It has been 45 years since I was drag racing my Yamaha YDS2 250cc bike on nitro mix fuel turning high 13 second times in the quarter. So while dated, I've tweaked various 2-cycle and 4-cycle engines to improve their performance.
Now I'm rebuilding a Lawn-Boy F-series engine. I can't help thinking about tweaking again.
The first thing I noticed was that the reed valve did not seal tightly shut when I tried to pull a partial vacuum with my mouth on the intake bore leading into the upper crankcase. So I pulled the reed, checked for flatness and obstructions, but found no apparent defects. Still leaking. Next I flipped it side for side. No better.
Years ago we commonly applied various grit lapping compounds to "salvage" plate glass pieces we'd get from glass shops, then use that abrasive-covered surface to hand-lap cylinder heads and cylinder barrel tops to flat surfaces so they could fit together forming a gas-tight thermally-conductive sealed with only a little aluminum-particulate paste from the bottom an aluminum paint can. I thought about cutting and breaking a little glass piece to bridge across the reed valve opening surfaces, and that still might be worth trying. Instead, I just applied some lapping compound to the reed sealing side, then vigorously rubbed it in circles over the aluminum sealing surface, mounting surfaces and stand-off pillar between them. This would be a good task assignment for a child to perform while watching a TV show. Boring and slow going. Eventually it cleaned up and was displaying a uniform wear pattern. I also worked on that plane with the edge of a single-edge razor blade. On reassembly, pushing air into the cylinder with my mouth caused the reed to audibly buzz like a quiet "duck call." Great it was making music. Better yet, when I tried to pull a partial vacuum on the enclosed volume between the reed and the outside bore, it momentarily sustained a vacuum and did NOT allow perceptible flow. It's not sufficiently gas tight to make a good balloon seal, but there's no way it's going to let a fog flow backward into the carburetor. I see in the old OMC Lawn-Boy shop manual that they suggested reed clearance of no more than 0.015 inch. I'll bet crankcase pressure applied to reeds installed that way never seal nearly as well as this tweaked reed now seals. Naturally, any air/gas/oil back-blast past the reed into the carb reduces intake charging per induction cycle. So this little trick increases induction charging volume per stroke with all subsequent expected benefits.
Ambient air pressure near this planet's surface is typically about 14.7 psi. When a Lawn-Boy piston rises, gas pressure within its crankcase is pulled well below external ambient air pressure by piston-movement-volume displacement. Seals into that space are the piston, piston rings, piston-covering transfer ports, crankcase parting line seal, two crank end seals and the intensionally-leaky reed valve leading to the carb. We want all those seals to be leak free except the reed valve. Form discussion postings indicate that crank end seals commonly develop leaks. As I looked at the crankcase seal-capturing bore length, it appears to me that bore is probably long enough at each end to accept two rather than one seal. Humm. If replacing those seals, how about soaking them over night in some automatic-transmission-seal softener, then reuse each with a second new seal stacked above or below? Increased friction from two seals at each end would be trivial but increased sealing might be significant. Low cost but potentially useful benefit? Has anyone re-softened old Lawn-Boy crank seals with seal-softening solvents like those sold as automatic transmission fluid additives? Experience teaches that those softeners DO enable old hard seals to sometimes but not always resume performing their sealing task. Just a thought.
At least 50 years ago, 2-cycle engine crankcase "stuffing" was popular. The idea was to reduce crankcase volume so it would pump more inducted fuel/air/oil mix through transfer ports above the piston. Cork and other low-density solids were used to fill any voids not periodically occupied by moving parts. It turns out that securing cork and work-alike substitutes to fast-moving parts without having it self destruct from acceleration loads or disconnect and be chopped up by moving engine parts and spit out and burned within the exhaust system requires tweaking-designer ingenuity. Factory designers understand these tricks well. So they don't leave much empty space that can easily be stuffed. More stuffing and better sealing translates into increased fuel/air/oil mix precharging into the volume above the piston before it begins its partial-stroke compression.
