Need help in figuring comp. ratio on a sbc.... - Chevelle Tech
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post #1 of 12 (permalink) Old Oct 15th, 19, 2:06 PM Thread Starter
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Need help in figuring comp. ratio on a sbc....

Guys, I acquired a machined sbc 400 block, crank , rods and pistons and am trying to figure out the compression ratio. In putting in the numbers into on line calculators , I am not sure of some of my figures. Based on my figures it came out to 8.7, not what I want. So, are my figures in the ball park. Bore is 4.185, that is stock, 4,125 plus .060 overbore. Stroke is stock at 3.750. Rods are stock at 5.565. They ask for head gasket bore, which I put in 4.185, not sure on that. They ask for compressed gasket thickness. I put in .040 as I do not know what a .040 gasket will compress down to. The block was not decked but rods were resized , so I put in .011 as a guess of how far below the deck the piston will be. Would another number be a better guess. Not finding any info on the pistons, they are Federal Mogul 400NP dished. I did a crude cc check and came up with 40 cc s to fill the dish . So I entered a plus 40 cc for that. Not sure if that is correct for that calculation .. Next for head cc i put in 58 cc as I plan to run a set of GM vortex heads. Are my figures in the ballpark or do you see something that needs correcting......thanks.

My plan for this engine is a low rpm high torque motor. Im thinking I want to be in the 9.5 to 9.8 ratio. The KB piston catalog list their 12cc D piston as being at 11.6 comp. when used with a 58cc head. That piston looks to be as dished as mine that held 40 cc s of fluid. I understand when I get a piston in a cylinder I can have better numbers to work with but trying to determine if the on hand pistons will need to be replaced or will they get me in my ballpark. thanks.......
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post #2 of 12 (permalink) Old Oct 15th, 19, 2:56 PM
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Re: Need help in figuring comp. ratio on a sbc....

That is a generic "destroked rebuilder" piston with a c/h of 1.540 which puts it about .045 down in the block that is not decked
Horrible choice for any kind of performance usage
Interchanges to a Sterling 400AP but I don't know the volume of the dish

The KB pistons are backwards from everyone else they use + for a dish & - for a dome

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post #3 of 12 (permalink) Old Oct 15th, 19, 4:36 PM
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Re: Need help in figuring comp. ratio on a sbc....

Quote:
Originally Posted by Wolfplace View Post
The KB pistons are backwards from everyone else they use + for a dish & - for a dome
KB is right, dish adds volume to the chamber dome subtracts.


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post #4 of 12 (permalink) Old Oct 15th, 19, 9:28 PM
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Re: Need help in figuring comp. ratio on a sbc....

Stock cast 400 pistons (400AP) have a 24cc dish, and as mentioned .045 deck clearance if std deck. With stock smog (76cc chambers) heads the compression would be about 8.2:1 - 8.3:1. A Vortec head would bring the compression to around 9:1. Sonic check the block before you spend too much on new parts - 4.185 is .060 over bore, that is a lot on a 400 - I personally do not like going over 4.165 (.030 over) and try to get away with 4.145 (.020 over) when ever I can!
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post #5 of 12 (permalink) Old Oct 15th, 19, 10:20 PM Thread Starter
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Re: Need help in figuring comp. ratio on a sbc....

Thanks all, I acquired the block , crank and rod/piston as already machined. A project a friend never got to. I ll see if he had the block sonic checked, but prolly not. I should have that done. 9.1 is better than 8.2 but looks like new pistons needed to get me up to 9.6 to 9.8......thanks....
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post #6 of 12 (permalink) Old Oct 16th, 19, 1:13 AM
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Re: Need help in figuring comp. ratio on a sbc....

pistons with a 12cc dish are popular for this type build. Look out for the compression height, it's supposed to 1.560, many most rebuilder type pistons are less, sometimes much less. you want a full height piston to build compression. You can find real-world specs for most pistons online if you look a little.

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post #7 of 12 (permalink) Old Oct 16th, 19, 1:32 AM
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Re: Need help in figuring comp. ratio on a sbc....

I wouldn't have the block sonic checked unless you know - really know - somebody at the shop. ask your self what's in it for them to tell you it's OK? Nothing except liability if the deal goes south.

