Rocker-Arm ratio questions - 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 03-04-2006, 10:13 PM
Registered User
 
Last photo:
Join Date: Jan 2006
Location: Louisiana
Posts: 73
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Rocker-Arm ratio questions

Exactly what all do rocker-arm ratios change about valve timing? I know that they change lift, but what about duration at .050 lift? How do I determin the best ratios for any application?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 03-05-2006, 03:41 AM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Pittsburgh, PA
Age: 40
Posts: 5,128
Wiki Edits: 16

Thanks: 0
Thanked 2 Times in 2 Posts
Rocker arm lift will change duration at the valve a little bit. If you are going from 1.5:1 to 1.6:1, its not a huge change and doesn't really affect overall engine RPM range too much. Since the ratio is faster, the point at which the cam reaches lift at the valve is sooner, so it adds a bit more duration.

The best way to choose ratios is by the head flow numbers. The heads will peak their flow at a certain lift. For instance, if you have a head that peaks its flow at .500" lift, and a cam that lifts .350, a 1.5:1 rocker will get you .525 lift. Changing to 1.6 rockers won't net you much power if any at all.

Sometimes the extra lift you get from higher ratio rockers will pay off with the added duration. Often times, heads will flow the same amount at lifts above their peak. Many times a head that flows 250 cfms at .500 lift will flow something like 252 cfms at .600 lift. The added 2 cfms doesn't account for any measurable power, but the added valve duration may add power. Basically the added lift doesn't add any power since it doesn't add flow, but the fact that its open longer by a couple degrees may give it more power.

In a carefully planned build, everything is set up properly for a good match. You would choose a cam with a duration that suits the head flow and power levels you wish to achieve. In a factory-stock setup often times extra duration can be simulated with higher ratio rockers.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 03-05-2006, 04:41 AM
malc's Avatar
Living At The Speed Of Life
 
Last photo:
Join Date: Jun 2003
Location: Espaņa Right Coast
Age: 63
Posts: 3,241
Wiki Edits: 0

Thanks: 86
Thanked 68 Times in 65 Posts
Go to ChevyHiPerf they have loads of cam and head info.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 03-05-2006, 09:12 AM
ztoy's Avatar
speed freak
 
Join Date: Dec 2002
Location: Circleville, Ohio
Age: 60
Posts: 229
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
great link malc!!!!!!!
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
SBC rocker arm alighnment help. 67 Deuce 4 Me Engine 3 02-16-2006 05:36 PM
Broken Rocker Arm Stud, Help to Remove RAW Engine 10 02-15-2006 02:23 PM
power loss from higher ratio rocker? KNanthrup Engine 5 08-26-2005 08:20 AM
rocker arm taps valve cover Echofive Engine 3 11-22-2003 05:15 PM
rocker arm help please windmaster Hotrodders' Lounge 2 08-05-2002 10:04 AM


All times are GMT -6. The time now is 12:39 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.