dvardaman85

I have decided that my cracked out 305 just isnt enough! but I dont need rediculous...

so i picked up a 4 bolt main 2 piece rear block to build

i will have it bored .030" over and i need to select a crank / piston set up, which is actually where i need help!!! what crank and piston combo i am lost and doesnt seem like that cheap compression calculator on summit can be very acurate.

oh and of course i am going to throw a wrench into the situation by saying that i already own these heads and need to use them

RHS-12303-01 they can be found on the summit website for reference

please help me

sofakingdom

iTrader: (1)

Compression calculators are RIDICULOUSLY simple. Doesn't matter if they're "cheap" or not. In fact, the one I use is free, because I WROTE IT.

They will all give exactly the same answer, if you give them the same data. That's the beauty of numbers, in general. It's why I LOVE math and science, and HATE psychology, political science, and stuff like that, that involves HUMANS. The one, you put the inputs in, crank the handle, and the same answer pops out, EVERY TIME. You don't have to worry about if the "7" in the one had a bad childhood due to an "emotionally unavailable" father, or the "3" in another thinks it's female instead of being satisfied with being biologically male, or the "8" somewhere else is Gen X or Gen Y. Numbers are EASY and SIMPLE for someone intellectually challenged like me. I'm sure they're even easier for someone intelligent like you.

The usual errors with compression calculations involve "assumptions", or simply leaving important data out. The most important data that usually gets left out is the deck clearance. "Forgetting" that can create MASSIVE errors; easily a full point in some cases. Another "overlooked" factor is the variation in head chamber volume cc's; ALOT of things can change that. Even, the brand and/or part # of the spark plugs; but mostly, how far the valve seats have eroded into the casting or been ground, and the shape of the valves (flat or tulip face, for example) or their thickness.

Basically it's IMPOSSIBLE to "calculate" an engine's compression ratio without ALOT of ACTUAL MEASURING. Looking stuff up in catalogs only gives a rough idea.

THose heads have 72cc chambers, nominal. They could vary somewhat from that but suffice to say the chambers are LARGE. This will make it hard to get the compression high enough. Use a rotating assembly kit that DOES NOT have cast, hypereutectic, or TRW/SpeedPro forged pistons in it. Those all add .020" to the already excessive .025" or so of stock deck clearance (which is how far below the deck the pistons are at TDC). Using any compression calculator of your choice, enter .025" (stock) into it and 72cc chambers, and see what your CR will be; then enter .045" (what you will end up with by using "rebuilder" pistons like the ones I mentioned above, instead of RACE ones); then enter .000", which is what your professional competitor (me) in the lane next to you will be running. Pretty easy to see how DRASTICALLY WRONG a "calculator" can be, by leaving out stuff.

What's your intended goal for this engine? What's the budget, which must include both the parts (the less important part) and the machine work (where your primary emphasis should be)? What converter and gears?

dvardaman85

gears 3.42, stall 2500 to 2800 not sure, machine work is $225 for Clean, Check, Bore, and Clearance (however machinist specified i must have the rotating assy available as so he could ensure perfect clearancing)

just a street car with some get up and go

would like to use this cam because of the small circle and because i have heard good results and a car that runs this cam. (summit pn : CCA-SK12-250-3)

if the block is not perfectly level or flat dont i have to get it decked or something? will that change my deck hight? semms like common sense but i could be wrong

i am no race car driver i just want a car i can put the easy efi system on and like to drive it on sunny days(and have fun doing it)

dvardaman85

oh and as for budget i think a couple thousand will be ok as long as i get the rotating assy (balanced) and the cam for that

if possible of course

dvardaman85

ok so what i guess i need to know whould i return the 72 cc's and get the same heads intake exhaust and valves, except 64cc's?

cause i need to do it now

Orr89RocZ

iTrader: (20)

zero deck the block with the rotator of your choice. That cam will compliment a 383 well and being that those heads are IRON, i'd shoot for 9.5-10.0 to 1. Flat top -5cc relief 383 piston will give 10 to 1 with those heads at 72cc.

-12cc mild dish will give 9.4 to 1... so basically you want to be in that range, 5-12cc relief/mild dish.

If you are going EFI, then put the cam on a bit wider lobe seperation to minimize overlap at idle, or just go a step down on the cam. Those systems rely on o2 sensor feedback at all times and that kind of overlap cam may not idle that smoothly in closed loop depending on the system's electronics/fueling code.

I've run big cams on factory TPI type systems in closed loop just fine but I am not familar with the EASY EFI type systems out there that tune for you.

dvardaman85

ok thanks

that being said would anyone object to me running 62 cc heads on a 383 seems like it would be over 11.5:1 with those heads

is that ok

Orr89RocZ

iTrader: (20)

with aluminum heads that would work with the right cam and good tune, but not with iron on pump gas.