What Kind Of Gain For Ported Heads

[Ormand]

Tom, I don't know where you learned your engineering, but it must have been at a school of liberal arts. The two terms, ACFM, and SCFM, have been in use in engineering texts for many, many years. There may be mechanical engineers out there who wouldn't recognize the terms, but if so, I hope they are not working on any projects requiring critical flow calcs.
As far as being "undefined" maybe you should learn to look things up. Here is a definition, which I found in thirty seconds.
Definition -- The term ACFM stands for Actual Cubic Feet Per Minute and is often improperly used by some air compressor vendors. For example, an air compressor vendor may state that a 200 HP air compressor is rated for 1000 ACFM at 100 PSIG. Taking this phrase literally, the natural assumption is that 1000 Actual Cubic Feet Per Minute of compressed air is leaving the discharge of the compressor at 100 PSIG. This is incorrect. The compressor vendor should have stated that this 200 HP air compressor is rated for 1000 Inlet Cubic Feet Per Minute (through the inlet air filter) when operating at a discharge pressure of 100 PSIG. The actual volume flow of compressed air leaving this air compressor is about 125 ACFM, depending on ambient conditions.
I've been a mechanical engineer since 1965, and the community that I have belonged to has certainly understood the difference between ACFM, and SCFM. The two are certainly interrelated, but they have entirely different uses.

 

[bison]

The big problem is people always refer to mass efficiency as volumetric efficiency. All engines are 100% volumetric efficient because we are dealing with a gas which takes up the entire volume of whatever it is contained in. Mass efficiency is the important topic. How dense of a charge can you get in the cylinder before the intake valve closes? How much of the inert gas can you get out of the cylinder before the exhaust valve closes? What can be done to increase the air density in the intake tract? Keep in mind peak MASS efficiency occurs at the engines peak torque output even though the airflow is not at its peak through the engine.

 

[tom h]

I don't know where you learned your engineering, but it must have been at a school of liberal arts ... ACFM, and SCFM, have been in use in engineering texts for many, many years...

My SAT Math scores were 800, what were yours?

Concerning "AFCM" I stand corrected. One definition I found (from Sullair) was :
Actual Cubic Feet per Minute - flow of air or gas delivered to the discharge connection of a compressor stated in cubic feet per minute under the conditions of temperature and pressure prevailing at the inlet of the compressor. ...

So if the inlet of the compressor is "standard conditions", ie 60ºF & 1 atm (sea-level), ACFM is same as SCFM.

But it's not a Unit I see used much, it's not even listed in my CRC handbook; the mech & facility engrs I have worked with nearly always SCFM ... they were not compressor designers per se, but they were buyers and specifiers of such components. And, every piece of semiconductor-chip manufacturing equipment I've ever spec'ed or bought, was always in terms of SLPM or SCFM (not "actual" LPM or CFM).

So I concede your point that while "ACFM" is a defined term as applied to air compressors, I stand by my point that SCFM is an environment-independent equivalent measure of the "quantity" of a gas, as useful as the mass. ACFM depends on the particulars of temp, pressure, & humidity which makes apples-to-apples comparisons difficult. I would rather know my cyl head will flow "220 SCFM" than "220 ACFM" but with the temp, humidity, & other conditions left unspecified.

What was it we were originally talking about ?

 

[Super6]

i'm lost ........but maybe if i read it about 200 times i might learn something:biggrin: ......................................................... Bob

 

[YI_work]

Not, not, not! Boost is the ratio of air pressure on the discharge side of the compressor to the pressure on the intake side of the compressor. You can increase boost by speeding up the compressor. How does that affect the efficiency of the heads and intake? The post by Bison is accurate and factual. Air flow VOLUME is determined by engine size and speed. Period. You can increase pressure and density and get more MASS, as Vader says, but to get more volume, you got to use more rpm, or get a bigger engine. When you are flowing higher VOLUME, then porting heads and intakes will help more than if you increase pressure/density without going to more cubic inches or rpm.

I just pulled apart my heads to polish the runners and touch the bowls up a bit. My motor is going back to stock except for a 204/214 cam. So that would be a waste of my time then right. I figured if I can improve flow I can improve hp no matter what.? I know that the engine needs to beable to use the volume, but I was thinking by making it more efficient would improve hp.

