Head Flow Under Pressure
Trevor Kertson; Silverton, OR: I am particularly interested in articles and tech answers regarding cylinder heads. This brings me to my question: Does anyone know what type of effect supercharging (turbo, roots, etc.) has on the way a cylinder head flows? I understand that, generally speaking, cylinder heads are tested at a certain depression (vacuum) of atmosphere, and this may not be identical from one tester (or manufacturer) to another.
I am aware of the wet-flow testing that is being done on some aftermarket heads, but I know this is done with vacuum and not pressure. Perhaps I am overthinking this whole thing, but I currently think that a cylinder head optimized for naturally aspirated operation could be optimized in a different manner for a supercharged operation.
Jeff Smith: Some enthusiasts think that using a supercharger or some kind of forced induction minimizes the need for a good cylinder head because the boost is a cure-all. While the inlet pressure does push the air and fuel in, anything that will help airflow for a normally aspirated engine will also improve power. It turns out that a weak set of heads, especially on the exhaust side, can have a detrimental effect on the supercharger’s ability to make power. I subscribe to what my friends like to call the Louie Hammel Funnel Theory. Louie is a very sharp internal-combustion-engine tech head who once described a supercharger as functioning like a funnel, directing a maximum amount of (in our case) air and fuel into the engine. But power potential is often limited by the exhaust side of the cylinder heads, which in this analogy is the restricted outlet of the funnel. If you want to flow more water (or air and fuel), you need to open up the restriction—the exhaust side. You can bolt on a very large supercharger, but if the exhaust ports are restricted in flow, the results will be disappointing.
The most common flow bench on the market right now is the SuperFlow 600 bench that is capa
You asked whether the cfm coming into the engine needs to be equal to the cfm coming out. It would appear on the surface that this would be true. But consider that the inlet side on a normally aspirated engine is only using atmospheric pressure to push the air and fuel. The air is relatively cool and is not under great pressure. Conversely, when the exhaust valve opens, the combusted mixture is very hot and still under significant pressure. This pressure helps to push the mixture past the exhaust valve. Later in the exhaust cycle, the piston also assists with pushing the exhaust gas out. That is why the exhaust valve can be roughly 20 percent smaller than the intake valve and still do a good job of venting the exhaust gas. But the valve is only a portion of the requirement. A very important element is the shape of the exhaust port. A very well-designed exhaust port can help evacuate the cylinder of spent combustion gases, which improves power everywhere, but especially at higher engine speeds where there is less time (both in exhaust lobe duration and actual physical time) to vent the cylinder.
Based on thousands of dyno tests on lots of different engines, a general rule of thumb is that if the exhaust port on a flow bench can flow roughly 70 percent of the intake-port flow at the same valve lift, the head will generally work well. Of course, this is a very general description with several caveats. For example, if the cylinder head has a weak intake port, even a poor-performing exhaust port can generate a 70 percent exhaust to intake (E/I) percentage. What we’re looking for is a balance between the intake and exhaust-port flow. For example, if you have a hero intake-port-flow number, like 280 cfm at 0.500-inch valve lift, the exhaust port has to perform as well. Using our 70 percent evaluation, the exhaust port should flow somewhere around 195 cfm or greater in order to achieve this percentage (280 x 0.70 = 196 cfm). Certainly, flow numbers exceeding this percentage are not bad, but it is important to keep this percentage in mind because it helps when deciding on camshaft specs. If the exhaust port performs at 70 percent or higher throughout the entire valve-lift curve, it’s a good indication that the engine will respond better to a single-pattern camshaft on which the intake and exhaust lobes are the same duration numbers. Conversely, a cylinder head with poor E/I percentages will generally work better in an engine fitted with a dual-pattern camshaft with more exhaust duration compared with the intake. Even cylinder heads with good E/I numbers will often produce better peak horsepower numbers with longer exhaust duration numbers mainly because of the limited amount of time available to push the exhaust out of the cylinder at very high engine speeds (above 7,000 rpm).
You also asked about flow numbers and the danger of comparing flow numbers of different heads flowed on different benches. Several years ago, we embarked on a quest to flow as many small-block Chevy cylinder heads as possible on the same flow bench while minimizing the variables between tests. While the most popular test depression is 28 inches of water, there are many other variables that can drastically affect flow numbers. The most obvious is fitting the exhaust port with a flow tube. This simulates the use of a header pipe and will often deliver 5 percent or more flow increases with no other changes. So when evaluating a cylinder head, you need to know whether the head was fitted with an exhaust-flow tube. It goes without saying that the intake port should always be fitted with a radius entry, but we still run across tests in which this is not done. Another favorite trick with flow-bench testing is to use a very large bore diameter. Again using our small-block Chevy example, if all we do is change from a 4.00-inch bore sleeve to a 4.155-inch bore in the flow bench, intake flow will especially increase by several percentage points. With a 250-cfm intake port, that could be worth 5 or more cfm. This is simply due to moving the cylinder wall away from the intake valve and allowing the air a less restrictive path in to the cylinder. This is not a flow-bench cheat. The flow bench is merely indicating what will happen if you place that cylinder head on a larger-bore engine. This is why big-bore engines are so popular in drag racing. The important thing to remember when comparing flow numbers is to make sure the heads you are comparing were all tested on the same bore adapter. If not, you need to take that into account in the comparison.
