Has anyone done a compression test on a non-FI car yet?
I am curious to see if there are any variances front-to-back on the engine after typical wear.
Thanks!
Wayne

'04 G35X Graphite/graphite, NAV, wood, premium

 

Check to FI section. I belevie it was done before the addition of a supercharger,to geta abase line. Or possible to check a plenums effect...

03.5 Sedan/B.Sil/Prem/Wntr/Aero/Tint/Rims/Hotchkis Sways--WHERE ARE OUR PARTS!
69 Chevelle Hard Top BB/4.11's/Posi

 

Gurgen did one. It went well I think

2003.5 DP 5AT Sedan
14.2 @ 97.18
Z-Tube
K&N Drop In & Airbox Mod
10 Wire Hyper-Ground
Crawford Plenum (V4)
UR Crank Pulley
Hotchkis Sways (M/M)
Polk EX-3500 Tweeters/Shock Sensor/H-Liner

 

Static compression [the old cranking method] is prone to error even with modern digital peak hold gauges. The problems are that the rings and piston haven't expanded with operating heat.

More of a gross diagnostic test to identify failure not an accurate test that will confirm whether all cylinders are within the oem 14 psi variance maximum.

Running compression test [running on 5 cylinders and measuring the 6th with fuel cut off of that injector] is more accurate of real world operating conditions.

They make wonderful pressure sensors that fit between sparkplug and head [like a thin washer with a shielded wire running out to data collector].........these are used in formula racing to measure the peak pressure from each cylinder to fine tune injectors.

 

Quote by Q45tech:
"The problems are that the rings and piston haven't expanded with operating heat. "

That's probably why every time I've done a compression test, it's on an engine that has reached operating temperature.

That's probably why the FSM states to "warm up engine thoroughly, then stop it."

You are SUPPOSED to do a test AFTER the rings have come up to temp.

If it is not accurate, why did the engineering gods at Infiniti give specific numbers for the compression range?

Please show me where they recommend a running vs. static test in the Factory manual.

'04 G35X Graphite/graphite, NAV, wood, premium

 

Because that's all the typical technican is taught how to do in trade school. They, nor dealer, Usually have multi ten thousands to waste on lab grade test equipment, just to do something ocassionally.

I have 6 digital pressure gauge [one cost $2,000] [they all are accurate to 1%] that would translate to 2 psi error IF temperature and a complicated calibration proceedure is used every 30 days.

Why factory says standard is 185 psi and minimum is 142.
They assume tech is comptent to adjust for local barometric changes [as barometer get multiplied by compression ratio].
Obviously the compression gauge will read very different in Denver than in California beach level.

They have been using EXACTLY the same specification for 15 years since 300zx and Q45 came out in 1990.......yet the exact CR of all engines is different. 14.7 x 10.2 = 150 + 14.7= ~~164.7 psi at sea level 60 F with 29.92" barometer.

Just a wide oem range to help techs but if an accurate corrected reading of worse than 142 psi guess owner would have a case, but then regional will have the factory ship the really accurate test equipment and an engineer.

The real variations occur with test rpm [300 rpm cranking]. And operator error and mechanical gauge error [usually > 3% and are only marked in 5 psi increments]..........why 3% of 165=5 psi x 3 for op error ~~ 14 psi allowed variation.
Most engines exceed 142 psi [corrected] while in warranty [70,000 miles] unless abused with oiled cotton or foam air filters or improper oil changes.

Now I'm speaking historically here as little data on 3.5 liter new design [not enough time /mileage] to say.
We'll know when the thousands of engines see over 100,000 miles.

 

Contradictory posts, Q

First you say that the margin of error is due to the lack of thermal expansion in the rings.

Then you say that the margin of error is due to numbering schemes on a dial face? Or equipment error? Are you the only one to own a digital gauge?

It is ridiculous to state that a 14 psi differential is allowable because of user/device error.

Using your (flawed) logic, a 2psi differential could actually be indicated as a 30 psi difference on the guage

160psi - 14psi= 146psi
162psi +14psi= 176psi

Additionally, a 25psi swing between cylinders could disappear in your margin of error:

185psi - 14psi= 171psi
160psi +14psi= 174psi

BTW, a 14psi differential is what most OEMs, both auto and motorcycle recommend as the MAXIMUM differential between cylinders. It is not just a Nissan/Infiniti number.


'04 G35X Graphite/graphite, NAV, wood, premium

 

Did you see a + or - in front of 14 psi? I said 14 psi variance as printed in FSM. The means highest to lowest reading as displayed on an ACCURATE LAB Instrument shall be within 14 psi TOTAL.

All I'm saying is [per FSM] any 142 psi cylinder would pass as long as no cylinder exceeded 142 +14= 156 plus the measured accuracy of the measing device [2% or 3%] so the 142 vs 14 + 4[for measurement accuracy] =160 psi.

So if 165 psi was nominal [average of all less weak cylinder] 165-14 minus 5= 146 psi lowest would be considered acceptable.

I didn't say they were my standards just what the FSM implies.
One could argue that the measurement error would wash since the 14 psi is a narrow range and just the 14 psi differential applies.
To me the 14 psi differential is more improtant than the actual 185 or 142 psi or anything in between.

We'll continue later tonight gotta go.