BTW, you'll need either 400 rods or or aftermarket stroker rods to avoid a serious session with a grinder opening up the bottom of the block. 350 rods will hit the oil pan and the skirts at the bottom of the cylinders. maybe also the cam lobes. the Scat stroker rods in either 5.7 or 6.00 length will eliminate a lot of the grinding in the block, but you can't assume anything. Trial builds with checking are where it's at.

these rods will work work with the stock style 1.560 height pistons, still have to check clearance at the pan rail, the cam tunnel and the cam lobes.

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post #8 of 12 (permalink) Old Oct 16th, 19, 8:12 AM
 
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Re: Need help in figuring comp. ratio on a sbc....

hi
If u are going to replace pistons and maybe rods do a price check on a full balanced rotating assembly .
Can have a hyper pistons with stroker I beams and cast crank all balanced ready to go ..at a fair cost .
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post #9 of 12 (permalink) Old Oct 16th, 19, 10:35 AM Thread Starter
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Re: Need help in figuring comp. ratio on a sbc....

I plan to stay with the stock block, crank and rods. The KB catalog shows their KB159 piston, 12cc D cup, l.561 compression height, for stock stroke and rods is at 11.6 compression ratio using a 58 cc head. I am planning to run a new set of vortex heads on this build but 11.6 is way over what is needed for a low rpm stump puller. Kbs next up piston, a flat top takes it to 12.3, and next up .150 dome goes to 14.1 compression.

Just trying to understand why their 12 cc dish piston is at 11.6 and my measured 40cc dish is at 8.2 c.r. I feel i am not measuring something right for the comparison. thanks all....

Sound info Tom, as I have just moved to a real small town and just now met the local machine shop. Small shop, only 2 guys, and the machinist is pretty backed up. Im on week 3 waiting on a bbc rebore and hone. Pretty sure a sonic check would be non productive. So, will risk a low po build on this one.
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post #10 of 12 (permalink) Old Oct 16th, 19, 1:59 PM
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Re: Need help in figuring comp. ratio on a sbc....

Quote:
Originally Posted by BanditRX View Post
I plan to stay with the stock block, crank and rods. The KB catalog shows their KB159 piston, 12cc D cup, l.561 compression height, for stock stroke and rods is at 11.6 compression ratio using a 58 cc head. I am planning to run a new set of vortex heads on this build but 11.6 is way over what is needed for a low rpm stump puller. Kbs next up piston, a flat top takes it to 12.3, and next up .150 dome goes to 14.1 compression.

Just trying to understand why their 12 cc dish piston is at 11.6 and my measured 40cc dish is at 8.2 c.r. I feel i am not measuring something right for the comparison. thanks all....

Sound info Tom, as I have just moved to a real small town and just now met the local machine shop. Small shop, only 2 guys, and the machinist is pretty backed up. Im on week 3 waiting on a bbc rebore and hone. Pretty sure a sonic check would be non productive. So, will risk a low po build on this one.
If you are using a GM Vortec head it should be 63-64cc not 58
If you are not decking the block the nominal height is 9.025 not 9" where most aftermarket pistons are speced for compression
All this stuff makes a difference
the 12cc dish with an undecked block, 040x4.200 gasket & 64cc heads will be 10.1 assuming the block is actually at 9.025 which is a huge assumption as the majority are not straight or the same side to side & corner to corner

I seriously doubt your 400np piston is 40cc.
24-25 sounds more reasonable
Also don't forget it is "destroked" at 1.540 as I said before which makes for really ugly quench
With a few assumptions,,,,, reasonable guess,,,,
Compression with a standard deck block, same gasket & 64cc head & your pistons would be 8.8

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post #11 of 12 (permalink) Old Oct 16th, 19, 2:24 PM Thread Starter
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Re: Need help in figuring comp. ratio on a sbc....

Thanks Mike, for some reason I got 58 cc in my head when I purchased the new vortex heads. My error. I did not know how to measure the dish in the pistons other than use a plastic plate on top of the piston with a small hole in it and count the fluid till it flatted out. Thats where the 40 cc came from. Your best guess , with out knowing for sure exact measurements , is 10.1 with 12cc pistons is much closer to the KB spects of 10.8 is mo better in the ballpark I am shooting for. Thanks much.
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post #12 of 12 (permalink) Old Oct 16th, 19, 3:54 PM
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Re: Need help in figuring comp. ratio on a sbc....

Are they Vortec heads for a 305? They might be 58cc
If you want the c/r under 10-1 with a 64cc head you'll need a 22cc dish, but you'd have to use 5.7 rods. Looks like the only offer 12cc pistons for stock rod length.

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