Sub piston induction,,how much is too much ,if any

[Rod Fonda]

Playing around with my .21,,Just wondering about the effects of SP induction,as I'm getting around a .005' gap under the skirt at TDC on my .21,,approx 180*/ 182* ex,124*/126* transfers ,pretty much where I want it,,a .005" shim puts the ex and transf up a bit too much for a cat,,I understand it messes with intake induction and maybe the pipe,,probably more so at half/part throttle than WOT, I'm assuming ? My .45 seems to start/ run well with about a .015" gap under the skirt or would it run noticeably better with no gap?

 

[Grimracer]

NONE!..lol.. thats the very reason I hated the OS46VM.

Grim

 

[Rod Fonda]

NONE!..lol.. thats the very reason I hated the OS46VM.

Grim

Couldn't they have added just a tad more skirt lenght lol,,was the OS like that out of the box ? or after you modded the timing,,I'l try the .21 as it is,then maybe add a .005" shim,,I'l try to not go too high on the ex port next time on the other liner I have lol,,cheers

 

[Buckshot]

I have a 1.01 that had part of the skirt break off (it was a one off billet piston). At TDC it has a hole approx. .25" wide and .125" tall in the lower corner of the exh. port. The motor turns a ton of RPM like this in a mono. My dad built several OPS motors were he machined the piston skirt so that it would sub-port. I'm not smart enough to know how much is too much but I know it works......sometimes.

 

[Rod Fonda]

I have a 1.01 that had part of the skirt break off (it was a one off billet piston). At TDC it has a hole approx. .25" wide and .125" tall in the lower corner of the exh. port. The motor turns a ton of RPM like this in a mono. My dad built several OPS motors were he machined the piston skirt so that it would sub-port. I'm not smart enough to know how much is too much but I know it works......sometimes.

Well that threw a spanner in the works,,that's a pretty big gap at TDC,So it runs better now than before the skirt broke? lol,,I know they used to do it in the old days with straight pipes,but most guys say no gaps with tuned pipes,,so I should be ok with my small gap haha,,cheers Will