does decarbing an engine hold water?
 

short answer: yes and no.

i figured i would put this to a test with some actual numbers to back it, the target car was an automatic powered S5 with an original engine at 106k miles on the clock. the car sat behind a body shop for several years until sold to a car flipper who found just the fuel pump was bad and then sold it to a customer who brought it to me for full maintenance and checkup.

the car runs, but not exactly well. so before continuing on with the tune up i wanted to see if i could improve upon the short life left in the engine. the results were both good and bad..

first i warmed up the car to operating temp and then ran the standard compression test:

http://imageshack.com/a/img829/1295/w8bz.png
http://imageshack.com/a/img594/9837/uyci.png

without doing anything besides warming the car up and doing the test the results were:
F: 72/73/77
R: 85/90/93
crank speed: 240RPMs

to find the RPMs i took 3 peak measurements, take the start and end time for 3 revolutions and subtract to get a cranking time for 3 revolutions:
4.742-->5.492= .75 seconds for 3 full revolutions.

divide .75 by 3(number of rotor pulses multiliplied by 1 second)= 4 revolutions per second
multiply 4 revolutions per second times 60= 240 cranking RPMs


now i ran 1 gallon of water through each rotor in 1 quart intervals 8 times, let the engine run for about 20 minutes and then performed the compression test again.

http://imageshack.com/a/img594/2397/ubc2.png
http://imageshack.com/a/img577/1702/qg4f.png

results after decarb:
F: 77/80/82
R: 82/86/89
cranking speed: 238RPMs

then if you want to average the results i prefer this online calculator for normalization:
RX-8 Web Compression Calculator

plug in the numbers, the cranking speed and you get a result averaged to 250RPMs
decarbed engine:
Front Rotor
First Chamber [98.6,120] : 80.82
Second Chamber [98.6,120] : 83.82
Third Chamber [98.6,120] : 85.82
Chambers Difference [<21.8] : 5

Back Rotor
First Chamber [98.6,120] : 85.82
Second Chamber [98.6,120] : 89.82
Third Chamber [98.6,120] : 92.82
Chambers Difference [<21.8] : 7
Rotors Difference [<14.5] : 7

compared to the results from the non decarbed engine:
Front Rotor
First Chamber [98.6,120] : 76.51
Second Chamber [98.6,120] : 76.51
Third Chamber [98.6,120] : 80.51
Chambers Difference [<21.8] : 4

Back Rotor
First Chamber [98.6,120] : 88.51
Second Chamber [98.6,120] : 93.51
Third Chamber [98.6,120] : 96.51
Chambers Difference [<21.8] : 8
Rotors Difference [<14.5] : 16


so as you see the numbers went both up and down, probably cleaning carbon out of the seal slots that was helping to build more compression but also probably getting ready to stick the seals inside the slot which is common on the automatic 6 port engines with this mileage.

the upside is the rotors are closer in line with one another so perhaps this might even improve upon power, but i can't comment on that as it would need further testing.

do i think it is still a good procedure to do? yes. carbon buildup is bad regardless as to whether it is helping or hurting compression. did it bring this engine back to life? no, it is still below the threshold of what i would consider to be good compression(85psi at 250RPMs), but it is closer to that line than it was before doing it.

the test is still somewhat non conclusive though, driving the car after the decarb for some time may yield even different results. but i didn't want this time to go to waste so i decided to share my findings. the results that the car does seem to idle better and respond to throttle quicker holds true in this event, if anything it should make life a little easier passing emissions when that step comes.