Calculating Dynamic Compression

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Does anyone have a formula for calculating static and dynamic compression?  I've found some pages that calc it for you and give you the result, but I want to be able to put it in Excel and play out a few different scenarios and be able to see them all change when I vary an input...all good, but I need the formulas.  I thought I found them online, but they don't make sense for a stock 440.

My goal in the end is a 440->500 stroker that runs on 87 octane.  But before I play there, I'd better have the stock 440 down, and here's what I get (and note I'm rounding whereas Excel stores lots of decimals):

Bore: 4.32
Stroke: 3.75
Cubes: 439.7
Rod Length: 6.768
Piston Compression Height: 2.067 (using some KB Hyper's as guinea pigs for this)
Piston Valve Reliefs: -5cc
Deck Height Clearance: 0.015
Deck Height: 10.725
Gasket Thickness: 0.039
Gasket Bore: 4.380
Quench Distance: 0.054
Cylinder Head Combustion Chamber Size: 83cc
Camshaft Intake Closing Angle: 53 (@ 0.006")

Compression, Static: 10.87
Compression, Dynamic: 9.41
Stroke w/Intake Closed: 3.19
Octane: 97.7

Uh, shouldn't the static compression be around 9.8-10?

For static compression, I'm calculating it like this:

=(Cubes + 8 *(PI() * ((Bore/2)^2) * DeckHeightClearance + GasketThickness * (GasketBore/2)^2 * PI() + CylinderHeadVolume * (1/2.54)^3 + PistonValveReliefs/(2.54^3))) / (8 * (PI() * ((Bore/2)^2) * DeckHeightClearance + GasketThickness * (GasketBore/2)^2 * PI() + CylinderHeadVolume * (1/2.54)^3 + PistonValveReliefs/(2.54^3)))

Hopefully the error is in the static part of the problem.  Any takers?   

[firefighter3931]

Does anyone have a formula for calculating static and dynamic compression?  I've found some pages that calc it for you and give you the result, but I want to be able to put it in Excel and play out a few different scenarios and be able to see them all change when I vary an input...all good, but I need the formulas.  I thought I found them online, but they don't make sense for a stock 440.

My goal in the end is a 440->500 stroker that runs on 87 octane.  But before I play there, I'd better have the stock 440 down, and here's what I get (and note I'm rounding whereas Excel stores lots of decimals):

Bore: 4.32
Stroke: 3.75
Cubes: 439.7
Rod Length: 6.768
Piston Compression Height: 2.067 (using some KB Hyper's as guinea pigs for this)
Piston Valve Reliefs: -5cc
Deck Height Clearance: 0.015
Deck Height: 10.725
Gasket Thickness: 0.039
Gasket Bore: 4.380
Quench Distance: 0.054
Cylinder Head Combustion Chamber Size: 83cc
Camshaft Intake Closing Angle: 53 (@ 0.006")

Compression, Static: 10.87
Compression, Dynamic: 9.41
Stroke w/Intake Closed: 3.19
Octane: 97.7

Uh, shouldn't the static compression be around 9.8-10?

I use an online calculator rather than plug in all those numbers and work it out by hand....less of a headache ! 

Ok, i plugged your scenerio into the calculator and it popped out 9.92:1 static compression. The gasket bore diameter will affect the final number somewhat....but not significantly.

What engine combo are you looking at ? 4.15 or 4.25 stroke ? What bore diameter ? What head type....iron or aluminum ? Open or closed chamber ? (Closed is much better)

As you've allready figured out the cam's intake valve closing point will largely determine how octane sensitive the build will be but the quench distance and head material are also major players. Other things that can help or hurt are the vehicles weight and gearing. It's a delicate balance when trying to sort out this type of a build.

Generally speaking, a flattop piston at zero deck with a closed chamber head is the best choice for a pump gas type build. This keeps the quench distance tight and reduces the chances for detonation. Aluminum heads are a real benefit in this typ of a build as well. The larger the displacement, the more relief area you will need in the piston top. Stroker pistons will have a "D" shaped dish that sits under the valve side of the chamber. The smaller the chamber....the more dish are you will need to dial in the ratio you're looking for.



