Greetings
@DeCaff2007,
Good news. After reviewing the history of your TBI 350 build, I have pretty
solid answers for the anomalous results of the compression test that you shared.
Sometimes, the more you dig, the fuzzier the big picture becomes. Usually this
happens when the recorded data leading up to the failure is sparse, lacks
the necessary precision, or the data is conflicting. (Due to uncalibrated
test equipment or adherence to procedures.)
This is when I find myself using the SWAG acronym in order to couch my
answer in such a way that you know that my answer is more of an informed
hunch based on similar experiences instead of specific, hard data..
Conversely, sometimes the more you dig the sharper the troubleshooting
big picture becomes. All the individual bits of info start to reinforce each other,
much like when you are piecing together a puzzle & the image starts to become
recognizable.
In a previous life, this was the confidence level required in order to get a
jet, previously grounded due to a serious IFE {In Flight Emergency} to pass
the formal review by the safety board, before they would release the aircraft
and authorize a FCF (Functional Check Flight) to be performed, so that ultimately
the aircraft could be returned to service.
Enough table setting, let's dig in and see what's going on with your 350.
****
First things first. Let's organize your compression test results to match
the physical layout of your engine:
You must be registered for see images attach
1) The measured pressure in 6 out of 8 cylinders are nearly identical -- this is *exactly* the desired result of a relative compression test.
The data suggests that the driver's side was tested first, and that the passenger side was
tested afterwards. (On the theory that the battery was discharging, and the test rpm was
starting to fade.)
IF the reverse is true (ie: the passenger side was tested 1st, driver's side 2nd)
the highest probability for these results is a small disagreement in the machined block
deck height from left to right. This is pretty common, and doesn't cause an issue for an
engine working at a normal, stock DD level of tune.
How to prove/disprove the above:
A) Easy: Rerun test on fully charged battery + jumper cables to second (running) vehicle,
which keeps the cranking battery voltage a constant from beginning to end.
B) Easy: Review deck height measurement recorded during assembly. (Straight edge & feeler gauge
is close enough, dial indicator if you have it.)
C) Invasive: Left and Right deck height data was not collected during engine assembly. Remove
cylinder heads and acquire data.
Note: Not recommended. The (3) 125 psi readings on the driver's side are
within 5% of the (3) 120 psi readings on the passenger side.
Also, getting results this even on a engine with only ball-honed stock bores on a ~90K mile
block and (7) original stock pistons is actually quite good.
2) In absolute terms, the compression numbers seem to be a bit lower than expected?
The commonly accepted response to this question would be that when running a compression
test the relative results between all the cylinders is key, and the absolute values are to be
taken with a grain of salt, since there are uncontrolled variables introduced during the test.
(Throttles left at idle impeding intake cycle leading to lower absolute compression reading,
what altitude was the test taken, late closing intake valves on performance camshaft, etc.)
But one forum member made the comment that possibly the motor was originally a lower
compression TBI 'HD' motor? I decided to dig a little deeper. Turns out that the light duty
TBI 350 featured 9.25:1 compression & 200hp, whereas the 8600+ GVW rated trucks got the
HD 350 with 8.75:1 compression & 190hp. Here's an older post with some part numbers
& specs: (
LD vs HD TBI 350s)
There was also some discussion about sodium-filled exhaust valves, which the HD motors
came with, but I needed more proof about the lower-compression HD theory. Also there was
a photo of the engine showing 4-bolt mains, which (supposedly) were reserved for the HD TBI
350s in the GMT400 production run, so it started to feel like this is what DeCaff2007 was working
with?
Looking closely at DeCaff2007's photo of his assembled short block, I then noticed a piston detail
that I was unfamiliar with:
You must be registered for see images attach
I've never seen this in the flesh, and my Google Fu was put to the test.
Finally, after almost admitting defeat, I found a single, solitary SBC devo who put up
a matching photo in a chevy-centric website, and identified these pistons with
the 45° identifying notch as having 18cc dishes in them:
You must be registered for see images attach
(Tip of the hat to "1983G20Van" over in the chevytalk.org forum for this 9-year old entry. Reading his stuff it sounded like someone I've
come to know in this forum...and then a photo confirmed my suspicions.
This is close enough for me. The compression results for this engine will be
a bit lower than expected due to the 1/2 point compression drop.
3) Why is the number #1 cylinder reading so much higher than the others?
This is the question that initially drove my interest in digging into DeCaff2007's
compression test results. I wasn't following this build until recently, so I was
unfamiliar with how the engine went together. Unfortunately, the engine build documentation
was scattered across at least 8 separate threads, and I was having trouble following
the flow after the fact?
