dynamic compression - 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 12-26-2006, 07:31 PM
Registered User
 
Join Date: Apr 2006
Location: FL
Posts: 96
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
dynamic compression

when i try to figure out my dynamic compression i'm getting a full 1 point difference betweent the one at keith black pistons site and the one from a post i made before. the keith black and the others say to add 15 to intake closing .050 #. this one goes off ADV intake closing. now i've read people say they can get a motor to run on pump gas with a 9.0 DC. i'm wondering if with the two different calculators if the recommendations are different?

with a roller cam closing at .05 +15= 58 with 10.5 compression i get 8.7DC
with ADV closing of 74 and 10.5 to1 i get7.6 DC

with a 232/240 hydroller and aluminum heads i find it hard to believe that i should run 9.6 to 1 compression for pump gas. when .050 + 15 is used.

but i'm also shocked that with the ADV number i need to use over 11 to 1 too.

crane says 9.5 to 10.75 to 1 for the 224/232 hydroller. i have to call them to find out what the recommended compression is for the 232/240.

what am i suppost to believe?

i edited it so it's makes more sense

Quote:
Originally Posted by 71C10
If in doubt go with Crane's compression recommendation.

Try the DCR calculator from this site:
http://members.uia.net/pkelley2/DynamicCR.html

You will find it more accurate than adding a generic 15 degrees to the .050 specs... and he has what I think are realistic "real world" recommendations.

    Advertisement

Last edited by 71javelin; 12-26-2006 at 10:28 PM.
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 12-26-2006, 08:37 PM
71C10's Avatar
U.S. Army Retired
 
Last photo:
Join Date: Sep 2005
Location: Texas
Posts: 266
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
I don't know where you got the 9.1 DCR on pump gas from, but if you use the cams advertised duration numbers in the KB calculator, you get the same DCR as from here: http://members.uia.net/pkelley2/DynamicCR.html at least with Comp Cams adv numbers...

From the site I originally provided:
"It is necessary to determine the position of the piston at intake valve closing to calculate the DCR. This can be calculated or measured (using a dial indicator and degree wheel). Since compression cannot start until the intake valve is closed, it is necessary to use seat times when calculating the DCR. Using .050" timing will give an incorrect answer since the cylinder is not sealed. At .050" tappet lift, using 1.5 rockers, the valve is still off the seat .075" and .085" with 1.7 rockers. While the flow is nearing zero at this point, compression cannot start until the cylinder is sealed."

I recently helped out on a pump gas SBC 383 dyno session that had a measured 10.99:1 compression ratio and a DCR of 8.41, its hot cranking compression was 200 PSI... it made 520HP, 530TQ through a set of (too small, but they are what the customer is going to use) 1.625 tri-y headers.

So I think its fair to say that DCR is a "useful" tool, if "used" properly.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 12-26-2006, 10:00 PM
Registered User
 
Join Date: Apr 2006
Location: FL
Posts: 96
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
yes you do, if you use the ADV number. but if you follow the KB calculator instructions. it says to use the .050 and add 15 you get a lot higher DCR.

how big was the cam and the closing intake number on that stroker and did it detonate?

i just checked the DCR with ADV #'s and with 11.17 to 1 i get 8.08 with the cam closing at 76 - 232/240 hydroller

with the 224/232 ADV #'s and 11.17 to 1 with a closing of 74 i get 8.24


where i got 9 DCR was from some guy building LS1 motors. but i'm thinking he's useing the .050 + 15 number to get the 9 DCR

what about the slow ramp from seat to .050 on the hydroller cam? since the 224@ .050 has the same ADV 292 as a hyd 230@.050 cam?

i'm also at sea level and it's hot in florida during the summer.

i've got the car on the lift and pulled the trans today and i'm pulling the motor and taking the pistons to get wacked tomorrow. so i need to find out what compression i should run. i was thinking 10.6 or 10.4 to be safe. but the only problem with that is my DCR will be around 7.6-7.8 with the 10.4 and the 10.6 to1 if i'm doing it right.
i just don't want this thing to detonate. been there done than

Last edited by 71javelin; 12-26-2006 at 10:18 PM.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 12-27-2006, 12:18 AM
71C10's Avatar
U.S. Army Retired
 
Last photo:
Join Date: Sep 2005
Location: Texas
Posts: 266
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by 71javelin
yes you do, if you use the ADV number. but if you follow the KB calculator instructions. it says to use the .050 and add 15 you get a lot higher DCR.

how big was the cam and the closing intake number on that stroker and did it detonate?