Each family of cylinder heads has its advantages and disadvantages. One reason the new-generation LS engines are so popular is because this family has tremendous flow potential on both the intake and exhaust side. This makes the heads very balanced components. The Mod motor Fords are also popular because of their multiple valves. Stuff two small intake and exhaust valves in the same-size cylinder bore, and you radically increase the flow area. This is commonly called the flow curtain and is a function of valve diameter x pi x valve lift. Let’s take an LS3 with a huge 2.16 intake valve at 0.600-inch valve lift. Valve diameter multiplied by pi equals the circumference of the valve: 2.16 x 3.1417 = 6.78 inches. Now let’s take that circumference and multiply it by the valve lift. That computes the flow area
One cylinder-head component that is not as easily measured is the shape of the combustion chamber. Newer-model heads tend to take advantage of chamber shape in order to improve combustion efficiency. There’s no test procedure for this, but CNC-machined chambers tend to be more consistent than as-cast chambers, and shallow, heart-shaped chambers tend to be more efficient than older, deeper chambers. Continuing with our small-block Chevy example, compare a ’60s-vintage 64cc “fuelie” chamber with an iron Vortec head and then with a current LS3 chamber design to get an idea of how the chamber has progressed. This can have a dramatic effect on engine performance, as a well-designed chamber requires less total timing. LS engines typically make best power with less than 30 degrees of total ignition timing. This may not seem important, but as you shorten the lead time of the initial spark, the engine reduces the negative work required to push the piston against increasing cylinder pressure as it approaches TDC. A difference of 6 to 8 degrees can be worth measureable horsepower, assuming, of course, that the chamber can take advantage of that shorter ignition lead time. We hope we’ve given you some information you can chew on to supplement your own fact-gathering efforts. As the famous rock song goes: “Is the knowledge gained worth the price of the pain?”
SuperFlow Performance; 800/471-7701; SuperFlow.com
Larry Soley; Istanbul, Turkey: I’m in Istanbul, Turkey, and it takes a while for me to get Car Craft. My wife forwards it from Wisconsin. The Apr. ’12 issue carried an article by Jeff Smith titled, “Junkyard Builder: Budget GM Rear Disc Brakes.” The closing sentence of the article (on page 74) reads, “Perhaps the biggest hassle in mounting this rear-brake conversion on an early car is pulling the rear cover to yank the C-clips so the rear axles can be removed.” Why do the axles need to be removed? There was no mention of this in the rest of the article. Thanks.
These C-clips fit in between the end of a C-clip axle and the inside of the side gears on
Jeff Smith: The axles have to be removed in order to remove the original drum-brake backing plates and to install the caliper mounting brackets. The drum-backing plates must be removed first—we just skipped that step.
A common complaint about rear-disc brake conversions is called piston knock-back, where excessive axle endplay pushes the caliper pistons into their bores, which creates a soft brake pedal that can cause major pedal travel before the rear-brake pistons regain their proper position. This is typically more of an issue with fixed-caliper brakes, but it’s worth discussing here because the fix is easy and cheap. We spoke with Tom at Tom’s Differentials, and he sells select-fit C-clips that can be used to set a minimal endplay clearance between the ends of the axles and the differential cross-shaft of 0.005 to 0.008 inch. These C-clips are high-strength forgings sold in various thicknesses. By varying the thickness of one or both clips, you can blueprint the endplay to the minimum spec and eliminate the problem of piston knock-back. The custom-fit clips are only $6 each.
Tom’s Differentials; 208/ 265-8111; TomsDifferentials.com
GM Crate-Engine Warranty Update
Kevin Shafer; Chevrolet (GM) Performance Parts specialist & Powertrain Contact Center consultant: In the May ’12 issue of Car Craft, Jeff Smith wrote an article called “Saturday Night Slayer.” The article uses the PN 10067353 universal 350. The article asks, “Who would give you a three-year /100,000 mile warranty and all new parts? GM does if you leave the engine stock.”
There are two things that are incorrect. The GM crate engine PN 10067353 only carries the three-year/100,000-mile parts and labor warranty if installed in ’72 to ’85 models that were available with a small-block. The engine could be used in a Monza or Monte Carlo but not in an S10 or a Vega. Jeff Smith intends to install this engine in a ’66 Chevelle, which is not covered under the 3/100 but would have the 12-month/12,000-mile “parts-only” warranty. Since you are using nitrous, the warranty would be void, and Jeff points this out in the article. But if a reader doesn’t use nitrous and installs it “out of application,” they would not receive the three-year/100,000 mile warranty. If the reader is installing the engine in an application outside of the ’72 to ’85 application, a wiser choice is to use the 290hp PN 12499529 application. This would carry a 24-month/50,000-mile parts and labor warranty in any street-licensed vehicle, as long as no power-adder was used. The 12499529 290hp engine is similar to the 10067353 engine, but with a different camshaft installed, so the basic foundation of the build would not be different.
The second item to clarify is how changing parts affects the warranty. On the 10067353 engine, the warranty would be void if you change items, because it is an “OE” engine. However, the warranty is not void if you change items on the 290hp engine, since it is a Performance Parts engine. In the article, Jeff changed the camshaft. If the reader changes the camshaft, the only portion of the warranty that would be void is if the failure were related to the camshaft or the change. For example, if the front-cover gasket starts leaking, this would not be covered, since the cover was removed for the cam swap. If the head gasket failed, this would be a covered warranty, since a camshaft swap does not disturb the cylinder heads. Of course, any warranty goes out the door with power-adders. The 12499529 290hp engine may be a wiser purchase for the majority of builders. This seems like a really cool “low-buck” article.
Jeff Smith: Thanks to Kevin Shafer, who works under contract with GM on warranty issues to make the GM warranty clear. I wasn’t aware that you could change the cam and still have the short-block under warranty. I also had not paid attention to the fact that the warranty applied only to the later-model GM vehicles. This makes choosing either the Goodwrench engine or the 290hp motor that much better of a deal. Keep in mind that the time factor often runs out long before the miles do. That means you should bolt the engine in the vehicle right away after purchase rather than waiting five years like some of us!
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