early 273 pistons.

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I cc'd my heads too, 58cc's. Took a clear CD blank and vaselined it to the level chamber. Took a turkey baster full of distilled water and weighed it then filled chamber up to the hole making sure there were no air bubbles, filled it so there was the slightest miniscus and then weighed the baster again. 58g short, so thats 58cc's. No drips out the ports so I believe that is pretty freakin' close. Nail head valves would pick up some volume.
 
That makes a difference. I ran the SCR with 58 cc chamber volume, -.020" deck height and a .021" think shim head gasket with 3.8" gasket bore, and came up with 8.75:1.

But I cannot see how the '68 2 BBL open chamber would be 9:1.... I cc'd 2 holes on one head off this '68 2 BBL 273 and they were 68 cc's. (675 heads). With the .021" shim gasket, .00" deck height, and 68 cc chambers, that works out to 8.1:1. I don't think your info on the later 2BBL 273 is correct.

BTW, I assumed 7 cc in eyebrow volume for these latest calcs. I've never actually measured them.
 
7cc's for the eyebrows? No way is it more than 12% of the combustion chamber volume: 1cc is about as big as a sugar cube! IIRC the entire 360 dish is a whopping 9.8cc's total. The eyebrow is only 1.350 wide, .500 tall and .095 deep (at deepest point). It (1) is about 40% of a 1.500 circle and the bottom is angled to a fillet so its not an easy thing to measure short of filling it with water and weighing the amount of water with the napkin piece you soaked it up with. My guess is less than 2cc's total. The pistons are still in the block so I cant get them level to fill and weigh water volume...OR fill eyebrows with clay, scrape top flush with piston crown, dig out clay and drop in a ml graduated burette with a known quantity of water (or mineral oil for a smaller miniscus) , see how far it rises with the clay bits in it and you have your volume in ml, which are the same as cc's.
As for the available head gasket, they are now 4.060-4.175 bore with maybe a .040 compressed volume for an additional 8.5cc's or more.
 
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OK a very good observation; thanks. I have just been roughly estimating. I too your measurement numbers and remeasured here and got just a hair larger. Then computed the volume and came up with 2.2 cc's, so with the fillet, I bet 2 cc's is very close. That is good data to have so many thanks for guiding me to the light!

Just for more info, I have measured the volume in the valve dimples in the 675 heads that came off of this '68 273 and they were 2 cc's total for an intake/exhaust pair.

Reworking the SCR with:
- 2 cc's of eyebrow volume
- 58 cc heads
- .028" thick gasket and 4.1" bore (Mr Gasket 1121G)
- -.020" below deck
Now SCR is 9.06
If gaskets become .040" thick then SCR is 8.77. Both are pretty good numbers!

If I rerun our '68 273 with these eyebrow numbers and the 68 cc measured chambers and 2 cc eyebrows, I get 8.5 with .021" shim gaskets, 8.1 SCR with .040" composite gaskets (Felpro 1008), and 7.9 with Felpro 8553PT gaskets.

Question: Are you gonna run a turbo with 9:1 SCR? IMHO, that is higher than I would want. The late 273 SCR numbers with the composite gaskets look to be a better choice, IMHO. Or use a small turbo and limit the boost a fair amount.
 
I already have .040 gaskets and a 72mm turbo as well as a turbo grind camshaft. I will limit the boost as much as I feel is safe with a wideband controller and a wastegate.
 
Ah, good on the wastegate... that is the way to go, IMHO. If you start with a low boost limit one, you can trick it into working at a higher boost.... you probably know that....
 
nm9stheham said:
Hey Pishta, I just ran some numbers on this, and with .040" thick head gaskets, 62 cc chambers, and estimating about 8 cc's volume in the eyebrows plus in the ring around the perimeter, I come up with a static CR of 7.92. Even with .022" thick head gaskets, the SCR is 8.25.

So this does look to be the (a) lower compression combination. If it is tight, it may be just right for your turbo setup.

That all falls out of the pin height and the standard rod length, 1/2 of the stroke, and standard deck height. I can look at a stock 2 BBL '68 piston if you like.
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The 273's had steel shim head gaskets. I imagine the compressed thickness to be .020 so the advertised 8/1 or 8.5/1 compression ratio is pretty close to the truth.
 
Interesting this thread should pop up again, because one member here reported some early 318 pistons that were close below the deck, rather than the more common .080" to .100" in the hole. So the early 318's were better CR too. I gotta suspect the changes top lower CR after 1970/71 all had to emissions.
 
nm9stheham said:
Interesting this thread should pop up again, because one member here reported some early 318 pistons that were close below the deck, rather than the more common .080" to .100" in the hole. So the early 318's were better CR too. I gotta suspect the changes top lower CR after 1970/71 all had to emissions.
Click to expand...
the 68 318 seemed to have the highest rated HP but looking at part numbers didnt seem to denote any changes in pistons. probably year to year testing got lower and lower results with more **** having to be hooked up and camshaft changed for emissions.
 
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