Compression Calculator Help - Page 2 - 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> Hotrodding Basics
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #16 (permalink)  
Old 11-15-2012, 01:40 PM
bentwings's Avatar
bentwings
 
Last photo:
Join Date: May 2002
Location: St.Paul, Minn
Age: 72
Posts: 1,798
Wiki Edits: 0

Thanks: 0
Thanked 16 Times in 15 Posts
As you can see there are many variables especially heads and gaskets.

Years ago when we were checking the CR of our TF motor the discussion got heated over who had the right numbers......several rod lengths, compression height pistons, ported heads with polished chambers etc. Well a long story short some money got laid on the table.

We assembled a block, crank, rod, piston on the motor stand. We found top dead center and set the pointer. Then we dialed the assembly to TDC. Since we ran pistons down the hole we cc'd the "deck clearance." This automatically took into account the small valve reliefs and the radius of the top of the piston. We then turned the motor over to BDC and again cc'd the cylinder. This gave us swept volumn and TDC volumn. From this we could calculate the CR using established vol of the gasket and head. However there is still room for error here so we added the gasket and cyl head. Torqued the head down to spec and using water for a fluid, we checked the vol at TDC and Again at BDC. Dividing BDC vol by TDC vol gives an exact CR as accurate as your equipment can give.


The result was the calculated value (mine) was within .01 of the measured value.

That steak was sure tasty haha.

Bottom line is that when in doubt check your CR as it is assembled....it's more work but very accurate.

    Advertisement

Last edited by bentwings; 11-15-2012 at 01:46 PM.
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #17 (permalink)  
Old 11-15-2012, 02:34 PM
gearheadslife's Avatar
MentalMuffinMan
 
Last photo:
Join Date: Nov 2012
Posts: 2,367
Wiki Edits: 0

Thanks: 137
Thanked 322 Times in 298 Posts
Quote:
Originally Posted by oldbogie View Post
Moving in that direction but not quite there. The big thing is they move the plug well inboard toward the valves so it has a more centered position relative to the bore diameter. This results in less burn time as the flame front spreads out to the bore limits rather than starting on a distant side then having to travel the entire bore diameter. The Swirl Port head used on the trucks shares a lot of common design with the L98 Corvette head. The truck head gets messed up with the added swirl vane. There is a cast iron version of the L98 head or if you will the Swirl Port head without the vane which are casting numbers 14096217 and 14101083. Castings 14096217 and 14101083 make part number 10125377 for 1986 to 1995 intakes using the 72 degree center bolts. Casting 14096217 is also used to make part number 10159552 for 1955-1986 intakes using the 90 degree center bolts. If you're interested these can be had quite cheaply on the used and rebuilt market for a bolt on replacement to Swirl Port heads or other applications. These heads also show up on some crate motors.

The Vortec is another step forward it borrows pretty heavily from the combustion chambers and port designs of the mid 1990s LT1 and LT4 heads as does the aluminum GMPP Fast Burn head but both the Vortec and Fast Burn use conventional coolant routing and will fit any GEN I block that has enough bore diameter to clear the valves.

Bogie
you don't off hand know the intake port CC of the gmpp fastburn alum.. would you?
Reply With Quote Quick reply to this message
  #18 (permalink)  
Old 11-15-2012, 03:22 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,078
Wiki Edits: 0

Thanks: 4
Thanked 535 Times in 452 Posts
Quote:
Originally Posted by gearheadslife View Post
you don't off hand know the intake port CC of the gmpp fastburn alum.. would you?
GM says they are 210 ccs, the ones I've touched measured 205. Either way these are big port heads so on a 350 they will like to wind up more RPMs to get the port velocity up to where they have enough velocity to ram feed the cylinder.

Bogie
Reply With Quote Quick reply to this message
The Following User Says Thank You to oldbogie For This Useful Post:
techinspector1 (11-15-2012)
  #19 (permalink)  
Old 11-15-2012, 05:10 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,282
Wiki Edits: 326

Thanks: 829
Thanked 1,143 Times in 942 Posts
Quote:
Originally Posted by Mr. P-Body View Post
Computing the dome volume from external dimensions is a tricky business unless it's a concentric shape. The engineers that made the piston have calculations that work, but "in the field", it's much easier just to measure it. At least one piston/ring assembly must be installed and attached to the crankshaft. Cam timing is not necesary at this point.

