Port Timing & Duration Specifications

W

Watcon

Location
Near Seattle WA
Nobody has an anwer because they don't really have / use bottom end power.. I think people confuse "mid-range" power and accelleration with bottom end.. Maybe Wax, Chuck and Zero can post a few torque curves for review since nobody is answering your question.

Bottom end = The power you need to tow your buddy's ski back.
Mid range = The accelleration and torque necessary to perform a backflip
Top-end = The peak rpm and speed necessary to outrun the river patrol when riding in the 5mph zone.

Best regards,
Randy
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
Most people think of bottom end as probably 250-1500 rpm.
Truth is, you hardly ever spend any time at that rpm.
When I hit the throttle, the motor goes past 3000 rpm within the first half second, before the ski even hardly moves.
 
Freakrider

Freakrider

X247
Location
Tampa Bay area
Wow #ZERO ! this is awesome info to read ! good reason to have a builder cut custom domes for MSV / max safe compression...

#ZERO said:
The tapered angle on the squish band is there to help the squish action; it forces the charge to where the spark plug is located and is generally one or two degrees more than the angle of the piston. A narrow squish band ratio allows for more of the total intake charge to be located in the combustion dome resulting in more power potential during spark ignition near TDC. The down side is that there is more piston/dome area that is exposed to combustion on a highly loaded engine and heat build-up can become an issue that could eventually lead to detonation. As RPM increases, there is less time for combustion and a shallow dome designs will take less time to scavenge and refill vs. a head with a deep dome. Most pump gas engines tend to like a wide squish band width and smaller diameter domes with more volume. Race gas engines like narrow squish band widths and larger diameter domes with less volume. The squish band area or ratio can vary with each engine and usually is between 30 - 60 percent of the bore size and the larger squish ratios require greater squish clearances to get equal velocities. There are many factors that come into play to get the correct maximum squish velocity or (MSV), such as; RPM, squish clearance, squish ratio, rod ratio, compression ratio, port timing, blending radius, diverging squish angles, fuel type, etc. and it can get quite complicated. You'll need a computer program such as the Bimotion advance head design software to calculate all the factors. This squish program enables you to calculate the MSV of turbulence in the combustion chamber. This is an important design tool for finely targeting the hit in the power band and to determine a link between power band and fuel. The squish ratio, dome shape, compression, MSV, etc., all work synergistically while at the same time are all a compromise. There's a fairly delicate balance that needs to be maintained and small changes are better than larger ones. Increased MSV will help speed along the flame front and complete the combustion process more efficiently, but fuel choice will limit MSV. There is a point where the friction from the squish action can be so great and happen so fast that it generates enough force/heat to ignite the charge. Some basic MSV numbers that have been around for some time range from 15 - 30 meters per second. This is divided into two groups, high MSV and low MSV. Lower MSV numbers, around 15 - 19 m/s, generally make power from peak torque to max RPM or mid-top power. Higher MSV numbers from 20 - 30 m/s generally make power up to peak torque and then start to go flat. Also MSV numbers that are too high will kill off over-rev and lead to detonation.

Here are a few things that can either raise or lower the MSV.

Factors that raise the MSV:
Lower Compression Ratio
Less Squish Clearance
Larger Squish Area Ratio
Higher RPM
Smaller Bore
Longer Stroke
Shorter Rod


Factors that lower the MSV:
Higher Compression Ratio
Greater Squish Clearance
Smaller Squish Area Ratio
Lower RPM
Larger Bore
Shorter Stroke
Longer Rod

Here's the Bimotion head software: http://www.bimotion.se/

Free squish velocity program: http://www.torqsoft.net/squish-velocity.html
Click to expand...
 
Last edited:
kevbo

kevbo

Location
dfw
egbrig said:
I see that most here always talk about bottom end, mid to top, and top end power rpm ranges. I'm currious as to what rpm everyone here equates these ranges to and if the ranges are tach derived or just feel. I'm just curious. WAX, ZERO this isn't for you to answer just yet.
Click to expand...

Its all from a riders perspective as to how the engine responds to throttle input. Our skis are not connected to anything solid like a car. Thrust occurs only when the engine is at high power/rpm. ANY power that the engine makes below stall rpm is felt as response more so than thrust. Low port engines make more power below the pipe so they are more responsive. High port engines suffer until the pipe starts working. Matching the pump load is very important especially with high ports.
 
E

egbrig

Thats funny Randy, my tach doesn't have numbers on it, instead its divided into three sections of: Pull faster I'm sinking /// This is strange, where's the water? /// Oh crap, how well jetted is this thing?
 
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