Can some one do some math for me??????

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

I get 10.65.

3.43 is what I came up with for your valve reliefs. I think you're getting different results based off what people are calculating for that.

110 CC is 6.71261 CI. The volume of your bore with the piston 1/2 inch down should be 6.50332 CI. The difference of 0.20929 converts back to 3.43 CC for the valve reliefs.

This is a flat top piston I assume?

Aren't you the madscientist? I thought you would have been good at math too

I think along with the rounding errors, depending on how far out you take it after the decimal, is how far out you take pi.

I used:
708.2 swept
12.2 gasket
67.0 head
3.35 piston
-9.4 deck height
-----------
781.35 total
73.15 total minus swept
-----------
10.68 (781.35/73.15)

Depending on the ring down placement and the actual piston diameter above the ring, there's going to be another 1-1.5cc there, so in reality it's more like 10.5-10.55:1.

I ASSumed the 3.32cc valve relief to be correct, however I even got a different result from yours when I calculate the volumes.
Pi*R squared*.500= 106.62cc
110cc-106.62cc=3.38cc making the Comp ratio 10.658:1

But yes, I would expect a scientist to be stellar at math as well

When I do the fill volume check on motors that have a dome above the deck hieght or positive deck hieght piston to figure out the dome volume I use a small needle nose end on my small hand held grease gun to seal the tops of the piston tops to the cylinder walls and I know thier is a small amount above the rings that I don't have filled so like already posted I could add another 1.0 to 2.0 CCfor that, I don't NHRA doesn't add any more to thier figures so I don't either

WRT "the actual piston diameter above the ring"

That's included in the fill volume, but not in the deck height, because it assumes the dome is actual bore size - which is never true.

How are you getting 73.15 for a chamber volume?

I get 67 for the chamber, 12.2 for the gasket and approximately 3.5 for the DISH that I am measuring. That would be:
67
12.2
3.5
for a total of 82.7

With a swept volume of 706.3 plus the chamber volume of 82.7 is 789 divided by 82.7 is 9.54


I can do math. Its just this one is pissing me way off.

Its simple math but we are all getting different numbers. Im losing the last bit of my mind thats left.

I always use the CR calculator on Diamond Piston's web site. Pretty much fool-proof... ==> diamondracing.net

Tim, the 10.65 is a good number. You are forgetting that the positive deck is reducing your chamber volume.

It doesn't say that's the chamber volume....... It's the total volume minus the swept volume.

Maybe he's at a higher altitude...

SPECIAL THANKS to Mike at B3 racing engines. I called him on the phone and he held my hand and walked me through my math mistake.

Porter has his numbers correct. Later tonight I will re-calculate the CR on this project out to the 3rd digit and then I'll be good to go.

It will be close to 10.5:1 or so.

Gotta remember to calculate the flat that comes out of the bore. That's where DP got the extra 9 cc's that I was missing.

Thanks to all that posted and especially Mike.

What do you do for a living?

Retired machinist, weldor and gunsmith. I still do some of all of it when I want to or need to.

Fix that carb yet............

10.6776658 LOL!

10.68

Sheeeeeeeeiiiiiiiiitt!!!

I'm slow as hell. It runs but don't drive yet. Front shocks will be here late this week. I'll get it lined up next week.

What a PITA.