Is 10 psi 10 psi???? (Sorta Twins vs. Single)
 

Okay, there is something I still don't understand with regard to turbos. Would a single turbo (say T-66) running 10 psi be any better than twin turbos running 10 psi? Obviously the T-66 is much larger and could move a larger volume of air. Does this have an effect?

Thanks for your input guys! I'm still learning!

yes, think of it this way... You stick a pressure gauge into your cheek (stop laughing :) ) then blow through a coffee straw till you get 1psi. Think about how much air is going through that straw. Then do the same thing but blow 1psi with a McDonalds straw. Think how much more air is moving through that straw at the same 1psi. A TON more! Air flow = power.

And the reason the straw is bigger is that there is less restriction on the exhaust side with a big turbo. The 10 psi at the intake port is still 10 psi with either turbo, but more air will flow in when there is less exhaust restriction (due to port overlap and the pressure of the exhaust that is left in the chamber even with no overlap). The bigger turbo provides less of a restriction on the exhaust side.

The other advantage of the "right" sized turbo is that the intake air will be cooler. 10 psi from overworked small turbos will be hotter than 10 psi from a larger turbo that is operating in a more efficient range (not overworked). This is also related to exhaust restriction in a sense. If the small turbos have to make 13 psi of really hot air to get 10 psi of cool air in the intake manifold after intercooling, there will be even more exhaust restriction than a turbo with a larger compressor that only needs to make 11.5 psi before the intercooler to get 10 psi of cool air in the manifold.

10 psi is not 10 psi in that sense. Or rather 10 psi is 10 psi, but the boost pressure isn't the only thing that matters.

-Max

it all gets down to temperature and cool is king. you can have 2 equal volumes of air at 10 psi but the cooler volume will contain more oxygen molecules. more O2 will allow more fuel to be injected and more power. the stock rx7 turbo is spinning at close to 150,000 rpm around 15 psi and it is very close to cavitating and is heating the air in a major way. a properly sized single turbo spins around 90,000 and makes efficient contact w the air molecules.
howard coleman

Very Interesting! I'm getting my freshly tuned, streetported FD with a T-66 back from the shop tomorrow. I was just curious as to how the process worked. I got an "A" in Physics and understand the whole pressure and heat concepts on paper but never had them applied as previously stated.

Thanks for the info...

Alan

Boost pressure is merely the strain being put on your manifolds etc

its all about CFM,

CFM = HP, not boost pressure

hehe, I wish I never started using the "straw" example :D

It's all about how big your straw is my son :o:

I *HATE* the straw example. It's FALSE.

With that example, compared to a bigger straw, you're increasing the VOLUME as well. People get stuck on this... take a basic physics course at your local college, please.

Howard hit it on the head.

CFM is related to efficiency, you want cool air being blown in, and the faster a turbo spins (relatively) the hotter the air gets, and is less dense, with the goal being a denser charge.

As for choking the exhaust, that is also a problem with small turbos, and why there is less power. Max Cooper is right about that.

But the intake isn't a straw that gets bigger or smaller by a straw change.

Actually the straw example is good. A larger turbo is changing the size. A T-66 is much bigger than the total area of the stock twins. Efficiency and CFM both play a part but are two seperate things. If both turbos produce 10 psi, at what cfm are they doing it at? The T-66 will have a much larger number. The T-66 will also heat up the air less. The amount of exhaust that can get out plays just as important a factor as the intake side. The less backpressure you have the more efficient each psi of boost you have. The stock twins are so bad in this respect.

no, it's not changing the size. porting would be changing the size. If you glued the small straw to the end of the Mc Donalds straw, THAT would be the right thing.

Yes, the T-66 is more efficient, hence a denser charge.

If you want to use the straw example, then port your motor, port your intake, and use 3 inch intercooler pipes. That's a bigger straw...

I don't like "the straw". It's a better example for why a ported engine might be more efficient. The question asked references 10lbs of boost, and that will never happen on a system with an open end, like a straw.

I mean who really cares how much air a turbo can push into the atmosphere. We're concerned with how much air we can squeeze through the port and into the chamber. And get it out again. The ports are the ultimate restriction, and for the most part not the path it takes to get there. No?

I think that's the root of the question that always seems to come up. Given that relieving exhaust backpressure with a better turbo/manifold design helps increase engine efficiency (particularly when compared to the twins) and the fact that a properly sized single generates less heat and therefore fits more air into the same space, what helps one turbo push more air through a given intake port than another when all other assumptions are equal?

That's what I get for replying from a busy desk at work.. you beat me to it.