But after I stopped and built a navigational cheat sheet in the form of a timeline, it became easier to
follow along. (See attached.) Sure enough, in reply #19 to the thread "Piston Ring Install Issues"
the original #1 piston was damaged after the assembly got jammed up:
(
Original #1 piston no longer serviceable)
After initially focusing on a suspected incompatibility of the bore chamfer vs the original ring compressor,
eventually the root cause was troubleshot to the oil control rings -- more specifically, deep groove vs
shallow groove. (
Incorrect diameter oil rings due to incorrect scraper spacing)
And eventually a single replacement piston was sourced on eBay. (
Shiny new Standard bore LD piston)
And by now you've guessed the punchline. It's for the TBI 350 put into all the <8600 GVWR GMT400s,
meaning that it has a smaller dish in the piston for that 1/2 point higher compression ratio.
In addition, since it's not a .030" rebuilder piston that normally have a 1.540" compression height (between
wrist-pin and top of piston) it's quite possible that this STD bore piston has the factory-spec 1.560" compression
height, adding even more squish to the mix?
To summarize, the 'higher than the rest' compression reading in the #1 cylinder accurately confirmed
that there *is* a physical difference between the piston in this cylinder vs the other 7.
Since I hadn't been following the build at the time, I was completely unaware of this going into this
compression test analysis, so I thought that it was pretty cool that the compression test had
enough resolution to make the 'unique to this build' artifact stand out.
Recommendation: If the engine runs quietly and there's no adverse tolerance stack up
in hole #1, run it as-is.
Since the majority of the 350 TBI motors out there run with 8 of these higher compression pistons,
this should not cause any issues in normal service. The possibility exists that the #1 cylinder
in this engine will be the first one to give the knock sensor something to listen to, but that's
why the design engineers put this error-correcting spark timing feedback loop on the engine block
in the first place?
In English, the total timing for all 8 cylinders *may* or may not be set by this cylinder, but again
in normal service this will be a non-issue anyway. Keep it cool and no problemo!
4) What about the slightly higher reading in cylinder #8? What does it mean, if anything?
Is this of any consequence in the big scheme of things?
In all the data above, I have confidence in the observed data and conclusions. But on
this one I am going to have to label it as a SWAG. In order to prove/disprove the following,
we would have to CC the combustion chambers of the aftermarket Flotek aluminum
cylinder heads...since the #8 piston looked to be one of the originals.
About the only thing I can dream up is what I am going to call the 'left hand' rule. That is,
what if this batch of Flotek castings has a single, slightly smaller combustion chamber than
the other 3, and this smaller chamber is the one closest to my left hand as I install the
head on the block?
On the driver's side, this smaller chamber would be on the #1 cylinder.
And on the passenger side, this same chamber that's closest to my left hand would end
up over the #8 cylinder. But I caution that this is pure conjecture on my part, until
such time as the cylinder heads were pulled and the chambers carefully CC'd.
Note:
Not Recommended,
for cylinder #8 currently measures well within reason.
****
Well, I don't expect very many people to still be reading this. But that's alright. You see,
I actually have more confidence in what a carefully done compression test can tell me about
a motor that's new to me than the average motorhead. And if you take it a step further and
proceed to perform a leak-down test on the outliers, you can really get actionable info from that.
But the vast, vast majority of the time we go for a leakdown test is because the compression is
way off in a cylinder or two, as in closer to zero than all the others. Having one cylinder
stand out because it has appreciably more squish than the other 7 was really unusual...and that's
why I decided to dig until I uncovered the root cause. As I've said elsewhere, I don't mind
a problem that I can fix...but I hate a mystery.
And if you consider yourself a student of engine building, if you weren't already doing so, you
can now justify making the following measurements while performing your 'trial assembly':
* Deck Height (for quench)
* Compression height (stock vs .020" less for non-blueprint rebuild slugs)
* CC'ing the combustion chambers (ideally they all match for the smoothest possible operation.)
****
Summary:
* This engine is perfectly fine. 6 cylinders are working as brand new. The 2 other cylinders
are bringing even a little more to the party than the others -- think of them as overachievers.
Assuming that the inside of the engine was cleaned enough after the 1st camshaft failure,
once the valvetrain gets the 'lash' (preload) dialed in for all 16 valves, this should be just
fine in front of that 5-speed manual.
To close this out, here I will paste some common-sense recommendations from an article on
this subject:
You must be registered for see images attach
"That's all I've got to say about that." - F. 'Road Trip' Gump
