zildjian4life218 said:
Can anyone tell me more about the pistons. I would like to know the specs on them like compression distance, and dome cc.
Just like every other part of life, if you want to know the correct answer, do it yourself.
Compression height can be measured with a dial caliper. Hook one tip of the caliper into the top of the wrist pin bore and the other tip onto the crown. Write down the measurement. Add the radius of the wrist pin bore (0.464" on a stock pin diameter SBC).
To find the dome displacement, first, get a cc kit like this....
http://www.summitracing.com/parts/SUM-911581/
While others may recommend a drug store flask or some other second-class, fosdick arrangement, this is the way to get a first-class measuring kit that will become part of your tool assortment and used with confidence for years to come. To recoup the cost of the tool, offer to cc heads for other racers or amateur builders. $35 for a set of V8 heads and the tool is paid for after doing 3 sets. $20 for a 4-banger head, $28 for a 6-banger.
With the motor sitting oil pan-down, choose any cylinder and turn the crank so that the highest part of the piston dome is just below the deck surface (like maybe 1/16"). Using the slide function of your dial caliper, measure from the deck to the main part of the piston crown right next to the bore, not on any part of the dome. Write it down. Just for example, let's say it is 0.371".
Use a heavy grease such as Vaseline to seal the perimeter of the piston crown to the cylinder bore, so that none of the liquid you are going to pour into the bore can get past the piston/bore interface all the way around the piston.
Alcohol will be "wetter" than water for this operation, so go to the grocery store and buy a quart of rubbing alcohol (doesn't matter if isopropyl or ethanol) and a small bottle of food coloring. Red, green, blue, doesn't matter, this is just to make it easier to see the fluid level in the burette. Pour a little of the coloring at a time into the quart of alcohol until you get an easy to see color saturation.
Grease the perimeter of the cylinder bore with Vaseline to seal your clear plastic to the bore. Position the feed/vent hole in the plastic at the very top of the bore, right next to the edge. If the plastic isn't drilled for a feed/vent, drill a 3/16" hole close to the edge of it. Leave enough plate between the hole and the edge of the plate so that you can seal the plate to the block deck.
Position the burette so that just the very tip end of the spout sticks into the hole, not enough to block the hole entirely, because this is also a vent hole, but enough so that the fluid will travel into the void you are measuring and not flow out onto the outside of the plate. Depending on the O.D. of the spout on the burette, you may have to make the feed/vent hole a little bigger, but I have never had to with a good quality burette.
Using a small funnel, fill the burette to the fill line with the colored alcohol. Turn the spigot and begin filling the void. Depending on the height of the dome, you should be able to fill the void on one filling, without having to stop at zero and re-fill the burette.
Now for the math:
Let's say that you positioned the piston down in the bore by the previously mentioned 0.371". If the piston had no dome at all and no valve notches, with the volume above the piston crown looking more or less like a hockey puck, then the volume above the flat crown to the block deck would be 77.548cc's. (.7854 x 4.03 x 4.03 x .371 x 16.387 = 77.548cc's).
Let's further say that you used 49.6 cc's to fill the void. Subtracting 49.6 from 77.548 would reveal a dome displacement of 27.948 cc's.
There, see how easy this stuff is? You just go through the steps and there's the correct answer!! You know it's correct because you did it yourself without relying on possibly bogus information from some fosdick guy like me. :thumbup:
Now, going further to figure the static compression ratio:
First, you want to cc all the chambers in the heads. Position a head on the bench with one edge slightly higher than the other so that your fill port in the plate will be the highest point. Seal the valve perimeters and spark plug threads with Vaseline. Pour as instructed above, with your plate sealed to the perimeter of the chamber with Vaseline.
Punching a number on the end of each head and detailing the volumes on a sheet of paper will provide you with good information for later. For instance, if you punched "1" on the end of one head and "2" on the end of the other head, then you could list the chamber volumes a 1/64.3/64.0/63.9/64.2 and the other head as 2/64.1/64.3/64.0/64.2 reading back from the number punched on each head.
We'll say that you determine an average volume would be 64.2. The easiest way is to deduct the dome volume from the chamber volume and go ahead and figure the c.r. from there as per usual. The kicker will be the piston deck height of the main flat part of the piston crown to the block deck. If the piston is down in the bore 0.012" for instance, you can't just figure that as a flat hockey puck type of volume, because the dome sticking up renders that invalid. It would actually look more like an o-ring seal, with the hole representing the dome. You can guestimate this volume by determining the average width of the ring around the dome or you can disregard it altogether if the piston deck is small enough. Let's say that taken on average, the width of this ring is 0.500". That would make it 4.03" on the O.D. and 3.53" on the I.D. If the piston deck is 0.012", then this would calculate to 0.58cc's. Call it a half cc or disregard it altogether.
To figure s.c.r. in this motor, we will need the cylinder cc's (627.08), chamber cc's (64.2 average), piston deck cc's (0.5), gasket cc's (8.64 on a 4.100" x 0.040" gasket) and dome volume cc's (27.95).
Deducting the dome volume from the combustion chamber volume, we will eliminate the dome and figure our calculations with a 36.25cc chamber.
627.08 + 36.25 + 0.5 + 8.64 = 672.47cc's filled with the intake valve open.
36.25 + 0.5 + 8.64 = 45.39cc's with the piston at TDC.
Dividing 672.47 by 45.39 = 14.81:1 s.c.r.