 

[VadersV6]

Ormand. Like Ive said in past posts on this subject, the entire forced induction community, from imports to domestic, street cars to drag cars to drift cars to bonneville cars, etc, would completely fall on its face if your theories were reality. Making 1200hp out of a 4 banger could never happen. Making 1700hp out of john mihovitz's 281 cubic inch modular twin turbo pro-5.0 car wouldnt happen. Making 7000hp on a Top Fuel motor wouldnt happen. Volume flow is a dead subject. It doesnt matter. The number of O2 molecules in the cylinder is everything. Like I said, looking at a compressor map, you can either make 600hp at 32psi, or 600hp at 18psi...with the SAME TURBO. Ask yourself HOW this can happen.
Boost is a measure of BACKPRESSURE in the intake tract. Its a measurement of how far above ambient the intake manifold pressure is. Why is the intake manifold pressure above ambient? If the heads and intake cannot flow a set mass of air into the cylinder fast enough, it creates backpressure. Open the heads and intake up, and it now moves the same mass of air though, but it takes less force to push it through. We're not dealing with 2 plenums and trying to let ambient pressure move from one tank to another like an n/a motor. We're pushing it through.

 

[tom h]

... The number of O2 molecules in the cylinder is everything. ... Boost is a measure of BACKPRESSURE in the intake tract ...

I concur ... a minor quibble is that fuel makes power, then we "all" need is somehow getting enough O2 mass into the combustion chamber for a stochiometric burn. But no question, it's the mass (or equivalently # of molecules) of fuel & O2 in the combustion chamber that sets torque, not "boost" or "volume" strictly speaking. And it's the mass of O2 & fuel burned per second, that defines the maximum available horsepower.

 

[bison]

I concur ... a minor quibble is that fuel makes power, then we "all" need is somehow getting enough O2 mass into the combustion chamber for a stochiometric burn. But no question, it's the mass (or equivalently # of molecules) of fuel & O2 in the combustion chamber that sets torque, not "boost" or "volume" strictly speaking. And it's the mass of O2 & fuel burned per second, that defines the maximum available horsepower.

We need to get all that hot inert gas out of the cylinder after combustion also. All those high hp 4 cylinders and 2000 hp turboed v-8's are doing this very well. Intake pressures are higher than the exhaust pressures in these applications. We will have a hard time achieving the same with the commonly used 3 bolt ex. housings at the inlet pressures we run on our 3.8 or larger engines. Lowering exhaust pressures between the engine and turbo will increase power more than one would expect. Intake pressure and mass flow on the intake is only one part of the equation. The exhaust flow increases surpass the intake flow as the cylinder gets filled with more oxygen.

 

[SloGN]

We need to get all that hot inert gas out of the cylinder after combustion also. All those high hp 4 cylinders and 2000 hp turboed v-8's are doing this very well. Intake pressures are higher than the exhaust pressures in these applications. We will have a hard time achieving the same with the commonly used 3 bolt ex. housings at the inlet pressures we run on our 3.8 or larger engines. Lowering exhaust pressures between the engine and turbo will increase power more than one would expect. Intake pressure and mass flow on the intake is only one part of the equation. The exhaust flow increases surpass the intake flow as the cylinder gets filled with more oxygen.

This is why we need to look at the exhaust valve/port and header selections along with the cam and A/R ratio of the exhaust housings to try and get the pre-turbine back pressure as low as possible even with a turbo impeding the flow. cause as you said the better we can clear out the enert gas means we have more area to put in a bigger charge. this will help in max power and it will also help in the part throttle area for better VE/ @ small throttle openings.

i'm currently working on doing backpressure tests with the stock exhaust manifolds. I have recnetly swapped out the stock headers for a set of the ATR knock-off headers from ebay . What i'm hopping to see is that the back pressure in the headers drop a few psig @ the same boost levels.

then it will be a challenge to try and lower it even more after that.



here is the info that i posted a while back on the back pressure tests

Mods to the engine/exhaust/turbo is in the sig. well here goes.


All the pressure reading were taken from the EGR port.

RUN 1 20 psi of boost in third gear i showed a BP of 33 psi made two runs to verify consistesy. this was thru the exhaust

RUN2 Next i proceeded to open the collecter on my test pipe. with the dump open @ the same 20 psi the BP was @ 36 psi Again to runs were made to show consistesy.

RUN3 i took the Test pipe competely off the car and ran just a open DWN pipe the boost went to 22 psi with 35 Psi of BP

Run4 i corrected the boost back down to 20 psi with the Dwn pipe and got 31.5 Psi of BP.

So from what i have noticed on my car just opening the dump on my test pipe yeilds no gains at all.


next. i'm going to check the Back pressure @ the o2 sensor location to see if there is an actual diffrence in pressure there than at the egr port. i doubt there will be any difference since the EGR port is tied in witht he rest of the exhaust system pre-turbo.

so 31.5psi/20 psi= 1.57 ratio just a open dwn pipe.

