PTS Flowbench Forum

Hi,
I'm a beginer in this world and I am interested to know if in the Turbo engines heads, is possible calculate the true CFM with a formula, because all flowbench I think, works as aspirated engines, but , if we put 1 BAR in the inlet manifold with a turbo ? what's the true CFM under pressure ?

Thanks for your opinions.
Regards.

Glad to see another Turbo Guy out there, even a beginner.

I have been doing Boost stuff for a while. (I did my first turbo engine in 1976-77 when I worked for Holley Carburetor). I have worked the last 34 years doing boost work for The Ford Motor Company. THINK ECO-BOOST. That being said, maybe I can help you out on your question.

You have Volumetric Flow through a head port (CFM) and you have MASS FLOW thru a head port. When you go to a boosted engine with a head that flows 300 cfm naturally aspirated you get a slight bit more cfm due to the boosting device adding a bit of velocity to the air charge. But basically the CFM stays about the same. Being GENEROUS maybe 5% more cfm flow. The MASS FLOW goes up greatly with increased boost pressure. You need a given amount of cfm flow in the head to not have a high restriction to flow and therefore be in a bad spot on the turbo map. Hugh MacInnis wrote a book called Turbochargers. He used cfm flow in his boost calculations.
Corky Bell, another Turbo Guy, wrote a book where he used Mass Flow. You could do a comparison using the same Turbo and engine and see if you come up with any real cfm difference.

With 7 psi of boost on a 310 cfm head on a 462 Pontiac engine you could make 40% more power, according to most people. We actually did better than that on the dyno engine at Steve Morris Racing.

We made 863 horsepower with only 13 psi of boost (inter-cooled) when the engine made 459 horsepower naturally aspirated. The increase in power was due to the increased mass flow from the supercharger combined with the cooling effect of the inter-cooler.

Increased Density (Mass Flow) makes the power at 1 BAR (14.5 psi of boost).

Tom Vaught

Hi Tom,
thanks very much your answer and for your kindness.
I think I can understund your explanation, you talk in firts time about Volumetric flow and second ( more important for me ) Mass flow.
For me is the mass flow the most important for gain, and second the volumetric flow for have enough flow and not have a restriction.
But, can you know the mass flow with a flowbench ?
I think , if we work in a head and increase the volumetric flow, the restriction for the mass flow is better, and we gain, but I am interested to know if is possible, to know the mass flow changes with diferents boos pressures... maybe I am a mad man ho want's know too.

I usually work in the trubos engines thinking in not put a lot of boost pressure, because with more pressure more problems ( intake temperature, the famous knock, intercooler dimensions and his charge loosts if is too big, etc etc ).
In the littles engines I work with 1 bar and in the other versions 1,2 bar / 1,4bar and in the lasts versions 1,5/1,8 bar but with a very special compression ratio, liquid to liquid intercoolers, water injection in the inkake......

Regards.

Would it be fair to say that in a turbocharged unit we are dealing with an engine which runs in two different states?
Up to a certain point the cylinder is in vacuum and pulls the intake charge in, then after that point the air is pushed in by the turbo.

Given this scenario, do you have to design your port or even complete air intake (thinking mainly plenum here) to be able to to support both states? Is this possible or do you err one way or the other dependant on application, ultimate power or where in the rev range you want the main of your power to be?

Am I too advanced for my own good, wrong or have I taken leave of my senses?

For me , is a engine, run always in two states, because, between 1500 rpm and 3000-4000rpm, you have vacum in the manifold and between 3000-4000rpm since 7000 rpm for example, in pressure, as Tom Vaught says, the interesting point is the mass flow.
The volumetric massflow , can be a restriction for the massflow, if you gain volumetric massflow, the restriction limit point is better and you can pass more massflow; you have a restriction also but better...

Regards.

Another forced induction lover here.

When you talk about applying 1bar of pressure, that can mean very different things depending on type of induction (turbo size, roots, centrifugal etc). The pressure is actually because the motor is a restriction on the charger. If you take 2 turbos, on 56mm and the other 62mm then charge the same engine with both at 1bar the 62mm is going to flow more at the same pressure all else being equal. I get bitter about the young kids bragging about their guage pressure when when the compressor wheel is the size of a half dollar and don;t realize more pressure doesn't always mean more power. Turbo size selection is at utmost importance.

It's my feeling that flowing a head naturally aspirated or forced induction will be applied, the airflow is still going in the same direction. Weather or not it's being pulled in by vacuum, or forced in with a blower, all "rules" should apply. We generally allow a bigger CSA and more port volume for forced induction heads since velocity becomes less important than a naturally aspirated motor - but you have to keep in mind that it's an N/A motor until above atmosphere is reached so just like N/A, bigger (more raw flow) isn't always better.