Another tweak I've been considering. Years ago I paid high prices to have certain combustion chamber surfaces ceramic coated to reduce combustion heat loss. The ideal gas laws (combined Boyle and Charles) describes how gas temperature reduction with unchanged volume causes lower gas pressure. Gas pressure from combustion heat pushes the piston down which is how we get crankshaft power out. We want high piston pressure to get more work out, so we don't want to uselessly reject heat into the head. We learned that some "barbeque" high temperature paints would stay affixed to the combustion chamber surface of cylinder heads. Run it for 100 hours, pull the head and that silly paint was still there. Amazing. Since then, moderately-priced super-insulating micro balls have been invented which are being added to paints applied to roofs, greatly reducing sun heating loads into those roofs, thereby lowering air conditioning loads. Those micro balls are sold by on-line vendors and even eBay sellers. It seems to me that we could clean a Lawn-Boy cylinder top, then paint it with super-high heat rated paint to which super-insulating micro balls have been added. Instant pauper's priced combustion surface insulation. We've all seen how you can boil water in a paper cup over a flame without burning the paper because liquid water's maximum temperature at atmospheric pressure is only 212 degrees F., well below the flash point of paper. I know it sounds crazy, but I think that insulation barrier would survive Lawn-Boy combustion chamber top surface heat exposure just as water-filled paper cups survive flames. These engines need cylinder walls to stay cool enough to sustain a molecular lubrication barrier between rings and cylinder walls. Engine power production is reduced by pulling combustion chamber heat DIRECTLY into cylinder heads. As a matter of fact, I suspect that this cylinder-top insulation strategy might enable better cooling cylinder walls because they are formed as a single piece with the head. If the head is not DIRECTLY heated from its combustion chamber surface, it can accept more heat from the cylinder which it can then reject with its top air fins. Similarly, but different are piston surfaces. Piston temperature during sustained engine operation balances heat absorbed from the top combustion chamber-exposed surface against heat rejected into evaporation-cooled inducted air/fuel/oil mix below the piston. Insulating the top piston surface would rebalance piston temperature to a cooler temperature range and slightly increase combustion chamber pressure against the piston top, slightly increasing output power. More of the same insulating paint.
We used to apply thin high-thermally conductive flat black paint to cylinder fins. Standard charts have existed for at least 75 years showing that flat black surfaces reject heat better than any other of the many surfaces those labs tested. Decades ago, Cycle World published an article stating that a racing team's tests showed that sustained peak horsepower could be as much as 5% higher by running thin flat-black paint as compared to bright shiny aluminum painted fins. Shinny aluminum fins may appear better but flat-black aluminum fins reject heat better. Shouldn't we be flat-black covering our Lawn-Boy fins?
Some fuel additives reportedly cause combustion chamber, piston and piston-ring bonded carbon deposits to slowly detach, one molecule at a time, eventually revealing previously hidden clean piston and ring surfaces. Has anyone EXPERIENCED this effect with any Lawn-Boy engine? I'm not looking for advertiser claims. Actual observations, if they exist, are what we should share. Lots of product advertisers claim their products can do this. Most are clearly wrong.
Those little Lawn-Boy foam air filters restrict fast flows. I see from their DuraMax engine series that engine developers increased air cleaner size about 50% compared to their F-Series air cleaner. I'd feel more comfortable with a much larger air filter. Consider this: At 3200 rpm, each full Lawn-Boy engine cycle is 60/3200 = 0.01875 second duration. The intake stroke must occur during less than 50% of that time. Intake cycle charging opportunities only occur during those intermittent periods which last less than 1/100th of a second. We want a tight seal AROUND air filters to prevent unfiltered air entering the induction tract. But we would benefit by larger air filter surface fitted at least a little further up-stream from the intake bore leading into the carb. I've seen engines forming a back-blast fog visible away from carburetors caused by intake valve sealing failures. I expect that keeps foam air filters wet which would help capture particulates from intake air streams. If someone swapped in a paper air filter, that wetting effect from back-blast through the carb would also wet that filter and greatly increase its restriction. I'd rather run better sealing valves.
