how far in the hole we're the 400's

[freightdriver]

i've got a 400 that had came with my challenger the motor was already built i've only got an old reciept from the engine builder and other then what i've done and the cam is the only specs i've got on it are the cam...

well anyways i was getting ready to put the top end back together afew days ago and noticed at tdc the pistons are about .120 or so in the hole according to the mic is this about correct from the factory? the pistons in it now are some forged flat tops and i'm not sure if its been decked... also i had seen a formula online for figureing out cr does'nt anyone have it??? Thanks Josh

[Killer_Mopar]

Yeah that sounds about right, I have heard of them beings over 1/4" down....simply werent a high compression engine.....

[Follicly Challenged]

Freightdriver; The only "true" way to do a compression ratio test is to pour a "downfill volume" using a burrett for CC calibration. Sans that, to do a "ROUGH  mathematical" calc., do this;

Remember this is ROUGH, but good enough for here, to get a ballpark on what you're working with,

(pie) X R2 X stroke = Volume

Then; Volume X 1000/ 61 = CC

For Example on a .030" overbore 400 engine, with the piston .120" downhole as you've indicated

Bore 4.370"/2 = 2.185  is the "R" or "Radius"
Stroke = 3.38"
Pie = 3.14 ( rounded off for this)

Now;

Radius X itself, ( 2.185 X 2.185 ) = 4.774 X 3.14 X 3.38 = 50.669 "cubic inchs of swept volume per cylinder.
Now to convert to CC.   50.669 X 1000/61 = 830.6 cc SWEPT VOLUME

Now to calculate C.R. using a swept volume PLUS the clearance volume, then divide by the clearance volume to get a compression ratio.

We'll "guess" here that you're 400 has a head volume(open chamber) around 85 CC, and that the head gasket is a volume of 10 cc, then add a downfill volume of 29.4 cc for the .120" downhole that your pistons are as you indicated, thats 2.45 cc per .010" of volume using the same above formula, for a TOTAL CLEARANCE VOLUME OF 124.4 CC

Therefore; SWEPT VOLUME + CLEARANCE VOLUME = TOTAL VOLUME, DIVIDED BY CLEARANCE VOLUME = COMPRESSION RATIO.
or  830.6 + 124.4 = 955/124.4 = 7.67:1 Compression ratio

Not good news I know, but if as you said, the pistons are indeed .120" downhole @ TDC as you've indicated, and Flat-tops, then thats approx where you are. The problem here is that when the engine was rebuilt it was far too common for rebuilders to access the "jobber" style rebuilder pistons that were indeed the .100 - .120 downhole as you've indicated. They wanted good running engines, unfortuneately, although indeed they run well, power output is "anemic" at best.

Sorry, to be the "bearee" of bad news, but I've seen this a hundred times before on "rebuilder" style engines, as I was an engine rebuilder years ago. Not that compression is "absolute" to make power, but lack of it will limit total output.

Hope this helps, Bob out.

[4Cruizn]

Great info Bob!!  Thanks!!      

[freightdriver]

well its got the 346's in it  oh well i'm goin to see about upping the compression on this thing alil bit more...

[Chryco Psycho]

http://www.csgnetwork.com/compcalc.html

[Follicly Challenged]

How you gonna go about putting some "wind" in it ? Any ideas ? Lots of people find that putting a set of closed chamber heads onto the lower CR motors "wakes 'em up" a bit.
Usually, 915's are expensive, 516's are a little more reasonable. Either way, make sure if you use either, that they have the larger 1.74" exhaust valve already installed. Most came stock with a 1.60" exhaust, Too small.

[Chryco Psycho]

the 516 heads can easily accept hard seats & 1.81 valve which is the common modification I make when using 516 heads 

[Follicly Challenged]

Yep, the 1.81" goes in too, seems to have been a great valve, for all iron BB heads, even without a bigger intake.

Ever try a 50 deg seat on the 1.74 ?

[greengochallenger]

So the only difference in the 915's and 516 are the valve size? For some reason I thought that the exhaust ports were different

[Chryco Psycho]

floor of the intake port has a rise in it on the 915 head

[Follicly Challenged]

915 is the same intake port as a 906,  ( "huber hump" port floor ), albeit the 915 is the closed chamber version.

516 has very flat intake port floor. Standard valve sizes on the 516 and 915 were 2.08/1.60, EXCEPT, as delivered on the '67 HP & GTX engines, where the 915 "stock" exhaust valve size was enlarged from the factory to 1.74".

All '68 and up heads retained these 2.08/1.74 valve sizes, just curious to note that the HP 67 915 head was the first to use that bigger stock exhaust valve.

The exhaust ports have changed slightly over the years, albeit, "slightly".

Hope this helps,

Damn I wish I could figure out posting photos !

[freightdriver]

i'm puttin goin to put the motor away for alittle while so i can ponder wtf i'm goin to do pretty much 2 options really throw it together again after checking all clearances and just run it for alittle while or just save up alittle more $ and do it once with a 440source 500 long rod kit and an updated cam then the 484 mp... my 346's already had 2.18's and 1.81's installed with alittle bit of porting so it should be a respectible motor either way i build it but the 500ci would make it soo much easier to exceed my goal of 11.49...

oh yeah started dissassembling the interior to clean,paint and rewire what was screwed with and found my buildsheet inside the seat    too bad it was missing the part i really wanted the paint codes which i know to be FY1 would've been nice though:P

[71383bee]

On average what head volume do the 516 heads run?

[Chryco Psycho]

78-80cc