Expert Engine Building Tips

Expert Engine-Builing Tips & Advanced Theories Straight From Darin Morgan of Reher-Morrison

Stephen Kim Jul 17, 2006 0 Comment(s)
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"At the Pro Stock level, rpm is the single biggest power maker. All we want is more rpm, and every addi-tional 100 rpm we can wring out of the motor is worth 20 hp. Although valvetrain technology is catching up, we still can't get enough rpm to take advantage of what our cylinder heads flow. The current limit of steel valve-springs is 83-85 cycles per second, which is about 10,000 rpm on a four-stroke engine. As for maximum rpm for an engine that runs for any appreciable length of time, 9,000 is really pushing it. If valve weight was cut in half we could gain another 1,000 rpm, but while it's feasible from a technology standpoint, it isn't feasible financially. There really isn't much to be gained by reducing spring pressure, either. It's true that high spring pressures rob power, but it's so minuscule, who cares? If you didn't have spring pressure, you'd have no power at all. At 9,800 rpm on the Spintron, a Pro Stock engine's valvetrain takes 126 hp to turn. Reducing spring pressure too much can result in a resonant frequency condition or spring surge, which actually increases that power loss to 137 hp. So if the springs go crazy from having too little pressure, it costs even more power. Since having the right spring pressure is crucial to making power, how much horsepower it robs is really a moot point."

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How much thermal coatings increase power depends on how well a motor is designed and built. "In all the testing we've done on our Pro Stock and Comp Eliminator engines, coating the piston crowns and combustion chambers has not proven to be worth lots of power, maybe 6-8 hp at most," Darin explains. "However, on an inefficient engine where thermal efficiency is lacking due to poor chamber design, cam selection, or poor inlet charge mixture motion, coatings can help quite a bit and give you 12-15 hp." In other words, a poorly designed motor--one with too much cam or too much port cross-sectional area--stands to benefit more from coatings than a properly built motor. There is a big difference between thermal coatings and lubricity coatings. "The Casidium coatings we use on our bearing and wrist pins have really saved us, and we put that stuff on everything. They let you get away with running half the oil that you ran before and allow tightening up the ring package as well."

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Big-Inch Strokers
Today's hot rodders are infatuated with cubic inches, and there's a huge trend to build larger and larger motors. However, with superior and readily available aftermarket block designs, it really doesn't make sense anymore to build a high-end motor with a stock block. "After you've put all that work into it, you still can't put a big bore or crank in it, and it's still a stock block," Darin opines. "If you want to build a 468- or 500ci deal and do most of the work yourself, you can probably get away with it. Now if you want to build a 900hp big-block, it's just not cost effective to use a stock block." As far as crankshafts are concerned, long strokes are inherently more unbalanced because their counterweights are farther away from the crank centerline. "You can compensate for a lightweight rotating assembly by turning down the counterweights and adding heavy metal to get the weight closer to the center of the crank.

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Bore VS. Stroke
"An oversquare motor is absolutely superior at high rpm compared to a long-stroke motor of the same displacement. Formula One and IRL motors are hyper-oversquare, meaning the bore is twice the size of the stroke. If I could do that in a Pro Stock motor, I'd do it overnight, and the power would go up accordingly. Once piston speed hits a certain range, frictional power losses sky-rocket. By decreasing stroke and increasing bore, you're not only dragging the rings up and down the bore that much less, but you also have less windage, the crank counter-weights get smaller, rod angularity decreases, deck heights get shorter, and the induction system package looks a lot better. With a bigger bore and a shorter stroke in a high-end engine, you don't need a tall-deck block. That lets you move the valve-train closer to the deck, shorten the pushrods, reduce the resonance frequency of the pushrods, and wind the motor higher with less valvetrain flex. In a high-end engine, you always use the biggest bore and shortest stroke you can get. However, there are some exceptions to the rule and you have to look at the entire engine package as a complete system."

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Compression Ratio
A higher compression ratio will always yield more pressure and power up to the point of the fuel's limitation, but more isn't always better. Engines generally reach a point of diminishing returns at about 1-1.5 points before detonation. "Pro Stock engines operate at 130-percent volumetric efficiency, so dynamic compression is very high even with a static compression ratio of 14.9:1," says Darin. "We can run them up to 16.0:1 and see no power change, but at that point they start nipping spark plugs and ringlands." Horsepower gains due to increasing compression hit a plateau a few points before detonation because a dyno can't simulate the rising loads and heat conditions an engine experiences in a car.

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Finding a Machine Shop
Getting quality machine work is always in your best interest. If you get sloppy machine work it means everything you do will be for nothing, but finding a good shop shouldn't be hard to do. "Just find someone whose reputation spans a decade or longer who builds high-end race engines." Find out who runs fastest at the track, and their machinist is probably the guy to go to. "I hate to say it, but while paying top dollar doesn't mean you'll get the best work, a shop that can get away with charging top dollar probably knows what it's doing."

Block Prep
Cleaning the block is an important step before assembling an engine, but there is much more involved in block prep-aration than spraying it down with soap and water. And knocking off sharp edges and casting flash isn't just for aesthetic purposes. "People wonder why they can build an engine in a clean environment and still end up with scratches on the bearings when they take it apart," says Darin. "Even though an engine has been run for a hundred-thousand miles, there can still be sand and lots of other nasty stuff embedded in there." So particular attention should be given to removing casting flash in the crankcase area. After washing the block, Darin suggests spraying it with WD-40 while it's still wet.

Sonic Testing
It's a procedure lots of people skip and never think twice about, but every block should be sonic checked to ensure adequate cylinder-wall thickness all around. Stock blocks can vary 0.040-0.080 inch in wall thickness and need to be checked for core shift everywhere, including all bosses, lifter bores, cam core plugs, and the pan rail. "We have to keep the foundry on their toes," says Darin. "Some GM DRCE blocks are awesome and are kept within 0.003-0.004 inch of tolerance, but we send high-end blocks back all the time."

Bushed Lifter Bores
Anything above 0.485-inch lobe lift and 284 degrees of cam duration is about as much as a stock cast-iron lifter bore can handle. The primary advantage of a bronze lifter bushing is its superior wear characteristics and resistance to load over steel. "Lifter bushings help straighten the bore and allow moving the intake bores 0.050-0.060 inch over to the side. That straightens out the pushrods on a spread-port engine. Instead of using an offset lifter and side-loading the lifter real hard, it allows you to keep the same pushrod angle while using a centered lifter cup, which disperses load over a wider area."


Reher-Morrison Racing Engines
Arlington, TX 76017


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