question about compression on a bbc - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 09-15-2006, 10:33 PM
inquiring_mind2's Avatar
a shade tree mechanic
 
Last photo:
Join Date: Jul 2004
Location: Illinois
Age: 43
Posts: 481
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
question about compression on a bbc

Hello,

I have a 427 bbc that has been bored 60 over with a 3.750 stroke crank, 6.135 eagle rods with 101 cc chambered closed port heads. The pistons are Speed pro domed with a dome of 35.5 cc. the valves are 225 188. I am trying to figure out what kind of compression I have. Any help would be greatly appreciated.

Thanks,

Chris

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 09-16-2006, 07:24 AM
Will I ever get it done?
 

Last journal entry: Really the Pits
Last photo:
Join Date: Oct 2004
Location: Tulsa, OK
Age: 57
Posts: 1,049
Wiki Edits: 0

Thanks: 1
Thanked 3 Times in 3 Posts
I come up with 14.7:1. This makes some assumptions. First, the pistons are at zero deck and the gasket has a zero thickness (or the pistons extend above the deck equal to the gasket thickness). Not likely and not good for the engine. It also assumes that the chambers are actually 101 cc, not just nominally 101 cc. Finally, the dome is the net volume. In other words, the volume of the dome less any valve reliefs.

It all makes a big difference. Let's say you have a gasket with a 4.34" diameter bore. It has a compressed thickness of 0.020". The pistons are decked at 0.010". Your heads actually measure out to 105 cc. Finally, you have valve reliefs not included in the 35.5 that amount to 5 cc. These small differences drop the compression to 12:1.

So, to get an accurate number, you need to get out your burette and check the combustion chambers. Check the piston domes. Measure your deck height. Measure your gasket diameter and verify the compressed thickness.

If you have Excel, I can send you a spreadsheet.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 09-16-2006, 07:45 AM
inquiring_mind2's Avatar
a shade tree mechanic
 
Last photo:
Join Date: Jul 2004
Location: Illinois
Age: 43
Posts: 481
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
the compressed gaket is .041 the the pistons have had .060 taken out out the so the I think it is the exhaust valves do not hit the top of the piston. The block has not been decked. I bought the gasket and pistons from jegs the the part number for the pistons are 844-L2268f60 and the gasket number is 375 1017-1 Sorry I don't havve excel. I wish I had the tools to check all of that out too. LOL The total valve to piston clearence is .110. I don't know if this will help out or not?
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 09-16-2006, 08:13 AM
Will I ever get it done?
 

Last journal entry: Really the Pits
Last photo:
Join Date: Oct 2004
Location: Tulsa, OK
Age: 57
Posts: 1,049
Wiki Edits: 0

Thanks: 1
Thanked 3 Times in 3 Posts
The advertised CR is 11.25:1. I would guess this would be close. The only way you are going to be more accurate is to measure everything. You've spent a lot of money and you have a stout engine. The difference between a good engine and a great engine is the details. If you use a burette to check the chambers, you will likely not only find that the volume is different than advertised, but varies from cylinder to cylinder. This is what blueprinting is all about. You make sure all the combustion chambers have the same volume.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 09-16-2006, 09:15 AM
72NOVA454
 
Last photo:
Join Date: Nov 2004
Location: chicago area
Age: 51
Posts: 922
Wiki Edits: 0

Thanks: 0
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by redsdad
I come up with 14.7:1. This makes some assumptions. First, the pistons are at zero deck and the gasket has a zero thickness (or the pistons extend above the deck equal to the gasket thickness). Not likely and not good for the engine. It also assumes that the chambers are actually 101 cc, not just nominally 101 cc. Finally, the dome is the net volume. In other words, the volume of the dome less any valve reliefs.

It all makes a big difference. Let's say you have a gasket with a 4.34" diameter bore. It has a compressed thickness of 0.020". The pistons are decked at 0.010". Your heads actually measure out to 105 cc. Finally, you have valve reliefs not included in the 35.5 that amount to 5 cc. These small differences drop the compression to 12:1.

So, to get an accurate number, you need to get out your burette and check the combustion chambers. Check the piston domes. Measure your deck height. Measure your gasket diameter and verify the compressed thickness.

If you have Excel, I can send you a spreadsheet.
Hi, saw your response. can you send me that excel spreadsheet please.

send to lwallace@pcsoc.com
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Boring 396 to 427,and compression question Montegogo Engine 13 02-02-2006 09:09 AM
compression question for 355 Mike_82 Engine 2 12-25-2005 02:23 PM
compression question, 82c10 Engine 1 12-14-2005 05:00 PM
A compression ratio question Centerline Engine 7 11-20-2005 07:52 AM
Compression Question Houman SS '70 Engine 3 03-10-2005 05:28 PM


All times are GMT -6. The time now is 11:54 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.