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Old 04-13-2011, 02:15 AM   #1 (permalink)
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347 compression HELP PLEASE!

Hey guys, I'm new here but I've been reading a lot and I can't find an answer....
I'm building a 347 stroker here is what I have so far:
Seasoned 302 block machined for 347
Carbed with 750 cfm double pumper
Scat cast iron rotating assbly
Srp premium forged pistons (dish)
scat I beam rods
"All" ARP bolts
DSS main girdle & Windage tray
Melling high volume oil pump
7qt moroso oil pan
Comp cam lifters roller
Victor jr intake
Comp cam xe282hr cam
Now according to scat if I use 58cc head my compression ratio will be 9.6.1 or I can use 64cc
and drop it to 9.0.1 (piston rings are set for max 150hp nitrous which is what I want N/A with nitrous
Is that compression good enough for 400 or 500 hp N/A ??? I'm going crazy here... I also don't want it to be a pig without the bottle... If not is that cam good for blower? Thanks anything helps a lot of money on hold sitting there...
All parts are new!
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Old 04-13-2011, 06:49 AM   #2 (permalink)
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Re: 347 compression HELP PLEASE!

Make sure you do the math yourself..When I was looking at rotating assemblies I noticed that many of the ones I was looking at on paper didn't add up when I actually did the math myself....Do you have the shortblock already?..If so measure how far down in the cylinders your pistons are and then add your headgasket thickness as well as the dish size of your pistons...Then you can decide on a cylinder head chamber that will put you where you want to be...
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Old 04-13-2011, 07:19 AM   #3 (permalink)
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Re: 347 compression HELP PLEASE!

Use the 58cc heads and keep the compression up. look into the AFR 185 heads.
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Old 04-13-2011, 10:06 AM   #4 (permalink)
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Re: 347 compression HELP PLEASE!

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Originally Posted by cmefly View Post
Use the 58cc heads and keep the compression up.
Absolutely!
Compression is your friend....it increases vacuum and aids in power and drivability.
Assuming 58 cc chamber,.038 gasket,5cc valve reliefs,and the piston at zero deck, you'll be in the 10.8-11:1 range. Now factor in the 70* abdc intake valve closing of the 282xe and you'll cranking compression around 175 psi. That should be fine on 89 octane,definitely safe on premium.

I don't think you'll make 500hp na,buy with the right head 450-470 should be doable.
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Old 04-13-2011, 10:34 AM   #5 (permalink)
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Re: 347 compression HELP PLEASE!

Your question is not answerable without knowing the volume of the dish on the pistons.

If you use Excel....tell me what version and give me your email address and I'll send you a file that will compute C/R for you.

Incidentally...you will NOT make 10.8:1 with a 58cc head and a dished piston, unless there is some form of quantum entanglement involved that we are not aware of.
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Old 04-13-2011, 10:37 AM   #6 (permalink)
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Re: 347 compression HELP PLEASE!

Oh...and I used that cam in my kid's 347, NOVI 2000 motor. It works fine.

454 RWHP @ 6 pounds.
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Old 04-13-2011, 11:49 AM   #7 (permalink)
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Re: 347 compression HELP PLEASE!

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Originally Posted by Mark O'Neal View Post
Incidentally...you will NOT make 10.8:1 with a 58cc head and a dished piston, unless there is some form of quantum entanglement involved that we are not aware of.
Yep-I missed the "dished" in parentheses.

To the OP,why are you using a dish piston. Is a blower definitely in the future?
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Old 04-14-2011, 12:56 AM   #8 (permalink)
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Re: 347 compression HELP PLEASE!

I will get that info asap on the reliefs of the dish... I was originally going for n/a with nitrous I don't want to go with blower. You say to do my own math ok what's the formula, and what is a good compression that will get me in the 500 hp range? Thanks for all the help guys big time!!!
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Old 04-14-2011, 01:02 AM   #9 (permalink)
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Re: 347 compression HELP PLEASE!