Find true TDC. Record deck clearance.

Using the dial indicator, lower the piston in the bore exactly 1". "CC" the cylinder in exactly the same manner as measuring the chamber volume in the head. Record the results.

Using the formula "Pi x R(squared) x H", find the "normal" volume of the 1" "cylinder". Where Pi = 3.14159, R = 1/2 bore size and H = stroke (in this case, 1"). Use the same formula to determine the amount "added" by the deck clearance where H = the clearance. Subtract the deck clearance volume from the actual volume. Subtract that from the "normal" volume of the 1" cylinder. To convert from "inch" to metric, multiply the results by 16.378 (how many CCs in one cubic inch). There is your "dome volume".

Doing this job isn't all that difficult. The tools aren't cheap. For a "once in a while" thing, I doubt it's worth the expense. A local machine shop can do the measuring for you at a reasonable charge if you do all the "prep" (subassembly, etc.). Call your local guys and ask. Of course, if you're up for it, buy or borrow the tools and do it yourself! You need a "CCing" kit, a dial indicator and base with at least a 1" stroke, a degree wheel and pointer and alcohol. Water will "do" in a pinch, but be sure to dry it up IMMEDIATELY when you're done. Nothing "thicker" than water. Jim


While I have a great deal of respect for Jim, let me make this a little easier for you fellows who have domed pistons. The piston needs only to be lower than the block deck to find the dome volume. Putting a short dome down in the cylinder one inch will work, of course, but you'll spend additional time refilling the burette, increasing your chances for error.

If, for instance, the dome is 0.200" above the edge of the piston, then you only need to put the piston down in the bore by 0.200", just enough to miss the clear Lucite, Plexiglas or Lexan cover plate. Hold the cover plate down on the block deck by hand, bring the piston up in the bore and push it up against the cover plate. Lay the cover plate aside and use your dial caliper to measure from the edge of the piston crown at the top ring up to the block deck. Let's say it measures 0.210". Write it down. Use Q-tips and Vaseline to seal around the edge of the piston to form a liquid-tight seal of the piston to the bore. Be careful not to push the piston down in the bore any further. Smear Vaseline all around the bore on the block deck and lay your cover plate down with the fill hole right at the edge of the bore and slightly uphill so the air will bleed out as you fill the void with alcohol from your burette.

Use math to figure the cc displacement of the void as if there is no dome on the piston, as if the piston were completely flat on the crown with no valve reliefs or dome. If we measured the bore at 4.031" (for instance) and we placed the piston down in the bore 0.210", then we have all the numbers we will need. Do the math thusly....
(.7854) x 4.031 x 4.031 x .210 x 16.387 = 43.91 cc's, call it 44.
Let's say you fill the void with alcohol, using 38 cc's to do so. Subtract 38 from 44 and find that the dome is worth 6 cc's. (Remember there are valve reliefs in the dome too, so it will take a lot of dome to make any real difference over a flat-top piston.) Here's a "for instance" of a 6cc dome piston....
http://www.kb-silvolite.com/kb_car/p...etails&P_id=54

I would advise against using water to find displacement in any voids. As Jim said, water ain't wet. You can improve the flow of it by adding a few drops of dishwash detergent and mixing it around, but alcohol is cheap and works very well. Go to Walmart in the pharmacy section and buy the largest bottle of 70% Isopropyl alcohol you can find, then go to the cake mix section and buy a bottle of food coloring so you can see the meniscus in the burette. Your choice, red, green or blue. It only takes a little food coloring to be able to see the fluid in the burette, so add a few drops at a time into your bottle of Isopropyl until you get the desired shade of color.