Ron

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Thanks Ron,
I see you always have great advice on here, so hat's off to you.  I guess the reasons why I like it in Excel better is I don't have to take their formula on faith, at any time I can look at how it's calc'd, and although it might take some effort to set it up initially, it's a lot easier in the long run, because you can just save whatever results you have, and copy/paste a bunch of different scenarios.  But that's me anyway.

As for the 500, I'm looking at Edelbrock Performer alumnimum heads (those are closed, right?), 4.15 stroke, and 4.380 bore.  I'm kind of worried that the new rod angle would be 17.6 degrees, which I've read should be below 17 degrees (stock is 15.9), or excessive wear is promoted at the piston major thrust surface, possibly resulting in piston breakage.  It also looks like, if we want to keep piston acceleration in feet per second per second below 100,000, then max safe RPM would be about 6,350, but I prefer hydraulic cams for simple street stuff, and don't need high RPM anyway.

I was looking at two cam manufacturers, really: Comp and Engle.  But I can't get the advertised intake closing point from Engle (but I think they have it @ 0.050"), and I'd really like to see that in here.  Let's see, I was also probably going to go with the Edelbrock Performer RPM intake, and I don't know about the carb, probably an 850.  Vehicle weight should be close to stock, but the rear gear may be either a 3.54 or 4.10 in a Strange S60 Dana, behind a Keisler 5-speed (but it'll be a few years before I can afford all these goodies!!!). Just trying to plan ahead. 

What do you think, Ron?

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All right, this is hard to believe, but...

I found an article at Car Craft (http://www.carcraft.com/techarticles/calculate_compression_ratio/index.html).  They only do static compression in the article, but...

I plugged in the numbers above in their simplified formulas, AND GOT THE EXACT SAME ANSWER: 10.87 for a stock(ish) 440.     Does one of my inputs (above) look out of whack or something?

[firefighter3931]

What do you think, Ron?

I guess my biggest question would be why build it to run on 87 octane. 


Ron

[firefighter3931]

I plugged in the numbers above in their simplified formulas, AND GOT THE EXACT SAME ANSWER: 10.87 for a stock(ish) 440.     Does one of my inputs (above) look out of whack or something?

You must be doing something wrong. Are you entering a negative number for the valve relief area or a positive number. A domed piston would be a negative (-) value while a dished/eyebrow valve relief would be a positive (+) value.

ex : a pop up dome with 12cc would be -12 in the calculator.....a standard flattop with 5cc valve reliefs would be +5cc

try using this calculator and see what you come up with :

http://www.rbracing-rsr.com/compstaticcalc.html


You can manipulate the numbers and play around with different gasket thicknesses, deck height masurements, chamber volumes and piston dimensions (flattop/dome/dish etc) to estimate the static comp numbers.

To determine the dynamic compression will require the intake valve closing point of your cam.


Ron

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So we'll consider that a triple check.  Thanks for the Ray Barton link, I love those guys.  Yes, the same answer, ok, so the equation is good.  The piston I thought was close to stock must not be, and you're right, it has -5cc's, which is a dome, which would raise it.  Sorry to take up your time on this one, just like anyone else here, want to make sure I have everything right.

I guess the reason I want to build it for 87 octane is that I want some room to play with, and besides, it's the cheapest gas.  I've read that dynamic compression only gets you set at low RPM's, but more octane will be needed at high RPM, and possibly detonation up there if you run what dynamic comp. tells you.  So if I set it for 87, I can probably safely run 89?

By the way, is it a concern with a 440 stroker when the rod angle exceeds 17 degrees?  Or, is there a way around that, such as different size rods?  Thanks.

[firefighter3931]

Good, i figured one of your inputs was incorrect which was skewing the results.  Using that calculator you can play with different parameters and come up with the needed parts combo...

I would just build it for 10.5:1 and run descent gas (92-93). It seems counterproductive to build a strong motor then handicap it with low compression. 

The 4.15/6.76 and 4.25/7.1 stroke/rod combos are acceptable for a street driven build, inmo. I'd have no problem running either. 



Ron

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Ok, thanks Ron