Static VS Dynamic compression (cam selection) - 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 06-04-2013, 11:43 AM
Registered User
 
Last photo:
Join Date: May 2009
Location: Wichita KS
Posts: 339
Wiki Edits: 0

Thanks: 0
Thanked 4 Times in 4 Posts
Static VS Dynamic compression (cam selection)

Am I using this dynamic compression calculator incorrectly?

The static seems correct form what I’m told. My LS series 5.3 Motor should be about 8.5:1

I chose an odd ball cam with almost no overlap. The engine is a street motor and will have a small turbo and high back pressures so I didn’t want a cam with a lot of overlap as the exh pressures will be higher than the intake pressures by quite a bit.

Anyway… What I’m trying to figure out is why this calculator is dropping my dynamic compression down to the 7:1 range? I was assuming a cam with little to no overlap wouldn’t loose much in terms of dynamic compression. I could see a cam with a ton of overlap and duration bleeding off compression, but I thought I wouldn't lose much compression going with this cam.

Any thoughts? Am I doing this wrong?

Cam specs (ZR1 OEM camshaft)

211/230 @.050 valve lift and .558/.552 lift with 1.7 rockers and a 122.5 centerline.


    Advertisement

Last edited by ForceFed86; 06-04-2013 at 11:48 AM.
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 06-04-2013, 12:40 PM
Registered User
 
Join Date: Jun 2010
Location: TX
Posts: 1,228
Wiki Edits: 0

Thanks: 0
Thanked 109 Times in 100 Posts
Ed Iskendarian once said: "The intake closing point is the most important valve event."

Example: My Comp camshaft has 39* valve overlap with a intake closing event of 58* ABDC.
The static compression ratio is 10.2: and the dynamic compression ratio is 8.5:1. NA dynamic compression ratio of 8.5:1 is the limit for 93 octane pump gas.

How is your valve overlap calculated? The valve overlap must be calculated using the seat to seat time or duration at .004" - .006" valve lift.

Degrees intake valve open + degrees exhaust valve close = degrees valve overlap.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 06-04-2013, 01:06 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 9,354
Wiki Edits: 0

Thanks: 20
Thanked 369 Times in 348 Posts
It all looks right, your IVC is 46.5 degrees, what did you expect? It should be a decent turbo cam but be sure to run a large enough exhaust housing.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 06-04-2013, 01:07 PM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
What calculator is that? Using the United Engine & Machine Co. Incorporated calculator I'm seeing about the same- maybe a tick lower but that's due to how that particular calculator works.

Last edited by cobalt327; 06-04-2013 at 01:13 PM.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 06-04-2013, 01:16 PM
Registered User
 
Last photo:
Join Date: May 2009
Location: Wichita KS
Posts: 339
Wiki Edits: 0

Thanks: 0
Thanked 4 Times in 4 Posts
It was an .XLS spreadsheet a friend sent me. I don't have a link. I'll try to attach it if anyone wants to play with it.

Running a t4 1.10AR t4 housing with an 83mm wheel.

I don't mind the lower compression, I plan to run alot of boost. Using e85 for fuel though so 7:1 seems a bit on the conservative side.

Thanks all...
Attached Files
File Type: zip MY 5.3_317 heads DCR Lookup LS1.zip (16.2 KB, 31 views)
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 06-04-2013, 01:27 PM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
Thanks for the link.

Is this a race-only deal or will you be driving it on the street a lot?
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 06-04-2013, 01:40 PM
Registered User
 
Last photo:
Join Date: May 2009
Location: Wichita KS
Posts: 339
Wiki Edits: 0

Thanks: 0
Thanked 4 Times in 4 Posts
Just to and from the track mostly. Occasional cruising.



Heres the build thread if your bored...


79 RX-7 LS 5.3 Turbo Build
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 06-04-2013, 01:47 PM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
That's a bad-*** ride! The only 'iffy' part I see is the lightened NASCAR-type rear gear- will it take the launch w/slicks?
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 06-04-2013, 01:56 PM
Registered User
 
Last photo:
Join Date: May 2009
Location: Wichita KS
Posts: 339
Wiki Edits: 0

Thanks: 0
Thanked 4 Times in 4 Posts
Quote:
Originally Posted by cobalt327 View Post
That's a bad-*** ride! The only 'iffy' part I see is the lightened NASCAR-type rear gear- will it take the launch w/slicks?
Thanks.

We'll find out...

I read about a few guys using them on Yellowbullet.com. They were also running heavier cars so I'm hoping I'll get away with it.

They are made of the shock resistant 9310 high impact steel. From what I hear it should be as tough as a standard gear even lightened. It's a soft metal though so It will wear quickly if I put a ton of street miles on it. I figured for $130 I could give it a shot. Local track is 15 miles from the house.
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 06-04-2013, 02:37 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,014
Wiki Edits: 0

Thanks: 4
Thanked 512 Times in 434 Posts
Quote:
Originally Posted by ForceFed86 View Post
Am I using this dynamic compression calculator incorrectly?

The static seems correct form what Iím told. My LS series 5.3 Motor should be about 8.5:1

I chose an odd ball cam with almost no overlap. The engine is a street motor and will have a small turbo and high back pressures so I didnít want a cam with a lot of overlap as the exh pressures will be higher than the intake pressures by quite a bit.

