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Turbo Drag Car - Ask Anything

Big Ports vs. Small Ports
Mike Smith; via CarCraft.com: I decided to do a motor build for my hot rod based on your Late Crate Update series. I called my local GM dealer to order the GMPP L92 CNC heads that are getting such rave reviews (PN 88958698). Imagine my dismay when they told me that those heads have been discontinued. A nationwide parts-dealer search showed none available anywhere, and every online store I have contacted has told me the same sad story—they are unavailable. At this point, I am stuck because I based my engine build budget on these heads. Are you aware of any future plans for GMPP to sell a comparable setup? How can they just discontinue the most cost-efficient head available with no warning? We deserve answers. Thanks in advance and I absolutely love your magazine.

Jeff Smith: While the heads were a very good deal for a CNC-ported cylinder head, there are other solutions. Rather than getting into why the head was discontinued, let's look at what those heads really delivered. In my engine buildup detailed in "The Garage-Built LS Stroker Part II" (Feb. '10, pg. 22), we flow-tested four LS heads, including the stock 6.0L castings, a set of West Coast Racing Cylinder Head CNC-ported 6.0L heads, a set of stock L92s, and the CNC-ported GMPP L92 heads. While the ported L92 heads took the win for the highest flow of all those heads tested, the results need closer inspection. Sure, these heads flowed 337 cfm, but that also occurred at 0.700-inch lift. How many street engines run a 0.700-inch- lift cam? For a street engine, even with the LS engine's power potential, this is akin to using huge 360cc rectangle-port cylinder heads on a 396ci big-block. A smaller, properly sized intake port produces much greater velocity, which will produce more torque in the usable rpm band for a street engine. Many enthusiasts get caught up in maximum flow numbers at peak valve lift. While peak flow numbers make for good bench-racing fodder, it's best to look at the midrange flow numbers of around 0.400 valve lift when considering a street cylinder head. Larger flow numbers in the midrange tend to support excellent midrange torque. Large- port heads tend to deliver impressive peak valve-lift numbers that may or may not be attainable. For example, the L92 heads can deliver some huge flow numbers at 0.700-inch valve lift, but few street engines achieve more than 0.600-inch valve lift, so the flow potential at 0.700-inch valve lift is superfluous.

Generally, a smaller port won't flow as much air as a larger port, but a cathedral-port head that will produce acceptable numbers of around 280 cfm at 0.600-inch lift will produce similar if not better overall power. Why is midrange power important? Because this torque is primarily responsible for accelerating the car down the quarter-mile, while peak horsepower helps create the ultimate trap speed. It's the combination of torque and horsepower that makes the car quick, but because virtually all street cars are under-geared for ultimate quarter-mile acceleration, the engine spends more of its time in and around peak torque as opposed to peak horsepower. So by building an engine with stronger torque in the midrange, you will have a quicker car that also is hammer-flat fun to drive.

We decided to go looking for a pair of cathedral-port heads with equal or better flow compared with the CNC-ported L92 heads. We found a set of Trick Flow Specialties Gen X 225 CNC-ported heads that offer a much smaller intake port (compared with the L92) yet with better flow. While the intake side of the TFS head offered measurable flow increases throughout the valve-lift window, the real difference was on the exhaust side, where at 0.400-inch valve lift, the TFS head flows an astounding 45 cfm more than the stock L92 head. This means you can run a single-pattern cam with the TFS head, while the L92 head requires a cam with as much as 12 to 14 degrees more exhaust duration. This also means the cathedral-port head at the same flow will automatically increase the intake port velocity. The downside of all this better technology is that the TFS CNC- ported heads cost $2,395.95 compared with roughly $1,200 for the stock L92 heads.

As a budget alternative, you could go with a CNC-ported production head. Several companies offer this service. One shop is West Coast Racing Cylinder Heads (WCRCH), which can deliver some impressive flow numbers for an affordable price. If you supply the cores, the WCRCH Stage 2 porting service will deliver a set of heads that flow slightly less than the CNC-ported L92 heads through the midrange valve lift on the intake but offer outstanding exhaust flow all the way through the lift curve with as much as a 21-cfm improvement at 0.400-inch. Again, this better exhaust port means using a single- pattern cam to take advantage of how well the exhaust works. The price for all this is $1,575, which is only a few bucks more than the L92 heads—and the potential for more torque in the middle is excellent. We hope this will get you to start thinking about cathedral-port heads as something more than just low-performance stockers.

More Info
West Coast Racing Cylinder Heads; Reseda, CA; 818/705-5454; ProHeads.com
Trick Flow Specialties (TFS); Tallmadge, OH; 330/630-1555; TrickFlow.com

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1 comments
The Don-#002
The Don-#002

good article, running a W2A myself as the FMIC was not a option i really considered.theres been heaps of discusion on the various types of recent

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