i just checked the DCR with ADV #'s and with 11.17 to 1 i get 8.08 with the cam closing at 76 - 232/240 hydroller

with the 224/232 ADV #'s and 11.17 to 1 with a closing of 74 i get 8.24


where i got 9 DCR was from some guy building LS1 motors. but i'm thinking he's useing the .050 + 15 number to get the 9 DCR

what about the slow ramp from seat to .050 on the hydroller cam? since the 224@ .050 has the same ADV 292 as a hyd 230@.050 cam?

i'm also at sea level and it's hot in florida during the summer.

i've got the car on the lift and pulled the trans today and i'm pulling the motor and taking the pistons to get wacked tomorrow. so i need to find out what compression i should run. i was thinking 10.6 or 10.4 to be safe. but the only problem with that is my DCR will be around 7.6-7.8 with the 10.4 and the 10.6 to1 if i'm doing it right.
i just don't want this thing to detonate. been there done than
Check the cranking compression before you yard it apart... what is it??? If it below 180-210 PSI, consider a complete build since, at least in my opinion too much compression is not your problem... if it is over 210 PSI then you could have have too much compression or too little cam to run pump gas...

CC the CLY head chambers before you circumcise the pistons, LOL Six degrees of duration difference for the same type of cam, one way or the the other is not going to kill ya... you can advance or retard the cam once you are that close.

The ADV duration on the Comp Cam in the 383 was 288-294 with a 110 lobe separation ground 4 degrees advanced (installed at 106 INT centerline). It has not been installed in the vehicle yet, so 93 octane on the dyno vs 93 octane on the South Texas (hot and humid) streets will probably not be an issue until Jul-Aug time frame.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 12-27-2006, 01:04 PM
Member
 
Last photo:
Join Date: Dec 2004
Location: San Antonio, Texas
Age: 50
Posts: 4,023
Wiki Edits: 0

Thanks: 0
Thanked 24 Times in 24 Posts
7.5 to 8.0 DCR is a good number for pump gas.

If you are using old cast iron heads with slow burn combustion chambers and dome style pistons then 7.5 is a good target dcr. With modern fast burn heads with flat top pistons (quench) then 8.0:1 is good. Aluminum heads take a little more.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 12-27-2006, 01:08 PM
Member
 
Last photo:
Join Date: Dec 2004
Location: San Antonio, Texas
Age: 50
Posts: 4,023
Wiki Edits: 0

Thanks: 0
Thanked 24 Times in 24 Posts
And.... different engines have different tolerance to DCR. Just because a modern LS1 can handle 9.0 DCR doesn't mean an old AMC engine can.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 12-27-2006, 01:35 PM
Registered User
 
Join Date: Apr 2006
Location: FL
Posts: 96
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
i had 240psi with the old setup and 11.9 to 1 with a little 224/232@ .050 roller.

the heads are indy and i have two sets of them. i had them ported and cc'ed the one set it 64.5cc and the other set i have are 63cc

EDIT i just cc the piston and i had 12.17 to one. i was guessing it had 8 cc for valve reliefs but it only had 6cc.

PS: i didn't build this motor. i got lazy and had someone build it.

Last edited by 71javelin; 12-27-2006 at 04:58 PM.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 12-27-2006, 03:28 PM
Hippie's Avatar
Analog man in a digital world.
 

Last journal entry: HEI comparison.
Last photo:
Join Date: May 2003
Posts: 2,253
Wiki Edits: 0

Thanks: 3
Thanked 5 Times in 5 Posts
Maybe this will help:

DCR combos that work

Most of these guys are regulars over at Chevy Talk, they use Pat Kelley's DCR calculator. I have seen cams with nearly identical .050" duration, LSA, Int CL and lift but vastly different advertised duration. Older cams will have large advertised numbers while more modern grinds have shorter numbers due to their more aggressive ramp profiles. For that reason I am more inclined to trust Pat's calculator. Maybe I'm wrong but at least I have real world examples to compare to using "apples" from the same tree.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 01-04-2007, 09:22 AM
my alter ego
 
Last photo:
Join Date: May 2005
Location: little kansas
Posts: 100
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Hippie, when I looked at the SBC combos I saw misleading info. a 4.030 bore with a 3.75 stroke isn't a 355, 358 but closer to 383. here is a calc I saw that gives an octane recommendation:

http://www.hotroddersauctions.com/Dy...Calculator.php
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
dynamic compression 71javelin Engine 13 10-15-2006 10:23 PM
Compression concerns. Siggy_Freud Engine 4 07-11-2006 11:06 AM
Dynamic Compression schatzy Engine 4 05-22-2006 02:11 PM
dynamic compression k-star Engine 8 06-25-2004 10:33 AM
camshaft selection and dynamic compression blndweasel Engine 7 03-13-2004 08:19 AM


All times are GMT -6. The time now is 09:08 PM.


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.