Compression ratio's effect on power - Ford Muscle Forums : Ford Muscle Cars Tech Forum
FordMuscleForums.com is the premier Ford Forum on the internet. Registered Users do not see the above ads.
Reply
 
LinkBack Thread Tools
Old 11-02-2005, 10:17 PM   #1 (permalink)
Registered User
 
Join Date: May 2004
Posts: 275
Compression ratio's effect on power

Is there a general ballpark rule of thumb for power gain for a given percentage increase in ratio? Say, going from a 8.5:1 to 10.5:1 with the same components, just jets and timing.
_________________
Eric
La Habra, CA
--------------------
1965 Econoline
408w SCAT crank, SCAT rods, Probe forged pistons, AFR205's in the works.

[ This Message was edited by: etcetera on 11/3/05 10:19am ]
etcetera is offline   Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
Old 11-03-2005, 05:08 AM   #2 (permalink)
Registered User
 
Join Date: Jan 2001
Location: Oregon
Posts: 2,386
Compression ratio's effect on power

I have seen the number "approximately 4%" mentioned for each full static compression point.

Going from 9.0:1 to 10.0:1 = approx. 4% increase in hp/tq


mstngjoe is offline   Reply With Quote Quick reply to this message
Old 11-03-2005, 07:38 AM   #3 (permalink)
Registered User
 
Join Date: May 2004
Posts: 275
Compression ratio's effect on power

Cool. I figured there must be a loose approximation out there. Joe probably can guess why I'm asking. I ended up with a very similar combo to one that is in one of the AFR articles, except with a 10.7:1 CR, vs the Turbo friendly CR in the article (they don't list it specifically).

Here's the combo
408w
Scat Crank and Rods
Probe -20cc forged pistons
Approx 10.7:1
Victor Jr
AED HO 950 (Lightly used)
Comp Cams XR286R (Lightly used)
Comp rollers
Scorpion rockers

Here's the article:
http://airflowresearch.com/articles/article46/A-P1.htm

Their turbo motor, without the turbo, made 560hp at 6000rpm and 538ft/lbs at 4900.

It's really tempting to dyno the engine to see if the numbers in the AFR article are close to reality.

_________________
Eric
La Habra, CA
--------------------
1965 Econoline
408w SCAT crank, SCAT rods, Probe forged pistons, AFR205's, Comp Cams XR286R, Victor Jr, AED 950

[ This Message was edited by: etcetera on 11/3/05 8:31pm ]
etcetera is offline   Reply With Quote Quick reply to this message
Old 11-03-2005, 12:31 PM   #4 (permalink)
Registered User
 
Join Date: Jun 2001
Posts: 86
Compression ratio's effect on power

My 2cents....I believe increasing compression has a diminishing improvement the higher you go. That is the improvement from 8~10 will be greater than 10~12 and so on. In my mind it takes the shine off CR greater than 10.5:1 for a streetable petrol car as the hassles start to greatly outweigh the gains.
speedar is offline   Reply With Quote Quick reply to this message
Old 11-03-2005, 01:53 PM   #5 (permalink)
Registered User
 
Join Date: Apr 2001
Location: Central West Virginia
Posts: 7,012
Compression ratio's effect on power

It's all about having adequate cylinder pressure.

A small cam bleeds off less cylinder pressure, so it can use a smaller compression ratio. A big cam bleeds off more compression, so it needs a higher compression ratio to maintain adequate cylinder pressure.

If your compression ratio is low for your combo, you have a lot to gain from an increase. If the engine already has adequate cylinder pressure, further increasing the compression ratio won't produce as large of a gain.

Adding cylinder pressure helps the low-end and midrange more than the top end.
__________________

66 Mustang, Toploader 4-speed
Pump Gas 306, 10.134@134.71
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220
n2omike is offline   Reply With Quote Quick reply to this message
Old 11-03-2005, 07:39 PM   #6 (permalink)
Registered User
 
Join Date: May 2004
Posts: 111
Compression ratio's effect on power

so what would be a good way to increase compression? is the only way changing the pistons and stroke?
__________________
1966 mustang 6 cyl to a 302 hi-po. 4 speed manual. 8 inch rear end with 3.91 gearing.
slyman2087 is offline   Reply With Quote Quick reply to this message
Old 11-03-2005, 07:44 PM   #7 (permalink)
Registered User
 
Join Date: Mar 2003
Posts: 700
Compression ratio's effect on power

Quote:
On 2005-11-03 19:39, slyman2087 wrote:
so what would be a good way to increase compression? is the only way changing the pistons and stroke?
Shaving or milling the heads and using a thinner head gasket will give you a little bit more compression but you run into clearance problems if too much is cut off.
mikes82GT is offline   Reply With Quote Quick reply to this message
Old 11-03-2005, 10:08 PM   #8 (permalink)
Registered User
 
Join Date: Feb 2004
Posts: 3,658
Compression ratio's effect on power

decking the block can raise it too.
thekingofazle is offline   Reply With Quote Quick reply to this message
Old 11-04-2005, 08:28 AM   #9 (permalink)
Registered User
 
Join Date: Apr 2002
Location: Merkel, Tx
Posts: 9,251
Compression ratio's effect on power

Diminishing returns. Go here
http://www.tprmag.com/issue/6/6_forged_perf.shtml

Click on (right side):

Percent Change In Efficiency & Power From Charge In Compression Ratio


_________________

1967 Falcon 4 door w/351C - Owner built, owner abused.
70 Mustang 302 / 02 Mustang V6 rag top, 04 SuperCrew parts hauler
http://raceabilene.com/kelly/hotrod

[ This Message was edited by: ckelly on 11/4/05 8:29pm ]
ckelly is offline   Reply With Quote Quick reply to this message
Old 11-05-2005, 03:44 AM   #10 (permalink)
Registered User
 
Join Date: Nov 2002
Location: maryland
Posts: 3,776
Compression ratio's effect on power

Mill heads and block.Or maybe some domed pistons
tonys10sec306 is offline   Reply With Quote Quick reply to this message
Old 11-05-2005, 09:09 AM   #11 (permalink)
Registered User
 
Join Date: Jun 2004
Location: Catonsville, MD
Posts: 502
Compression ratio's effect on power

both overbore or boring & stroking will increase ratio. for instance keeping the same combustion chamber head while going from having a 302 to having a 331 will up the comp ratio.
__________________
Dave Shea
S/ST 1818 S/P 1018 H/R 1564
igo1090 is offline   Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Ford Muscle Forums : Ford Muscle Cars Tech Forum forums, you must first register.
Please enter your desired user 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 On
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -7. The time now is 03:29 PM.



Powered by vBulletin® Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Friendly URLs by vBSEO 3.6.0
 

Garage Plus vBulletin Plugins by Drive Thru Online, Inc.