Correct my misconception, please.

Almost. The piston is not changed, the exhaust ports on the cylinder just come down a bit closer to the crankcase.

On a reed engine, the reed doesn't close until the crankcase pressure increases above atmospheric. Due to the intake restriction this happens well after TDC. The engine can't start compressing the mixture in the case until the reed closes. The more it is compressed the better the scavenging, and the more power you get out. The idea with SPI is to relieve any remaining vacuum in the crankcase so that compression of intake charge can start as soon as the piston comes down enough to close the SPI opening of the exhaust ports. This results in more pressure in the crankcase when the piston uncovers the transfer ports.

SPI allows a smaller venturi on the reed engines for excellent fuel draw without suffering a huge power loss, and the high crankcase vacuum also gives spirited reed action.

The SPI opening is pretty narrow, so I have heard it opined that not much air can get in. Remember that the piston velocity is zero at TDC, and still very low for the few degrees each side while the SPI is active, so the SPI covers a fair range of crank angle/time even with that narrow slit.

As Rusty says, it helps some even on rotary valve engines. The intake timing and venturi size is only ideal for one RPM and one atmospheric condition. SPI allows some compensation when the engine is operating away from what the designer had in mind, and the venturi sizing is always a compromise between fuel draw and power even under the best of conditions...SPI allows a little smaller venturi for ease of use without costing as much power.

The air that gets sucked into the crankcase during SPI has no fuel in it, so if the venturi were making ideal mixture, then SPI would cause a lean condition. Not a problem on a non-throttled engine, we just set the needle as needed to compensate without even worrying about what mixture the venturi is delivering, or how much SPI is diluting it. Of course it gets all screwed up if you try to throttle it. I think this the biggest reason that the Tatone carbs for the tee-dee never made much headway. SPI is fighting any attempt to throttle the engine. SPI is going to cause trouble with pretty much any way you try to reduce the power output, either via the intake, or the exhaust.

Throttling aside, another downside to SPI is that it increases pumping losses in the engine. The piston is pulling a higher vacuum in the crankcase than it would with a more open venturi. It takes work to move against that force, and at the top, the SPI relieves the vacuum, so that work is not returned on the downstroke. The payoff is better fuel draw and a lot less fussy engine.

kevbo wrote:Almost. The piston is not changed, the exhaust ports on the cylinder just come down a bit closer to the crankcase.

That's true on all the open exhaust engines. But when newer designs came out with the slit exhaust and dual bypass/boost flute, the exhaust ports were all the same height, and a standard piston would not allow SPI. The new way to achieve SPI was to have a short skirted piston.

I see, interesting!

So A guy could make a holder for a Sure-Start piston, trim the skirts, and convert the engine to SPI with a lighter piston?


I have never seen a slitted SPI engine. All the slitted engines in my engine box are non-SPI. I have a couple of old product engines with narrowish single ports and no SPI.

kevbo wrote:I see, interesting!

So A guy could make a holder for a Sure-Start piston, trim the skirts, and convert the engine to SPI with a lighter piston?

I have wrapped a piston in electrical tape and chucked it in the drill press. I held the con rod with one hand and held a flat needle file against the skirt. Some people have used an exacto knife to thin the inside of the skirt too.

Here's a better way:
http://coxengines.ca/cox-.049-sub-induction-cylinder-piston-spi.html