AnywayÖ What Iím trying to figure out is why this calculator is dropping my dynamic compression down to the 7:1 range? I was assuming a cam with little to no overlap wouldnít loose much in terms of dynamic compression. I could see a cam with a ton of overlap and duration bleeding off compression, but I thought I wouldn't lose much compression going with this cam.

Any thoughts? Am I doing this wrong?

Cam specs (ZR1 OEM camshaft)

211/230 @.050 valve lift and .558/.552 lift with 1.7 rockers and a 122.5 centerline.

My DCR calcualtor shows a 6.62 against a SCR of 8.39. I'm guessing that there's another 15 degrees which I added on to get to the closing point of .006 which will be exacerbated by the 1.7 rocker ratio holding valves open more per degree than would be seen with a lower ratio rocker like those of the GEN I SBC. Given that you'll have a lot of blower pressure against the rising piston this may not be all that bad. The weakness of huffed motors is the cam effects when there is no to little intake pressure. DCRs in the sixes are pretty common with competiton motors running a lot of intake pressure which you say is an operational goal.

Bogie
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 06-04-2013, 02:38 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,161
Wiki Edits: 326

Thanks: 794
Thanked 1,087 Times in 899 Posts
Couple of questions....
1. Is that a fuel filter hose-clamped to a crossmember and lines run next to the driveshaft? If so, I'd send you home for a re-do.
2. Where's the rest of the cage? I don't see a windshield bar or halo bars. You can't go quicker than 10.00 in the quarter or faster than 135 with a rollbar.
3. You are going to build a metal cover for the cell, right?

Last edited by techinspector1; 06-04-2013 at 02:44 PM.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 06-04-2013, 02:42 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 9,354
Wiki Edits: 0

Thanks: 20
Thanked 369 Times in 348 Posts
Quote:
Originally Posted by techinspector1 View Post
You can't go quicker than 10.00 in the quarter or faster than 135 with a rollbar.
more than once...

And I would definitely watch the fuel line, fluid control means a lot even when your car isn't crashing and just running down the track as it should.
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 06-04-2013, 03:03 PM
Registered User
 
Last photo:
Join Date: May 2009
Location: Wichita KS
Posts: 339
Wiki Edits: 0

Thanks: 0
Thanked 4 Times in 4 Posts
Quote:
Originally Posted by oldbogie View Post
My DCR calcualtor shows a 6.62 against a SCR of 8.39. I'm guessing that there's another 15 degrees which I added on to get to the closing point of .006 which will be exacerbated by the 1.7 rocker ratio holding valves open more per degree than would be seen with a lower ratio rocker like those of the GEN I SBC. Given that you'll have a lot of blower pressure against the rising piston this may not be all that bad. The weakness of huffed motors is the cam effects when there is no to little intake pressure. DCRs in the sixes are pretty common with competiton motors running a lot of intake pressure which you say is an operational goal.

Bogie
Man if the DCR is that low Iím surprised it gets out of its own way. Iíve had it out and about locally and wastegate turned down as much as possible. Even out of boost with the 3:10 gear and 29Ē tires it accelerates decently. Felt pretty strong at 9lbs. Assuming it stays together I had planned to crank the manifold pressure up to the mid to upper 20ís once I get the tune all dialed in.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 06-04-2013, 03:19 PM
Registered User
 
Last photo:
Join Date: May 2009
Location: Wichita KS
Posts: 339
Wiki Edits: 0

Thanks: 0
Thanked 4 Times in 4 Posts
Quote:
Originally Posted by techinspector1 View Post
Couple of questions....
1. Is that a fuel filter hose-clamped to a crossmember and lines run next to the driveshaft? If so, I'd send you home for a re-do.
2. Where's the rest of the cage? I don't see a windshield bar or halo bars. You can't go quicker than 10.00 in the quarter or faster than 135 with a rollbar.
3. You are going to build a metal cover for the cell, right?

Appreciate the heads up.

That picture isn't the best. The rx7's had a high spot there where the factory storage bins were. Is there a regulaton for how close the fuel lines can be to the driveshaft? I didn't think mine were that bad? I'll look again and get a measurement. They are very close to the same spot the OEM lines were. Lines are also SS. The filter is up out of the way (again hard to see).


Yes, the steel box around the cell does have a cover.

I know I need to address the cage. Trying to get the car road worthy and tagged at the moment. I had planned on dialing in the suspension on low boost to get a general feel for the car. Shouldn't break 10.0 on low boost. Hell if the DCR is really 6.5:1 It may not break 10's without some serious pressure.
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 06-04-2013, 04:16 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 29
Posts: 9,354
Wiki Edits: 0

Thanks: 20
Thanked 369 Times in 348 Posts
Quote:
Originally Posted by ForceFed86 View Post
Man if the DCR is that low Iím surprised it gets out of its own way. Iíve had it out and about locally and wastegate turned down as much as possible. Even out of boost with the 3:10 gear and 29Ē tires it accelerates decently. Felt pretty strong at 9lbs. Assuming it stays together I had planned to crank the manifold pressure up to the mid to upper 20ís once I get the tune all dialed in.
that DCR isn't as low as it sounds. There have been MANY engines built that way from the factory from way back when, and they all did fine.

Don't confuse SCR and DCR, there's a big difference.
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
Static CR vs Dynamic CR seandiddy Engine 6 06-28-2011 05:51 PM
How much static compression for this cam? z-adamson Engine 16 06-26-2011 02:42 PM
Cam Selection and Dynamic CR Arcuden Engine 10 03-23-2008 12:03 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 07:41 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.