SSR, yes, it is two different scenarios once boost comes in, but either way the charge is still flowing in the same direction. It's just that in a positive pressure situation, that more air is "waiting" at the back of the valve to enter when above atmosphere rather than it having to be drawn in by the piston.

I what would happen if we applied our vacuum source in blow configuration into the plenum, rather than drawing it through the port if it would closer mimic a positive pressure situation? One would probably have to set the manifolds, and throttle to the equation to be accurate, but then also would have to mimic the positive pressure and any cam overlap to the mix since the exhaust pressure is so great in a turbo system.

Until we experiment, we are just going with what we know works in my little world.

-John

Hi John,

I am with you with the turbo dimensions, but I see also guys ho put a big compressor wheel for run with a low boost pressure with a camshaft not at 100% for this use and the turbo lag is horrible, I use the OEM turbo for the littles engines modifications and I change the boos pressure 0,8 bar standard to 1 bar with this turbo... after, for big engines, yes I change the turbines dimensions but I try always tobe in a not very big compressor wheel and turbine for the lag. Also I know a guy ho loves make his engines with a big compressor wheel because the engine works as a aspirated engine, progrssive and all in the high rpm... here we say : for choices we are colours...

I like also the think of test with the plenum and accelerator or carburetor, yes for me is very important for know as all work mounted, not only the head... and can be interesting of course test with pressure in the inlet for a "real" simulation, but in the exhaust, with the pressure generated between the turbo and the exhaust valve for me now is a problem.
I think the better in this cases is try to gain CFM for have a better limit with the massflow and be sure all the ports are the same CFM... with this is sure the head is better and with security before test the engine.
The air speed is also a very important point, I read sometimes is better arraive at 100 meters / second but no more.

Regards.

Forced_Firebird wrote:
I what would happen if we applied our vacuum source in blow configuration into the plenum, rather than drawing it through the port if it would closer mimic a positive pressure situation? One would probably have to set the manifolds, and throttle to the equation to be accurate, but then also would have to mimic the positive pressure and any cam overlap to the mix since the exhaust pressure is so great in a turbo system.

Until we experiment, we are just going with what we know works in my little world.

-John

I'm not too sure what you are saying there ^^.

**If you suck through an average shaped port then the air will favour the short side, if you blow through it will favour the long side**

Do you think this is true? If so then does it have any complications, or would it change the way we shape a turbo port or measure it with our benches?

I think maybe it depends on your overall boost pressure, you may see +14.7psi in the plenum, but will you still be seeing that in the inlet port? Here you still might be seeing vacuum until (throws a number in) say, 28psi.

For sure inlet plenums must be designed with some 'equal port share' in mind, but I think they too are to be questioned and could work differently at different boost pressures. The low boost OE style ones don't seem to have anything which guides the air into the ports, so I'm guessing that at a certain boost pressure or air speed we need to be thinking of doing something about this.

It's very difficult to find some empirical evidence on this subject so I am either;

Wrong

People are keeping it to themselves

It's too advanced (hardly likely!)

SSR wrote:

Forced_Firebird wrote:
I what would happen if we applied our vacuum source in blow configuration into the plenum, rather than drawing it through the port if it would closer mimic a positive pressure situation? One would probably have to set the manifolds, and throttle to the equation to be accurate, but then also would have to mimic the positive pressure and any cam overlap to the mix since the exhaust pressure is so great in a turbo system.

Until we experiment, we are just going with what we know works in my little world.

-John

I'm not too sure what you are saying there ^^.

**If you suck through an average shaped port then the air will favour the short side, if you blow through it will favour the long side**

Do you think this is true? If so then does it have any complications, or would it change the way we shape a turbo port or measure it with our benches?

I think maybe it depends on your overall boost pressure, you may see +14.7psi in the plenum, but will you still be seeing that in the inlet port? Here you still might be seeing vacuum until (throws a number in) say, 28psi.

For sure inlet plenums must be designed with some 'equal port share' in mind, but I think they too are to be questioned and could work differently at different boost pressures. The low boost OE style ones don't seem to have anything which guides the air into the ports, so I'm guessing that at a certain boost pressure or air speed we need to be thinking of doing something about this.

It's very difficult to find some empirical evidence on this subject so I am either;

Wrong

People are keeping it to themselves

It's too advanced (hardly likely!)

I'm just theorizing, but...

We all know that airflow is what's sought after.

I agree about the SSR (love the handle BTW).

But here's the catch. It's NOT really about the boost pressure, but rather the airflow - I generally use pressure numbers for 2 reasons: 1. Place the compressor map to desired power levels 2. To map the fueling. Let's use a simple example of twin vs single turbo. At the same boost levels using identical turbos, do you think the single turbo will flow as much as the twins?

Another case in point. A local is talking about his 21psi setup where I know the turbo is past it's efficiency and he wonders why he's not making any more power. Why? Not because of the amount of boost, but rather the ability for the compressor wheel to move the volume needed to make more. Boost levels go up but power output becomes stagnant only increasing inlet temps.

