The 7cc myth.....

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pishta

pishta

I know I'm right....
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I read that the 4 valve relief 'eyebrows' of a stock 273/318 piston were a given 7cc's of lost compression when doing calculations. Im here to debunk it: I took some modeling clay and filled one of the reliefs of a stock 273 piston them rolled a deep socket over it to make sure it was nice and filled. I then cut the excess off and was left with a totally flush piston top in that area. I pealed the clay out and dropped it in a 10ml syringe filled with 5ml of water. It displaced exactly .6ml of water, raising it up to 5.6ml (or 5.6cc) SO: the eyebrows are a paltry .6cc's each, for a grand total of 2.4cc's. If you factor in the area of the ridge at the circumference (.75mm X 292mm) , your only adding another .21cc's . So you get 2.6cc total 'dish' for a stock flat top 273 with 4 eyebrows. Now you can factor in your compressed head gasket where C is the compressed gasket in mm {.040" is 1.016mm) (C x 6647)X.001 to get ml/cc. That happens to be 6.75cc's but dont forget your bore size is the bore of the gasket, not the piston so lets look at a 4.080 MP .040 head gasket: 4.080 to mm is 103.63mm; that equals 8434 square mm x 1.016 x.001= 8.5cc's. For a grand total of 11.1cc's plus any negative deck height (0.58mm~4.25cc) you have added to the 58cc's of (my) stock 64 head. Stuff starts stacking up, eh?
 
That is good work, Pista. BTW, I usually used 6 cc's for these so the 7 cc thing is not so much a myth... just the guess-of-the-day. From now on I will use 2 or 3 cc... I swear! LOL
Thanks again!

Edit to add: Yes I always work in the gasket bore diameter. Actually, I don't work it in.... the program does it. If you are not using the Pat Kelley calculator, you are missing a very easy to use SCR/DCR program. Get a few factors memorized and you can run a piston/head/gasket, cam combo in 2-3 minutes. It ain't rocket science with a program like that.
 
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I still have trouble finding where's Waldo. And your doing math for fun? Yikes!
 
I had a few minutes and it gave me a reason to take a piston out of that old 64 273 block. Had std Clevite 77 rod bearings in there with a date code of 1974, a felpro head gasket and no cylinder ridge, but a lot of carbon in the gasket ring. Even had valve seals still intact. Exhaust valves look pretty toasted.
 
You are WWWWAAAYYYYY too scientific for me LOL. That's some good thinkin there
 
Heck, over the course of 8 cylinders we just lost the CC's of 2 weed eaters or one decent small chainsaw.:D
 
TXDart said:
Don't forget top ring height .
Click to expand...
on it....ooh, another .6cc.. (volume of .325 tall cylinder at 3.625 (cylinder) minus the .325 tall cylinder volume at the piston top at 3.605, or just the open space on top of the top ring)
 
I never heard that myth but I'm going to do the math.
 
pishta said:
on it....ooh, another .6cc.. (volume of .325 tall cylinder at 3.625 (cylinder) minus the .325 tall cylinder volume at the piston top at 3.605, or just the open space on top of the top ring)
Click to expand...
LOL,
I was guessing .4 to .5cc,no 273's here...So what compression is it?
 
i just cc'd intake valve on my 308 castings, about 2.5 cc's in the dip on the valve face, i think i'll get some flat 202s
 
Clay? 2.5 is like 2.5 sugar cubes.....get some stainless nail heads.
 
Stainless undercut nailhead valves have to be lighter than fat sodium filled tulip exhaust valves or big ski ramp stocker tulips. I got some old valves to weigh but I think that is not much of a factor.
 
Sorry, just thinking out loud. You know they are not sodium filled. That is really race stuff. I could check the weights also.
 
66fs said:
So what's the difference in weight between tulip or nail head valves?
Click to expand...
A good question as weight lowers peak RPM for valve float.

I found 2 cc's in the dimple's in the valves of our 675 heads off of a '68 273. More 'anti-compression' in the cylinders....

Soooo, does anyone have a 4 valve relief, low compression 318 piston to measure the volume in those reliefs?
 
Pishta is gonna like this... I used a burrette and alchohol to measure the 4 eyebrows total on one of our '68 2 BBL pistons..... 1.2 cc's the first time and 1.1 cc's the 2nd time. I did see that the eyebrows varied from side to side so it may vayr some, but 1-2 cc's total looks like a pretty solid number. The bevel around i right at 1 cc so that edge pushes it up to the 2-3 cc range.
 
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Yeaaaa, IDK about that. Alcohol?

By the time you set everything up, and start measuring, the amount of evaporating alcohol into the air and down the old choke pipe may have an effect....
:rofl:
 
I knew someone would comment on that LOL.. .but good thinking. It was 65F and humid in the garage... and filling each eyebrow was quick; it took all of 40-50 seconds to do them all. I used alcohol because water in such a small volume would have problems forming a nice flat surface due to surface tension, even with a water wetter to break the surface tension.

For a combustion chamber, I use water and a water wetter.

If anyone has a done-for, 4 eyebrow 318 piston, that is next.
 
I was more thinking that alcohol would disappear down my throat, but,....
 
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