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.
Octane
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.