Your thoughts?
John
Now I'm rebuilding a Lawn-Boy F-series engine. I can't help thinking about tweaking again.
The first thing I noticed was that the reed valve did not seal tightly shut when I tried to pull a partial vacuum with my mouth on the intake bore leading into the upper crankcase. So I pulled the reed, checked for flatness and obstructions, but found no apparent defects. Still leaking. Next I flipped it side for side. No better.
Years ago we commonly applied various grit lapping compounds to "salvage" plate glass pieces we'd get from glass shops, then use that abrasive-covered surface to hand-lap cylinder heads and cylinder barrel tops to flat surfaces so they could fit together forming a gas-tight thermally-conductive sealed with only a little aluminum-particulate paste from the bottom an aluminum paint can. I thought about cutting and breaking a little glass piece to bridge across the reed valve opening surfaces, and that still might be worth trying. Instead, I just applied some lapping compound to the reed sealing side, then vigorously rubbed it in circles over the aluminum sealing surface, mounting surfaces and stand-off pillar between them. This would be a good task assignment for a child to perform while watching a TV show. Boring and slow going. Eventually it cleaned up and was displaying a uniform wear pattern. I also worked on that plane with the edge of a single-edge razor blade. On reassembly, pushing air into the cylinder with my mouth caused the reed to audibly buzz like a quiet "duck call." Great it was making music. Better yet, when I tried to pull a partial vacuum on the enclosed volume between the reed and the outside bore, it momentarily sustained a vacuum and did NOT allow perceptible flow. It's not sufficiently gas tight to make a good balloon seal, but there's no way it's going to let a fog flow backward into the carburetor. I see in the old OMC Lawn-Boy shop manual that they suggested reed clearance of no more than 0.015 inch. I'll bet crankcase pressure applied to reeds installed that way never seal nearly as well as this tweaked reed now seals. Naturally, any air/gas/oil back-blast past the reed into the carb reduces intake charging per induction cycle. So this little trick increases induction charging volume per stroke with all subsequent expected benefits.
Ambient air pressure near this planet's surface is typically about 14.7 psi. When a Lawn-Boy piston rises, gas pressure within its crankcase is pulled well below external ambient air pressure by piston-movement-volume displacement. Seals into that space are the piston, piston rings, piston-covering transfer ports, crankcase parting line seal, two crank end seals and the intensionally-leaky reed valve leading to the carb. We want all those seals to be leak free except the reed valve. Form discussion postings indicate that crank end seals commonly develop leaks. As I looked at the crankcase seal-capturing bore length, it appears to me that bore is probably long enough at each end to accept two rather than one seal. Humm. If replacing those seals, how about soaking them over night in some automatic-transmission-seal softener, then reuse each with a second new seal stacked above or below? Increased friction from two seals at each end would be trivial but increased sealing might be significant. Low cost but potentially useful benefit? Has anyone re-softened old Lawn-Boy crank seals with seal-softening solvents like those sold as automatic transmission fluid additives? Experience teaches that those softeners DO enable old hard seals to sometimes but not always resume performing their sealing task. Just a thought.
At least 50 years ago, 2-cycle engine crankcase "stuffing" was popular. The idea was to reduce crankcase volume so it would pump more inducted fuel/air/oil mix through transfer ports above the piston. Cork and other low-density solids were used to fill any voids not periodically occupied by moving parts. It turns out that securing cork and work-alike substitutes to fast-moving parts without having it self destruct from acceleration loads or disconnect and be chopped up by moving engine parts and spit out and burned within the exhaust system requires tweaking-designer ingenuity. Factory designers understand these tricks well. So they don't leave much empty space that can easily be stuffed. More stuffing and better sealing translates into increased fuel/air/oil mix precharging into the volume above the piston before it begins its partial-stroke compression.