THANK YOU!!! On the compressor side, the turbo may be a bigger straw BUT when turbo A is a 5 in. diameter straw, turbo B is a 6 in. diameter straw, and the engine is a 1 in. diameter straw, if the charge isn't any cooler, IT DOESN'T MATTER!

Even when you port, the combustion chamber is a fixed size, you are simply minimizing pressure losses through the engine.

For those of you who think 10 psi is 10 psi no matter what size the turbo , can you tell me why fuel maps have to be changed when a larger turbo is bolted onto a stock block , replacing the twins ? , this very same thing was experienced by a guy down here , everything else was left the same before the single was added and you could actuslly hear the engine knocking when the single hit 10psi , the exhaust side a/r was 0.91.
And another thing , if you look at a compressor map it always specifies a certain flow per minute , this is always greater the larger the compressor , so what exactly is this telling us , that the 92lbs per min put out by the T76 is the same as the 35 llbs per min put out by the T04XX ? .

For the people who hate the straw example.

Next time you have your engine apart (if you ever have ?) take time and lok at the effective area's we are talking about, take time and look at the length of the restrcition offered by 4 fairly large intake runners and 2 peripheral exhaust ports.

Then go look at you pissy little turbo, factory intake manifold, undersized delivery piping, restrictive air filter and then come and talk to me before you start sprouting wisdom about how you hate the straw example.

Any engine even if its a stock 10A ! Will benifit from a more free folwing (bigger straw!) turbo, you just need to make sure you upgrade other parts along the way that will cause restriction in the system (too small a straw !) Even a shitty std 12A Turbo engine from Japan with minute primary ports benifits greatly when adopting these priciples WITH NO EXTRA PORTING !

Sorry to shout or come across as rude but how many of you huys have done your own experiments on this ? the engine in a turbo car is one of the LAST things in the package that needs attention on this side of things, it is not a NA engine and it is not mandatory for massive porting work to make flow increases. A majority of flow restricitions come from bolt ons that go onto even a stock motor and this includes the turbo my friends !

SMALL STRAW, excellent example indeed and 10psi is not 10psi, the McDonalds straw v's a normal one is a classic representation of pressure (alone) not being a true representaion of FLOW rate due to far less restriction.

For my practical example of factory ported engines benifiting from larger "straws" go find an old racing beat book, showing power (flow increases) increases given when free flowing exhausts and intake systems are employed (all at constant atmospheric pressures !) big increases from 110bhp to over 146bhp. these same increases in efficiency happen on turbo cars without increasing boost pressures ! You need larger turbo, larger more efficient headers, intake manifold, IC delivery piping, air filter.........

Its all pretty basic stuff boys ;)

Yes, you don't understand the basics. It's physics. Marcel, you are missing the point totally. None of us said that 10psi with a large turbo is the same as a small one.

Rice, no matter what size turbo or intake, your intake ports aren't going to *know* that you have a bigger sized intake. The ports are the smallest part of the intake, and that is what the air is pushing against. If there was no resistance from the engine, the air would flow right through (for example, if there were no rotors).

Also, no one said a car wouldn't BENEFIT, we are saying you just don't understand the concept and use it in your bad example.

N/A or not, air still flows into an engine the same way no matter what pressure, but with no turbo, the air is just less dense. when a turbo is pressurizing, it isn't RESTRICTING the intake, it is FEEDING it. There's a difference.

I think the McDonalds one is only something in your book, because in books I've read like in Corky Bell's, he doesn't mention Mc Donalds straws.

You're now comparing free flowing exhausts and intakes versus one single component we were talking about, a turbo. Doesn't EVERYTHING benefit from that?

This is about pressure and efficiency with the different turbos, don't change the subject ;)

I understand alot more than you do grass hopper :D

When you get up to speed come talk to me ;)

My tip is dont read Corky Bell's book thats is just the start of where you are going wrong ........ LOL

Do a flow bench test on all the ports !

Then hook all your other system components to a flow bench and see what flows you achieve

:)

Your theory is demolished by looking at old RB data on stock engines fitted with better flowing parts (remember stock engines, std ports !) higher flows mate, all at the same pressure, while retaining the same std stock MOST RESTRICTIVE ports.

What you say makes no sence in theory or in practice.

The facts are MANY systems can be improved before you need consider touching the porting on the engine and ALL will result in much greater FLOW at the same pressure, the TURBO is NO DIFFERENT, size does matter and it does make a difference.

My tip is stop reading certain publications and go do some tests of your own befroe you bless me with your wisdom :D

Re: Is 10 psi 10 psi???? (Sorta Twins vs. Single)
 

YES YES YES !

Less restrction, less back pressure to intake pressure ratio all = better Ve% and more flow for same pressure ratio.

