Welcome to the North American Subaru Impreza Owners Club Friday September 19, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
NASIOC
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Built Motor Discussion

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 07-18-2008, 01:43 PM   #1
NITROS
Scooby Specialist
 
Member#: 41561
Join Date: Aug 2003
Chapter/Region: MWSOC
Location: Chicago
Vehicle:
04 GT3076 2.1
Stroker = Broken 3rd

Default Camshaft Terms Explained: Purchasing guide.

I had a hard time finding definitions for these terms on nasioc. I found this on a honda site and felt it would benefit us here. I couldnt find anything in the faq. Probably a good idea to have this added there.

http://northwesthondas.com/phpBB2/viewtopic.php?p=847

No quote for better search results.

I will explain below what you will wanna take in mind when purchasing the perfect cam for your application.

Duration-
Is the number of degrees of the crank rotation that the valve is held
open by a cam (don’t forget that the cam spins at half the speed that
the crank spins at). A general rule of thumb is that the more duration
the cam has, the more top-end power it will create, and this does
cause you to lose low-end power. The more duration the cam has the
more the valve will stay open during the cranks rotation. The longer
the valve stays open the longer the time the cylinder can be filled.
This is important at high rpm, since there is less and less time for
the cylinder to fill.

Lift-
Is the height that the valve is lifted of the valve seat. Usually the
more lift (with in reason) as the higher the valve is lifted the more
flow that can go by. Look at it this way, if you have your window open
half way you let in less air then if the window is open of the way.
The draw back to having more lift is that the valve opening and closing
speeds become high increasing the chance of valve float. This is why
you should upgrade to stiffer valve springs if the cams lift is
moderately aggressive.

Overlap-
Is the point at which both intake valves and exhaust valves are open
at the same time (at the end of the exhaust stroke and the beginning
of the intake stroke). Overlap is important, because having both
intake and exhaust valves open at the same time creates better
scavenging of stale exhaust to occur. This is because the flow from
the head into the cylinder (from the intake valves) creates a good
push to get rid of stale exhaust out of the cylinder. Just like
anything though, too much of a good thing is bad. If you get to much
overlap the flow from the intake valves will push gas out of the
cylinder before it has been used, thus wasting fuel, which equates to
power. One more draw back of to much overlap is reversion into the
intake ports. Reversion is when fuel/air is pushed back into the
intake ports. This reversion causes intake charge dilution at low rpm
as the backflow in the intake port gets in the way of cylinder filling.
Two things will occur because of this, cylinder pressure becomes poor
at low speeds because of the incomplete filling and the fuel/air
intake charge becomes diluted because of air rushing back into the
intake ports causes low speed misfire. This is why aggressive cams
that have big overlapping, create a ruff loopy idle. This misfiring
causes the motor to skip a beat in rhythmic fashion at idle and low
rpm. Usually firing once every four revolutions when the cylinder gets
enough fuel to touch off. This occurrence is called 8-storking,
because of the misfiring and the car skipping a beat it fires every
other stroke of the 4-stroke cycle.

Thanks to bugeyes for the following:
A common misconception is that a camshaft with heaps of lift is the way to go.
Its not so simple.

As flow is measured through a heads port the air flow increases as lift is increased, however there is a point where flow starts to flatten out as lift is continued to be increased.
This point where flow starts to flatten out is largely proportional to the valve size.
This is why you may see chevy's with 2"+ sized intake valves running 0.650 lift, it actually is of some benefit to them, but not to us.
For our small valve Subaru's we benefit from 4 valves per cylinder.
Four valve heads typically benefit from high flow rates at low valve openings but to run cams with excessive valve lift is only flogging at the valve gear and creating power loss through having to run high tention valve springs.

Typically you will only see subaru cams with a maximum lift of about 10 to 11mm of lift.

What you should be concerned about is not lift but the duration both advertised and at 0.050".
* Registered users of the site do not see these ads.

