miling block part II....compression ratio?

[Old Guy with 87 GT]

i'm currently having my moter rebuilt and was curios about what kinda comprssion ratio i'm going to come up with.......

i had my engine 0 decked and the decks milled to bring the pistons up higher in the hole
.....i didn't have any specific numbers on the last post ....but got some more info thursday

the engine is bored to a 306 with speed pro forged pistons....flat tops with 4 valve reliefes .......i chamfered the edges of the valve reliefes ,but not enough to effect there ratio much .........according to speed pro there advertised compression with a 63.5cc head is 9.2 -1 ....(part # L2482f )

my e7ets are milled 020 witch should bring them to around 60.5 cc s
....according to my engine builder he milled my block so the pistons are .030 in the hole ......took like .030 off one side and .040 on the other ,reducing deck hight to around .035 shorter then stock........im using the felpro performance head gaskit ...think there like .040 crushed thickness ...not sure

my best educated 'guess' would be around 9.7 to 1 .....but i'm not sure how to calculate it .....any help would be appreciated

.........my engine builder could calulate it for me ,but he's pretty busy so and i didn't ask him to come up with an exact number ......as long as it's not higher then 11-1 it's not a big concearn .....but i'm still curios

also any body with experience doing this when building a moter ,feel free to comment on the plus/minuses of 0 decking ........i'm still pretty new at this so if there's anything i should know chime in

other specifics ..
e-cam (already checked for valve clearance)
had moter balenced
i'll be using the stock crank and rods(returned and resized)
arp head stud kit and arp rod bolts added
using plasma moly rings(file fit) and clevite bearings
everything else new (in short block)
see sig for other bolt on specifics

thanks in advance ...james

[Jeff Chambers]

I think your engine builder is a little off. A factory motor has the slugs .010-.020" in the hole. The nominal deck height is 8.200" to 8.210" (technically its 8.206"). Subtract from that half the stroke 1.500", rod length 5.090" and compression height 1.600", and it leaves the pistons at 0.020" maximum in the hole, 0.016" nominally and 0.010" minimum. I've always found them to be close to the .010" though.

With things zero-decked, a .043" thick gasket and 60.5cc chambers you're looking at just a hair under 9.5:1.

I've got a compression ratio calculator on my site under the Tech Articles section if you'd like to run the numbers yourself.

[302 LX Eric]

I agree.

Here's another quick link for calculating compression ratio...

CR calculator

Also, I wouldn't be afraid to ask your engine builder to calculate your final compression ratio...that should be routine.

E

[Old Guy with 87 GT]

ok ....you can tell i'm pretty novice

i might have the decimals at the wrong place ...... he said it was three thousants in the hole ........origanally i told my builder to 0 deck and that i wanted the pistons as far up as i could safly get away with ,he mentioned leaving a tiny amount incase i ever had to resurface my deck .....

could this have meant .0030 in the hole ?
or are my pistons farther down then the stock ones??......the moters assembled and the tdc pistons are flat with the top ,theres no noticable diiference in the deck and piston tops .........this is what i thought 0 decked was

i'm kinda lost now and probly sound like an idiot ,
.....the guy building this motor does alot of sprint car work and has alot of experince and peaple that swear by him ...i trust his work,..... but the guys got a pretty small operation and has been very busy so i just haven't really sat down and asked him to teach me all this stuff .......lord knows i allready asked a milion questions ,...thats why i didn't ask for exact numbers ,its not that big of a concearn unless its to high ............9.2 -10 to1 is what i was aiming for ,...the speed pro flat tops were chaep enough that the compression was kinda left open to whatever it came out to

i was just curios though ,... thanks for the insights and links and if you guys got any other opinions or insights,then post away ....i'm always open to learning

[lx mike]

lord knows i allready asked a milion questions ,...thats why i didn't ask for exact numbers ,its not that big of a concearn unless its to high ............9.2 -10 to1 is what i was aiming for

Ask 2 million questions if you want. thats the only way you will know what you have unless you do it yourself.


as for your compression ratio, you hit your target range. only way to know exactly is for your engine bulder to measure eveything. only then will you know the exact number but i'd say your in the range you wanted

[Old Guy with 87 GT]

i got the chance to check out both compression ratio calculaters .......theres a few specifics that i'm not positive about with the new pistons though......i'm thinking it'll come out to around 9.4-1..... 9.5 -1 .....my pistons have a little bit bigger valve reliefes and i'm not sure of the ring hight

i'm going to see my engine builder tonight so i'll see if i can clear things up with the deck hight .....i'll see what i can find out and update ........2 million questions.........i'll probly ask three million by the time i'm done

thanks for the help

[Old Guy with 87 GT]

update ,i just got the decimals in the wrong place up above

typed in my info and it calculated out to 9.46 to 1 ......thanks for the info guys

[srv1]

OG, i have a question for you. Why not invest in some GT"P" heads? i was told stock, unported "P" heads milled .030" bring a stock engine like ours without decking the block, around 10:1. Jeff can clear this up. I think he is using them. I just bought a set of "P" heads. i was told .030" is cutting it really close with stock cam w/ 1.7's, but i hear alot of people are doing that. well it sound pretty sweet setup you have. let me know when you get all the figures.

James

[Old Guy with 87 GT]

i've had many discussions about the same subject ,....your correct about the compression ,the p heads have like a 58cc chamber stock

i haven't gone with them cause of budget reasons ,....this is how i see it .....i can take a set of e7ets (25$ each)....put good seals in em(60)do a valve job on em(50$) ,mill em,(50$) port em (free)and and put good springs in em(110).....then they'll run pretty good (320$)

where as if i buy the p heads new ,then do work to em i'd be within 500-600$- range,then i'd have to buy headers 175$ ........(700-800$).......they'll run better then e7s but it would seem that the margin is to small to justifie the expence ......most of the cars i've seen run p heads aren't going that much faster with a mild combo like mine ........it be more economicle to spend a few humdred more and get a set of afrs..........witch at this piont in time i can't afford either

btw ,what are you doing with your old e7s?

[srv1]

OG, i dont think the margin is small. people are running unported "p"'s hitting high 12's. yeah you got to buy headers and that does suck. I may take the motor in my car now and put it in my tbird. i have 89 shortblock sitting in my garage that i may make a 306, with speed pro forged pitsons, Stage 2 Summit rods, and polish the stock crank and rebalance it with a girdle. Why do you ask?

[Old Guy with 87 GT]

ya ,i can see where your coming from ..........

thing is peaple are running ported milled e7tes and hitting high 12s too ,.....difference is peaple with e7s only paid about 400$ to hit high twelves ........now the p heads do have more potencial if you port and mill them ,but by the time you buy the heads,headers and put down money for machine work your going to be in the 700-900$ range ,depending on your resources ,....well at that price your in the price range of a better out of the box head,like the roush180 or the or wndsor jrs ,witch will run low twelves out of the box..........and you don't get stuck with usless headers if you upgrade

don't get me wrong ,i'm not downing the heads ,there good heads and if you buy em used ,there a good value ........but for me ,if i upgrade to better heads then the e7s it would be worth waiting a couple more weeks till i could afford better heads .....like afr 165s...

[Old Guy with 87 GT]

ps......i was wondering about your e7ets caus a friend of mine has been tinkering with em ,and i told him i'd keep my eyes open for used ones

let me know how you make out with building that 306.......i had to regrind my crank and go with over size bearings .....200,005 thousand miles will do that to a crank .........i went with the origanal rods but put arp bolts in them .....from what i gather the bolts are the weak spot ,change them and the origanal rotating assembly are good to 300, 350 at the wheels easy