SPI.............

Surfer_kris wrote:

rsv1cox wrote:Often I read here referances to SPI Split Port Induction but I do not see how the term applies to two cycle Cox or other two cycle model engines.  Does it apply to the boost ports along side the induction flutes?  

I get sub-piston induction as applied, but SPI?  Maybe one in the same?

SPI stands for "sub piston induction" nothing else. Many two-stroke engines of other makes have that too.

"Nods" A good example is the chainsaw engine. The only real difference between SPI in our Cox engines and the chainsaw or other types is the type Cox uses is "Fresh air SPI" which introduces fresh out side air into the crankcase to allow for a richer mixture and better fuel transfer to increase power while the Chainsaw's SPI accualy introduces the whole fuel/air charge. It is reliable but is not the best for performance. Another name for this is "Piston Port Induction" (Bandit)

rsv1cox wrote:Thanks, obvious answer.  Same abbreviation applied to two different applications.  Just wanted to be sure.

What does SPI do in the engine i was always curios

Trevor

TopBannana wrote:

rsv1cox wrote:Thanks, obvious answer.  Same abbreviation applied to two different applications.  Just wanted to be sure.

What does SPI do in the engine i was always curios

Trevor

In my belief SPI in a cox engine is used to introduce fresh air into the crankcase at top dead center. This allows for better fuel transfer as well as allow for better enrichment of the Fuel/air mixture in the crankcase so it builds more power this is done because the engine can only pull so much air through the reed and veturei so it helps boost it up more by allowing the engine to get one last puff of fresh air so it can burn that richer mixture. Since SPI is used to introduce fresh air, it cannot be used with a muffler or it will breath its own exhaust and will loose substantial power. So if you wanna use a muffler on an engine that has SPI like a TD, you will have to switch to a cylinder with no SPI. (Bandit)

There are cylinder shims you can buy Bernie carries them they stack up under the cylinder till you have created a small gap such as 1mm or hair less there is a actual sweet spot I have forgot the number eyeball with an existing SPI engine will help. You only need 1 glow plug shim if you shim the cylinder for muffler.

The upstroke: On the piston upstroke the crankcase pressure is relieved in part by the action of the piston moving up while it compresses and burns the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carburetor/venturi.

TDC and the Downstroke: As the piston goes through top dead center, the skirt clears the bottom of the exhaust port. The crankcase vacuum created by the rising piston  causes fresh air to rush into the crankcase to relieve any residual crankcase vacuum. As the now descending piston forces the freshly sub piston inducted air to combine with the existing fuel/air mixture,* it gives the crankcase a momentary belly full of normally* pressurized charge ready to be the next cycle. The increasing pressure of the descending piston forces the mix to scoot up through the bypass ports until sufficient pressure is relieved. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has this useful effect at high RPM.

*Obviously, surprising the premix from the venturi with bonus air leans out the mixture. The goof with his fingers on the needle valve has to open it more to let in more fuel to balance the mixture. More to burn, more fun.

*Ambient. Atmospheric pressure.

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed and injected by the descending piston. All that, plus the extra fuel from the richer needle setting to balance the F/A ratio, now creates a larger charge in the crankcase than if there had been no sub induction. Sort of a simple turbo charge, without the turbine.
Rusty

RknRusty wrote:The upstroke: On the piston upstroke the crankcase pressure is relieved in part by the action of the piston moving up while it compresses and burns the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carburetor/venturi.

TDC and the Downstroke: As the piston goes through top dead center, the skirt clears the bottom of the exhaust port. The crankcase vacuum created by the rising piston  causes fresh air to rush into the crankcase to relieve any residual crankcase vacuum. As the now descending piston forces the freshly sub piston inducted air to combine with the existing fuel/air mixture,* it gives the crankcase a momentary belly full of normally* pressurized charge ready to be the next cycle. The increasing pressure of the descending piston forces the mix to scoot up through the bypass ports until sufficient pressure is relieved. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has this useful effect at high RPM.

*Obviously, surprising the premix from the venturi with bonus air leans out the mixture. The goof with his fingers on the needle valve has to open it more to let in more fuel to balance the mixture. More to burn, more fun.

*Ambient. Atmospheric pressure.

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed and injected by the descending piston. All that, plus the extra fuel from the richer needle setting to balance the F/A ratio, now creates a larger charge in the crankcase than if there had been no sub induction. Sort of a simple turbo charge, without the turbine.
Rusty

Couldn't of said it better myself! (The Bandit is impressed!)

TDbandit wrote:... this is done because the engine can only pull so much air through the reed and veturei so it helps boost it up more by allowing the engine to get one last puff of fresh air so it can burn that richer mixture....(Bandit)

Righto, thus the residual vacuum exists to grab that last little bit of fresh air. An accurate and less tiresome way of saying it than my prosaic explanation.
Rusty