We're not talking about hooking up different components, we are talking about turbos.

You beat around the bush, avoiding the whole subject of what was previously discussed. 10 psi on twins versus 10 psi on a T-66.

And now you're agreeing bigger turbos make a difference. Good. We all agree.

I think you're just stubborn. Don't worry, people use to think the earth was flat :D

No NO, I was trying to use examples for people to understand.

Fitment of a T66 involves using a much better manifold, less charge plumbing (wich is usually more efficient too) this all combined with a better/bigger flowing exhaust turbine and compressor section ALL add up to increasing the flow rate through the "std" engine at the given pressure ratio you choose to examine.

Though again I resort back tot he straw example, if you hav e a restriction in the system (from air filter to rear muffler and everything in between) and its significant it can eliminate any potential gains assosiated with a big turbo.

Which I am sure we both agree on ?

10" pipe breathing through a McDonalds straw or exhailing through one will limit its capacity ;) Facts are the ports on the engine even if stock or ridiculiously small (as in 12at example) can flow stupid amounts of power as the flow restrictions they present are small when in comparison to the rest of the restricitions in the system.

When they operate at pressure ratios that are tripple to what are experienced in NA form these losses or restricitions are not such a handicap as many people think when they are isolated in one spot (such as intake port), BUT and this is the big but (Oprah sized !) when conditions go back do to "free air" such as at the intake or post turbine area, flow restricitions and pressure drops are CRITICAL and can influence any potential flow gains.

I can see the problem some might be facing though and that is thinking of the engine as a straw being fed by a 10" pipe ?

The BIG issue here is the engine is operating in a much higher pressure environment, and can flow amounts that are almost alien to people who are used to NA flows and sizes needed to make power.

Though pressurised systems do reat favourably TOO to increased porting and less resistance via bigger runners more timing bigger charge delivery pipes etc etc, this is very critical when using low pressure ratio such as up to 15-18psi etc.

Still pressure drops and resistance do happen, its just understanding the effects I suppose, I understand why its confusing for lots of people though. Hard topic to get your head around.

Can you explain why the ports flow much better than one would think?

And I do realize I don't know shit, I'm glad I can instigate this release of knowledge ;)

They are working in a pressurised environment.

And no dont ever say you dont know shit, that was not my intention despite my badly chosen words sometimes !

Under a pressurised environment quantities that can be flowed (power made) are far greater than under the normal atmosheric environment or low pressure environment (low boost), but the same gains can be seen if you adopt these low pressure principles of greater flow rates (IE heavy porting, less restrictive intakes etc etc).

Pressure changes everything and I think this is again where some peoples ideal falls through in that they think only a certain amount can "flow" through that and thats it. This to an extent is true, but there are many other parts to the "system" that will have greater mcuh greater effect on the flow capacity especially if they are near the low pressure ends of the system, IE very restrictive muffler/exhaust or poor intake or very restricitve compressor set up for example.

There are lots of things to consider, but to answer the poor guys origninal question the short answer is yes it will give more power due to the reasons of being more efficient as explained.

Std ports can make big power, provided you have a BIG turbo feeding them. That BIG turbo is feeding them BIG CFM.

a good example is 87GTR's engine
stock standard 13BT block, with a T04E, E6k rady rady rady make 440 engine HP at the same boost pressure he made 260-270HP with the standard turbo.

Porting basically gains a % of the HP that is already there, you could put HUGE ports on a std turbo and gain 20% more power, 20% of the 250HP you are making already, giving you 300HP, or with a BIG turbo making 500HP, porting will gain you 100HP

yes porting makes extra power, but nothing unleashes the full potential of a rotary like ditching the factory restrictor (opps i mean turbo) and replacing it with one that will actually flow what the engine requires

When you port an engine you aren't getting near as much of a gain from the port being physically larger than you are by the fact that the port is open longer. There is more time for more air to enter. Yes the port is bigger and flows better but it is more that it is open for longer. This is true to a point within reason. To go back to the turbo thing, lets look at the T-II turbos. Measure the outlet size on the turbo and then compare it to the size of the engine port runners. It is half the area! It matches up with 2 port runners in size. Even the 3rd gen turbos outlet don't equal the total area for the engine port runners. The straw example applies to the straw (turbo) feeding a bigger straw (intake manifold and engine). 10 psi is only 10 psi in pressure value alone not in cfm. A smaller turbo may have to hit 12-15 psi just to equal the amount of air in cfm that the larger turbo hits at 10 psi and that is not even considering heat. It is the amount of air not the pressure it is at that makes power. CFM is everything.

