stang_killa94

I didnt know where to post this, so if its in the wrong section please move.

So tomorrow in class we're gonna figure out compression ratio and i was wondering if its hard? I SUCK at math and im afriad i wont catch on. So i was wondering if you guys knew how to get the compression ratio so maybe i could give it a try.

Mindgame

If you can multiply, divide, subtract and add then you've got it covered. Nothing to it, just have to convert everything to "like units". The formula for calculating static compression ratio (SCR) looks like this....

SCR = (cylinder volume + combustion chamber volume + compressed gasket volume + piston volume ("+" values for dish & "-" for dome) + deck height volume) / (combustion chamber volume + compressed gasket volume + piston volume + deck height volume)

You can convert any way you want... cubic centemeters or cubic inches. If the gasket volume is known (most manufacturers list that) then cylinder and deck height volume are the only ones that need conversion. Then you just (cid/8)*16.39 for your cylinder volume and work the cylinder volume formula ((pi/4)*dia^2*h)*16.39) for your deck height volume.

-Mindgame

stang_killa94

I dont stand a chance

OldSStroker

The top thread in Advanced Tech is a "sticky" called Automotive Calculations on CamaroZ28.Com . Click on it and click on the link in the first post. 6th item on the list should help you out.

LT1 Tech has the same "sticky".

Remember that "Piston Top Volume (CC)" is positive for valve reliefs or "dish" and negative (-) for a domed piston. "Deck Height (inches) is positive if piston is down in the hole, and negative (-) if it sticks out of the block.

Sure you have a chance.

Mindgame

killa,

You can do it.

Maybe I misunderstood... did you want to know "how" to work through the formula or were you just looking for an online calculator?

If you can do (A+B+C+D+E)/(B+C+D+E) then you can calculate a compression ratio. If you want me to break it down I can do that. If I can teach my 14 year old nephew how to calculate a compression ratio, I'm sure I can teach you... if you're willing that is.

-Mindgame

442olds

http://www.et-motorsports.cc/tech/en...uild_sheet.xls

This spreadsheet will calculate it for you. Just fill in the information denoted by the yellow boxes.

rskrause

Think of CR as the ratio of the volume above the top ring of the piston at BDC to the volume above the top piston ring at TDC.

Rich

stang_killa94

Ok well we finished up part of the chapter and so far all i know is this formula

V1+V2+V3+V4
____________
V2+V3+V4

V1 = Cylinder Volume
V2 = Deck Clearance
V3 = Head gasket
V4 = Cumbustion chamber CC


Does that sound familiar? I was looking at you explaintion Mindgame, and itd completely different.

rskrause

It would have to be: volume above piston at crown at BDC/volume above piston crown at TDC. Your equation doesn't make sense because V2 and V3 aren't volumes, they are linear and the difference in volume from BDC to TDC isn't accounted for.

Rich

Mindgame

Well, you've got some of the variables there. Need piston volume, or are you shortcutting and adding that in?

Not so different from what I explained.

Me
You
(V1+V2+V3+V4)/(V2+V3+V4)

Gotta do some converting in there. Also need to crunch the cylinder volume formula a couple times.

-Mindgame

stang_killa94

Hmpm............well what has to be converted and how do i do it? If you dont mind

rskrause

The volume of a cylinder is L × pi × r^2 where L = height of the cylinder. So, the volume of the gasket, if it had a compressed thickness of 0.040" and a bore of 4.125" for example, would be 0.04x3.1417x2.0625^2 = 0.53ci. You can figure the space above the flat part of the piston the same way.

Rich

Mindgame

There ya go.

Then you just need to make sure everything is converted to the same units. For in³ to cm³, multiply by 16.387. Add everything up, do your division and you're done.

-Mindgame