Compression tested my C7. Found 250 psi on all cylinders. What does that mean? It looks really good to me with all cylinders being so balanced. But that is higher than I expected. I thought it was supposed to be at least 175 psi but 14.7 x 11 = 161.7. Obviously I’m wrong about something.

I did the test because I’m noticing it break up above 5,000. Ecs kit is boosting to 8 psi. Stock motor. Could it be plugged cats? I’m taking to borescope to those next. Plugs gapped to .026”. It was breaking up worse when the gap was 030”. How do I test the coils? I don’t have dad old Sun Machine anymore LOL

 

The first thing is trusting your equipment…is it accurate ??…a cause of high compression could be carbon buildup on the top of the pistons so being that you have a borescope you can check that easily…GM top end cleaner could help that…plugged cats would not cause higher compression…if the cats were clogged air can’t get into the cylinder because the air can’t get out of the cylinder doing a compression test…you’d also have multiple cylinder misfires depending on what bank is clogged…with plugged cats the car will fall flat on its face on acceleration…old school vacuum gauge testing, incylinder pressure analysis (my preferred method) or a pressure gauge installed into your upstream O2 sensor will show a plugged cat…an old Sun machine is still a great tool to test secondary ignition…I use a Picoscope with a capacitive pickup on these “coil near plug” ignition systems (see below)…the secondary waveform on my ‘01 shows a good secondary waveform…”firing KV”is 15.84 at idle (circled and black line ) and spark KV is where the spark jumps across the plug (red line)…upward curve is because as the fuel is burned in the cylinder the coil needs more “juice” to burn the mixture as it leans out…a very sharp upward and long slope is a real lean mixture such as a clogged injector…a downward slope means a rich mixture which is more “conductive” electrically…fuel hydrocarbons are electrically conductive…is a science to interpret these waveforms but it tells a lot…you’ll be lucky to find a shop that uses a scope…some dealerships have them but only use them for NVH (noise, vibration, harshness) testing…nothing else…I’d find a diagnostic shop in your area that does this type of testing.

 

Good point about the diagnostic equipment, I was gonna say
1. probably a bad compression tester, never trust a single tester always use at least two and never a borrowed or rented unit

2. Breakup in boost- what is the a/f ratio and what is the timing set to. Have you verified the timing and a/f ratio is accurate. Does the wideband seem to work normally, can it identify the stoichiometric of closed loop hovering around 14.7 switching. Does it richen up as it should in boost to 11's roughly and stay down.

Without this info it is difficult to look at the coil ability to fire under boost and rich mixtures. For example if we know the a/f is good and timing is good then I could say look for loose grounds, check your battery, alternator situation, voltage drop. Check the connection of the coils. Look at the plug wires, are they old? Probably old plug wires. Etc... Stuff related to the delivery of spark. But if the engine is running lean because the fuel pump is dying or the injectors are clogging up because of bad gas or something the wideband would show... you would be wasting your time looking at the battery and coil situation... see what I am saying

 

You guys are most certainly on point about the calibration of the gauge but I did use it years back on my old Nissan sentra and it was measuring about a 175PSI. I can verify it up to 125 psi. I did throw the boroscope down all the cylinders and I do see carbon but I'll let you be the judge

 

If 175 psi IS (is it?)the minimum, what is considered max and normal? If it is just carbon build up I might be able to get some results with a seafoam smoke show.

The air fuel is about right based on the fuel trims and I did have a wideband in the midpipe it showed 12.5 at WOT. WOT spark is near 13 deg. But the tune is not right in torque tables so I'm about to bring it to Meyhem mitorsports in Raleigh to diagnose and retune. But if i can diag it, I will.

 

12.5 is a little high for 8psi with such high compression. I would expect 11.2 to 11.5 maybe max 11.8 range on 93 octane.
13 degrees is about right.

Maybe look at fuel pressure at wot.
Otherwise start replacing ignition components. Use a quality plug, iridium, .028" gap or so. I think your compression is high which is blowing out the spark due to aged ignition components, assuming it isn't fuel related.
Don't touch spark plug with human hands. Use new gloves. Keep anti seize away from the business ends, if any is used.
You need a clean plug, never touched, proper gap, iridium ideal for E10-E85. This, some new plug wires, proper coils, will go along way to spark delivery. Try to reduce the heating of the coils may also help. IDK what your exhaust is like but some shields or heat sinks will help keep them cooler from radiated heat might help. Plug wire insulating covers/sheath too.

 

Do these look ok?

 

They look fine !!

 

those look pretty good. It is tough to say HOW good without some closer up pictures. If they all look the exact same, that is the most important thing.
Make sure there is no crusting, crumbles, melting, little specks or flecks.

They are iridum which is ideal for your application. However be careful when you gap and check the gap, the iridium are notorious for breaking or being damaged by the gap tool. You prob alrdy know that I hope. Since you pulled em out why not replace them anyways? Start fresh. New plugs, new gap. I never pull out plugs unless I am replacing them. I tune the engine on cheap plugs then pop in new iridiums and never remove them again for 50k to 80k miles in < 800rwhp forced induction applications daily driver.