I'm sorry I started talking about straws! ;)

The straw example is not the most accurate, but it gives a person a picture to see how the flow of air is important and that restriction of any sort is what reduces the efficiency (heat, port, turbine, runner, manifold design).

Actually, the straw example isn't too far off considering the engine could be your body and the straw could be the turbo. You have to inhale air to help your muscles convert ATP and work. Work creates heat. Heat has too be cooled be means of fluids and air (breathing included) otherwise efficiency and reliability of the body can be compromised. The body exhausts energy (heat, for example) partly through breathing, exhaling.

You can knickpick all you want, but the big picture is illustrated. I'm also glad that you guys corrected the details that could've been taken away as inaccurate.

10psi is 10psi. It's pressure per square inch . The volume or mass may be different with some influence from temperature, but the pressure remains the same.

So talking about relative atmospheric pressure isn't the best way of talking about power resulting from a given turbocharger unless you're trying to discern low boost from high boost.

J

I think you're still missing the point that the ports aren't open all the time.

A turbo isn't a restriction when it is the start of the system. if seperates ambient air from pressurized air. the throttle plates at half throttle=restriction.

:eek: I think I opened up a can of worms! This is interesting reading though. The reason I asked was that I bought my FD (first FD, last car was 90 Vert) with a T-66 and basically every other modification already done. I'm supposed to pick up my car tomorrow (crosses fingers) with it tuned and boostin' about 10 psi. In one of my other posts, someone told me that I would be wasting my time if I only ran 10 psi on an upgraded turbo. I was just thinking about that and that's what spurred on my question.

Does anybody care to guess what kind of numbers I'll have at 10 psi?:confused: I know what my mechanic told me, but I want to hear from you guys.

rwhp?
1/4 et?

Most all the significant mods are in the sig. I'm sure that I've missed some. Basically, everything is done except a lightened flywheel and an electronic boost controller (manual currently).

Thanks for the input guys!

Alan

thats where you are looking at it wrong though, the turbo isn't the start of the system, the important part of the turbo is the last piece of the system, the exhuast side of the turbo, restricts how much CFM the motor can exhale, by exhaling more, it can inturn inhale more (higher VE)

then the compressor side starts to play a part which is the start of the system.

When the end of the system isn't limiting flow, then you can worry about opening up the start of the system

I'd honestly like to see how much Hp could be made from a Std S2 compressor with a T04 exhuast housing/P trim wheel on the back

If the throttle plates are half closed then you probably have little or no boost anyways so where's the problem?

As a general rule (but there is always an exception!) for every 1 psi of backpressure you can free up on the turbo exhaust side, it is equivalent to 2 psi more on the intake side! HWO hit a good point. Too many people try to upgrade to a really large compressor when half of their problem is a really small exhaust side. The T-II compressor is still too small for really high numbers though but there would be an improvement.

~300 rw SAE on a dynojet, same as stock twins. At 10 psi, these "huge restrictions" don't show up. I still stand by the only gains you'll see if you otherwise leave the motor untouched will be from better exhaust flow and lower charge temps. Dyno and draw your own conclusions.

Personally on a 13B I don't think a T66 compressor will be more efficient than the stock twins at 10psi. The compressor map doesn't look too good until you start running some real boost. Therefore, I think any gains you'll see will be the result of the better exhaust flow.
If you only want to run 10psi I bet you could gain power by going to a smaller compressor which is effiecient under the operating conditions. It's not just a case of bigger compressor = cooler intake temps.

Hence i am not a fan of hybrid turbo's, the exhuast side of the factory turbo is the MAIN thing limiting the Hp output, by putting a larger compressor into it, all you are doing is forcing more air in, this is great for the people who think PSI = HP, but for the people who know that CFM = HP its not a nice equation

I believe a hybrid at 15psi making 330HP, is probably putting more strain on engine components than a Full T04 at 10 psi making 400HP

Big turbos have big compressors that are designed to flow big CFM. This has come up in this thread several times. But the compressor size increase's effect is not one of increased CFM, it just compresses the air. Its up to everything down stream to change the CFM. Here's my example of this:

Suppose for a minute that you could take the T66 compressor, and power it with the stock TII turbine -obviously not possible, but humor me. The T66 spools up and makes for this example 15psi. Because this compressor is more efficent at this pressure than the stock turbo it makes a cooler charge. Thats it though, every thing is still the same because the "straw" that the compressor sees is still the same size. But a cooler charge is only going to give you 10-20% maybe. So at 15psi you make 320-350 vs 280-290, hmm sounds like what the typical compressor upgrades would give, go figure. Like RICE has pointed out, the biggest restriction in this "straw" is not the porting, but the turbine. On the stock turbo I'd bet the inlet pressure is at least twice the boost pressure at any given time. Thats a worse restriction than the worst possible NA OEM exhaust. The T66 makes bigger power because it's turbine flows much more efficently, lowering the turbine inlet pressure considerably.