20psi/33psi= 1.65 ratio thru the exhaust

20psi/36psi= 1.8 ratio with the dump open on the test pipe.


The only thing that i can think of why the Backpressure went up by just opening the test pipe is that it is creating turbulence in the pipe by the air having turn and exit the dump collector. thus having to have more pressure to get the air to flow out or maybe not more pressure bu more engery to push the air out even tho it's a shorter way out.


The only thing that i can see that would make the Backpressure lower by opening the dump collector is the fact that the car has a real restrictive exhaust.

 

[2QUIK6]

To put in simple terms, if your intake manifold pressure is greater than the air pressure in the cylinder at BDC before the compression stroke....you have something to gain by porting heads.
Because all of that boost you're producing at the turbo outlet...isn't making it into the cylinders....

Now with that said, you must also consider that there may be some exhaust gas pressure that wasn't fully exspelled on the exhaust stroke or it came back in the cylinder while the value was open due to excessive exahsut pressure from other cylinders...which is why you want the freest flowing exhaust on a turbo car...

That's why uping the boost doesn't always equal making more power...
And as mentioned...there's probably some low boost point where porting the heads does not change the air pressure at BDC but I'm sure thats well below any boost levels anyone on this board is running

 

[tom h]

... All the pressure reading were taken from the EGR port. ...

Do you mean an Exhaust Gas Temperature (EGT) port
And if so, is this in the typical location, near the O2 sensor?

Interesting to know the amount of backpressure is in the 30s PSI (for your combo) ... this is the pressure at turbine's inlet. The pressure difference across the turbine is what's going to spin it, so a healthy amount of backpressure from the turbine is required.

It also explains why a 1-2 PSI reduction in back pressure from after-market cat-back exhaust systems is often not significant, relative to the 30+ psi BP from the turbine alone.

I never measured pressure drop across my stock catalytic converter but it must have been enormous (10-20 PSI ??), as removing it increased my 1/4 MPH trap speeds from about 98 to 102 MPH !

 

[SloGN]

Do you mean an Exhaust Gas Temperature (EGT) port
And if so, is this in the typical location, near the O2 sensor?

Interesting to know the amount of backpressure is in the 30s PSI (for your combo) ... this is the pressure at turbine's inlet. The pressure difference across the turbine is what's going to spin it, so a healthy amount of backpressure from the turbine is required.

It also explains why a 1-2 PSI reduction in back pressure from after-market cat-back exhaust systems is often not significant, relative to the 30+ psi BP from the turbine alone.

I never measured pressure drop across my stock catalytic converter but it must have been enormous (10-20 PSI ??), as removing it increased my 1/4 MPH trap speeds from about 98 to 102 MPH !

No i used a calibrated pressure gauge to measure the exhaust pressure in the EGR port. the egr port is tied in directly with the #3 exhaust port and that is tied in with the exhaust system pre-turbine.

 

[bison]

Vader it would be good to know the rpm at the pressure readings you obtained. Sometime this year i will do some similar testing with different exhaust wheels. GTQ vs. P, etc.If yolu can get a higher inake than exhaust manifold pressure you will be able to play with the exhaust duration and timing also too make even more power. I dont think its possible to get these numbers through a 3bolt housing trying to push 600+hp worth of exhaust through it. To achieive the higher intake than exhaust pressure a 4 bolt tangential exhaust housing with a big a/r of nearly 1.2 will be required on our size engines. Just throw a litlle N2O at it to turn it on .

 

[John Wilde]

Great conversation, thanks to everyone that posted!

 

[VadersV6]

Vader it would be good to know the rpm at the pressure readings you obtained. Sometime this year i will do some similar testing with different exhaust wheels. GTQ vs. P, etc.If yolu can get a higher inake than exhaust manifold pressure you will be able to play with the exhaust duration and timing also too make even more power. I dont think its possible to get these numbers through a 3bolt housing trying to push 600+hp worth of exhaust through it. To achieive the higher intake than exhaust pressure a 4 bolt tangential exhaust housing with a big a/r of nearly 1.2 will be required on our size engines. Just throw a litlle N2O at it to turn it on .

Im not the one who did those tests :smile:
Brent

 

[bison]

Im not the one who did those tests :smile:
Brent

My bad. It was SloGn.

 

[tony1917]

heads

WOW!

1983 T-Type Riviera
231 Carbbed Turbo HotAir
Stock Heads#293 no porting.Blueprinted to factory spec
Stock Turbo T3 Blueprinted to factory spec
NHRA National Record Holder
BF/S Stock That's front wheel drive
13.60 @ 102 @ 22psi with c16
3860lb
Tony Massari