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Discussion Starter · #1 · (Edited)
trying to get some new heads soon for my 351w and from reading other threads, it looks like i need to know what my cuurent comp ratio is in order to make a correct purchase. am i right so far? to make things more difficult, the motor is a 74 and the current heads are from 78. from looking online, in 74 they came with flat top pistons. well mine arent flat tops so that makes it even more confusing. below is a picture of the top of the pistons. i dont know if that will help at all. but if anyone can lead me in the right direction, that would be great.

thanks

 

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trying to get some new heads soon for my 351w and from reading other threads, it looks like i need to know what my cuurent comp ratio is in order to make a correct purchase. am i right so far? to make things more difficult, the motor is a 74 and the current heads are from 78. from looking online, in 74 they came with flat top pistons. well mine arent flat tops so that makes it even more confusing. below is a picture of the top of the pistons. i dont know if that will help at all. but if anyone can lead me in the right direction, that would be great.

thanks

only 1969 came with flats , all others are dish . you need to measure or find the specs for pistons you have
 

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You need to know which pistons you have. Yours look like the .19 inch dish which I think was around 20 cc's. I've only had 2 351W's so I don't know the history of all of it. They threw in marine applications too which added to it all. I will guess that you have 69-70cc heads. That would have made your engine an 8.1 compression job.

You can cc the head yourself with a piece of plexiglass. Drill a hole in the center, lightly coat around the combustion chamber with vaseline, stick it down, get a horse syringe from your vet, inject water in there and keep track of the cc's. Obviously, you need the valves and a spark plug in the hole.

Check your piston to deck height. Bring one up to tdc, throw a straight edge on the block and use a feeler gauge under it to check the height of the tallest part of your piston. I'm not for sure what that would measure. Maybe something in the .015 range?

Once you know all that you can go play with the calculators. Even Summit Racing has one. Somebody must have gotten java savvy on their website.
Compression Calculator - SummitRacing.com
 

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Agree with all below but wanted to point out something before you get too excited about new heads. With a dished piston like you have I doubt you're going to find any head with a small enough combustion chamber to bring up the CR much at all - assuming that's your plan.

If you're just looking for a replacement head those can be had any day at a salvage yard or online ad. What is your plan for a build, performance, stock or something in between? Of course if you're going blown you're ahead of the game if they're forged pistons! :tup:

John
 

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Discussion Starter · #5 ·
well the motor is completely back together right now, so measuring the dish wont be possible at the moment. the ONLY thing that i know about them is they are still a 4'' bore piston. so the motor has not been bored at all.

john i dont want to just put replacement heads on it. i am trying to get some power out of the motor to go along with my cam, intake, carb and headers. not looking for a track car or anything wild. but i would like a nice street cruiser with a decent amount of power to play with
 

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Those are the early bathtub dish pistons from '69-'74. The '69 had flats (with eyebrows) for the M-code 4v engine only, and only one year. All other years were H-code 2V, and the M-code 4V's 1970-up became 351C. Anyway, a '74 engine would have those pistons and D0OE heads with 60-61cc chambers for about 9.5:1 compression. You can figure from there. BTW, the flat-tops in '69 gave 10.7:1, used a different cam and ignition timing curve to help avoid detonation, and you can see it takes a dish that large to drop it 1.5 points.

David
 

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Discussion Starter · #7 ·
ok, that is kinda helpful i guess. haha. im just a bit confused on how i now use that info to purchase heads...i know i dont have 74 heads, i actually have 78 heads on it right now (what came with it). so lets just assume that i have no heads at all. and im strickly going off of having the "bathtub" pistons. what do i now look for when purchasing heads? if it makes a difference, i will most likely be buying a empty set of heads so that i can put on my own springs and rockers of my choice.
 

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Discussion Starter · #8 ·
ok after reading more. i have a little more info to go off of. the stock 74 block (if using stock 74 heads) was 9.5 to 1. the 79 heads that i have on there right now are 67.5-70 cc heads. i guess im still not any farther than i was before. but is there a way to figure out what my current cr is with that info?
 

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Discussion Starter · #9 · (Edited)
using the calc on fordmuscle.com i used 4'' bore, 3.5'' stroke, combustion chamber size of 68cc (for my 79 heads), -.19 for the piston volume (got .19 from hottarods post), and .047 for gasket thickness. hit calculate and its saying 10.25:1.

AGAIN I HAVE NO IDEA IF I DID THAT RIGHT THOUGH
 

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My understanding is that in 69 & 70 the 351W 2V had 9.5:1 compression with 60cc chambers. In 71 they increased the deck height, which brought the comp. ratio down to 8.5:1, or so. If Kenny's current heads have 68cc combustion chambers, and with the tall deck (74) block, why not just get any 60cc head and get on with it.

You could go around the block, over and over again, talking about which head is better, AFR, TWS, RHS, Edelbrock, etc., but unless you buy sets of each and try them back to back, how will you ever know the difference. You could probably buy Edel E-Street heads for half the price of AFR's, but pulling that big car street light to street light, would it make a real world difference? The AFR guys will say AFR all the way, The twisted Wedge guys will say TWS is best. Just about anything will be better than what is there now and 10 - 20 HP at 6,000 RPM won't be a big deal cruising on the street. What do you want it to do?
 

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Discussion Starter · #11 ·
im not arguing about which brand head i should get. im more concerned about my compression ratio. i need it in the pump gas range. and if im using the comp ratio calculator right, with my current pistons which hottarod says have a .19 dish to them, then if i run 60cc heads it will put me at 11.38:1. that isnt pump gasable is it? i only have the options of 87, 89 and 91 octane.
 

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I think you should have entered - 20 cc for piston.
.039 compressed head gasket and .015 deck height(average guess).
70 cc chambers
4" bore and 3.5 stroke.

I get 8.13 on Ford muscle and on Summit. 60 cc heads would bump you up to 9.19 assuming the other numbers are correct. 60 cc heads and flat tops with valve reliefs would put you up around 10.48. I used 8 cc's for valve relief in case you need to fly cut.

Keep in mind that this is static compression ratio and not motoring or dynamic. The cam affects that.
 

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Discussion Starter · #14 ·
ok after changing it to what you are saying yea i got the same thing as you. now the heads i plan to get, which i know everyone hates on but i dont care. are 58cc chambers. its what i can afford and its my car anyways. so with the 58cc chambers it should bump me to 9.09:1. which gas do i need to run with that
 
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