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

  • CHR Technical Library

    by Published on 04-29-2011 12:41 AM  Number of Views: 13747 

    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.

  • Recent Activity


    1948 Thames

    Thread Starter: oldcoyote

    ..... here'a a picture of "Timmy", my 1948 English Ford Thames 1/4 ct. panel van at a recent Cars and Coffee event in Kernersville, NC ....... at...

    johnboy Yesterday, 09:22 PM Go to last post

    1969 small block CE Warranty Block

    Thread Starter: Yammer

    New to the forum. Hi all I purchased a 1969 all numbers matching Z28 a few years back which I have recently sold and my wife keeps tripping on a...

    Yammer Yesterday, 07:25 PM Go to last post

    FFR 33 Ford Hot Rod

    Thread Starter: wrp

    Had the car about three years now. Initially put it on the road as a roadster. This winter we decided to add the roof, hood, side skirts and front...

    cffisher Yesterday, 04:33 PM Go to last post

    For Don Shilady

    Thread Starter: 34_40

    :LOL::eek::)Here you go Don, just for you. Post those pics of the 1915 Speedsters or if anyone has a pic or two of a funny car with the body up......

    34_40 Yesterday, 03:27 PM Go to last post

    New build thread; 48 Plymouth

    Thread Starter: falconvan

    I'm really enjoying the old truck I just finished; my wife and i took it to the car show last week and most people had a lot of good things to say....

    falconvan Yesterday, 01:28 PM Go to last post

    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...

    Scrubline 02-14-2019, 08:02 PM Go to last post
    jerry clayton-17169

    Top Speed

    Thread Starter: vara4

    If I had a top gear of 0.80 and a rear end gear of a 4.10 what would my top speed be at 6,300 rpm's???? Any one know how to figure this out???? I got...

    jerry clayton 02-14-2019, 02:57 PM Go to last post

    Chevy engine block ID assistance request

    Thread Starter: Willey

    Hi All. This is my first post and would like to ask for some assistance with identifying an engine block I was given by a family friend about 10...

    Scrubline 02-14-2019, 01:29 PM Go to last post

    Water in oil

    Thread Starter: Beard1976

    I have a 85’ Chevy Silverado with a 327 in it. Its bored 0.060 over. Steel heads that have been ported n polished and shaved. Alum intake. 11.5:1...

    Beard1976 02-14-2019, 04:56 AM Go to last post

    the Official CHR joke page duel

    Thread Starter: john gemmer

    this page for any jokes you have to post try to keep them clean this was sent to me today just thought I would share:( :p :D :LOL:...

    johnboy 02-12-2019, 09:26 PM Go to last post

    1971 Camaro RS

    Thread Starter: 40FordDeluxe

    I've had a soft spot for these cars since I was in high school. One of my best friends had a 72 RS he bought from his brother when he was 16. I was...

    40FordDeluxe 02-12-2019, 05:20 PM Go to last post

    350 with tbi check engine light

    Thread Starter: leva56

    Put motor in 57 chevy truck with the tbi setup. After driving 3-5 miles check engine light comes on go to manually check codes obd 1, it will only...

    DennyW 02-11-2019, 11:38 PM Go to last post

    Custom driveshaft

    Thread Starter: Scrubline

    Please excuse my venting, but why is it everybody wants up to $350 to make me a 15" driveshaft? It doesn't seem to be that labor intensive or time...

    DennyW 02-11-2019, 01:25 PM Go to last post

    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...

    40FordDeluxe 02-11-2019, 12:53 PM Go to last post
Links monetized by VigLink