Welcome to Club Hot Rod!  The premier site for everything to do with Hot Rod, Customs, Low Riders, Rat Rods, and more. 

  •  » Members from all over the US and the world!
  •  » Help from all over the world for your questions
  •  » Build logs for you and all members
  •  » Blogs
  •  » Image Gallery
  •  » Many thousands of members and hundreds of thousands of posts! 

YES! I want to register an account for free right now!  p.s.: For registered members this ad will NOT show

 
  • Static compression ratio, domed pistons, how to figure the math.

    Static compression ratio, domed pistons, how to figure the math.
    We'll use a 454 Big Block Chevy for this explanation. Let's say we're using the stock bore and stroke of 4.250" and 4.000" and production cast iron heads with 122cc chambers. The pistons are down in the bore 0.025" (piston deck height) and have a dome volume of 28.7 cc's (get this info from the piston mfg or figure the dome volume yourself: See Determining Piston Dome Volume). To figure static compression ratio, you need 5 different values...

    1. cylinder volume in cc's
    2. combustion chamber volume in cc's
    3. piston crown volume in cc's (+ or - or flat with eyebrows)
    4. piston deck height volume in cc's (volume above the piston crown up to level with the block deck with the piston at TDC)
    5. head gasket volume in cc's

    Let's take these one by one so that you and others will know how to figure SCR with a domed piston....

    1. We know that the bore is 4.250" and the stroke is 4.000", so we multiply (.7854 times 4.250" times 4.250" times 4.000" times 16.387) and find that there are 929.88 cc's in the cylinder.

    2. Anyone building a motor should use a burette and colored alcohol to "pour" the chambers and determine the exact volume in each and every chamber (yes, they differ one to the other, but can be equalized prior to assembly), but I doubt that many of you fellows do that. These heads are advertised at around 119cc's, but production heads will usually pour a little bigger, so let's call them 122 for this exercise.

    3. OK, we'll do a little play acting here. We will visualize taking a sharp instrument and slicing the dome off the piston, leaving only a flat plane crown with no eyebrows. So, here we are holding a lump of aluminum that displaces 28.7 cc's. We are now going to jam that lump up into the combustion chamber to reduce the chamber volume to 93.3 cc's. This will result in a piston crown value of zero.

    4. We have no idea what the piston deck height is on this motor, but let's just use a theoretical value of 0.025". So, we have a bore of 4.250" and the piston is 0.025" in the hole at TDC. Remember, the piston is now a flat-top, so we will simply multiply (.7854 X 4.250" X 4.250" X 0.025" X 16.387) and find 5.81 cc's.

    5. I'm going to assume that the decks of the block and heads are flat enough to use a Mr. Gasket shim head gasket, #1131, 4.370" X 0.020". This will put our squish at 0.045". Again, figuring cc's just like anything else that's round and has a thickness......(.7854 X 4.370" X 4.370" X 0.020" X 16.387) and find 4.92 cc's in the head gasket.

    Now, we add all our values together....
    929.88 + 93.3 + 0 + 5.81 + 4.92 and find a total volume of 1033.91 cc's.

    Now, we will drop out the cylinder volume and add the other 4 values together....
    93.3 + 0 + 5.81 + 4.92 and find a compressed volume of 104.03 cc's.

    Now, we will divide the larger volume by the smaller volume (1033.91 / 104.03) and find 9.94:1 static compression ratio.
    This article was originally published in forum thread: Technical Repository started by techinspector1 View original post
    Comments 4 Comments
    1. Brent Mills's Avatar
      Thanks for the efforts, nicely done.
    1. vara4's Avatar
      Trying to confuse me again, HUH Tech HE! HE! HE!
      Kurt
    1. glennsexton's Avatar
      Am I the only one who gets this message?

      vBulletin Message

      glennsexton, you do not have permission to access this page. This could be due to one of several reasons:

      Your user account may not have sufficient privileges to access this page. Are you trying to edit someone else's post, access administrative features or some other privileged system?
      If you are trying to post, the administrator may have disabled your account, or it may be awaiting activation.

      Log Out Home
    1. pat mccarthy's Avatar
      well that only works when you have the cc of the dome nice but if you do not have the cc of the dome then there is abit more to the job
  • Recent Activity

    HOSS429-21036

    random stuff

    Thread Starter: HOSS429

    i started such a thread during our down time but it seems it`s not coming back so without further adu here it is again .. there are times i wish to...

    HOSS429 Today, 12:41 PM Go to last post
    v8nutz-40923

    Project Sebring GT Spyder

    Thread Starter: Hotrod46

    Well gang, it's time it introduce my latest project. I've been looking forward to this for quite some time, but other projects kept getting in the...

    v8nutz Today, 09:05 AM Go to last post
    NTFDAY-1709

    55 Wagon Progress

    Thread Starter: MP&C

    This is a customer car, the owner and I discussed shaving the fender seams quite a while back, and she asked about it again last week, so I guess...

    NTFDAY Yesterday, 08:37 AM Go to last post
    Navy7797-36108

    1940 Ford Pickup

    Thread Starter: Navy7797

    Here's the start of my new project. Picked this truck up a few weeks ago and now have it down to the frame to start the repairs. See anything I'm...

    Navy7797 Yesterday, 07:00 AM Go to last post
    johnboy-11976

    WTB: Classic car

    Thread Starter: NeverEnpough

    Hi everyone, Looking for a driver quality vehicle. Prefer something from the list below with the Chevelle being number one on it. I might consider...

    johnboy 01-16-2021, 08:18 PM Go to last post
    wrp-40096

    34 Ford Three Window Build

    Thread Starter: wrp

    Bolted together the mold and frame for the 34 Ford Three Window. Expect to start laying the glass soon.

    wrp 01-16-2021, 05:20 PM Go to last post
    rspears-21970

    Grandpaís Grand Prix

    Thread Starter: falconvan

    Hey all, havenít been on in a while and getting ready to start a new one. The young man that plays drums in my band inherited this 77 Grand Prix when...

    rspears 01-16-2021, 02:37 PM Go to last post
    glennsexton-10307

    Iím getting more new parts

    Thread Starter: TooMany2count

    Looks like on January 18 Iíll be in the hospital in Central Florida to finally have my hip replaced. My doctor up in St. Louis said it wouldnít be...

    glennsexton 01-16-2021, 12:13 AM Go to last post
    firebird77clone-14225

    You guys are the worst

    Thread Starter: firebird77clone

    In the BEST possible way of course. Having trolled this site as long as I have, I KNOW what cars CAN be. AND so, I have tried to raise my...

    firebird77clone 01-15-2021, 05:55 PM Go to last post
    FordyG-42440

    New guy from Utah.

    Thread Starter: FordyG

    Just thought I would drop by and say hello, I am somewhat new to the hot rod world with my own project but grew up around them, I am currently doing...

    FordyG 01-15-2021, 04:48 PM Go to last post
    wrp-40096

    32 Build

    Thread Starter: wrp

    Friend of mine is building a 32 Ford Roadster from the ground up. I am working as best as I can with him, learning about building Hot Rods in the...

    wrp 01-14-2021, 08:30 PM Go to last post
Links monetized by VigLink