Last edited by NITROS; 07-20-2008 at 06:35 PM.
NITROS is offline   Reply With Quote
Old 07-20-2008, 04:28 AM   #2
kellygnsd
Scooby Guru
 
Member#: 32669
Join Date: Feb 2003
Chapter/Region: SCIC
Location: Rancho C
Vehicle:
2007 2.34LR, EFR7670
LINK G4 hybrid STi

Default

Centerline and profile may be good terms to add also.

Another thing on the overlap too is the rapid evacuation of exhaust gases actually help to pull in a fresh charge even after the intake stroke reaches BDC position.
kellygnsd is offline   Reply With Quote
Old 07-20-2008, 08:43 AM   #3
bugeyes
Scooby Specialist
 
Member#: 64494
Join Date: Jun 2004
Location: Australia
Vehicle:
2001 V7 sti

Default

I just want to add to lift.

A common misconception is that a camshaft with heaps of lift is the way to go.
Its not so simple.

As flow is measured through a heads port the air flow increases as lift is increased, however there is a point where flow starts to flatten out as lift is continued to be increased.
This point where flow starts to flatten out is largely proportional to the valve size.
This is why you may see chevy's with 2"+ sized intake valves running 0.650 lift, it actually is of some benefit to them, but not to us.
For our small valve Subaru's we benefit from 4 valves per cylinder.
Four valve heads typically benefit from high flow rates at low valve openings but to run cams with excessive valve lift is only flogging at the valve gear and creating power loss through having to run high tention valve springs.

Typically you will only see subaru cams with a maximum lift of about 10 to 11mm of lift.

What you should be concerned about is not lift but the duration both advertised and at 0.050".
bugeyes is offline   Reply With Quote
Old 07-21-2008, 10:50 AM   #4
9vapors
Scooby Guru
 
Member#: 92582
Join Date: Aug 2005
Chapter/Region: Tri-State
Location: NY
Vehicle:
*

Default

If you get Crower (not Brian Crower) cams, get ready for the cards to be incorrect...
9vapors is offline   Reply With Quote
Old 08-08-2008, 11:20 AM   #5
dub c
Scooby Specialist
 
Member#: 145069
Join Date: Apr 2007
Chapter/Region: W. Canada
Location: Calgary, Alberta
Vehicle:
Keep It Sleazy

Default

Thanks for the info!
Any quick info on installing them?
dub c is offline   Reply With Quote
Old 08-08-2008, 12:20 PM   #6
gandapanda
Scooby Newbie
 
Member#: 51476
Join Date: Dec 2003
Chapter/Region: MWSOC
Location: Naperville, IL
Vehicle:
05 FXT
06 s2000

Default

Good info, Walter. Perhaps some information regarding the characteristics of turbo vs. NA cams would be beneficial. With FI, more/equal lift but less duration and overlap would be the key.
gandapanda is offline   Reply With Quote
Old 08-09-2008, 10:54 AM   #7
kellygnsd
Scooby Guru
 
Member#: 32669
Join Date: Feb 2003
Chapter/Region: SCIC
Location: Rancho C
Vehicle:
2007 2.34LR, EFR7670
LINK G4 hybrid STi

Default

Quote:
Originally Posted by dub c View Post
Thanks for the info!
Any quick info on installing them?
Installing cams is anything but quick
kellygnsd is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not 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
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Purchasing parts on eBay, a guide for j00! Unabomber Newbies & FAQs 38 05-16-2010 04:22 PM
Bicycle Noob: Guide my purchase please. ScubaruImpreza05 Off-Topic 25 02-24-2008 10:27 PM
Long-term purchase - how does an Impreza stack up? slippers2k Impreza Forum 3 05-26-2006 04:52 PM
Help with study guide--Business/MBA students/grads or those in purchasing and supply Bonjo2 New England Impreza Club Forum -- NESIC 12 04-05-2006 04:35 PM


All times are GMT -4. The time now is 10:13 PM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.