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Expert Engine Building Tips

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

Stephen Kim Jul 17, 2006

The numbers are just beyond comprehension: nearly 1,400 hp from just 500 ci. No blowers. No alky. Not even a secret pact with the devil to make it all happen. At 2.8 hp per cube, a Pro Stock motor trounces the specific output of even a Nextel Cup engine. To maintain a competitive edge in this cutthroat arena, the dyno rooms wail nonstop. Success is measured in fractions of a pony at an outrageous cost of $38,000 for each additional horsepower found on the dyno. Despite the daunting task of improving upon the pinnacle of domestic V-8 engineering, the team at Reher-Morrison Racing Engines in Arlington, Texas, miraculously extracts another 10-15 hp each year.

Leading that foray is cylinder-head and induction specialist Darin Morgan. Having both a father and a stepfather who built engines for a living, as a kid Darin got a double shot of paternal gearhead influence. He started spending ountless hours on the flowbench at the age of 16 and took a job with Bob Glidden building motors a few years later. He then jumped ship to Larry Morgan's shop and eventually ended up at Reher-Morrison's R&D department. Although his official job duties involve developing cylinder heads, his expertise in overall engine design and theory ranks second to none. So here's his take on some misconceptions running rampant out there, along with some insight on advanced engine theory.


Rod Length
"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero diff-erence in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."


Setting Clearances
"Going too tight on clearances such as piston-to-valve, piston-to-head, piston-to-wall, and main and rod bearings will kill you every time," says Darin. "On the other hand, there are no drawbacks to being too loose except for a little more oil up top as bearing and side clearances increase. That can be counteracted pretty easily so it's not a big issue." If the ring tension, package, or design or cylinder hone is inade-quate, looser clearances will exacerbate oil control problems; in that situation it's not the fault of the clearances. Reher-Morrison's mantra is that loose is good and looser is better. Factory engines have tight clearances because their operating temperature and rpm range are such that components aren't stressed as much, thermally or mechanically, to need extra clearance. Racing engines see much more stress since they start running hard at 6,500 rpm, where stock engines are already out of steam.


Cylinder Wall Finish
Honing is a lot of science and a little bit of black art. One particular hone isn't best for all blocks or ring packages, but smoother isn't necessarily better for a street engine or a race engine. Different types of blocks vary on the Rockwell hardness scale. Harder blocks want a softer finish and softer blocks prefer a harder finish. Of all the mistakes an engine builder can make, going too smooth is more detrimental to power than anything else. There must be a balance of having enough surface area to properly seal the rings, but not so much that it compromises oil retention and ring lubrication. "Too smooth a finish will increase surface area to the point where there's not enough oil retention, and ring wear and frictional power loss will go up significantly," Darin explains. "That's why racing engines usually have a very deep plateau finish to hold a lot of oil with as little surface area as possible for the rings to ride on." The difference between the right and wrong hone on a high-end engine (4,500-5,000 fps mean piston-speed range) is 25-30 hp. The power loss from having the wrong hone isn't as noticeable on a street engine, but ring wear will go up dramatically.


Second Ring Gap
On a typical naturally aspirated motor, the top ring gap should be bigger than the second ring gap. "There are situations where opening up the second gap yields extra power," says Darin. "That's because a lot of blow-by is getting past the first ring, trapping too much gas between the ringland, creating ring flutter, and unseating the second ring." Preventing this condition requires a properly designed piston with an accumu-lation groove, in which case a bigger second ring gap isn't needed. However, nitrous and forced-induction motors can benefit from a bigger second ring gap.


High octane means slow burn, and you should always run the minimum octane level you can get away with. Octane is simply a fuel's resistance to detonation, and a higher-octane fuel must be com-pressed more to burn at the same rate as a lower-octane fuel. Likewise, a faster-burning fuel always makes more power up top. "An engine is tuned to fuel, not the other way around," says Darin. "If you pick up power on race gas, it means your engine combo is more suited to that fuel. Sometimes you can find that magic fuel that burns better for your combination, but it's not one size fits all. When they changed our fuel in Pro Stock, everything about our motors--including combustion chamber design, compression ratio, and cam specs--had to be changed."




Iron VS. Aluminum
When pushed to the extreme, it's pretty safe to say that the only advantage of an aluminum block over an iron block is weight savings. "Keeping all else equal, we take a 30-45hp hit in our aluminum motors compared to our iron motors." The rate of thermal dissipation is higher, but that only accounts for 10 percent of total power loss. "The bigger issue is that cylinder-wall retention is impossible to maintain. With aluminum, the cylinder walls can move around, ring seal is terrible, and leakdown is always worse." However, if a thicker cylinder wall is used in the block, the trade-off wouldn't be as bad and the weight savings could make up for the difference in power. Reher-Morrison compensates for this by putting bigger cams in its aluminum motors and by winding them out a little higher.


"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."


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."


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.


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."


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.


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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