heilo,i would like to ask about dump valves for cossies,so which is the correct ?single piston or dual? thanks a lot.

 

Single. Duals were designed for cars with air flow meters.

 

can also run diaphragm (sp) ones

 

^^ no sp, that's correctly spelt mate

If not running a dump valve with its own filter then imo the dual piston jobby is better for a cossie. With a normal diaphragm piston at idle and off boost the vacuum is great enough to hold the valve fully open allowing none filted air into the engine....a dual piston wont do this.

 

NO, the vacuum actually helps the valve stay shut even more as it sucks the actual diaphragm onto it's seat with a diaphragm one, it needs both boost pushing one side and vacuum on the other to overcome the spring pressure.
tabetha

 

Thats you told Stu

 

With throttle closed at idle and also at gearchange periods(ie off boost), how?!
Yes as soon as the throttle is opened the vacuum pressures are harmonised either side of the throttle housing so the vacuum in conjunction with the spring will keep the diaphragm closed but as soon as the butterfly is closed enough to increase vacuum plenum side the valve will open and stay open until vacuum that side is decrease enough again to allow the spring to over come it. Ignor Vacuum drawing on the diaphragm and think of vacuum from plenum side via the small pipe. If in doubt, look at any cossie at idle running a single/diaphragm dumpvalve.

 

GRRRRRRRR

 

https://passionford.com/forum/showth...ght=dump+valve

https://passionford.com/forum/showth...ght=dump+valve

https://passionford.com/forum/showth...ght=dump+valve