Another tweak I've been considering. Years ago I paid high prices to have certain combustion chamber surfaces ceramic coated to reduce combustion heat loss. The ideal gas laws (combined Boyle and Charles) describes how gas temperature reduction with unchanged volume causes lower gas pressure. Gas pressure from combustion heat pushes the piston down which is how we get crankshaft power out. We want high piston pressure to get more work out, so we don't want to uselessly reject heat into the head. We learned that some "barbeque" high temperature paints would stay affixed to the combustion chamber surface of cylinder heads. Run it for 100 hours, pull the head and that silly paint was still there. Amazing. Since then, moderately-priced super-insulating micro balls have been invented which are being added to paints applied to roofs, greatly reducing sun heating loads into those roofs, thereby lowering air conditioning loads. Those micro balls are sold by on-line vendors and even eBay sellers. It seems to me that we could clean a Lawn-Boy cylinder top, then paint it with super-high heat rated paint to which super-insulating micro balls have been added. Instant pauper's priced combustion surface insulation. We've all seen how you can boil water in a paper cup over a flame without burning the paper because liquid water's maximum temperature at atmospheric pressure is only 212 degrees F., well below the flash point of paper. I know it sounds crazy, but I think that insulation barrier would survive Lawn-Boy combustion chamber top surface heat exposure just as water-filled paper cups survive flames. These engines need cylinder walls to stay cool enough to sustain a molecular lubrication barrier between rings and cylinder walls. Engine power production is reduced by pulling combustion chamber heat DIRECTLY into cylinder heads. As a matter of fact, I suspect that this cylinder-top insulation strategy might enable better cooling cylinder walls because they are formed as a single piece with the head. If the head is not DIRECTLY heated from its combustion chamber surface, it can accept more heat from the cylinder which it can then reject with its top air fins. Similarly, but different are piston surfaces. Piston temperature during sustained engine operation balances heat absorbed from the top combustion chamber-exposed surface against heat rejected into evaporation-cooled inducted air/fuel/oil mix below the piston. Insulating the top piston surface would rebalance piston temperature to a cooler temperature range and slightly increase combustion chamber pressure against the piston top, slightly increasing output power. More of the same insulating paint.
We used to apply thin high-thermally conductive flat black paint to cylinder fins. Standard charts have existed for at least 75 years showing that flat black surfaces reject heat better than any other of the many surfaces those labs tested. Decades ago, Cycle World published an article stating that a racing team's tests showed that sustained peak horsepower could be as much as 5% higher by running thin flat-black paint as compared to bright shiny aluminum painted fins. Shinny aluminum fins may appear better but flat-black aluminum fins reject heat better. Shouldn't we be flat-black covering our Lawn-Boy fins?
Some fuel additives reportedly cause combustion chamber, piston and piston-ring bonded carbon deposits to slowly detach, one molecule at a time, eventually revealing previously hidden clean piston and ring surfaces. Has anyone EXPERIENCED this effect with any Lawn-Boy engine? I'm not looking for advertiser claims. Actual observations, if they exist, are what we should share. Lots of product advertisers claim their products can do this. Most are clearly wrong.
Those little Lawn-Boy foam air filters restrict fast flows. I see from their DuraMax engine series that engine developers increased air cleaner size about 50% compared to their F-Series air cleaner. I'd feel more comfortable with a much larger air filter. Consider this: At 3200 rpm, each full Lawn-Boy engine cycle is 60/3200 = 0.01875 second duration. The intake stroke must occur during less than 50% of that time. Intake cycle charging opportunities only occur during those intermittent periods which last less than 1/100th of a second. We want a tight seal AROUND air filters to prevent unfiltered air entering the induction tract. But we would benefit by larger air filter surface fitted at least a little further up-stream from the intake bore leading into the carb. I've seen engines forming a back-blast fog visible away from carburetors caused by intake valve sealing failures. I expect that keeps foam air filters wet which would help capture particulates from intake air streams. If someone swapped in a paper air filter, that wetting effect from back-blast through the carb would also wet that filter and greatly increase its restriction. I'd rather run better sealing valves.
Your thoughts?
John