I agree that if you're going to be serious about knowing what you're doing, you need the proper tools. While there is absolutely a need for a dial indicator and magnetic stand, quite a lot of measuring can be done with a dial caliper and you'll use it more than you will the dial indicator. Go to ebay and bid on a used Starrett, Brown & Sharpe or Mitutoyo 8" dial caliper. Used is fine for your purposes. I recently bought a used Mitutoyo 12" with wooden case on ebay for 85 bucks. With an 8", you can measure the block deck height on most small blocks. With a 12", you can measure the block deck height on any block. A 6" caliper is the most common and you will use it every day, but you can't do block deck height with it. This would be a good representation of a caliper I would recommend....
Mitutoyo 505 676 Dial Caliper 001" 0 8" Stainless Steel w Case | eBay
For instance, on a Gen I SBC, we know that the main bearing bore is ~2.641". Half of that figure (radius of the hole) is 1.3205". Hook one jaw of your dial caliper on the I.D. of the end bearing bore at each end of the block and hook the other jaw of you dial caliper onto the block deck. The key here is to get the caliper lined up parallel with the cylinder bore in "X" and "Y" axes. Let's say you measure at 7.6695". Add the radius of the bore (1.3205") to your
measurement and find a block deck height of 8.990". Since the blueprint dimension from the factory is 9.025", you have discovered that the block is 0.035" shorter than blueprint. That's probably not the only thing you will find. I suspect that you'll end up with four different figures from all 4 corners of the block.

As far as a burette, there are members on this board who will recommend a graduated device of some kind from the drug store. Here is an example of a 60cc catheter syringe that, while being better than having no measuring device at all and possibly doing a pretty good job at checking domes, will not be as precise as a burette.....
60ML EXEL (26304) Catheter Tip Syringe Only (Qty. 5) | 60 ml Syringes | Syringes Only | Syringes & Needles | Med Lab Supply

I am of the opinion that if you're going to do something, do it properly just as a professional would do it, with a graduated 100cc burette (with on/off spigot) and a stand so you can easily do cylinder heads....
Powerhouse Cylinder Head CC Graduated Burette 0 100cc Stand Plexiglass Plate | eBay.
If you put the word out that you will do a set of V8 heads for 35 bucks, you'll be able to pay for the equipment pretty quickly and begin making a little extra cash for yourself.

Last edited by techinspector1; 11-15-2012 at 05:37 PM.
Reply With Quote Quick reply to this message
  #20 (permalink)  
Old 11-16-2012, 05:52 AM
Registered User
 
Join Date: Mar 2012
Posts: 139
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Tech Inspector you the man. Gotta a question for u. I dont know a hole lot about performance parts but Im trying like hell to learn. Anyway those KB pistons you showed (http://api.viglink.com/api/click?for...13530672408284) are those strictly designed to raise compression ratios. Correct me if Im wrong, do they make the so called dome, raised so there is less area in the combustion chamber therfore creating more compression??

Last edited by allansmith859; 11-16-2012 at 06:00 AM.
Reply With Quote Quick reply to this message
  #21 (permalink)  
Old 11-16-2012, 11:33 AM
gearheadslife's Avatar
MentalMuffinMan
 
Last photo:
Join Date: Nov 2012
Posts: 2,367
Wiki Edits: 0

Thanks: 137
Thanked 322 Times in 298 Posts
Quote:
Originally Posted by oldbogie View Post
GM says they are 210 ccs, the ones I've touched measured 205. Either way these are big port heads so on a 350 they will like to wind up more RPMs to get the port velocity up to where they have enough velocity to ram feed the cylinder.

Bogie
so about the same as my brodix trackI
Reply With Quote Quick reply to this message
  #22 (permalink)  
Old 11-16-2012, 12:42 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,282
Wiki Edits: 326

Thanks: 829
Thanked 1,143 Times in 942 Posts
Quote:
Originally Posted by allansmith859 View Post
are those strictly designed to raise compression ratios. Correct me if Im wrong, do they make the so called dome, raised so there is less area in the combustion chamber therfore creating more compression??
Yep, you are right on the money, except they create less VOLUME rather than less area. Once you have the displacement of the dome, you can deduct that figure from the chamber volume and then figure the piston at a zero value for figuring the static compression ratio.

In other words, if you were using that piston I linked with a 76cc chamber, you would deduct the 6cc's of the piston dome from the chamber volume and use a zero value for the piston as you do the calculations for SCR. If the dome is 6cc's, then deducting that from the chamber volume would yield a 70cc chamber for calculating.
Reply With Quote Quick reply to this message

Recent Hotrodding Basics 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
Compression calculator prostcelica Hotrodding Basics 2 06-22-2012 01:46 PM
Dynamic Compression Ratio Calculator 4 U McGurk General Rodding Tech 1 07-04-2011 01:04 AM
Which CR calculator is right? adam83 General Rodding Tech 3 01-06-2010 10:13 PM
compression calculator oklowride Engine 6 02-21-2006 01:06 AM


All times are GMT -6. The time now is 12:21 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.