I think the slit type is supposed to reduce the likelihood of the fuel flashing off and starting a fire. But there's more to it than that. The Wikipedia entry on Cox engines is certainly comprehensive, but I can't vouch for its authenticity.
http://en.wikipedia.org/wiki/Cox_049
Hi guys,
I can vouch for the authenticity of that Wikipedia page
http://en.wikipedia.org/wiki/Cox_model_engine, because I wrote it.
It is an accumulation of the knowledge I have learned from various sources including but not limited to:
As mentioned at the bottom of the wiki article.
Now, to answer the original question... The answer isnt a simple one... What is more important than the type of exhaust port the cylinder has, is what engine the cylinder and piston were designed for. ie the bypass porting inside the cylinder, whether or not it has SPI (Sub piston induction) and whether or not the cylinder has a tapered grind. Tapered grind refers to it being smaller at the top to tighten piston fit at tdc. The only cylinders that are [intended to be] tapered grind are the Tee Dee, Killer Bee and Venom, (and some Black Widows.)
As for the porting, the Tee Dee porting has two boost ports or grooves that extend beyond the original port grooves. This apparently assists with improving the atomisation of the air fuel mixture by creating a vortex.
The Killer Bee whilst being a slotted exhaust, also has SPI, a tapered grind and one bypass boost port on each of the two bypass ports. This particular way of porting was thought to be better than the Tee Dee porting for reed valve engines, hence why it was used on the Venom. (which the prototype used a Tee Dee cylinder).
Confused yet? The slit exhaust was designed to stop fires and was used after they tried various screens attached to the cylinder and found them to be too difficult/costly to manufacture.
Most slit exhaust cylinders you will see on bee type engines will have no SPI, and have one or two bypass ports and my or may not have a bypass booster on each bypass depending on when it was made and what engine it was intended for.
So before you decide which one you will use I would look at all of the above factors.
Cheers