Time area targets

[Colbyc]

I write this in hopes of gaining some knowledge from those who have much more than I do. As I look at the countless hours spent reading on here and other books as well as sae papers I’m still at a stand still due to the lack of life experience. I’m working on a 61x 61x platform. I’ve found that in stock platform this particular motor has these specs at stock 6300 rpm
Exhaust open: 89 duration 182
Mean duration in degrees 134.5
Time area of .0001565 sec-cm^2/cm^3
Time angle area of: 5.936 deg- cm^2/cm^3

Transfer a: open 119, duration 122
Transfer b: open 121, duration 118
Boost port same as b
Total transfer time area of: .0001248 sec-cm^2/cm^3


I have found that these numbers align almost perfectly with things that Gordon Blair and naitoh namura presented in there research papers,
After many phone calls and hours spent reading forms on here I’ve found the members here like different durations much greater than those recommended by Blair and namura. I understand there are many different set up’s for different styles of riding but I was hoping that some could fill the missing link I was looking for.

Wax recommended a 196 exhaust and 128 intake, if keeping exhaust area constant and just raised duration ( not do-able I think?) you would have a time area of .00019 sec-cm^2/cm^3 much greater than that recommended by Blair

Which leads me to my questions, when determining how much to increase duration and area what factors do you use to determine the limitations? Is it just years of experience and trails to know what combo works best? How much is to much on exhaust width percent of bore? Is 65% still the rule of thumb or out dated? I realize there is so much more to equation such as maximum squish velocity, octain used, style of riding, ignition advance, the pipe used and the resonance tuned by water and the list goes on but I was just hoping to focus on time area targets to shoot for.

Any insight would be greatly appreciated and please correct me if I’m wrong in areas as I’m trying to learn and want to learn the correct way. Thank you all for your time

 

[kevbo]

The main difference is a jet pump requires max torque near its full throttle RPM since its load squares with RPM. A vehicle on wheels needs to make just enough torque at its shift point where a jet pump requires peak torque at its full throttle RPM. There is some compromise between throttle response and peak power. You often need to lighten the pump load so it turns a little faster and/or use a lower than ideal port in order to get enough throttle response. My experience says higher ports and RPM pull harder and go faster. High ports also require careful tuning for decent response and good fuel since it takes only a few seconds to stick a piston.

 

[Colbyc]

Thank you all for you response. You all have given me a nudge in the right direction. I now realize that field testing means more than any calculation one can do.
I hope not to put off the vibe of just give me your calculations and hard earned time as that wasn’t the goal and understand the years it takes to get where you all are. I was looking for a nudge which you all have graciously provided.

It never really crossed my mind as to what max torque would be with a pump, I just always assumed you matched pump to motor combo but now I see where you have to have an entire package

Is there a program that you guys recommend as I hold your opinions to the highest regard.

Most everything I have is 701 set ups to practice on. Just with calculations I have tried is taking the exhaust width to .75% of bore (a lot in my opinion correct me if I’m wrong) and taking the exhaust to 196 if I turned the motor at 7300 it would put the time area target back to close to original, and raise the transfer to 128 to have a good blow down. Is that to extreame to start testing on a recreational use ski. Obviously setting up cr squish and all those other factors.

 

[kevbo]

I would back off those numbers for your first attempt. Start with a stock 62t cylinder and get a good pipe. Spend time learning how to tune the carb and pump first. Once that is perfect you can cut the cylinder a little at a time, until you ruin it. At that point you will have more tuning experience than 99% of the jet ski community.

 

[Quinc]

More info for ya:

Free porting templates:

Yamaha 701/760 Freestyle/Freeride porting templates

This is an idea I have had in my head for a while and it recently became apparent that the original Sport Port Templates by Racelogic, are no longer available. In an effort to fill the gap left by this I have created a similar product. I did not want anything to do with plagiarizing Racelogic's...

www.x-h2o.com

 

[Colbyc]

I would back off those numbers for your first attempt. Start with a stock 62t cylinder and get a good pipe. Spend time learning how to tune the carb and pump first. Once that is perfect you can cut the cylinder a little at a time, until you ruin it. At that point you will have more tuning experience than 99% of the jet ski community.

Ok that’s what I was wondering.
I feel like I’m decently competent in tunning motors and have dealt with a lot of different two strokes from bikes to snowmobiles and jet skies, but obviously have so much more to learn but I do agree to start slow and then go bigger (which is hard for me as I’m in patient and want all the power now lol)

Have any of you guys tried shaving the piston top down to alter port timings to get an idea before shaving on the cylinder?
Is that a method you would recommend?

What about crank case compression ratio? I have not takin the time to measure it but i haven’t read anything where people alter the ratio.

After your recommendations I think I’ll go to 65% of bore on width and go from 182 to 185 and see what time area is then and try and turn the motor to get it back in range, as well as alter the transfer

 

[Quinc]

Best thing is to save your time and money and just get a NOS kit for it.

 

[Colbyc]

Best thing is to save your time and money and just get a NOS kit for it.

Agreed, but then I don’t learn anything

 

[kevbo]

Buy a cylinder thats been ported for the superstock class. That is the max for an OEM cylinder. Most riders want better response with pump gas than race porting generally provides.