9550

after doing all that math, i realized how this is true. the shims affect the installed height and thus the closed pressure, and also help compress the spring even more during opening so the pressure is increased as well.
so like if normal installed height is 1.8 and the spring length is 2.08, the spring is compressed .28" and results in a certain pressure. if you add a shim of lets say 0.15, now the spring is compressed .43" and the installed pressure is greater. same thing for opening pressure.
yay, i learned something, i think

9550

hmm, so basically, i know that the comp cam springs are 322lb per ". i just need to get my ford springs tested to see what their rate is and hope its close to 322?
if the springs are what i think they are, the M-6513-A50's, it appears their rate is 327.2lb per ". is there more to it than that?

9550

and now i think i just put the whole puzzle together:

these install heights i am seeing listed 1.75 for comp cam springs, and 1.8" for ford springs, they are there to show what the pressure will be at that install height, not necessarily what my install height will be when i put the springs on.
this is where the shims come into play i now assume.
i install a spring. measure the height. add shims as necessary to create an install height of my desired pressure. bingo?

so the comp cam card says to use comp springs 986-16. the 986-16 springs say installed height 1.75" is 132lb. these springs have a rate of 322lb per inch.
my ford springs have a rate of 327.2 per inch. using the rate of 327.2 and the ford install height of 1.8 and pressure of 123lb from their literature, i can calculate that the spring length is 2.17". now, to create an installed (closed) pressure of 132lb recommended by comp cams, i take 123/327.2 and get .376. then 2.17-.376 = 1.794. i install my springs, measure, and add shims as necessary to create an install length of 1.794. boom. done.
am i a genius, or way off?

now however, if i set up the ford springs to match the closed pressure of the comp cams, my open pressure will be off a bit. (327.2-322)*5.12= 26.6lbs higher. yikes. is that too much?

whats the standard move here, shim to get the desired open pressure, the desired closed pressure, or compromise between the two (slightly lower closed/open pressure or slightly higher closed.open pressure)?

sorry for all the babbling
i would delete all the previous posts, but maybe theres value in them somewhere.

thanks again, i hope i got this all correct now

unfortunately, i already see the next problem coming: how the frick do i measure the spring heights!

barnett468

you absolutely positively must measure the trapped length otherwise just run what you have and pray for the best.

barnett468

yes.



yes



yes



yes



yes or different height retainers, uh oh...



yes but you need my formulae to determine open pressure.



no comment, lol. please reread my post and set your pressures to that if it is a roller or 300 open if its a non roller, closed is from 118 - 127 on either.



no, i already told you that's what it should be for a roller cam in a previous post.



use a calculator, determine the right spring and it will come out correctly. the 322 may very well work but as mentioned, until you know EXACTLY what you real actual true trapped distance is nothing matters.



the spring heigts are noted in the catalog but recheck with a 15.00 venier caliper from home depot.

9550

thanks for the reply. i know its a pain to quote all that stuff.

now, i think i pretty much understand it all except for one thing-
you mention repeatedly how the trapped height is the key to everything. i dont quite understand why. seems to me the key to everything is 1. the spring rate. 2. the required open/close rate. 3.the spring length. using those, you determine what the trapped height should be, install and adjust if necessary.
seems what youre saying is check the trapped height and then calculate everything and see if youre good. maybe its 6 in one hand, half dozen in the other?

im going to go pick up a caliper today. you said to get a vernier caliper. did you mean specifically a vernier caliper, or will a dial caliper suffice? seems a vernier caliper may be a little more difficult to use (figuring out the readings). it appears a vernier caliper will last longer and has no parts that can break though.



thanks again for your patience

barnett468

then unfortunately, i don't think i can help you anymore.



nope



the trapped height is only adjustable by a max of around .060". just comp alone has over 50 different spring rates. if you have the wrong springs you can not adjust the trapped height enough to make them work.



nope.



its sounds like you do not know what one is. look for a photo on line. many have either a digital read out or a dial read out.





no problem, i'm going to get drunk now.

9550

drunk sounds like a good idea.

resolution to this whole thing is that im going to buy the comp springs and drop the heads off at a shop.

heres a link to calipers wiki. http://en.wikipedia.org/wiki/Calipers
it appears vernier, dial, and digital calipers are different. but you, as i suspected, were grouping them all the same, which is why i wanted to clarify. i was going to pick up a dial caliper, but wanted to be sure there wasnt some particular reason you suggested vernier caliper, just in case.

thanks again for your help. i was hesitant to have some unknown machine shop do my work, but having talked with you, i was able to talk to this guy semi intelligently enough to tell he knew wtf he was doing.

barnett468

like i said, your other springs might be ok. i never looked to see what they were. i left that for you to determine just for fun.



yes, you are correct, and your original term was also correct. i was simply dizzy from answering all your questions and doing all the calcs in my head [because i don't like to use a calculator] that i didn't csatch what you called them, sorry about that.



lol, no prob.

9550

so, new issue and a heads up for people just in case:

new springs are ordered comp cam 986-16 as recommended by comp cam. of course they want you to use their springs. whatever. i think i realize i dont actually have to use their springs, but in an effort to make everything easier and faster, i bought them.
havent gotten them yet, but now im looking around the web, and it appears to use these springs with my gt40 heads, theres some head machining required. theres a step around the stem base where the springs sit that will apparently need to be machined flat, and maybe the stem guide OD may need to be cut thinner, and different stem seals need to be gotten. i already bought replacements seal for what i had, may be no good and have to be returned. returning them isnt a huge deal, but its just a hassle.

so, im thinking this all sounds like a pain in the ***, plus, i dont think i want to machine the heads just to make the comp springs work. i would rather get some other springs that will do the job.

am i overreacting and this is not a big deal or? seems that step is there to keep the single spring ford spring in place and prevent it from dancing around. if i machine the step away, am i unable to use ford springs later down the road if need be?

feel free to add any thoughts/comments. im kinda bummed out right now mainly because my schedule is blown i think. might take a page out of barnett's book and go get hammered.


so, again, just an fyi, anybody thinking of using the comp springs, research a bit to see if theres extra machining involved.