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Cox Engine of The Month
Correct my misconception, please.
Page 1 of 1
Correct my misconception, please.
On another thread someone mentioned having the SPI piston and cylinder on a Medallion .049 and getting higher RPM. My understanding of the SPI set is that by shaving the bottom of the piston, this helps get a cleaner air mixture into the cylinder when the piston is at TDC and the lower slits are uncovered on the cylinder. Since the Medallion uses a venturi to mix the air and fuel, I don't see the advantage, other than a slightly lower rotating mass.
Please correct my understanding.
THE Mark
Please correct my understanding.
THE Mark
batjac- Diamond Member
-
Posts : 2336
Join date : 2013-05-22
Age : 61
Location : Broken Arrow, OK, USA
Re: Correct my misconception, please.
Hey, MK-II, no misunderstanding. SPI doesn't do as much for front rotary valve engines. I did put a new style piston cylinder on my Medallion and the main performance gain was due to the dual bypass ports combined with a boost flute accompanying each port. The original Medallion was a benign single port #2 cylinder just like the ones on many Babe Bees. SPI makes some difference on front rotaries or the Tee Dee wouldn't have it. But not as much as on a reedy.
SPI raises the volume, thus the pressure of the crankcase fuel/air charge. On a Reedy the valve feeding the crankcase on the upstroke doesn't completely equalize the case pressure with atmospheric pressure, so when the skirt opens briefly at TDC the remaining vacuum pulls air in through the gap to finish filling the crankcase volume. On a front rotary, the crankcase is exposed directly to outside air through the venturi and gets a bigger gulp. Because the timing doesn't allow it to quite finish quenching, SPI can help with that last little bit. Tee Dees were contest engines, so every little bit counts. On the models they sold with mufflers or exhaust restricting throttles, SPI is eliminated to prevent the engine from drawing hot wet exhaust into the crankcase instead of fresh cold air.
SPI raises the volume, thus the pressure of the crankcase fuel/air charge. On a Reedy the valve feeding the crankcase on the upstroke doesn't completely equalize the case pressure with atmospheric pressure, so when the skirt opens briefly at TDC the remaining vacuum pulls air in through the gap to finish filling the crankcase volume. On a front rotary, the crankcase is exposed directly to outside air through the venturi and gets a bigger gulp. Because the timing doesn't allow it to quite finish quenching, SPI can help with that last little bit. Tee Dees were contest engines, so every little bit counts. On the models they sold with mufflers or exhaust restricting throttles, SPI is eliminated to prevent the engine from drawing hot wet exhaust into the crankcase instead of fresh cold air.
_________________
Don't Panic!
...and never Ever think about how good you are at something...
while you're doing it!
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...and never Ever think about how good you are at something...
while you're doing it!
My Hot Rock & Blues Playlist
RknRusty- Rest In Peace
- Posts : 10869
Join date : 2011-08-10
Age : 68
Location : South Carolina, USA
Re: 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.
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- Silver Member
- Posts : 91
Join date : 2013-03-05
Re: Correct my misconception, please.
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.kevbo wrote:Almost. The piston is not changed, the exhaust ports on the cylinder just come down a bit closer to the crankcase.
_________________
Don't Panic!
...and never Ever think about how good you are at something...
while you're doing it!
My Hot Rock & Blues Playlist
...and never Ever think about how good you are at something...
while you're doing it!
My Hot Rock & Blues Playlist
RknRusty- Rest In Peace
- Posts : 10869
Join date : 2011-08-10
Age : 68
Location : South Carolina, USA
Re: Correct my misconception, please.
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.
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- Silver Member
- Posts : 91
Join date : 2013-03-05
Re: Correct my misconception, please.
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.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?
Here's a better way:
http://coxengines.ca/cox-.049-sub-induction-cylinder-piston-spi.html
_________________
Don't Panic!
...and never Ever think about how good you are at something...
while you're doing it!
My Hot Rock & Blues Playlist
...and never Ever think about how good you are at something...
while you're doing it!
My Hot Rock & Blues Playlist
RknRusty- Rest In Peace
- Posts : 10869
Join date : 2011-08-10
Age : 68
Location : South Carolina, USA
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