My thought is if you are using a turbo that can outflow the engine (positive pressure), you would think that the air IS actually being "blown" into the engine, the only way any vacuum would be seen is if the piston was already on it's way down before the valve was opened.

Am I in left field with this thinking?

Artigas,

There is a fine line when deciding on the application of a specific turbo between being "too big". You have to remember, the closer you are to 1:1 ratio on exhaust to intake manifold pressure, the better. The same reason that causes the compressor to be too small as I mentioned above, the turbine can become a restriction giving you the same outcome (high EGT's, choke points). This causes undesirable intake/exhaust pressure ratios.

As far as spooling time, there are many ways to correct it that are rather simple for someone who builds turbo systems. One is a ball bearing journal. You can have on in your hand a literally blow into it and watch the wheels move. Truly amazing, but is also very expensive.

Divided pulse manifold/turbine housing. This is also great. I have recently worked on 2 different cars. A BMW inline six and a Honda 4cyl. Both using the same turbo from a Deisel truck (Holset HX35), both have divided turbines/manifolds. The BMW sees full boost at about 3500rpm under light load, the Honda sees it at 4000 under light load. The BMW redlines at 6800RPM the Honda at 7200RPM. You would think that because of the two different sizes/styles of engines there would be a big difference in the spool time, but it's not the case in real life. This same turbo runs a large diesel engine to 30psi.

Finally the quick spooling device. This allows you to have a large turbine, large compressor and not choke and not lag, either. You feed all cylinders to one half of the twin scroll turbine until a desired pressure level is reached, then open the other half to prevent choke.

Well, enough rambling. Got to get back to welding these headers. Sorry to get off topic.

-John

Forced_Firebird wrote:
I'm just theorizing, but...

We all know that airflow is what's sought after.

I agree about the SSR (love the handle BTW).

But here's the catch. It's NOT really about the boost pressure, but rather the airflow - I generally use pressure numbers for 2 reasons: 1. Place the compressor map to desired power levels 2. To map the fueling. Let's use a simple example of twin vs single turbo. At the same boost levels using identical turbos, do you think the single turbo will flow as much as the twins?

Another case in point. A local is talking about his 21psi setup where I know the turbo is past it's efficiency and he wonders why he's not making any more power. Why? Not because of the amount of boost, but rather the ability for the compressor wheel to move the volume needed to make more. Boost levels go up but power output becomes stagnant only increasing inlet temps.

My thought is if you are using a turbo that can outflow the engine (positive pressure), you would think that the air IS actually being "blown" into the engine, the only way any vacuum would be seen is if the piston was already on it's way down before the valve was opened.

Am I in left field with this thinking?

Artigas,

There is a fine line when deciding on the application of a specific turbo between being "too big". You have to remember, the closer you are to 1:1 ratio on exhaust to intake manifold pressure, the better. The same reason that causes the compressor to be too small as I mentioned above, the turbine can become a restriction giving you the same outcome (high EGT's, choke points). This causes undesirable intake/exhaust pressure ratios.

As far as spooling time, there are many ways to correct it that are rather simple for someone who builds turbo systems. One is a ball bearing journal. You can have on in your hand a literally blow into it and watch the wheels move. Truly amazing, but is also very expensive.

Divided pulse manifold/turbine housing. This is also great. I have recently worked on 2 different cars. A BMW inline six and a Honda 4cyl. Both using the same turbo from a Deisel truck (Holset HX35), both have divided turbines/manifolds. The BMW sees full boost at about 3500rpm under light load, the Honda sees it at 4000 under light load. The BMW redlines at 6800RPM the Honda at 7200RPM. You would think that because of the two different sizes/styles of engines there would be a big difference in the spool time, but it's not the case in real life. This same turbo runs a large diesel engine to 30psi.

Finally the quick spooling device. This allows you to have a large turbine, large compressor and not choke and not lag, either. You feed all cylinders to one half of the twin scroll turbine until a desired pressure level is reached, then open the other half to prevent choke.

Well, enough rambling. Got to get back to welding these headers. Sorry to get off topic.

-John

Yes I agree, I was just using the term 'boost pressure' because it would be recognised as to what I was speaking about, it is also a measure of restriction and it is mass flow the engine wants. It depends on where you take your measurement from too....
I dunno which set up (twin or single) will flow the most, you tell me! I do know that twin is no good on less than six cylinders and that if the original set up is single people try twin, if it is twin, they try single! It seems that single turbo is king for the highest power outputs though, twin probably gives better driveability.

As to whether you are 'pushing or pulling' depends on whereabouts in the rev range you are*, where you take the measurement from and the 'boost pressure'*..... as these* are constantly changing.

I thought that diesel turbos were no good for petrol engines due to the hotter temps?