Cdi performance

[Street Ryderz]

So ramping the piston at exhaust as well as the transfers maintains the stock blow down and aids smoother flow at all three ports. You loose some duration of the comp. and comb. portion of the stroke. I think that is where low end torque is lost, but you can regain some of the bottom end power loss with modifications increasing the compression.So with the smoother flow, advanced port timing with 3 ramps and comp. mods. you can widen the potential range. Is my thinking right on this one?

Not really the best way of doing it imo because only a portion of the transfers actually open and will allways be obstructed!The exhaust side is only blocked for the return wave from pipe,So if the ramps are all exactly the same in depth yes it would maintain the stock blow down but at the same time the loss of compression is harder to get back!Also and this could be just me but the ramps make uneven pressure on the crown of piston and could also create hot spots that may promote detonation!

 

[gary55]

Ya I thought about that part too. You go through all the trouble setting the squish then screw up 1/3 of it in ramping

 

[Deleted member 12676]

You all are missing something very important.
Blowdown degrees have no significance except in relation to max RPM and exhaust port size.
When I put the ramps on my engine it did not limit it in any way, there was only benefit.
You can use my free spreadsheet calculator to determine if your blowdown timing is limiting your RPM.

 

[Street Ryderz]

You all are missing something very important.
Blowdown degrees have no significance except in relation to max RPM and exhaust port size.
When I put the ramps on my engine it did not limit it in any way, there was only benefit.
You can use my free spreadsheet calculator to determine if your blowdown timing is limiting your RPM.

If you cut ramps or raise the transfers it does not benifit anything but a loss of intake charge the exhaust port has to be raised to match or slightly more to gain anything!If you input the correct figures into say Lambretta's port timing calculator you will see that the duration of blow down is greatly affected by very minute change in height of the transfer port also as the rpm incresses and the time of duration decresses is where you want more duration to offset the lesser time!

 

[Deleted member 12676]

OK smarty pants, tell me the formula for determining peak power RPM that utilizes the blowdown time, RPM, and blowdown area.

(nothing is as dangerous as a little knowledge.)

 

[Deleted member 12676]

this is a good example of why I rarely frequent this forum any more.
People open their big mouths when they should shut up and listen.

 

[Street Ryderz]

OK smarty pants, tell me the formula for determining peak power RPM that utilizes the blowdown time, RPM, and blowdown area.

(nothing is as dangerous as a little knowledge.)

You yourself said that your blow down should be at 30 to support 10,000 rpm!

 

[Deleted member 12676]

that was back when I was still in the swamp of the old basic ideas. I've since matured.
yes lower blowdown does reduce your "possible" peak RPM but that doesn't mean it will limit your present RPM.
you have to use a good formula such as what I use in my spreadsheet to see if lowering your blowdown degrees will limit your present RPM.

 

[Purple Haze]

I thought about ramping the piston, but won't this affect squish, quench, etc? I think the piston top acts as part of the chamber, would changing the shape change the swirl? Someone please enlighten me on this.

 

[Deleted member 12676]

don't do it unless you've already raised the exhaust port for more RPM.
No it won't have much of any total effect on the squish because of its small area.