Thus big turbos flow big cfm because they pose less restriction to the system

And I forgot to read the second page.... you already covered it!!

After talking to my friend, he explained the same thing. It does make sense too, I'm running a S5 hybrid with a T04E wheel and housing, and I know I'm not near 300 HP.

Yes, the exhaust is a big restriction and will allow the engine to breathe much better. The gray area is whether or not the compressor section can flow more in addition to what you get from the cooler charge temp. I say no, if the compressor is compressing and can maintain your desired boost, then it isn't a restriction -it may be inefficient and blowing alot of heat, thus lowering flow, but the flow isn't restricted at the compressor other than by the temp.

mass flow = density * cross section area * velocity

Keeping the engine the same, at the intake port at a certain rpm, the CSA and velocity will be constant, the only thing that can move is the density. Density can change with pressure or temperature. We've already said pressure is constant, so the only thing making a difference on the compressor side is the temp. What did I miss?

This is why I bet money that 10 psi on the T-66 makes no more power than 10 psi with healthy stock twins. Higher boost and the T-66 will make more power because of less exhaust restriction and cooler charge air.

If you have the same engine and same IC pipe system you have the same amount of space to fill up with air. The difference in how much o2 is in that air. The larger compressor wheel grabs more air and compresses it into the same space, but since it grabed more air it got more o2.

Say you have a box that holds 100 cubic feet of air, you can have a small turbo grab 150cubic feet of air and compress it down to 100 or you have have a big turbo grab 200 cubic feet of air and compress it down to 100.

The size and design of the wheel determine how much and how efficiently it grabs the air. Then that air is compressed into the IC/engine system.

STEPHEN

??? :confused: In that example, if you keep temp the same, the pressure HAS to rise and the wastegate won't allow that. If temp is constant, and you compress 150 ft^3 of air to 100 ft^3. you will need to compress it 50%. Likewise, 200 ft^3 to 100 ft^3 will require a 100% increase in pressure.

Who said the temps would be the same??? Large turbo's temps are generally lower. You system doesnt create o2 the only way to get more o2 in the system is to take in more air

Also, the pressure inside the compressor housing of a turbo where all the air is actually being compressed is not the same as the pressure in your IC system.

STEPHEN

And I'm saying that the only way for it to get more air in for a given pressure is to reduce temp. What other way is there?

And you're right the temps of course won't be the same, the amount they rise will be proportional to how much the air is compressed and how efficient the compressor is -I just used that as an example to show that you can't just "fit" more air somewhere without something budging (pressure or temp). We've established that lower temp will get more 02 molecules into your motor, the question is:
If you're already reaching and sustaining desired boost, what can a more efficient compressor do besides lower temp?

Will it give less losses through the compression? Sure, but that shows up in the efficiency which shows up in the outlet temp.

Now, if you've got a free flowing turbine section and a compressor so small that it cannot reach your desired boost level, even with the wastegate fully closed, THEN it would seem the compressor is a restriction.

The compressor wheel grabs the air and compresses against the housing creating a huge amount of pressure then as the compressed air travels thru the "snail" to cools off and expands. The amount that it cools and exands is dependant on the size and length of the "snail". Then at that point you'd basically come out with a certain volume of air that has more O2 in it then that air would pass into your IC system.

The difference in the stock twins and a T78 at 15psi cant be explained soley off of charge temps. The temps arent THAT much lower to make a 100hp difference. If you take a T78 in the summer it would have hotter charge temps than stock twins in the winter.....however the T78 would still make more power.

STEPHEN

If the system was open, yes. ...but it isn't. Other than lowering temp or raising pressure, how else can you pack more 02 between your turbo outlet and your combustion chamber?

I'd love to see a T-78 put 100 hp over a good set of stock twins at 15 psi.

But true, it would make more power, because of the exhaust section.

dumb question: simplistically is the spooling of the turbo the difference in pressure between both sides of the turbine wheel?

mike

the difference in pressure between both sides is what equates to HP in a way.

the less pressure on the exhuast side the more HP it'll make

stock T2 turbo at 15psi generally has 40-60psi exhuast manifold pressure ............................ VERY BAD

For a given set of ports at a given boost pressure, reducing backpressure will increase the amount of air that flows through the engine. Temp is very important, too, of course, but it isn't the only factor in the amount of flow (quantity of air molecules) that you get.

-Max

True, but is that not a contribution from the exhaust side?