| Author |
Message |
    
Fred
| | Posted on Monday, April 14, 2003 - 9:23 pm: |       |
Bogie,
I read your response as to how the majority of flow through a chevy head favors the inboard side of the valve and how there is a swirl motion (like a coil) as the cylinder charges. I can see, with your help how car mfg engineers put alot of design into setting up an engine. How unshrouding the valves could be a bad thing though I don't know how many articles I read that it was a good thing.
I'm almost intimidated to modify anything since with me it's like pot luck as to whether it will perform as well without alot of inefficiencies i.e. 1.6 rockers, changing duration of cam and so on.
Is it really just hit or miss when you hear about guys trying this cam with that manifold and those headers? I understand (I think) how to set an engine up for higher RPM operation, which I consider the brute force approach but could you take a street vortec sb engine and improve it's performance, get better power, same RPM range, and be just as efficient? I hate to harp on efficiency but the vortec heads are a prime example of efficiency. No larger valves, no larger ports and yet out perform earlier head designs.
Have I lost you yet or are you getting my drift?
Thanks as always, you are informative as ever. |
Bogie
| | Posted on Tuesday, April 15, 2003 - 2:54 pm: | |
Fred, I know you probably never read anything that said "shrouding can be a good thing". Like I said in the introduction of my paper, I will say things that differ from the accepted norm, this is one of those exceptions. This is not to say that shrouding is good or bad. I intended to convey the thought, that if you understand what's happening in the cylinder, it's possible to manage the airflow using the configurations available to your benefit.
It's important to understand that flow benches tend to be unidirectional, that’s to say the configuration has the head to be tested at one end and a fan at the other. Many years ago Smokey Yunick suggested that for a flow bench to be truly representative of what’s going on inside the engine, it needed to be “motored”. This of course would entail having all the mechanism necessary to operate a piston in a cylinder under the test head. This, then, really leads you to testing with the valve being properly sequenced, thus a cam and valve train would also be necessary. While this would do much better at allowing you to test the dynamic relationship between piston speed, valve event, flow pulsing and port flow; one can see how difficult this becomes.
So instead we test by sucking air through the port with a fan. The port (intake) is usually dressed with a wad of clay to smooth the entry flow, like that really represents the operating configuration of your engine. Then there’s always the old problem, “that when observing a system, you change its nature by virtue of performing the observation”. Here’s an example of what’s meant by that statement; on the flow bench we test areas of flow within the port by inserting a probe to measure pressure. OK, so how often do you stick probes into the ports of your engine while it’s operating? The probe itself disturbs the air flow, thus to some extent (large or small) the tested result.
Anyway, where I’m going is to say that in a substantial measure the published data concerning ports, valves, and combustion chambers, while pointing in a general direction is largely incomplete, insufficiently definitive and to the unwary and unthinking, not as useful as it needs to be. Plus, one needs to realize that the guys who are successful competition engine builders/head porters are not going to tell you everything they know, and believe me they understand both the limitations of and the interpretations from their test data.
All of the above being said, it doesn’t discount good information from reliable sources. I had suggested reading Dave Vizard, I’ll add “How To Build Max Performance Chevy Small Blocks On A Budget” to my short list. And if you have a computer, get a copy of “Desk Top Dyno”. It ain’t the end-all, be-all answer, but it’s very informative and a hell of a lot of fun to mess around with different engine configurations.
If you just follow some of the things you read in this column and many of the popular magazines and books, you will certainly see more than a little “hit and miss”. Too many people get caught in the “Monkey See, Monkey Do” syndrome. Trust me this is not limited to hot rodders; at my daytime job as an, aerospace engineer, I see plenty of this. We call it “Boiler Plate” engineering, this is where the unwary pick up someone else’s previous work and apply it to a new job without analyzing the subtle differences. Over the years it’s cost billons of wasted dollars, caused hundreds of missed opportunities, and resulted in scores, if not hundreds, of deaths. So the fact you’re feeling squishy about taking a die grinder to a set of heads is a good thing, it indicates you respect what you don’t know. God knows, I wish I knew more, my list of “F…… ups” is plenty long. In fact I made one in my combustion chamber paper, which I'm surprised no one caught. That is the midget Offy is a pent head not a hemi. However, a 4 valve pent flows very much like a 2 valve hemi so I didn’t steer you quite into the bushes.
I’m hoping to get into the porting article itself this week and that will begin to explain a lot of things in more detail. But let me cut to the quick for brevities sake. The Vortec head, the Fastburn, many GMPP and aftermarket heads for the Gen I, as-well-as similar high performance heads for the Gen II and Gen III small blocks (you can throw the Ford GT-40 head and late model wedge MoPars in there as well) are coming from the factory with shapes, dimensions and tolerances that previously were the domain of the head porter. These new heads are very, very good out of the box and are tough to beat with anything that ever preceded them, ported or not. This, my friend, is the result of Federal Clean Air and Fuel Mileage standards meeting people’s insatiable appetite for performance.
Basically, all I’m going to do is explain to you how to emulate the Vortec with earlier heads. But you can’t get to a Vortec with the old heads because all we can reasonable do is remove material. To match many of the contours of these late model heads, material needs to be added in specific places. With cast iron heads the process of building up material is touchy and expensive, so it’s more practical to simply replace the head if your budget and/or rules allow. Your slightly better off with aluminum, in that it can be welded without a lot of heat-treat and stress relief issues, which can make the cost of modifications less than replacement. |
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