Home
Basic Dirt Bike How-To's - Video
Dirt Bike How-To's - Video
Living The Moto Life - Video
Bike Tests | Shoot-Outs - Video
Forums
What's new
Latest activity
Log-In
Join
What's new
Menu
Log-In
Join
Navigation
Install the app
Install
More options
Close Menu
Forums
MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Motorcycle Myth # 5 - You need big ports to make big horsepower
Reply to thread
JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an
alternative browser
.
Message
[QUOTE="Rich Rohrich, post: 1451527, member: 16241"] Each engine has a sweet spot velocity where the engine is most happy and does the best job of cylinder filling usually around 300 fps through the intake port. As strange as it seems to some folks air has mass and it takes some time to accelerate it. Once you accelerate the mass of air it will try and stay moving (body in motion and all that physics most of us ignored in high school). Low rpm combustion is improved due in part to greater charge turbulence from the increased velocity and will require less ignition timing for best torque. This in turn makes the engine much more tolerant of lean jetting and much less octane sensitive. Problems start to occur when the rpm, port size, valve, and bore sizes combine to generate velocities where sonic boundaries are crossed. It's relatively easy to reach a point where the flow stops completely due to shock waves in the port or valve throat. So it's a system that can be manipulated but too much of a good thing is still too much. In many cases the overrev (power past the torque peak) is improved with a smaller port because the higher port velocity makes it more difficult for intake flow reversals to occur once the piston passes BDC and begins to rise while the intake valve is still open. When the ports get bigger they get lazier and it takes more time to get the air up to speed so reversals at lower rpm are more common. So big ports will tend to move more total air and show bigger numbers on a flow bench and possibly the dyno, they also tend to be more sensitive to getting the Intake Valve Close cam timing perfect and you can only get it right in a much more narrow rpm range. Big ports also tend to show dips in the torque curve in places where the cam timing (and by extension the intake and exhaust lengths) are "out of time" Increased RPM to make usable torque and less area under the torque curve will be the result as the ports get too big. In other words they get peaky. MotoGP engineers have been battling this phenomena since they switched to four-strokes. It became more complicated when they dropped the displacement limit to 800cc, and they have had more leeway as they have moved back to a 1000cc limit. [/QUOTE]
Verification
Which ocean is California closest to?
Post reply
Forums
MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Motorcycle Myth # 5 - You need big ports to make big horsepower
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
Accept
Learn more…
Top
Bottom