Compression Testing and Dynamic Compression Calculation - Am I doing it Wrong

[XH29N0G]

I did a compression test today on two cylinders after a long drive - so the engine was well warmed up (180 F).

I tested only two cylinders (1 and 5) and then thought I would ask for help.  

Both cylinders behaved the same with the 1st pulse at 120 PSI and stabilizing at 230 PSI (after 5-7 pulses) which seems high to me.  Is it?  Did I do something wrong?

Procedure:  Cranked by key with the carburetor open by flooring the accelerator - this would put gas in the manifold ... do not know if this complicates things.

I also did a calculation of static CR and dynamic CR using http://wallaceracing.com/cr_test2.php and http://www.wallaceracing.com/dynamic-cr.php
It is supposed to have 10.1 compression.  With these specs I calculated something in that neighborhood ~10.5 using a Wallace racing calculator but this does not change the dynamic value.  The Dynamic calculator predicts a lower value than I measured (around 170 PSI instead of 230 PSI).

The specs of the engine are:
     3.91 stroke,
     4.28 bore,
     6.7 rod length,
     5088 intake lobe (comp cams with 112 LSA and 105 CL),
     0.039/4.414 Felpro 1009 gaskets,
     deck at -0.004,
     Diamond 51910 pistons,
     RPM heads 84 cc (60929).  

Valve opening at ABDC calculated as (advertised duration)/2 + LSA - (LSA-CL) -180 (or 280/2 + 112-7 -180 = 65 degrees

Any thoughts will be appreciated.

[Challenger340]

3.91 stroke,
      4.28 bore,
      6.7 rod length,
      5088 intake lobe (comp cams with 112 LSA and 105 CL),
      0.039/4.414 Felpro 1009 gaskets,
      deck at -0.004,
      Diamond 51910 pistons,
      RPM heads 84 cc (60929)

Something wrong here ?
What Engine is this ?
The Diamond Piston #51910 is a 1.72" Compression Distance ? which if used with a 6.7" Rod ? and 3.91" Stroke ? would stick out of a 383 block ~.400" or be down the hole .375" in a 440 Block ?


No matter, I'll just assume the .004" Deck Height and 4 CC Flycut ?

The Comp 5088 "XFI" lobe installed on the 105* C/L should indeed close around 65* ABDC for calculation purposes for use on the Wallace Calculator. There are better actual true wave factor programs out there.... but as a "free" internet calculator based on Static C.R. it is fine.
But,
the problem correlating actual test results to the Wallace Calculator, using a Std Shrader Valve type Compression Tester,(in my experience anyways), can be the "accumulated" excess pressures the gauge reads ?

All 8 Spark Plugs out, Ignition dis-abled ?
Wire the throttle plates wide open(IMO, using the accelerator pump could be skewing things even more as in what's called a "wet" test)

Then understand, that if using a std Compression Tester the "accumulation" of excess pressure on multiple bumps on the Schrader Valve itself can vary to distort readings ?
(I utilize a "volume tester" for actual gases trapped on ABDC Cam Events when looking for psi, as a way to correlate what the Schrader valve is accumulating)
Nonetheless,
if using just a std C.R. tester as a stand alone ?
I would humbly suggest to try....
max only 4 bumps per hole, and recording the 2nd bump reading, to then be added to the average of the 3rd and 4th bumps in the TOTAL calculation, as close enough(again, in my experience anyways).... WITHOUT going out and buying a "Swept Volume" Tester to get an EXACT correlation.

For example on your 5-7 bumps ?:
if you are seeing 120 psi on the first bump, 150-160psi or so on the second bump ?
and,
say another 60 psi or so to the 230 psi Total in 4-5 MORE Bumps ? (In my experience anyways)that is shrader valve accumulation taking root, so divide that extra 60-70 psi by the number of bumps past the 2nd bump to get an "average"(3 or 4), then add THAT to your 2nd bump reading.

Again here, this is in "my experience only"... your results may vary according to the differences in Compression Testers ?, and the Valving to trap compression contained therein ?, which can vary widely ?

Just a suggestion based on what I've found, hope it helps
Bob

PS;
the FelPro #1009 is 9.9 CC's Compressed Volume because of the MW Valve Reliefs common in all BB Mopar Head Gaskets
170 psi is getting "up there" a bit for pump fuels depending upon elevation ?
Is the Engine "pinging" ?

