What if thread

[andrewddrennon]

Some what if questions?

What happens when you advance timing past factory specifications....just wondering if anyone could give me a advanced explanation of what goes on in the combustion chamber if you advance the timing to far. Im aware of overheat, bearing failure from to many rpms, possible pre ignition. My real question is what happens when you advance the spark so far that that fuel isnt compressed enough to combust. Wondering if anyone has ever jacked with a old 2 stroke j/rude to see how much horsepower you could get out of one, if you had a better cooling system, bearings, crankhaft, lubrication, higher compression, higher octane fuel. The stuff needed to run say 10,000 rpm without fail....for a run or two. This is a what if.......im very aware you could burn up many engines before finding any success

Do people race larger outboards on bass boats anywhere?

 

[durban]

Re: What if thread

every thing is based on the crank shaft u need to start there first .

 

[Chriswalden]

Re: What if thread

There is a fine line in engine timing with any engine. Look at like a graph with HP along the side and engine timing in degrees before and after TDC center along the top. As we start from the lower left hand corner of the graph you have low HP with a retared timing. As you increase that timing your horse power will increase well into 2+ degrees BTDC. There is a peak power output of any engine, and increasing much above and beyond manufacturer's recommended timing will actually yield less power. The earlier the ignition occurs in the combustion chamber, the less efficient the engine becomes because you are igniting the fuel air mixture before it has the opportunity to compress. Therefore you actually begin to fight the piston on its compression stroke. This is where you start getting into hot reeds, feathered reeds, burning cylinder ports and a host of other engine damage. Hope this helps.

 

[F_R]

Re: What if thread

There is a fine line in engine timing with any engine. Look at like a graph with HP along the side and engine timing in degrees before and after TDC center along the top. As we start from the lower left hand corner of the graph you have low HP with a retared timing. As you increase that timing your horse power will increase well into 2+ degrees BTDC. There is a peak power output of any engine, and increasing much above and beyond manufacturer's recommended timing will actually yield less power. The earlier the ignition occurs in the combustion chamber, the less efficient the engine becomes because you are igniting the fuel air mixture before it has the opportunity to compress. Therefore you actually begin to fight the piston on its compression stroke. This is where you start getting into hot reeds, feathered reeds, burning cylinder ports and a host of other engine damage. Hope this helps.

That's a pretty good explaination. I look at it in a simple light. It takes TIME to light a fire, whether it be in your fireplace or a combustion chamber. You want that fire to be going good as the piston is at the top of the stroke, and continues to burn as it pushes the piston down. For that reason, you need to light the fire before the piston is at the top----timing advance. But light it too soon, and the fire tries to push the piston down while it is still on the upstroke. That causes high presure within the combustion chamber (think very high compression). That extra high compression causes the fuel to explode rather than burn. That is the ping or knock you hear. Old timers called it "spark knock" in old cars because the driver had control of the spark timing and if he advanced it too far, it would knock. Now they call it pre-ignition. Yes, I'm actually old enough to remember the spark advance on the steering wheel but i was too young to drive.

Anyhow, things haven't changed all that much. Too much spark advance still causes pre-ignition, which will try to push the piston down while it is still on the upstroke, actually slowing the rpm rather than increasing it. It also still causes the fuel to exlode rather than burn, and that is what melts and burns the metal.

BTW, high octane gas ignites slower than low octane. That's why it doesn't detonate in high compression engines. High compression makes more power IF the fuel doesn't detonate. Also the reason there is no advantage in running high octane fuel in a an engine that doesn't require it for high compression.