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 04-28-2011 11:41 PM  Number of Views: 14657 

    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


    Electrathon - A Different Kind of "Hotrod"

    Thread Starter: J. Robinson

    If you have been following my roadster build thread, you may have noticed a mention of something called Electrathon. This is something that I got...

    34_40 Today, 03:44 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:...

    rspears Yesterday, 07:18 AM Go to last post

    Followed Me Home II

    Thread Starter: rspears

    I've been toying with the idea of another build, and have been watching for a stalled project and considering other options, but with no time frame...

    rspears Yesterday, 06:49 AM Go to last post

    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 04-16-2021, 05:33 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...

    34_40 04-16-2021, 03:39 PM Go to last post

    1971 c-10

    Thread Starter: glennsexton

    Some here may remember a few posts I made years ago concerning a 1971 Chevy C-10 that I bought for my son in 2004 when he graduated from high school....

    glennsexton 04-16-2021, 01:50 PM Go to last post

    61 impala sport sedan

    Thread Starter: falconvan

    Hereís my next project; itís coming home in a few weeks. Iíve known this car for a long time. A good friend of mine bought it several years ago and...

    falconvan 04-16-2021, 07:14 AM Go to last post

    Acceptable Piston to Deck Height Tolerance

    Thread Starter: ZipTie

    I have a new GMPP Ė 19419042 SBC, ZZ4/6 Short Block Assembly. I measured the piston to deck height using a magnetic deck bridge w/ a dial...

    34_40 04-15-2021, 04:46 PM Go to last post
    Bob Parmenter-32

    1940 ford hood strut conversion kits

    Thread Starter: Robert Hilliard

    Are there any?

    Bob Parmenter 04-15-2021, 06:57 AM Go to last post
    Bob Parmenter-32

    Sharing some family news

    Thread Starter: 53 Chevy5

    This past Sunday, my dad went to be with the Lord, his health and physical body had really gone downhill in the last 3 years. My mom had taken such...

    Bob Parmenter 04-15-2021, 05:05 AM Go to last post

    It's beem a while to say the least

    Thread Starter: pepi

    Looking around things seem to be rocking along for the most part. A good thing for sure, still messing around with cars. Driving, and all that...

    40FordDeluxe 04-15-2021, 04:36 AM Go to last post

    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...

    40FordDeluxe 04-15-2021, 04:34 AM Go to last post

    1927 T Farm

    Thread Starter: wrp

    Out scouting around today and ran across an orchard that produced among other things, 1927 Fords. Picked up 5, Couple were steel bodied. One had a...

    40FordDeluxe 04-15-2021, 04:17 AM 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...

    40FordDeluxe 04-15-2021, 04:15 AM Go to last post

    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...

    40FordDeluxe 04-15-2021, 04:10 AM Go to last post

    Checking in

    Thread Starter: Mike P

    I still read the forum everyday but haven't been posting to much lately so I figured I'd do a catch-up post. My hand continues to improve but...

    40FordDeluxe 04-15-2021, 04:07 AM Go to last post

    Updating my Corvette

    Thread Starter: NTFDAY

    This spring Ian and I will be doing some updates, this is one of them, headers and sidepipes

    40FordDeluxe 04-15-2021, 03:58 AM Go to last post

    Shock/Strut Oil

    Thread Starter: rspears

    Not a hot rod question, just a Dad trying to help his daughter. Her 2019 Yukon was stinking to high heaven, giving her headaches so she took it back...

    40FordDeluxe 04-15-2021, 03:56 AM Go to last post

    1939 Ford Tudor Delux for sale

    Thread Starter: Navy7797

    I have a 1939 Ford Tudor Delux that I'm just no going to get around to rebuilding, its up for sale. located in Seguin TX No motor no trans....

    Navy7797 04-14-2021, 02:17 PM Go to last post
Links monetized by VigLink