CLICK BELOW TO SEE ALL OF THE STORIES COVERING THE BUILD OF PROJECT DANGER MOUSE
DANGER MOUSE PART 1
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DANGER MOUSE PART 4
DANGER MOUSE PART 5
DANGER MOUSE PART 7
DANGER MOUSE PART 8
DANGER MOUSE PART 9
DANGER MOUSE PART 10
DANGER MOUSE PART 13
DANGER MOUSE PART 14
DANGER MOUSE PART 15
DANGER MOUSE PART 18
DANGER MOUSE PART 20
DANGER MOUSE PART 22
DANGER MOUSE PART 23
DANGER MOUSE PART 24
DANGER MOUSE PART 25
No matter how much we love them, our engines are still basically inefficient air pumps and need all the help they can get to make power. Like any other type of pump, if it leaks, it won't work. And since it's the pistons that are doin' the pumpin', if they're allowed to flop around in the bores, they'll leak and little power would be the result. Not only would power suck, the pistons, along with the rest of the engine would quickly destroy itself.
Your rings may look like ordinary bands of steel, but they are far from simple in their design and construction. They work to locate the pistons in the bores while sealing the cylinders for power and directing oil to the proper spots to keep the engine alive. Ring thickness, tension, shape, material, and profile all play a crucial role in cylinder sealing, power making, and oil control.
The Fire RingThe top ring's main job is sealing the combustion process and many refer to it as the "fire" ring. The top ring works in a dual capacity by holding high vacuum as the piston moves down its bore and by keeping cylinder pressures high when the combustion process begins. Ring gap is critical and, if left uncontrolled, much of the combustion gasses would simply escape into the crankcase. Anytime you allow cylinder pressure past the rings, (otherwise known as "blow by"), you are losing horsepower.
Utilizing a two-piece, overlapping ring design with their gaps placed 180 degrees apart; gapless rings are engineered to eliminate as much blow by as possible. Obviously, a small amount of combustion gasses will always leak past the top ring, but that is helpful in creating the proper seal for the second ring. Performance top rings will usually be marked to identify which side faces up in the bore. That's because the inner diameter of the top ring is tapered to force the rings out and increase the seal against the cylinder walls as the piston goes up the bore. If installed upside-down, the top ring will lose power.
The Second RingThe second ring serves as a back up to the top ring in sealing the crankcase from the combustion process. But, more importantly, it acts as an oil scraper to keep any oil that has migrated past the oil control rings from seeping into the combustion chamber. The second ring usually features a smaller gap than the top ring, due to the lower amount of heat and correspondingly less expansion it will see. In almost every case, cast iron is the material of choice for the second ring and it will typically not have a face coating.
The fact that the top ring absorbs the majority of combustion abuse allows a cast iron ring to be used in the second groove of almost any engine. Second rings are designed several ways; with the reverse-torsional taper face being the most common. The taper face allows the ring to ride over the film of oil on the upstroke and scrape oil off the cylinder walls on its way down. The other basic second ring design is the wiper ring that has a groove cut on its bottom face to wipe oil down.
The Oil RingThe last ring set is just as important as the rest, yet often ignored. The oil rings are critical to maintaining engine life, as well as controlling detonation. Oil rings are very thin chrome-faced stainless steel bands with stainless steel expanders in between. Oil rings do not need to be gapped, but their fit should be checked prior to installation to make sure the ends don't butt together. Both standard and low-tension oil rings are available. However, low-tension oil rings should only be used in engines equipped with an external vacuum pump. Otherwise, excess oil may seep into the combustion chambers where it could ignite and cause detonation.
Performance RingsOriginal equipment engines are usually equipped with uncoated cast-iron rings and considering how long most factory engines last, many good things can be said for their durability. The cast-iron ring's biggest advantage over other sets is its low cost and wide availability. Another advantage is that a cast ring is very easy on cylinder walls and has great wear characteristics. The main disadvantage to a cast-iron ring is that it may break if subjected to the rigors of high-compression and/or detonation. Cast-iron rings can safely be used in the top groove of engines up to 11:1 compression.
The first upgrade to a cast-iron ring is applying a molybdenum or "moly" coating to the ring. A moly coating slows the cylinder-wall wear rate and reduces friction. Moly also has the highest melting point of any of the face coatings used, but can flake off if the engine experiences severe detonation. Moly-faced rings work extremely well in high-rpm or high-load conditions. The main disadvantage of moly-faced rings comes into play in dusty or highly abrasive conditions because its porosity can trap dirt in the face of the ring. This is a good reason to always run an air cleaner.
