Re: difference in 115 and 140 crossflow
Ok, let's talk about that but let's first establish that we're talking about stock port timing and stock heads on a late model 115 that has the bubble back exhaust. And it's on a performance hull that will respond to increases in propeller rpm (more later).<br /><br />The stock steel reeds will start to flutter about 6300 or so. You may get to 6500 on a long run. It'll feel like you hit a wall. It just wont go any more. Install Boyesens and you'll gain some, to about 6600-6700. Then take out the exhaust stuffers and it'll go to about 6900. At that rpm it can't get enough air in to go any higher. It may make a real loud popping noise that makes you think it's about to fly apart.<br /><br />At that point you take the intake manifold and cut the center wall out so each cylinder can pull from both carb throats. Instant 7400. <br /><br />BTW -- we've stopped trusting the stock flywheel a long time ago. <br /><br />It's important to realize we have to have a hull that responds to prop rpm. Every motor has a hp & torque curve. At 7400 we passed the hp peak quite a ways back. If we used 115 as a baseline, at 7400 I'd be surprised if it would be much more than a true 75 hp. But the performance hull lifts and has very little drag. Basically you've convinced the prop that the boat weighs nothing because of the extreme low drag.<br /><br />As for how long you can do that, I don't know. We ran one like that for 2 years. We'd drag race it, drop the motor an inch or so, switch drivers and run circle races. Sold the boat but never hurt the motor. Between races we had the motor on a ski boat set up at 6500.<br /><br />Another obvious thing I forgot -- that wasn't with cast pistons.
Ok, let's talk about that but let's first establish that we're talking about stock port timing and stock heads on a late model 115 that has the bubble back exhaust. And it's on a performance hull that will respond to increases in propeller rpm (more later).<br /><br />The stock steel reeds will start to flutter about 6300 or so. You may get to 6500 on a long run. It'll feel like you hit a wall. It just wont go any more. Install Boyesens and you'll gain some, to about 6600-6700. Then take out the exhaust stuffers and it'll go to about 6900. At that rpm it can't get enough air in to go any higher. It may make a real loud popping noise that makes you think it's about to fly apart.<br /><br />At that point you take the intake manifold and cut the center wall out so each cylinder can pull from both carb throats. Instant 7400. <br /><br />BTW -- we've stopped trusting the stock flywheel a long time ago. <br /><br />It's important to realize we have to have a hull that responds to prop rpm. Every motor has a hp & torque curve. At 7400 we passed the hp peak quite a ways back. If we used 115 as a baseline, at 7400 I'd be surprised if it would be much more than a true 75 hp. But the performance hull lifts and has very little drag. Basically you've convinced the prop that the boat weighs nothing because of the extreme low drag.<br /><br />As for how long you can do that, I don't know. We ran one like that for 2 years. We'd drag race it, drop the motor an inch or so, switch drivers and run circle races. Sold the boat but never hurt the motor. Between races we had the motor on a ski boat set up at 6500.<br /><br />Another obvious thing I forgot -- that wasn't with cast pistons.