I suspect just by looking at the plugs the gap is a little large for the compression you have. Just a guess. That gap looks larger than .032" in the pictures. I suspect .035" which might be blowing out at your compression. If you check it, be careful, vewy care fue.
I would go .028" just to be sure its done right one time. If the spark still blows out (misfires, chokes on boost) then there is some underlying voltage/coil/spark delivery issue. Unless the engine is running too lean (wideband wrong, or 12:1 air fuel ratio is insufficiently rich). 12.5:1 scares me a little bit. That is the fringe of acceptability at 9:1 compression. I suspect you have a bit more than 9.5:1 if you are cranking out 170-200psi.

 

I’ll double check but I set the gap to .027-.026 ish. This is my second set of plugs and wires. What is ideal compression supposed to be? I tested my gauge to 130 psi and it read 125. I don’t have the means to test it to NIST traceability, but I’m calling it good enough, LOL. I’m heading to parts store to get some seafoam in the manifold and give the neighborhood a smoke show.

 

There is no "ideal" compression...some OE's have the compression listed in service info and some do not...my 01 doesn't have it and I looked in Alldata for your 2015 and it's not there either...below on this 2005 Trailblazer 4.2 liter the compression is 215 psi.... on our cars "the minimum compression on any one cylinder should not be less than 70% of the highest reading cylinder"...that's it !!...the only thing listed is no cylinder below 100 psi...go to Auto Zone and rent one for a few hours... I should just sell you my compression gauge because I don't even use it anymore...LOL !!..GM’s labor time for doing a compression test is 2.5 hours...I'll do a "relative compression" test using an amp clamp on a battery cable...5 minutes at the most and I can do the test without even opening the hood...just from the drivers seat...won't give me a psi reading but I know if one cylinder is low or not.

 

I just tested one cylinder on my 05 tahoe 5.3 and it read 180 psi. I think my gauge is good. If 215psi is good on a trailblazer, maybe 250psi is normal for the gen 5 LT1. Regardless, I did the sea foam smoke show and cleared the codes for rich from it. I put in the other set of plugs (more ngk 6510) and I'm about to full send it after the game.

 

OK !!…just us know if it’s “breaking up” and need some direction…I’d definitely have the scope hooked up for this diagnosis !!

 

I have some results but I also want to acknowledge what you’re advising I do here. I did not even think of diagnosing this like an avionics tech would troubleshoot a faulty pulse coming out of a bad RADAR transmitter. Thank you for pointing me to critical info on what the spark pulse should look like. I do have an HP digitizing oscope, and I went ahead ordered the probe to clip on the plug. King, it does look like there is crusting on the plugs.

Here s the good news. The alternate set of plugs took care of the misfire. Even when GAPPED to .030! But I wonder how long a set of plugs should last in my set up? The compression test after I did the sea foam in 1 and 3 cylinders showed 240 psi. 10PSI drop. I wonder if my tune is fouling plugs. I want to monitor the way it runs with both sets of plugs using the Oscope. I want to see if I can see a difference in the pulse formation between the good and bad plugs.

 

I guess you’re talking about TDR with your Radar RT ??…LOL !!…you’ll need a capacitive pickup for your scope that clips over the lead or a “paddle probe” that is placed on top of the coil pack to read secondary ignition…great informative video on ignition waveform analysis !!

 

To keep plugs clean, there are a few checks necessary
1. It must be tuned properly to run leaner than 14.7:1 when at cruise and idle.
Here is an example map showing my a/f tuned values for vacuum


Notice anything over roughly 55KPA for idle and cruise is lean, much leaner than 14.7:1. This is what will keep the plugs looking brand new, clean, almost white.
In Chevrolet LS applications they still develop some darkening due to the angle incidence. But in Toyota/Nissan apps (top down inline engines) the plugs stay pretty much white and clean like new.

2. You must have a fully intact OEM pcv system. No catch cans or venting/breathers. This will keep the crankcase clean which has a long term influence on plug and ring/cylinder carbon cycling and buildup. The PCV system helps keep the oil clean which in turn helps keep the cylinder and engine oil orifices clean.

3. Use a high quality air filter, similar or superior to OEM. Debris from air will 'carmelize' or carbon coatings forming on objects due to heat and pressure of combustion.
For example pollen and fungus, present everywhere in the world, are forms of life which contain myriad substances as carbon, potassium, iron, molybdenum, sulfur, oxygen, sodium, etc... basically the entire first two rows of the periodic table. When they enter the engine they will be burnt under pressure and this creates myriad conglomerates- partially reacted byproducts with randomly oriented covalent bonds which attach to the rings, piston, plugs, forming hard diamond like or sticky tar-like coatings, which over time collect into visible gunk, residue, carbon deposits, etc... and may lead to stuck rings, poor piston ring performance, increased wear

 

When I used to repair corporate jet avionics, radar, we just held the scope probe close to the pulse transformer. It would not give you the exact amplitude but the waveform was correct.

Not to close or ZAP!

 

in regards to plug lifespan I had a 1000 rwhp porsche turbo and ran a .025 gap and 35 psi boost. swapped plugs every 5k miles whether they needed them or not. plugs are cheap compared to what detonation and misfires do. just my .02