Say you have two engines. Engine A is 2000cc. Engine B is 1500cc. Both engine's use the same cylinder head, which at 28 inches of water flows 250cfm at .500" lift. Both engines are run on an engine dyno with RPM held constant at 7000rpm both with a cam with .500" of lift. At this rpm, at that lift there is a calculatable volume of air being ingested into each engine, will engine A actually displace more than engine B? They are both sucking through the same cylinder head at the same speed with the same flow.

Discuss.

 

I would say yes... displacement is dependent on the physical characteristics of the engine. If they are using the same head I guess the bores or strokes must be different, so could we assume that Engine B has higher compression, since it has the same amount of air in it but lower volume?

 

Smaller engine (B) won't draw as much air in because it's smaller displacement. The flow numbers of the head aren't the whole story. That just says how much air flows if a limitless dP of 28 inches is imposed. It ain't gonna flow that much if the engine isn't big enough to take it in.

 

The crappier the head, the closer the amount of air the two engines would draw. If the head consists of a pinhole that flows a maximum of 1 cc a minute...well, you can attach a 2000 liter or a .1 liter bottom end to it, but either way it won't be getting more than 1 cc a minute.

However, if the head is good, in that it offers minimal resistance, then no, the bigger engine will draw more air. You're talking about a pressure 28" of water (whatever that means), but when you attach a bottom end, there is no 28" of water...the bottom end is what's creating the pressure. A small displacement is going to make for a lower pressure difference, or the pressure difference won't last as long becasue the cylinder will get filled much quickly.

Imagine a perfectly flowing head...hell, imagine no head at all. The cylinders are just open to the atmosphere. The 2000cc engine is going to draw in 2000cc of air, while the 1500cc is going to draw in 1500cc....and yet they use the same "head".

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">Smaller engine (B) won't draw as much air in because it's smaller displacement. The flow numbers of the head aren't the whole story. That just says how much air flows if a limitless dP of 28 inches is imposed. It ain't gonna flow that much if the engine isn't big enough to take it in.
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This is right along the lines of the answer i was looking for. But the question still remains, if both heads are flowing the same amount, forget the 250cfm@.500, and one engine is asking to displace 2000cc, how does it get there while the other engine asking to displace 1500cc with the same head?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Lsos &raquo;</TD></TR><TR><TD CLASS="quote"> You're talking about a pressure 28" of water (whatever that means), but when you attach a bottom end, there is no 28" of water...the bottom end is what's creating the pressure. A small displacement is going to make for a lower pressure difference, or the pressure difference won't last as long becasue the cylinder will get filled much quickly.

Imagine a perfectly flowing head...hell, imagine no head at all. The cylinders are just open to the atmosphere. The 2000cc engine is going to draw in 2000cc of air, while the 1500cc is going to draw in 1500cc....and yet they use the same "head". </TD></TR></TABLE>

28 inches of water is industry standard for cylinder head flow, and air "acts" like water in a scientific way. The bottom end is creating a NEGATIVE pressure. Your next section makes no sense. I understand if there were no cylinder head that the 2000cc engine would displace 2000cc. BUT there is a cylinder head. And if it flows 250cfm@28" im asking are both engines actually displacing their respective figures. Is this getting you thinking yet?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">
This is right along the lines of the answer i was looking for. But the question still remains, if both heads are flowing the same amount, forget the 250cfm@.500, and one engine is asking to displace 2000cc, how does it get there while the other engine asking to displace 1500cc with the same head? </TD></TR></TABLE>

That's the thing, both heads aren't flowing the same amount. The bigger engine is creating a greater negative pressure for a longer amount of time than the smaller engine, thus creating more flow.


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">
28 inches of water is industry standard for cylinder head flow, and air "acts" like water in a scientific way. The bottom end is creating a NEGATIVE pressure. Your next section makes no sense. I understand if there were no cylinder head that the 2000cc engine would displace 2000cc. BUT there is a cylinder head. And if it flows 250cfm@28" im asking are both engines actually displacing their respective figures. Is this getting you thinking yet?</TD></TR></TABLE>

I'm asking you to imagine an extreme to better visualise what's going on. If you take off the cylinder head, it's the equivalent of an extremely bored out cylinder head. If you wish, imagine that we put a cylinder head on there, but bore it out so much that it's essentially a plate with four large holes. If you put it on a bench for testing, you will get a flow number for 28" of water...let's say it's 5000cfm. However, if you put it on top of an engine that displaces only .01 liters, well, that whole 28" of water goes right out the window. Now it's more like a fraction of an inch of water. Just becasue the head can flow 5000cfm, doesn't mean it's going to. In fact, put it on a 0 liter engine...a wall. How much pressure difference do you have now? None...and thus no flow.

