized t3 housing. .63 and the wheel is a stocker from the SVO/XR4TI... so I'm looking into upgrading the cold side down the road. I believe that the hotside will flow well up top, but (after some discussion with ryancivic2000) the cold side will become inefficient fast, even with the 60trim setup.

(this is all for a 1.6L z6)

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by splitime &raquo;</TD></TR><TR><TD CLASS="quote">

Nod, I know personally I'm starting out with a decent sized t3 housing. .63 and the wheel is a stocker from the SVO/XR4TI... so I'm looking into upgrading the cold side down the road. I believe that the hotside will flow well up top, but (after some discussion with ryancivic2000) the cold side will become inefficient fast, even with the 60trim setup.

(this is all for a 1.6L z6)</TD></TR></TABLE>

It was mainly because you wanted to rev so high.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RyanCivic2000 &raquo;</TD></TR><TR><TD CLASS="quote">It was mainly because you wanted to rev so high.</TD></TR></TABLE>

Honestly, the hypothetical revlimits are there because the motor "can" go that high... dyno and tuning will tell me my final limits. I only wish to rev as long as I'm making power.

But yeah... reving high is nice... extending the powerband as long as practically possible.

 

Nice discussion guys, always good to read a technical discussion on honda-TECH. Especially nice because there is no flaming, just people helping each other out to understand.

fyi - R = 8.31 J/[mole * deg K] ^_-

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by KeyserSoze &raquo;</TD></TR><TR><TD CLASS="quote">fyi - R = 8.31 J/[mole * deg K] ^_-</TD></TR></TABLE>

Don't come in here with all your technical mumbo jumbo that we don't understand.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">Don't come in here with all your technical mumbo jumbo that we don't understand. </TD></TR></TABLE>

HAhahaah, sorry I was brainwashed by school, I can't get the crap outta my head!!!!

 

good info in here...one thing i was wondering...did anybody ever make a t25/t3 hybrid or only t3/t4?

 

Sorry to break up the discussion guys, but I'm making this so I can keep up on it when I get home.

So the only reason a SC61 would make more power on my car at 7psi as opposed to my TD04 14G (T28ish) at 7psi is because the SC61 is more effecient than my TD04 at 7psi?

So it starts to come down to temperature. Is it temperature as the air is leaving the compressor or as it's entering the intake manifold?

IE 1: 1 turbo has the charged air coming out at 250*F while Turbo 2 has it coming out at 275*F. Both at 10psi. In the 1st scenario Turbo 1 is more effecient.

IE 2: The charged air from both turbos is routed through intercoolers where the charge temperatures are dropped to 150*F right before entering the intake manifold. Both turbos producing 10psi at the MAP sensor. Since both are at 10psi and 150*F are they going to make the same power?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Everyones Hero &raquo;</TD></TR><TR><TD CLASS="quote">Sorry to break up the discussion guys, but I'm making this so I can keep up on it when I get home.

So the only reason a SC61 would make more power on my car at 7psi as opposed to my TD04 14G (T28ish) at 7psi is because the SC61 is more effecient than my TD04 at 7psi?

So it starts to come down to temperature. Is it temperature as the air is leaving the compressor or as it's entering the intake manifold?

IE 1: 1 turbo has the charged air coming out at 250*F while Turbo 2 has it coming out at 275*F. Both at 10psi. In the 1st scenario Turbo 1 is more effecient.

IE 2: The charged air from both turbos is routed through intercoolers where the charge temperatures are dropped to 150*F right before entering the intake manifold. Both turbos producing 10psi at the MAP sensor. Since both are at 10psi and 150*F are they going to make the same power?</TD></TR></TABLE>

It's the temperture once it reaches the manifold that matters. Your second example doesn't really work because if the air went in hotter to begin with it won't come out as cool.

 

If the temperatures at the t/b are the same, then yes...unless the exhaust backpressure has changed, which it will have, and will be the main result of the power increase...

 

When you have a t3/t4, the compressor side will flow more PSI at a given turbine RPM than the t3 compressor would.

Anyways, a guy over at team-integra explained to me that basically the only reason a bigger turbo will produce more hp at the same PSI, is due to exhaust backpressure. Seems like the exhaust backpressure couldnt be enough to cause the big differences we see between straight t3's and t3/t4's, so anyone care to comment on some real physical facts or numbers from the backpressure issue.

Also, if you look at a compressor map like this gt28rs map, which I plotted for a b16:



You can see that the turbo is capable of producing 1 bar from 10lb/min all the way up to 35lb/min. So, PSI is really determined by flow, not the other way around. The lines plotted represent the flow of the b16 at a given rpm. Basically, the engine flows a certain amount of cfm, which in turn spins the turbo. Then of course, this increases the flow of the engine by forcing more air in until the wastegate comes in and regulates boost.

Thats my take on the whole issue

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RyanCivic2000 &raquo;</TD></TR><TR><TD CLASS="quote">It's the temperture once it reaches the manifold that matters. Your second example doesn't really work because if the air went in hotter to begin with it won't come out as cool.</TD></TR></TABLE>

Exactly, so if your intercooler is efficient enough to overcome the ineffeciency of the smaller turbo so that the charge temps are identical AND the PSI comes out the same, the only possible factor that would effect horsepower is exhaust backpressure (unless Im missing something big here).

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by z6 &raquo;</TD></TR><TR><TD CLASS="quote">good info in here...one thing i was wondering...did anybody ever make a t25/t3 hybrid or only t3/t4?</TD></TR></TABLE>


yes its called a t28