Slowmoe
Member
Hi all,
First of all, the majority of my two stroke experience is with Yamaha Banshees, so some of this may or may not apply.
In the Banshee engine, you have two cylinders, which are roughly 175cc each, 350cc total and the engine produces approximately 35-40 hp at the crank. With mild to extreme engine modifications you can coax an extra 5-60 (yes 60) hp out of the standard 350cc engine. This is through the use of anything from aftermarket exhaust to porting, big carbs, methanol etc.
The Rotax 717 is roughly twice the size and, in stock form, produces roughly twice the power. It seems that a substantial amount of performance may be exracted with a calculated array of modifications. Since aftermarket exhaust is out of the question (to the best of my knowledge), I wanted to focus on two things; compression and port timing. Again, my experience with these engines is very limited, so bear with me.
After doing a brief search for compression ratio, it appears that the stock compression ratio (UCCR) for the 717 is a very mild 6.2:1. For comparison, pump gas is still feasable at a ratio of 11:1, leaving a pretty big gap there. My suggestion is to either mill enough off the cylinder deck or cylinder head to achieve .045" squish, or sending it off to someone like mull engineering to mill and rechamber (recommended) the head. On the Banshee this modification results in a horsepower increase of 5-10 hp, so I would assume that 7-12 hp increase per engine across the powerband would be achievable at a cost of roughly $75-125 per engine plus shipping. Oh yeah, the increased compression would dictate richer jetting, so that would need to be factored in.
On the Banshee, increases in port timing (opening the intake/ transfer/ exhaust ports sooner) will yield very significant increases in midrange and top end power. Now, not being an expert in Seadoo, my next suggestion is based solely on assumption; either remove 1mm from the piston crown in the exhaust port area or add base gaskets to raise the cylinders 1mm (this would require additional milling of the decks to the order of the thickness of the additional gaskets). On the Banshee this typically nets 6-10 hp, so I would assume that 10 hp at each engine would be achievable. The downside to this modification is that the increased port timing and subsequent shortning of the power stroke would decrease low end power, which would be offset by the increased compression. The cost of additional base gaskets and milling of the cylinder decks shouldnt run more than $200 or so per engine.
So, in a nutshell:
1. Mill and rechamber heads on both engines: $400 (with shipping)
2. Additional base gasket(s) and associated cylinder decking $400
3. Jetting for both engines $50-75?
That's $850-875 and, if my assumptions are correct should get you an increase of (conservatively) 15-20 hp on each engine with little to no loss in bottom end (due to compression increase).
Thoughts?
First of all, the majority of my two stroke experience is with Yamaha Banshees, so some of this may or may not apply.
In the Banshee engine, you have two cylinders, which are roughly 175cc each, 350cc total and the engine produces approximately 35-40 hp at the crank. With mild to extreme engine modifications you can coax an extra 5-60 (yes 60) hp out of the standard 350cc engine. This is through the use of anything from aftermarket exhaust to porting, big carbs, methanol etc.
The Rotax 717 is roughly twice the size and, in stock form, produces roughly twice the power. It seems that a substantial amount of performance may be exracted with a calculated array of modifications. Since aftermarket exhaust is out of the question (to the best of my knowledge), I wanted to focus on two things; compression and port timing. Again, my experience with these engines is very limited, so bear with me.
After doing a brief search for compression ratio, it appears that the stock compression ratio (UCCR) for the 717 is a very mild 6.2:1. For comparison, pump gas is still feasable at a ratio of 11:1, leaving a pretty big gap there. My suggestion is to either mill enough off the cylinder deck or cylinder head to achieve .045" squish, or sending it off to someone like mull engineering to mill and rechamber (recommended) the head. On the Banshee this modification results in a horsepower increase of 5-10 hp, so I would assume that 7-12 hp increase per engine across the powerband would be achievable at a cost of roughly $75-125 per engine plus shipping. Oh yeah, the increased compression would dictate richer jetting, so that would need to be factored in.
On the Banshee, increases in port timing (opening the intake/ transfer/ exhaust ports sooner) will yield very significant increases in midrange and top end power. Now, not being an expert in Seadoo, my next suggestion is based solely on assumption; either remove 1mm from the piston crown in the exhaust port area or add base gaskets to raise the cylinders 1mm (this would require additional milling of the decks to the order of the thickness of the additional gaskets). On the Banshee this typically nets 6-10 hp, so I would assume that 10 hp at each engine would be achievable. The downside to this modification is that the increased port timing and subsequent shortning of the power stroke would decrease low end power, which would be offset by the increased compression. The cost of additional base gaskets and milling of the cylinder decks shouldnt run more than $200 or so per engine.
So, in a nutshell:
1. Mill and rechamber heads on both engines: $400 (with shipping)
2. Additional base gasket(s) and associated cylinder decking $400
3. Jetting for both engines $50-75?
That's $850-875 and, if my assumptions are correct should get you an increase of (conservatively) 15-20 hp on each engine with little to no loss in bottom end (due to compression increase).
Thoughts?
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