better info than posting flow number is a velicity chardt that show actual flow through the lift cycle. if those port are that good there should be no drop off in filling at or near max lift. in other words there will be no dead spot in the lift cycle of the port. it is useless to lift to .600 if the velocity and filling drops at .580. the net loss of filling negates a meaningless flowbench number. i haven't seen a chart that shows that, but i am curious what the actual usefulness of these huge ports is.
a 96 cobra port starts at 222 cc's. it can be ported to even larger cc's and be made to flow ludicrous numbers. but in the lift cycle of an actual cam, filling stops increasing long before max lift is obtained, thereby making any higher lift useless.
that's my real concern with these street heads lifting to .700 and quoting those numbers as actual usefull street hp and torque.
i do however agree that the ls7 and new truck heads are out there on power. for mr the jury is out on ls1,ls2 as being that good. your opinion is countered by other chevy experts opinions. one thing this conversation has managed to accomplish is to reopen my 4v consideration.
I was impressed at first that you displayed maturity and were willing to admit that your input on this subject was your simply your opinion (billfisher -“i can only base my information on reading others findings”) and that you did not have any discernable experience on the subject of cylinder head development. Then, you couldn’t leave well enough alone, and your desire for an attempt at self-admiration took over and led you to make your last post in a desperate attempt to impress your piers here. And of course, you again, felt the need to end your post with an arbitrarily negative and unnecessary comment towards Chevy (GM).
I am going to delve deep into my psychic abilities and go out on a limb here that you probably spent most of the day yesterday scouring the internet searching for some sort of relevant information that you could copy and paste, then fumble over and change up the wording (and proper spelling) that might give you the opportunity to make a last ditch effort to sound as if you actually had experience with cylinder head design and development.
I was again impressed by your last post…..that is….until I got about 9 words into it when I read stuff like this: “velicity chart”, “useless to lift to .600 if velocity and filling drop off at .580”, and “the net loss of filling negates a meaningless flow bench number.”
Mr. billfisher, allow me to share with you the
reality of cylinder head D&D. Perhaps you should listen closely, because it sounds like you need it.
Before any flow testing can be performed, the flow bench must be calibrated for that day's air conditions. Then you set the cylinder head on top of a bore fixture with the appropriate bore for the particular application. A dial indicator is then placed on the valve. We then use a threaded actuator to push open and hold the valve at the target lift being tested. Some people will use lighter rate valve springs in order to make the valve actuation easier, but the lighter springs, specifically the opposing spring from the one being tested, can be sucked open and skew the results. This is more common than you can imagine. I use the actual spring that will be used in the application. Once you record and “sync” the flow data, then you can plug the lift/flow data into FlowCom and/or a data analysis program. V = 1096.7*Sqrt(H/d). Once velocity has been calculated, the volume can be calculated by multiplying the velocity by the orifice area times its flow coefficient.
A cylinder head with a relatively small intake runner and high velocity characteristic can be a good or bad thing. Contrary to the popularly perpetuated internet myth, if you work a cylinder head so that it shows an exceptionally high velocity profile which results in sacrificing even moderate amounts of low lift and mid lift airflow (CFM), you'll lose power. Even if it shows gains at the upper lift range. That’s just the nature of the beast. Conversely, high flow (CFM) capability is very important at upper and peak valve lift, but not at the expense of the low lift velocity that initially gets the intake charge moving and keeps it moving even as the valve closes. Increased velocity or flow will not necessarily get you more charge after IVC if the motor is not tuned for that specific point in RPM, but with some careful intake runner pulse tuning we can optimize the velocity/flow to IVC relationship. The speed at which flow begins as the valve begins to lift is critically important. That speed creates gas inertia that helps fill (or empty on overlap) the cylinder as thoroughly as possible. A thoroughly stuffed cylinder normally equates to power, but the proper runner volume for a given application that has a relatively small cross section and a relatively high flow, will produce good results on a street/strip motor. Maximum effort drag race heads are given a slightly different approach and will typically require a larger runner volume. For instance on the LS2, the stock 211cc intake runner’s cross section works well for a street/strip low cube (under 350”) motor, but for a high HP motor or a maximum effort drag race head, you can focus more or your time in the runner to increase it’s volume. This can be done by changing the geometry of the entire port….SSR (short side radius), ceiling radius, SSR apex, strategic removal of material, etc. The right combination of runner volume and flow will lend itself to increased port airspeed. This will help to eliminate fuel separation, reversion, promote swirl, and increase the air/fuel intake charge. All which equates to performance and power. There are little tricks to manipulating flow characteristics. You can partly control the lift to flow characteristics by way of valve grinding. A inner radius cut (back-cut) can lend itself to improve the effect on low lift and mid lift performance, whereas valves without a back-cut tend to accentuate the very high-lift numbers. It all boils down to the motor combination and its purpose.
In spite of the perceived limitations of a flow bench, in skilled hands it still provides an excellent source of information in order to guide the engine designer/builder in deciding how to go about modifying the head and other engine components. Head flow data is also essential to provide accurate port flow coefficient data that is required for the use of data analysis software. With that data analysis software and flow data teamed together, it provides a powerful way to finitely understand a running motor and it's unique characteristics. Details which are just about impossible to view any other way. A flow bench is a valuable tool if used for what it is that can help fine tune a combination, but obviously it is not the ultimate authority.
My opinion has been countered by other Chevy experts huh? I cordially invite them to come aboard in this thread and participate in a discussion of cylinder head technology.