[XH29N0G]

Bob,

Thank you for the detailed response.  I will have to go back to square 1 in order to do the test the way you describe.  I am headed on a trip next weekend so, it will be a couple of weeks before I can do this. 


Some clarification on the procedure I used (I now know it is not ideal) and on questions you asked.

• 
       I ran the test until the gauge did not rise any more.  It was somewhere between 5-10 bumps.  I think more likely 5-7.
  
•      I only noted the first bump and then where it stabilized so will have to go back afterwards. 
  
•      I think my gauge is a  standard Schraeder type valve.  It is nothing special.  (Bosch-Actron CP7826)
  
•      I had only those two plugs out and ignition disabled. (other plugs still in)
  
•      I put the throttle down at the start of each test, so both accelerator pumps emptied into the manifold.So a wet test.
  
•      The engine is a stroked 383. 
  
•      The diamond PN# was my guess.  The pistons were changed to diamond during the build.  I did not get the PN#. 
  
•      I agree they would need to be 1.31 CD to work.
  
•      Pinging?  Not with the current total advance (32 degrees) (It pings at 34/36 in high gears under load).

I  appreciate you taking the time to describe the issues and approaches with compression gauges and calculating cranking compression. 

I am guessing that the 'accumulated excess pressure' of the schraeder valve testers related to the adiabatic compression of the gases in the cylinder and this is transferred through the valve to the gauge (I am thinking something like: (P_1/P_air)=(V_2/V_2)^L (where L is an adiabatic constant ... something like 1.4 or 1.3) and (V_2/V_1) would be the dynamic CR. 

I also think I understand what you are saying about volume testing, but am not familiar with this (no need to explain now, I'll do some research).

[/list]

[1974dodgecharger]

does the psi change that much on each bump? I did the same thing as you drove it and warmed it up to 180 then did my test and I was consistent 130psi on each bump and I did 3 bumps for driver side.  Mine did not vary much on each bump 130 to 140 psi and 3 to bumps to each one to make sure the number was consistent.

[Challenger340]

Bob,

I am guessing that the 'accumulated excess pressure' of the schraeder valve testers related to the adiabatic compression of the gases in the cylinder and this is transferred through the valve to the gauge (I am thinking something like: (P_1/P_air)=(V_2/V_2)^L (where L is an adiabatic constant ... something like 1.4 or 1.3) and (V_2/V_1) would be the dynamic CR. 

I also think I understand what you are saying about volume testing, but am not familiar with this (no need to explain now, I'll do some research).

[/list]

I think you possess a FAR greater working understanding of Thermodynamics than I ever will !
Are you an Engineer ?
Now I kind of feel like "goober" trying to explain "cipher'in" to Andy on the old Andy Griffiths TV show ? LOL !
Something tells me you will figure it ?

Mine is an old Cylinder Volume tester tool from the Roundy-round(stock car) tech pits, just a big burette for measuring air really.
I think it's Snap On... but could be anybody because all snappy used to do was buy stuff and paint their logo ?
We had a Shop break in back in the day where a bunch of our stuff got stolen. The guy broke in stealing stuff all over including the local Track.
Later, when the cops finally pinched the guy, we got back our stuff with other stuff including the volume tester. When I called the Tech guys at the track to tell them their tester was in my bin stuff, they said "keep it" as Insurance had already bought them a new one, so I made a $20 donation to the track fund and kept it.

They would just back off both Valves(sbc) and do a sweep from BDC to TDC to measure cylinder "volume", and make sure nobody was running cheater sized engines.
But obviously the tool doesn't care where it starts to see "volume" directed it's way with a valve closing ABDC.
We haven't used it in awhile(no time), but it was fun back in the day as a "learning" thing to see the volume changes in "trapped" air by moving Cams around 2*, 4*, 6-8- even 10*.

later
Bob

[BSB67]

Bob,

Thank you for the detailed response.  I will have to go back to square 1 in order to do the test the way you describe.  I am headed on a trip next weekend so, it will be a couple of weeks before I can do this. 


Some clarification on the procedure I used (I now know it is not ideal) and on questions you asked.

• 
       I ran the test until the gauge did not rise any more.  It was somewhere between 5-10 bumps.  I think more likely 5-7.
  
•      I only noted the first bump and then where it stabilized so will have to go back afterwards. 
  
