.049 motor difference

The later cylinders on all Cox .049s were made with the slits instead of the open exhaust ports. These are actually more aggressively ported internally and the slits are a fire prevention measure. The slits don't seem to inhibit performance much, if any, because the old style ports were extremely oversized for the job. The new type exhaust ports are all the same height, whereas the old ones had a lower height on the high performance ones to create an SPI gap at TDC. With the slit ones, SPI is achieved by way of a short piston skirt. If you have a slit cylinder with SPI, you'll see the gap under the skirt at TDC through the bottom slit.

RknRusty wrote:The later cylinders on all Cox .049s were made with the slits instead of the open exhaust ports. These are actually more aggressively ported internally and the slits are a fire prevention measure. The slits don't seem to inhibit performance much, if any, because the old style ports were extremely oversized for the job. The new type exhaust ports are all the same height, whereas the old ones had a lower height on the high performance ones to create an SPI gap at TDC. With the slit ones, SPI is achieved by way of a short piston skirt. If you have a slit cylinder with SPI, you'll see the gap under the skirt at TDC through the bottom slit.

O ok.
so it's safe to use a exhaust on the newer 2 slit models then?

Safe?...Yes

Hurt performance? Maybe

Better to marry up a exhaust collector / muffler with a NON SPI cylinder piston set for max RPM and Torque in my opinion

The Sub Piston Induction (SPI) is intended to introduce a final fill of fresh air to the crank case as the bottom of the raising piston un-shrouds the exhaust port

When a exhaust collector, or muffler is surrounding the exhaust ports with SPI, the final fill to the crank case is now hot oily exhaust instead of fresh air

So far this is Theoretical for me as the few SPI engines I test ran with and without the car exhaust collector Bernie sells needled and sounded the same to me... I don't fuss with a tachometer on my little engines, so I can not say if or if not there is a RPM drop... I suspect in the air the torque and RPM would be lesser for this combination

The 2 slit ones are more likely to have no SPI (Sub-Piston Induction) so they will potentially perform better than the one that is milled out with SPI...if you are buying new.

To check for SPI hold the engine up to a "strong" light source (don't hurt yourself) when you slowly rotate the engine through look through the exhaust ports, specifically the bottom slit. If light passes at top dead center the engine has some SPI. No light, no SPI.

Phil

Thanks guy's very helpful.

so all the 2 slit models are no spi and the ones with one large opening are spi models?

well just looked at 2 motors and the big single slot version seems to only have one port inside the sleeve and the 2 small slit cylinder has 2 of the ports each the same size as in the one big slot cylinders.

so the 2 slit version seems to be s better ported motor from my experience with modern day nitro motors in rc's.

reptile wrote:Thanks guy's very helpful.

so all the 2 slit models are no spi and the ones with one large opening are spi models?

no.

Sure start motors were built to NOT have SPI, however manufacturing tolerances being as they are some have SPI and some do not.

If the lower slit is wider than the upper slit the cylinder is probably machined to have SPI.

Phil

reptile wrote:Thanks guy's very helpful.

so all the 2 slit models are no spi and the ones with one large opening are spi models?

There have been some slit cylinders that are different, like the pre-Estes Killer Bee. Surestarts are the most common and are the ones Bernie sells and they are all the same. Any Surestart slit cylinder can have SPI if you use an SPI piston, i.e. a short skirted piston. Look at the slit cylinder/piston sets Bernie sells, and you have the SPI or non-SPI option. Only the piston is different. And just because older ones have wide un-slit ports doesn't mean it's SPI. Re-read my earlier post; it depends on the height of the exhaust port. The position of the bottom of the port is slightly lower in the cylinder so the piston skirt rises above it at TDC opening the SPI gap.

reptile wrote:well just looked at 2 motors and the big single slot version seems to only have one port inside the sleeve and the 2 small slit cylinder has 2 of the ports each the same size as in the one big slot cylinders.

so the 2 slit version seems to be s better ported motor from my experience with modern day nitro motors in rc's.

Other than the Tee Dee #4, that is correct.