machine4321

ok gonna make this short .

need to make f20b lower comp .
it sits at 11:1 right now , specs are as follows

bore 85mm (3.346 )
stroke 88mm
rod length 5.709
C/H 1.22
wrist pin .866
head cc is 54cc

now i can get forged evo pistons for a good price. there specs are
these are a 10:1 flat top piston (flat top would be nice )

bore 85.344 (3.36)
stroke same
stock evo rod lenght is 5.905
C/H 1.377
wrist pin is the same
head cc is 45cc

now i would have to bore a bit to fit these in right , but i was concered about the C/H but seeing as the evo rods are longer by .2 i dont know if that will make up for it enough . also the head volume is gonna throw things off abit . im thinking that the comp might be to low in the end . if like to be around 9.5:1 but i dont think i can do it with these and might have to stick with the f22a 11:1 pistons "this one " used in his f20 (there hard to find and kinda costly )


what are your thoughts on all this stuff , im still learnign as i go

tokesGTR

Compression height is the distance from the center of the piston pin to the top of the crown of the piston. The EVO piston is .15" taller than your F22 piston, and it will not work. The piston would be sitting .15" out of the hole, which is not going to work.

machine4321

yes i know but look at the diff in rod length . wound the .2 inches not make up for the .15 , and tjhat would help in getting the comp lower then 10:1


thanks

tokesGTR

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by machine4321 &raquo;</TD></TR><TR><TD CLASS="quote">yes i know but look at the diff in rod length . wound the .2 inches not make up for the .15 , and tjhat would help in getting the comp lower then 10:1


thanks</TD></TR></TABLE>

Maybe you're not understanding what compression height is. The compression height on your F20B is 1.22 inches. That means there is 1.22 inches from the center of the small end bore of the rod (the top of the connecting rod) to the top of the cylinder bores. If you try and put a piston with a 1.37" compression height in there that is going to push the piston .15" out of the hole.

machine4321

**** i under stand , i was thinking c/h was related to the rod lenght ,
so regardless of rod length the c/h stays the same . poopy lol

JDogg

i can get you some custom wisecos for the f20b.... takes about 6-8 weeks to get em usually.

machine4321

race eng has the 11:1 f22a in stock at .20 over , i need to stop being cheap and just shell out the money . due to the shorter rod on the f20 they lower comp a bit . and the f20 head is bigger


i dont thikn i could afford custom pistons, i know yours were big bucks .

btw you get them yet

JDogg

i ahve put the NA project on hold for a while... doing some experimenting with some other stuff right now....

but shoot me an IM, i can get you custom stuff for not much more than shelf, i wouldnt advise f22 pistons b/c of they also have different compression height and the valve reliefs wont be big enough

machine4321

according the the specs thread on the lude section , the vlaves are the sam esaize , although tey might not be deep enough , the shorter stroke should take care of that .

also i though the c/h was the same

JDogg

f22 has a higher compresison height, if you use an f22 piston the piston will stick about 0.050 out of the hole. stroke dosnt change the need for valve reliefs, or how deep they should be.

but f22 single cam heads have different valve angles than the f20b/h22 head does, so if you use f22 pistons with a big cam they will probably hit the piston, not to mention the shape of hte dome wont work well w/the combustoin chamber