Now put it on top of a 200 liter engine. When that thing starts swinging you're going to get a huge pressure difference, and it's going to suck in almost all of that 200 liters.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">This is right along the lines of the answer i was looking for. But the question still remains, if both heads are flowing the same amount, forget the 250cfm@.500, and one engine is asking to displace 2000cc, how does it get there while the other engine asking to displace 1500cc with the same head?</TD></TR></TABLE>The thing is, both heads WON'T be flowing the same amount. They can't flow any more than the capacity of the engine to take in.

Think of 250cfm@28" as a way of expressing the flow resistance. When a piston moves down in the cylinder, it may or may not produce 28" of suction. A larger displacement engine produces more suction, smaller engine produces less. Any engine would produce more suction if you just spin it at faster RPM.

And you're going to install that head into an engine, along with an intake manifold, throttlebody, air filter, & all that junk. Each of these contributes its resistance to the total. So you consider the amount of suction produced by the bottom end, and the total of all those resistances at the top.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">28 inches of water is industry standard for cylinder head flow, and air "acts" like water in a scientific way.</TD></TR></TABLE>28 inches of water is a pressure difference of just about 1.01 psi. Its nothing more than a different way of expressing pressure or suction.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">
28 inches of water is a pressure difference of just about 1.01 psi. Its nothing more than a different way of expressing pressure or suction.</TD></TR></TABLE>

Ah, but there is no such thing as suction. Only negative pressure.

Anyway, i am getting at something else here that i dont have time to post tonight, ill type it up tomorrow. Thanks for all the replies. We do have a few intelligent people on this board!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">
Ah, but there is no such thing as suction. Only negative pressure.
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In this case, suction is the result of the negative pressure. It's what we're interested in.

 

Obviously those of you who pointed out that the flow bench is a steady state vacuum device were right on the money. Obviously the bigger displacement engine would create a larger pressure differential when the inlet valve opened and more hp/tq/displacement would obviously be the end result. But what about cylinder head flow? What exactly do flow benches tell us? Why do i care that the cylinder head flows 250cfm @ .500" lift? I was trying (in a roundabout sort of way) to highlight the fact that it is my belief that your cylinder head *is* your engine, yet at the same time, so many people on this board get hung up on "overcamming", flow numbers (or lac k of) and immediately bash setups that dont make sense to them on paper.(or in this case, on the screen.) I came across this great article which i shall duly copy and paste here.

Flow Bench Fallacies
by David Reher

Our era is often referred to as the Information Age, but not all of the available information is necessarily useful. I am beginning to think that flow benches should be labeled with a government warning: "Caution! Excessive reliance on flow numbers may be harmful to your engine!"

I'm kidding, of course. Used wisely, a flow bench can be a useful tool in engine development, just like a timing light or a dynamometer. Unfortunately, some racers believe that a flow bench is the ultimate answer machine. When the subject is cylinder heads, the four words I dread to hear are, "What do they flow?" Novice racers and magazine writers share a fixation about airflow. The mistaken belief that "more is better" is often the false assumption that produces an underperforming engine.

I learned this lesson myself when my partners Buddy Morrison and Lee Shepherd built our first flow bench in the mid-'70s. It was a great contraption that could just about suck the windows out of our rented shop on Arkansas Lane. While this homebuilt test bench boosted our racing program, it certainly didn't make us engine experts overnight - even though we initially thought we had found the key to the vault of knowledge.

We had been racing 287-cubic-inch small-blocks in various Modified and Comp classes before we decided to make the move to Pro Stock with a 331ci engine. (Students of Pro Stock history will recall that the '70s was the era of weight breaks for various engine and chassis combinations.) We were determined to be "scientific" in our approach, and reasoned that a 15 percent increase in engine displacement demanded a 15 percent increase in airflow. We dutifully enlarged the ports, increased the valve diameters, and hit our airflow targets. We set off to conquer the world of Pro Stock - but our pride and joy was a dog.
After struggling to even qualify in our initial outings, we pulled an old pair of Modified heads off the shelf. Lee worked on the ports for an afternoon, we bolted them on our Pro Stock short-block, and we qualified fifth at Englishtown in our next race.

If you went strictly by the flow numbers, those heads would hardly enough air to satisfy a respectable big-inch bracket racing engine - and yet they were magic on the race track. That was when I realized that cfm isn't everything. It's a lesson that I have seen repeated countless times in the last 25 years.

A flow bench measures air movement in a very rudimentary way - steady-state flow at a constant depression (vacuum). Obviously the conditions that exist inside a running engine are quite different. The flow bench can't simulate the effects of the pistons going up and down, the reversion pulses as the valves open and close, the sonic waves that resonate inside the runners, the inertia of the fuel droplets, and all of the other phenomena that influence engine performance in the real world. When you flow test a cylinder head, you are simply measuring how far you can move the liquid in a manometer.