After moly-faced rings comes cast rings with a hard chrome facing. The chrome, like moly, is applied to the ring and although it displays better adhesion characteristics and fewer tendencies to flake than moly, it cannot withstand the same high temperatures and does not have the high resistance to scuffing like a moly-faced ring. Chrome is very hard making it the best choice for engines run in highly abrasive environments. Chrome-faced and moly-coated cast rings both require a specific bore finish to seat properly and may not actually last any longer than an uncoated ring.
Racing RingsDuctile-iron rings rank considerably higher on the performance ladder than cast-iron rings. Ductile-iron rings can also be coated for good break-in and wear characteristics, but they cost more. Ductile iron is probably the most widely used ring material in racing engines today because it can take the abuse of higher compression and detonation. That's because ductile iron does not shatter or break like cast iron. If you were to bend both a cast-iron and ductile-iron ring by hand, the cast ring would snap, while the ductile ring would merely flex.
Ultra-high-performance racing rings are usually manufactured from one of two different steels: stainless or H-11 tool steel. Both of these steels are very durable but can take a long time to seat and can wear the cylinder walls. Most of these rings are chrome-faced, although some are available nitrided. Stainless and tool steel rings work well in engines that will see high heat from turbos, superchargers, or large doses of nitrous oxide. Steel rings also have the highest tension.
A widely used racing ring material is HF479 iron composite. Most ring manufacturers are currently offering HF479 race rings specifically geared towards very high-compression and/or nitrous racing engines. The HF479 material is a special ductile iron with Ni-chrome plasma facing that delivers unmatched wear and flaking. Although HF479 rings may be the best race rings going, they're not available to fit standard rings grooves or in small bore sizes which makes them less than ideal for your street car.
Gapping For PowerOddly enough, even "gapless" rings still have end gaps. To be effective, the proper ring gap is more dependent on what usage the engine will see rather then on the rings themselves. Generally, a top ring gap of at least 0.004-inch per 1 inch of cylinder bore is sufficient on a normally aspirated street engine. If you do the math on a typical small-block that has been bored 0.030 inch over, (0.004 x 4.030) you'll find that a top ring gap of 0.016 inch is what to use. However, it's wiser to increase that factor by about 0.002 making 0.018 inch your minimum top-ring gap. This compensates for piston and bore tolerances that could be a little off. As engine demands increase, so should the ring gap.
Since the second ring is not responsible for the majority of combustion sealing, it will not see as much heat and consequential expansion. Therefore, the second ring can run a smaller gap than the top ring. The same small-block street engine described earlier would do well with a second ring end gap between 0.010 and 0.012 inch (0.0025 inch per inch of bore).
Ring sets for mild-performance engines can be purchased in pre-fit packages, which will not require grinding the end gaps to fit. But any serous engine will run custom, file-fit rings. Generally, a file-fit ring set will be 0.005-0.015 inch oversize, requiring the builder to check, custom file, and then recheck each individual ring set in each specific bore. Many aftermarket companies offer piston ring grinding tools that can make the task of file-fitting rings much easier. Total Seal offers its own battery-powered ring grinder that makes custom ring filing fast and even kinda fun. The old fashioned method of dragging the ring across a mill-file clamped in your bench vise is highly inaccurate. If you're planning on file-fitting your rings, a ring grinder is a good investment.
The most important factor to keep in mind when choosing a ring set or deciding how much gap to run, is your engine's ultimate destination. Don't buy the high-dollar racing ring set for your mild motor because it may not make any more power, or last any longer than a stock ring set would. However, don't try to save a few bucks here either if you're building a motor that'll see some abuse, spend a few extra bucks on a good ring set. Many of today's street engines might see a dose of nitrous oxide, too; and as such could make good use of a ductile-iron ring set. The difference in price is negligible, but the difference in performance is immeasurable.
Dyno Testing Part 20This month's test is being compared to Part 19-Test 34 from last month. For Part 20-Test 35, Speed-O-Motive replaced the standard-gap moly rings we were running before with a set of ductile iron, file-fit gapless top rings from Total Seal. The power increase we found was outstanding, considering that the gapless rings only cost about $100 more than standard set we were using. We also learned a few things about tuning for the lack of a ring gap and how it affects power (see: Tuning The Gap).
Danger Mouse specs from Part 19-Test 34 (last test from last month):355 cid, 10:1 cr, 4.030-bore 4-bolt Motown block, 3.48-stroke Lunati crank, 5.7-inch Lunati rods, Edelbrock Victor Jr. heads (64cc chambers, 215cc runners, 2.08-inch intake valves, 1.60-inch exhaust valves), Victor Jr. intake, Mighty Demon 650 carb, COMP Cams Xtreme Energy solid roller camshaft installed at 105 intake CL (242/248 at .050, 280/286 adv, .600/.606 lift with COMP 1.6:1 rockers, 110 LS), 38 degrees total ignition advance Danger Mouse specs for Part 20-Test 35:Same as above, but with Total Seal ductile-iron gapless top rings (PN M3690 35)
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