I'm hoping someone can run some compression ratio numbers for me. I'll supply the data and you do the math. I can't get the numbers to come up with what I think they should be.

Here goes:
4.0705 bore
3.313 stroke
.055 gasket with 4.150 bore
67 cc chamber
110 cc 1/2 inch down fill
Piston is .044 positive out of the bore

Run the numbers and see what you get.

Thanks in advance.

10.65

I get 9.95

I thought it should be 10.8 per so.

It's definitely pissing me off.


Thanks for the math.

I got 8.69 in the wallace calculator which feels wrong. I don't know the real math behind calculating so I made some guesses.

I went with 3.32 as the valve relief because a 4.0705 x .5 cylinder should be 106.68 CC, the 0.044 into the deck height made me question it.

Maybe it should be -0.044? that would come out to about 10.66

That's one issue I have. The actual downfall is 110 cc's so the flat top piston has valve notches that are about equal to a 3.5 cc dish.

Guess I should have shoved it .060 out of the deck.

10.63

used -3.5cc for the piston dish
-.044 for deck

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

I get 8.68 if you do pistons in the hole .044 but out of the hole .044 is 10.65

I get 10.68, not accounting for the area above the rings.

Piston to head is .011?

hopefully it's one of those open chamber heads? I was wondering the same thing...I kind of doubt it's an old open chamber head, with that small of chamber though.

I get 10.72 without accounting for extra bore size of the gasket, so 10.63-10.68 sounds real close to me. I got 3.32 for the valve relief. 1/2" x 4.0705= 106.68cc. Subtract that from your 110cc and you get 3.32

That 3.32 is what the DISH Is even though the piston is out of the bore.

The 1/2 inch down fill is 106.59513 so that makes the piston think it's a dish even though it's positive out of the deck.

I'd have to post all my math but I get 9.95 and if that is the case, there was NEVER an actual 10.5:1 340. It's frustrating for sure.

my moody calculator says 9.55 to 1

We are all using the same numbers with different results.

Unreal.

[s][/s] I guessed at the top ring being down .200" from the top of the piston. The calculated fill volume for your bore is 106.22 cc. 110-106.22=3.88 effective dish. A lot of different answers here for a very important dimension while building an engine. I use the Diamond Pistons calculator.
Instructions: Enter a value in the un-shaded boxes below as they pertain to your engine.
Note: For pistons BELOW the block @ TDC - value in box must be a POSITIVE number (.000)
Note: For pistons ABOVE the block @ TDC - value in box must be a NEGATIVE number (-.000)
Note: For a DISH piston - value in box must be a POSITIVE number (0.0)
Note: For a DOME piston - value in box must be a NEGATIVE number (-0.0)
Note: Manipulate the DOME/DISH & PISTON to DECK volumes to get the ratio you want.

Enter the number of cylinders of your engine
>

8
Bore
>
Finished bore of your engine
>

4.0405
Stroke
>
Stroke of your engines crankshaft
>

3.313
Rod Length
>
Ctr. to Ctr. length of your connecting rods
>

Gasket Bore
>
Head Gasket bore diameter
>

4.150
Comp. Gasket
>
Compressed Head Gasket thickness
>

.055
Block Deck Height
>
Center of main bearing bore to top of block
>

0
Top Ring Down
>
Top of 1st ring to top of pistons (FLAT)
>

.200
Chamber Volume
>
Cylinder Head chamber volume in CC's
>

67
Dome/Dish Volume
>
DOME (-) or DISH (+) vol. (theoretical) in CC's
>

3.78
Piston to Deck
>
Top of BLOCK to top of piston (FLAT) @ TDC
>

-.044
Click "Calculate" for dimension results.
Calculate
Total Volume
>

>

770.01
Cylinder Volume
>
Total cylinder volume in CC's =
>

696.14
Clearance Volume
>
All effective vol. above block deck @ TDC in CC's =
>

73.87
Gasket Volume
>
Volume of your head gasket in CC's =
>

12.19
Top Ring Volume
>
Total volume above the top ring @ TDC in CC's =
>

0.79
Deck Volume
>
Volume of area above the pistons @ TDC in CC's =
>

-9.24
Piston Top Land
>
Piston top land diameter for volume calculations =
>

4.002
1/2 Stroke
>
1/2 stroke of your engine's crankshaft in inches =
>

1.657
Compression Ht.
>
Ctr. of piston pin to top of piston (FLAT) in inches =
>

NaN
Cubic Inches
>
Cubic inch displacement of your engine =
>

339.84
For the given dimensions, your compression ratio =
>

10.44

Try using the 3.32 CC for the valve relief inputs and use a flat top piston above the deck for the rest of the math the last two 340 NHRA legal stocker motors I built ended up with right at 11.56 to 1 measured compression ratios

Can you run my numbers they way you are talking about and tell me what you get?

Like I said, I get 9.95:1 and when I get some time tonight I'll post my math to verify. I'm not too concerned with crevice volume at this point.

I guess my other question is what happens at an NHRA event and you have a pull apart your engine after you set a record or something. Does NHRA calculate crevice volume? Do they just drop the piston .500 and do a down fill and calculate the CR?

I have done the down fill for years and for some reason, I'm way off the reservation on this.

I'll bet money that NHRA will use the measured deck hieght or above deck measurments from the manufactures blue print specs along with the head gasket thickness and if your total measurements don't exceed the stock specs your golden Hence the 426 street Hemi motors in both stock and SS getting away with round top pistons that calculate out to have 12.+ to 1 compression ratios using the one inch down fill volume methods

I got exactly the same as the first reply 10.65366:1

Im sure rounding errors come in to play depending on what calculator is used