The bigger you make a port, the more it flows. That's hardly shocking news. Bolt a sewer pipe onto a flow bench and it will generate terrific flow numbers. So should we use ports as big as sewer pipes on our race cars? The flow bench says we should - the time slip says something completely different.

If airflow were everything, we would all use the longest duration camshafts we could find - after all, more duration means more flow. In fact we know that there is a finite limit to how long the valves can be open before performance suffers. That is because the valve events have to be in harmony with the rest of the engine.

The same principle applies to cylinder heads. Simple airflow capacity should never be the first consideration in evaluating cylinder heads. Characteristics that are far more important include air speed, port cross section, port volume and shape, and the relationship between the size of the throat and the valve seat. If these attributes are wrong, you can work forever on the flow bench and not overcome the fundamental flaws.

Here is a do-it-yourself example: Turn on a garden hose and the water will dribble out a couple of feet. Now put a nozzle on the hose and the water will spray across your backyard. The water pressure and volume haven't changed, but the velocity has increased dramatically. Now think about the air and fuel going into your engine's cylinders. Which would you prefer: slow and lazy or fast and responsive?

An engineer will tell you that an engine requires a prescribed amount of air and fuel to produce "X" horsepower. In a perfect world, that may be true - but we race with imperfect engines. The shape and cross-sectional area of the runners are absolutely critical to performance. For example, I have two sets of Pro Stock cylinder heads that produce nearly identical flow numbers, yet one pair produces nearly 150 more horsepower at 9,200 rpm than the other. The flow bench can't tell the difference between them, but the engine certainly can.

There are software programs that claim to be able to predict an engine's performance based on airflow numbers. Unfortunately, a critical shortcoming of many of these programs is that they are based on inaccurate information or false assumptions. A computer is an excellent calculator, but it is not an experienced engine builder. The software doesn't know whether a port's short-turn radius is shaped properly, whether the flow is turbulent at critical valve lifts, or whether the flame speed is fast enough. Racers have a tendency to believe that computers are infallible, so they accept the software's solutions as gospel, when in fact they may be badly flawed.

Textbooks would lead you to believe that an exhaust to intake flow ratio of 80 percent is ideal - yet a typical Pro Stock head has exhaust ports that flow less than 60 percent of the intake runners. You can improve the exhaust flow tremendously with about 40 minutes of work with a hand grinder - but the supposed improvements will just about kill the engine's on-track performance. I know because I've been there.

We have also learned that low-lift flow (meaning anything below .400-inch valve lift in a Pro Stock engine with a .900-inch lift camshaft) is relatively unimportant. Think about the valve events in a racing engine: From the point when the valve first moves off its seat until it reaches mid-lift, the piston is either going the wrong way (that is, it is rising in the cylinder) or it's parked near TDC. The piston doesn't begin to move away from the combustion chamber with enough velocity to lower the pressure in the cylinder until the valve is nearly halfway open. Consequently it is high-lift flow that really matters in a drag racing engine.

The shape of the combustion chamber also has a significant impact on performance. A conventional chamber with deep reliefs around the valve seats and a relatively flat valve seat angle can produce terrific flow at .200 to .300-inch valve lift. Today a state-of-the-art chamber typically has 55-degree valve seats and steep walls that guide the air/fuel mixture into the cylinder to enhance high-lift flow.

This doesn't mean that every racer needs state-of-the-art Pro Stock cylinder heads - along with the high maintenance they require. The heads have to match the application. Conventional combustion chambers and 45-degree valve seats are just fine for a dependable, low-maintenance racing engine that will run a full season between overhauls.

The classic Hemi combustion chamber is capable of producing impressive flow figures, but it's not going to make impressive power. Engine technology in all forms of motorsports is converging around smaller, high-efficiency combustion chamber designs. You can see the result in lower brake specific fuel consumption (BSFC) numbers, which indicate improved engine efficiency. Twenty years ago, a racing engine with a .48 BSFC was considered very good; today's competition engines produce BSFC numbers in the neighborhood of .35. This means that a given quantity of fuel is being atomized and burned more effectively to produce more power. A cylinder head's combustion efficiency can't be measured on a flow bench, yet it has a huge impact on performance.

I am not against flow benches; in fact, we use computerized flow benches daily at Reher-Morrison Racing Engines. What I am against is over reliance on flow numbers as the primary measurement of a cylinder head's performance. A flow bench is a valuable tool that can help a racer fine tune a combination - but it is not the ultimate authority.

 

Interesting article, but I think both people that were replying to you already understood what it pointed out...hence our confusion. Still, a good thread for people that didn't know.

 

Youre right Lsos, i managed to get the 2 intelligent people surfing HT on the day i posted this...