This is for some of those, like myself, that are trying to learn what works, how it works, why it works, so forth and so on.
I thought it better to make a new Post, seeing as all the others seem to be a big clusterfuck of who knows what, who don't know shit, my head outflows yours, who worked for who......blah-blah-blah!
This is a very good read. For some of those that will read and respond, read closely, David Reher 'IS' an engine guru, not a wannabe. With that said, he gives some good information, but, he does NOT give away all of his hard earned 'secrets' so to speak. Only a fool will tell you everything that he knows and has learned through his many years of sweat and tears!
I, like most everyone else here, simply want to know what works for a given combination and don't have the time or the money to burn on a 'guess'!
Flow Bench Fallacies
by David Reher
Our era is often refered to as the Information Age, but not all of the information is necessarily useful. I am beginning to think flow benches should come 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 dynomometer. 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.
Their mistaken belief that "more is better" is often the false assumption that produces an under-performing engine. A flow bench measures air movement in a very rudimentary way - steady-state flow at a constant depression (vacuum). Obviously, the conditions that exist in a running engine are quite different.
The flow bench can't simulate the effects of 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 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 the port, the more it flows. That's hardly shocking news.
Bolt a sewer pipe on to a flow bench it will generate terriffic 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 always use the longest duration cams we could find - after all, more duration means more airflow. 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.
That 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's a do-it-yourself example: Turn on a garden hose and the water dribbles 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 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 prescibed amount of air and fuel to produce "x" amount of horsepower. In a perfect world, that may be true - but we race with imperfect engines. The shape and cross sectional area of the runners is 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 9200 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 if the 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 softwares solutions as gospel when in fact they can be badly flawed. Textbooks would have you 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 also have learned that the low-lift flow, (meaning anything below
.400 valve lift in a Pro Stock engine with a .900 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 is parked near Top Dead Center. The piston doesn't begin to move away fom the combustion chamber with enough velocity to lower the pressure in the cylinder until the valve is nearly half-way open.
Consequently, it is high-lift flow that really matters in a drag race 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 terriffic flow at .200 - .300 vlave 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 numbers, 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 the over reliance on flow numbers as the primary measurement of a cylinder heads'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.
Thanks to David Reher and NHRA's National Dragster that first ran this story.