Why do two strokes have pea shooter exhausts?
Discussion
I've been wondering this for a while, and I suppose it has to do with back pressure in the exhaust?
The subject comes to mind lusting over RS250's, RGV Gammas, NSR125's and RS125's etc etc. They're often equipped with two small end cans with tiny exits. Why not just one big can, with a big exit, as per four stroke sports bikes? I know two stroke exhausts are a different kettle of fish with their massive expansion chambers, but do they use a small exit to keep internal pressure high to prevent blowback into the cylinder when the port is open?
On the two stroke throttle I've tried (I've not yet ridden one, sadly), a slight twist of the throttle grip produced an instantaneous response. It revved round so freely, you'd have never guessed it was a single cylinder. Compared to the KTM Duke 390 with an aftermarket exhaust I was looking at the other day. That sounded amazing, but each twist of the throttle, in neutral and from a closed throttle, felt like there was an pressure to overcome. From an open throttle, it'd rev freely but from a closed throttle it lagged and the change in gas pressure from the wide exhaust exit was really obvious, almost like closing the throttle created a vacuum of atmosphere into the end of the can. I know the two aren't exactly comparable because the four stroke Duke has fuel injection and the two stroke was on carbs so would rev more freely, but I hope I have conveyed what I mean about the 'inertia' which must be overcome on the Duke.
And I'm guessing because I've not seen a 2T with a 4T style exhaust that there is a reason they're designed the way they are. I know many 4T exhausts have a wide exit with a sleeved down pipe inside, but some look like you could roll a pool ball down them!
Pea shooters:
The subject comes to mind lusting over RS250's, RGV Gammas, NSR125's and RS125's etc etc. They're often equipped with two small end cans with tiny exits. Why not just one big can, with a big exit, as per four stroke sports bikes? I know two stroke exhausts are a different kettle of fish with their massive expansion chambers, but do they use a small exit to keep internal pressure high to prevent blowback into the cylinder when the port is open?
On the two stroke throttle I've tried (I've not yet ridden one, sadly), a slight twist of the throttle grip produced an instantaneous response. It revved round so freely, you'd have never guessed it was a single cylinder. Compared to the KTM Duke 390 with an aftermarket exhaust I was looking at the other day. That sounded amazing, but each twist of the throttle, in neutral and from a closed throttle, felt like there was an pressure to overcome. From an open throttle, it'd rev freely but from a closed throttle it lagged and the change in gas pressure from the wide exhaust exit was really obvious, almost like closing the throttle created a vacuum of atmosphere into the end of the can. I know the two aren't exactly comparable because the four stroke Duke has fuel injection and the two stroke was on carbs so would rev more freely, but I hope I have conveyed what I mean about the 'inertia' which must be overcome on the Duke.
And I'm guessing because I've not seen a 2T with a 4T style exhaust that there is a reason they're designed the way they are. I know many 4T exhausts have a wide exit with a sleeved down pipe inside, but some look like you could roll a pool ball down them!
Pea shooters:
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