I was reading through some of the archived topics about stroking. In one post someone said Skunk 2 stage 1's work good on b16's because of the shorter stroke and not good on b18's because of the longer stroke. I am curious as to why that is and an explaination of where cam duration figures into it and the factor that keeps the piston stationary at TDC. Any links to good info is always welcome.


[Modified by mobrep, 8:04 PM 10/8/2002]

 

bump

 

A stroked motor has a longer stroke (duh...heh heh) which, along with increasing displacement and decreasing the R/S ratio, also gives the piston a longer dwell time at TDC (and BDC). When the piston is at and near to TDC, the cam is actuating the valve, which, if not designed correctly, could cause the valve to open fully at a point where the piston is still close enough for the two to touch.

You can adjust cam timing, changing the moment at which the cam begins to open the valve, which also gives a margin of error where the CAM TIMING can cause the valve to open too early, causing the valve to basically be in the way of the piston during it's upstroke.

Hope I explained it clearly...

X2

 

"stroked motor has a longer stroke (duh...heh heh) which, along with increasing displacement and decreasing the R/S ratio,"

i knew that. der


"also gives the piston a longer dwell time at TDC (and BDC)"

i had seen posts saying the that it was the other way around. (shorter stroke = longer dwell time) thats one thing i was curious about.


"the cam is actuating the valve, which, if not designed correctly, could cause the valve to open fully "

in that sence. do you mean the duration might be too long. therefor opening the valve earlier than needed and keeping it open too long? atleast that is what i'm getting out of it.

also, is there no way to increase the rod stroke ratio with different rods? say for example in a b16 using a b17 crank with b18 rods?

i've done some searchs but nothing of much help has come up.

 

Well, I am not an engineer... duh... I will do some more research and let you know... but logically, a longer stroke would produce a longer dwell time, but higher piston speed. Hell, if I'm wrong, someone tell me so !!! But I'll do some research and get back to this...

X2

 

Sounds good. If you have any good links, please post. I can always use some good reading material.

 

Don't have an actual link, but check out these sites:

Theoldone.com and any Mustang boards....

 

Well, we are all here to learn, right?

Here is the correction: A longer stroke than rod (for example: r/s ratio of 1.4:1) will shorten TDC dwell time, increase piston speed, squish velocity, and more load against cylinder wall, and rod & crank bearings.

 

Quick 200k Mile Motor has it right in his last post, a slower moving piston in the B16 will well londer at TDC than the B18.

Piston speed corrolates directly with the amount of velocity you can attempt to have a port vlow at, once the barrier is reached (e.g. to much lift, duration, flank, or a combination of) then you have a basis for the cam and the final specs are worked from there to obtain the best flow ratios , the faster moving piston speed of the longer stroke combined with the added volume that the engine displaces accomidates for the aloowance of a higher velocity goal in the port.

Cut and dry of it is that the higher the velocity threshold can be placed, the more lift, duration, flank, etc. you are able to run.

For instance the stage 2 cam could be beyond the velocity threshold for the B16 but not the B18, making it the best cam for the B18 application.

On the other hand, the stage 1 could be well below the velocity threshold of the B18, making less power. But it could be just short of the velocity threshold of the B16, making it the perfect candidate for the longer rod.

Cam timing, like mentioned before, allows for much more dictation of where and how the power is applied.

 

i think rod ratio is a main factor to decide the dwell period.
and yes, longer stroke will increase piston speed since piston speed = stroke X 2 X rpm /(1000 * 60)

 

spoon_ek9 , you're right, r/s does dictate piston speed, we were just leaving that unsaid, since it is obvious.