•      I think my gauge is a  standard Schraeder type valve.  It is nothing special.  (Bosch-Actron CP7826)
  
•      I had only those two plugs out and ignition disabled. (other plugs still in)
  
•      I put the throttle down at the start of each test, so both accelerator pumps emptied into the manifold.So a wet test.
  
•      The engine is a stroked 383. 
  
•      The diamond PN# was my guess.  The pistons were changed to diamond during the build.  I did not get the PN#. 
  
•      I agree they would need to be 1.31 CD to work.
  
•      Pinging?  Not with the current total advance (32 degrees) (It pings at 34/36 in high gears under load).

I  appreciate you taking the time to describe the issues and approaches with compression gauges and calculating cranking compression. 

I am guessing that the 'accumulated excess pressure' of the schraeder valve testers related to the adiabatic compression of the gases in the cylinder and this is transferred through the valve to the gauge (I am thinking something like: (P_1/P_air)=(V_2/V_2)^L (where L is an adiabatic constant ... something like 1.4 or 1.3) and (V_2/V_1) would be the dynamic CR. 

I also think I understand what you are saying about volume testing, but am not familiar with this (no need to explain now, I'll do some research).

[/list]

Nothing in the "system" is adiabatic. Neither the system in the engine, at the schraeder valve, or in the gauge.  There is nothing in thermodynamics, or any where else, that would explain what would appears an extra 40 psi at the gauge.  Maybe a few kPa in the absolute. The theory may be interesting, but not particularly useful for what we're doing at this level.

On pump gas without detonation, plus or minus a little, 170 psi is about all you are going to run w/ a cast iron head, about 190 psi with aluminum.  Some guys are claiming 200 - 205 psi, but that is not an accident.

I've seen both questionably high readings and low readings.  It has always turned out to be either a bad gauge, or bad valve.   

[XH29N0G]

BSB  Thanks, I will look into checking the valve and gauge too.  

Bob, Regarding the thermodynamics, I am mostly self taught with respect to hard chemistry so could easily be (and have been) wrong and defer to BSB there.  Profession - not engineer, but professor in geochemistry (rocks and fluids - not much in gases) so partly there, but not.

Thanks both for the input.  I will keep reading and learning. More importantly driving and enjoying the car.

[Challenger340]

I dunno shiat about "adiabatics" ? and probably just enough about thermodynamics to be dangerous ?  
Just being honest here.

Anyways,
I just ain't paid much attention to the smaller final bumps on my Compression Tester ? other than note after 2 bumps, then just averaging the other 2 in ? (4 total)
which,
seemed to be pretty darn close on my Compression Tester... the times when we backed it up with a Volume test and then calculated "mathematically"(swept vol. plus clearance vol. divided by clearance vol.)

I'd like to know just "what" is responsible for the variances in the final bumps on different tester's ?
I always just figured it was something in the variances between shrader valve sensitivities ? but I am all ears if anybody knows better that would be GREAT !

Agreed on the 170 psi BSB, which I tend to stay at as a MAX most times for street stuff, even on the aluminum (maybe just a bit higher), due to the operating temp variances which can skew burn rates and tune, inconsistencies in fuel quality etc. ? just safer for me I find.
But then again, I don't run iron heads up past 150 psi as a rule either for the same reasons, just a Chicken-shiat I guess.

[BSB67]

I dunno shiat about "adiabatics" ? and probably just enough about thermodynamics to be dangerous ?  
Just being honest here.

Anyways,
I just ain't paid much attention to the smaller final bumps on my Compression Tester ? other than note after 2 bumps, then just averaging the other 2 in ? (4 total)
which,
seemed to be pretty darn close on my Compression Tester... the times when we backed it up with a Volume test and then calculated "mathematically"(swept vol. plus clearance vol. divided by clearance vol.)

I'd like to know just "what" is responsible for the variances in the final bumps on different tester's ?
I always just figured it was something in the variances between shrader valve sensitivities ? but I am all ears if anybody knows better that would be GREAT !

Agreed on the 170 psi BSB, which I tend to stay at as a MAX most times for street stuff, even on the aluminum (maybe just a bit higher), due to the operating temp variances which can skew burn rates and tune, inconsistencies in fuel quality etc. ? just safer for me I find.
But then again, I don't run iron heads up past 150 psi as a rule either for the same reasons, just a Chicken-shiat I guess.

My numbers are close to the edge at 1000ft ish, good cooling, reasonable ignition curve, cool plugs, cold manifold, 160°F thermostat....