Oh and the bad news is the shortblock is already assembled, I hope I don't have to change the pistons? : / just trying to figure out what to do now about the cc's of the heads I choose... I want 185 afr's but the whole compression thing is killing my build time... Plus my brain! lol anyway, thanks again guys...!
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Old 04-14-2011, 01:09 AM   #10 (permalink)
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Re: 347 compression HELP PLEASE!

Mark thanks my email is onielrod7@aol.com thanks! WOW the people on this site are great guys! I appreciate all the help....
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Old 04-14-2011, 09:12 AM   #11 (permalink)
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Re: 347 compression HELP PLEASE!

I am sure Mark will get you setup with a formula for calculating everything..One thing that you need to calculate since the shortblock is done is how far down in the cylinder the piston is...You will need a piston stop that bolts across the cylinder..I made one from a scrap hunk of angle iron long enough to span across the cylinder and simply threaded a bolt into the middle section that would contact the piston....Bring the piston up close to TDC and screw the bolt down till it just touches the piston..Then mark your balancer in line with your timing tab in the front cover..Next turn the engine backwards until it contacts the piston stop again..Now mark your balancer again..Half way in between your two marks will be true TDC..This is also a great way to verify if the TDC marked on your balancer is accurate or not...If not make note of where TDC actually is on your balancer so you will be able to set the timing properly when the engine is running....Once you have found true TDC you will need to check how far the piston is below the deck surface(if any)..You can use a dial indicator or feeler gauges to measure the difference between the deck surface and the edge of the piston....Here is a pic of the crude but effective piston stop that I made using a piece of angle iron,two sockets and head bolts.....

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Last edited by frdnut; 04-14-2011 at 09:17 AM.
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Old 04-14-2011, 10:55 AM   #12 (permalink)
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Re: 347 compression HELP PLEASE!

Quote:
Originally Posted by 347FOX View Post
Oh and the bad news is the shortblock is already assembled, I hope I don't have to change the pistons? : / just trying to figure out what to do now about the cc's of the heads I choose... I want 185 afr's but the whole compression thing is killing my build time... Plus my brain! lol anyway, thanks again guys...!
Don't let it bother you....it's not bad news for us.....

It will cost you about 40 horsepower.....but, if I'm in the next lane, it's good news for me!:lol
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Old 04-14-2011, 12:59 PM   #13 (permalink)
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Re: 347 compression HELP PLEASE!

Actually, don't let it bother you as your HP losses by using less than optimal compression (roughly 1.0:1 less) will only lose you roughly 16 hp based on accepted formulas and rules. You can easily lose far more than that in basic fuel, ignition, and cam tuning. Lower static compression losses can often be recovered with altered ignition timing and/or cam timing to result in the same dynamic compression, cylinder pressure, and torque. Carry on, match your components in the whole vehicle to work together, and don't go cheap when it comes time to tune your engine and chassis for your power. You're fine.

David
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Old 04-14-2011, 07:40 PM   #14 (permalink)
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Re: 347 compression HELP PLEASE!

Here's one of the may calculators available online to give you a ballpark idea of your compression ratio.
United Engine & Machine Co. Incorporated

If the shortblock is together order your 58cc heads and put it together.
Changing pistons at this point isn't cost effective IMO.
You can always mill the heads to 54cc and pick up a half point of compression.
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Old 04-15-2011, 10:26 AM   #15 (permalink)
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Re: 347 compression HELP PLEASE!

Quote:
Originally Posted by PSIG View Post
Actually, don't let it bother you as your HP losses by using less than optimal compression (roughly 1.0:1 less) will only lose you roughly 16 hp based on accepted formulas and rules. You can easily lose far more than that in basic fuel, ignition, and cam tuning. Lower static compression losses can often be recovered with altered ignition timing and/or cam timing to result in the same dynamic compression, cylinder pressure, and torque. Carry on, match your components in the whole vehicle to work together, and don't go cheap when it comes time to tune your engine and chassis for your power. You're fine.

David
It'll do....but you use your calculations...I'll use a dyno.

It's 40+

But perfect for a blower....
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