Compression ratio? - 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 02-13-2011, 01:22 PM
Madmax305's Avatar
Registered User
 
Last photo:
Join Date: Jan 2010
Location: Brea, Ca.
Posts: 249
Wiki Edits: 0

Thanks: 1
Thanked 2 Times in 2 Posts
Compression ratio?

Hey guys, I have a GM performance 290hp motor with 72cc heads in my El Camino. It has 8.5 to 1 compression. A while back I replaced the heads with RHS Vortec with 64cc chambered heads. The stock pistons are flat tops? Can anybody give me any idea of my compression ratio with the RHS heads installed? The head gaskets were the ones RHS recommended, sorry that's all I know about the gaskets or the motor.

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 02-13-2011, 01:29 PM
FastOrange's Avatar
Registered User
 
Join Date: May 2010
Location: Canada
Posts: 236
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
my guess is like 10-10.5:1
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 02-13-2011, 01:58 PM
bigdog7373's Avatar
Of course it's fast
 
Last photo:
Join Date: May 2010
Location: florida
Posts: 2,673
Wiki Edits: 0

Thanks: 13
Thanked 49 Times in 49 Posts
I would say 10 or lower.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 02-13-2011, 02:55 PM
Registered User
 
Join Date: Jun 2010
Location: TX
Posts: 1,216
Wiki Edits: 0

Thanks: 0
Thanked 108 Times in 99 Posts
Pump gas

A engine with 10.5:1 CR is on the borderline of detonation when using 93 octane pump gas if the initial timing advance is set where the engine will run best. Aluminum heads can tolerate 1/2 to one point higher compression ratio than cast iron heads with the same combustion chamber volume. When the engine get some time on it, the problem with detonation on 93 octane pump gas will increase.

Rule of thnub is the first three numbers of your compression ratio indicates the minimum octane rating of the fuel that must be used. 9.3:1 CR = 93 octane, 10.5:1 CR = 105 octane, 11.0:1 CR = 110 octane, 12.0:1 = 120 octane


There is no scientific reason for it but that is just the way it is.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 02-13-2011, 04:51 PM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 9,471
Wiki Edits: 0

Thanks: 29
Thanked 487 Times in 442 Posts
Taking a rough stab at it using an online calculator and making an educated guess at some of the dimensions you don't have - 64cc head, 7cc valve relief, common .041" thick composition head gasket, and an undecked block which puts the piston .025" down the bore at TDC results in a 9.6-1 compression ratio.

I use the calculator at www.wallaceracing.com They have a huge calculators page
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 02-13-2011, 04:56 PM
Madmax305's Avatar
Registered User
 
Last photo:
Join Date: Jan 2010
Location: Brea, Ca.
Posts: 249
Wiki Edits: 0

Thanks: 1
Thanked 2 Times in 2 Posts
Thanks for all the info guys. Thank you ericnova72 for the calculations. I was hoping for about 9.5 but 9.6 is even better.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 02-13-2011, 11:24 PM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,153
Wiki Edits: 0

Thanks: 5
Thanked 380 Times in 374 Posts
The GM 350 290Hp crate motor does not have flat top pistons
The pistons have a dish , approx dish volume is 12cc

if you were to shave the 64cc heads by .030" (59cc finished) and install with a .015" felpro 1094 gasket, you would have a true 10.00:1 cr.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 02-14-2011, 12:34 AM
Madmax305's Avatar
Registered User
 
Last photo:
Join Date: Jan 2010
Location: Brea, Ca.
Posts: 249
Wiki Edits: 0

Thanks: 1
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by F-BIRD'88
The GM 350 290Hp crate motor does not have flat top pistons
The pistons have a dish , approx dish volume is 12cc

if you were to shave the 64cc heads by .030" (59cc finished) and install with a .015" felpro 1094 gasket, you would have a true 10.00:1 cr.
So any guesstimate on my compression ratio with the above information?
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 02-14-2011, 12:40 AM
SSedan64's Avatar
Registered User
 
Last photo:
Join Date: Dec 2007
Location: Macon, GA.
Age: 50
Posts: 6,016
Wiki Edits: 0

Thanks: 0
Thanked 46 Times in 46 Posts
With Eric's Info and the 12cc Dish Pistons = 9.00:1Cr

No milling and a .015" Thick Shim Gasket = 9.50:1Cr
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 02-14-2011, 12:40 AM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 9,471
Wiki Edits: 0

Thanks: 29
Thanked 487 Times in 442 Posts
Figuring with an undecked block so piston is down the hole at TDC .025", with .041" thick composition gasket, 64cc head and 12cc piston gives 9.1-1.

Calculator used is on Automotive Calculators page at www.wallaceracing.com
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 02-14-2011, 12:43 AM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 9,471
Wiki Edits: 0

Thanks: 29
Thanked 487 Times in 442 Posts
Use of an .015" steel shim head gasket instead of the common .041 composition gasket will get you back to 9.6-1 with a 12cc dish and the 64cc head.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 02-14-2011, 12:49 AM
Madmax305's Avatar
Registered User
 
Last photo:
Join Date: Jan 2010
Location: Brea, Ca.
Posts: 249
Wiki Edits: 0

Thanks: 1
Thanked 2 Times in 2 Posts
Thanks for the quick replies. I was hoping it would be a bit higher, but 9.1-1 is not so bad with 91 octane.
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 02-14-2011, 03:55 AM
SSedan64's Avatar
Registered User
 
Last photo:
Join Date: Dec 2007
Location: Macon, GA.
Age: 50
Posts: 6,016
Wiki Edits: 0

Thanks: 0
Thanked 46 Times in 46 Posts
Does your 290HP Crate engine have a standard 4.00" Bore? If so that's why Eric's & my calculations differ by .10
I used a 4" Bore.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 02-14-2011, 10:25 AM
Madmax305's Avatar
Registered User
 
Last photo:
Join Date: Jan 2010
Location: Brea, Ca.
Posts: 249
Wiki Edits: 0

Thanks: 1
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by SSedan64
Does your 290HP Crate engine have a standard 4.00" Bore? If so that's why Eric's & my calculations differ by .10
I used a 4" Bore.
Yes, it has a 4.00 bore x 3.48 stroke.
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 02-14-2011, 12:45 PM
Registered User
 
Join Date: Jun 2010
Location: TX
Posts: 1,216
Wiki Edits: 0

Thanks: 0
Thanked 108 Times in 99 Posts
Pencil and paper measument

Quote:
Originally Posted by SSedan64
With Eric's Info and the 12cc Dish Pistons = 9.00:1Cr

No milling and a .015" Thick Shim Gasket = 9.50:1Cr

5738 CC engine displacment
5738 / 8 = 717.25 CC cyl. vol.

717.2 CC / 85 CC chamber vol. = 8.44 : 1 CR

Assuming (without actual measurment):
64 CC combustion chamber vol.
+9 CC gasket, reliefs, deck vol.
+12 CC dish vol.
_______
85 CC total chamber vol.
That engine could run good on Cool Aid.
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:
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
compression ratio compared to compression PSI squids1 Engine 14 12-28-2010 11:36 PM
Compression Ratio Help KillerK10 Engine 5 02-10-2010 05:02 PM
383 compression ratio? 72GMCTruck Engine 10 10-09-2008 06:14 AM
compression ratio 84 GMC Sierra Pickup Engine 9 10-25-2005 05:19 AM
compression ratio? Dans83Olds Engine 3 06-07-2003 06:49 PM


All times are GMT -6. The time now is 09:48 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.