porting for 4mm crank

[Alex29573]

hey guys why would someone say that their 4mm cylinders are ported for a short rod?

 

I dont get it, wether a 4mm crank is short or long shouldnt change the porting...

 

The only thing i see that changes is the pin height on the piston but both are a +4mm stroke.

Edited January 20, 2010 by Alex29573

[Burt Reynolds]

They arent giving enough information. It probably has to do with the whole setup. If he is using a spacer plate and stock pin piston and short rod,and you go and use a long rod and no spacer,and cut the heads,the timing will be different.

 

If he is running no spacer,and has the head cut,then you could run a long rod and a 795.

[Alex29573]

thanks, thats what i was thinking

[Wildcardracing]

Rod length will affect the port open duration so yes it does matter.

[Alex29573]

can you explain what you mean by that??

 

 

dont you get the same outcome whether you use:

 

- short rod with regular pistons

- long rod with 795series (higher pin height) pistons

 

(assuming both cranks are a 4mm stroke)

[Burt Reynolds]

can you explain what you mean by that??

 

 

dont you get the same outcome whether you use:

 

- short rod with regular pistons

- long rod with 795series (higher pin height) pistons

 

(assuming both cranks are a 4mm stroke)

 

 

My point was the cylinders need to be set up like he had it.So if the first guy is running a spacer,then wether you go long rod or not you have to keep the cylinders at the same height.

[Burt Reynolds]

Rod length will affect the port open duration so yes it does matter.

 

 

Do you mean at what time the port opens or how long the port is open?

[broncbob]

Do you mean at what time the port opens or how long the port is open?

 

I believe the port timing is expressed in degrees, and in what degree they open or close on crank shaft location. You have to think of port timing as the cam on a two stroke engine. You can change the specs on the "cam" or port timing to get different things out of your engine. The ports are also like the valves on a four stroke as well, you can change them to affect how well air moves through the engine.

 

You now know every thing I know about porting. There is lots of information and theory about the subject, just do some digging.

[Alex29573]

guys Ive been reading a lot on what durations people port shearers for and i don't understand why on a stock cylinder the usual exhaust duration is around 198/200 and then on a cub the durations are usually a little lower like 196..

does anyone know why the cubs are ported with less duration?