Abnormally High Compression
Mike Andresen
changed plugs, I did a compression check on the engine. The results were
170, 180,190, 190, 190, 200, 200 and 210 psi. My manual says that the
engine has a 9.5 to 1 compression ratio. That should give a 140 psi
reading (9.5 x 14.7 psi).
I assume that an increase in cylinder pressure is from a buildup of carbon?
Is there any way to get rid of it? Is this a terminal engine illness?
- Mike
Jim Treber
Where did you get that formula for finding correct compression?
The reading on your compression guage doesn't necessarily reflect the
compression ratio. For example, changing to a different camshaft won't
change the compression ratio but will definetly change your guage reading
because of a change in overlap. Follow what the factory manual states for
specs for your compression guage for your car.
mark lyon
: My Chevy Camaro Z-28 just hit 130,000 miles. Over the weekend when I
: changed plugs, I did a compression check on the engine. The results were
: 170, 180,190, 190, 190, 200, 200 and 210 psi. My manual says that the
: engine has a 9.5 to 1 compression ratio. That should give a 140 psi
: reading (9.5 x 14.7 psi).
140 psi is the right answer only if the temperature remains constant.
When a gas is compressed rapidly, it heats up, which raises the pressure
even more.
If air is assumed to be an ideal gas, with a gamma (ratio of heat
capacities) of 1.4, and if the compression is done fast enough so that
no heat is lost to the surrounding cylinder wall, then the following
equation is true:
P*V^gamma = constant or Pi*Vi^1.4 = Pf*Vf^1.4
where Pi, Vi are the initial pressure and volume, and Pf,Vf are the final
pressure and volume. Rearranging a bit,
Pf = Pi * (Vi/Vf)^1.4 = 14.7 psi * (9.5)^1.4 = 340 psi.
And since some gas is lost around the rings, and some heat is lost
to the cylinder wall, you'd expect the actual pressure in the cylinder
to be a bit less than that.
Mark Marr-Lyon
o993...@unicorn.it.wsu.edu
: I assume that an increase in cylinder pressure is from a buildup of carbon?
John T Ibrahim x2593
I too have done a compression test on my car ('92 sentra se-r,2 liter 4 cyl).
The first three were about spec (180 psi) and the fourth one 170 psi.
Compression ratio 9.5 : 1.
My question is, if the fourth cylinder was at 180 when the car was new,
is 10 psi enough to notice difference in power? My car does lack the
throttle response it used to have, particularly in fifth gear. Is this
a cause?
In article <shimaoka_earl-1...@earls.patch.tandem.com>,
Earl Shimaoka <shimao...@tandem.com> wrote:
>In article <Mike_Andresen-P2...@mac-andresen.sat.mot.com>,
>Mike_Andre...@email.mot.com (Mike Andresen) wrote:
>
>> My Chevy Camaro Z-28 just hit 130,000 miles. Over the weekend when I
>> changed plugs, I did a compression check on the engine. The results were
>> 170, 180,190, 190, 190, 200, 200 and 210 psi. My manual says that the
>> engine has a 9.5 to 1 compression ratio. That should give a 140 psi
>> reading (9.5 x 14.7 psi).
>>
>> I assume that an increase in cylinder pressure is from a buildup of carbon?
>> Is there any way to get rid of it? Is this a terminal engine illness?
>
>I don't know what your engine is spec'ed at but I believe that what you are
.
. (deleted)
.
>The high cylinder pressure numbers are of no concern if you're not experiencing
>any knocking or pre-ignition. The higher the pressure you can generate during
>combustion without the knocking/pre-ignition, the better as your engine will
>make more power.
>
>
>Earl
Brad Pitt
>Mike Andresen (Mike_Andre...@email.mot.com) wrote:
>: My Chevy Camaro Z-28 just hit 130,000 miles. Over the weekend when I
>: changed plugs, I did a compression check on the engine. The results were
>: 170, 180,190, 190, 190, 200, 200 and 210 psi. My manual says that the
>: engine has a 9.5 to 1 compression ratio. That should give a 140 psi
>: reading (9.5 x 14.7 psi).
>
>140 psi is the right answer only if the temperature remains constant.
>When a gas is compressed rapidly, it heats up, which raises the pressure
>even more.
>
>If air is assumed to be an ideal gas, with a gamma (ratio of heat
>capacities) of 1.4, and if the compression is done fast enough so that
>no heat is lost to the surrounding cylinder wall, then the following
>equation is true:
>
>P*V^gamma = constant or Pi*Vi^1.4 = Pf*Vf^1.4
>
>where Pi, Vi are the initial pressure and volume, and Pf,Vf are the final
>pressure and volume. Rearranging a bit,
>
>Pf = Pi * (Vi/Vf)^1.4 = 14.7 psi * (9.5)^1.4 = 340 psi.
>
>And since some gas is lost around the rings, and some heat is lost
>to the cylinder wall, you'd expect the actual pressure in the cylinder
>to be a bit less than that.
>
>Mark Marr-Lyon
>o993...@unicorn.it.wsu.edu
>
>
>: I assume that an increase in cylinder pressure is from a buildup of carbon?
>: Is there any way to get rid of it? Is this a terminal engine illness?
>
>: - Mike
Mark's calculations are correct, but can only be applied to your test if you are
checking the compression while the engine is running.
I assume you were checking the pressures by removing one plug at a time
and cranking the engine over without it starting. If this is the case, you were
checking the static compression. An engine with a static compression ratio
should a reading on your guage of 165 to 170 lbs. This range can be
influenced by things such as camshaft duration (overlap), piston ring/cylinder
bore wear, valve seat sealing and excessive combustion chamber deposits.
The numbers you give vary by almost 20%. This indicates poor piston ring
sealing, or poor valve seat sealing, and possibly a flat cam lobe (very small
chance). A flat cam lobe would show in a loss of power and idle quality.
The 20% variance may be something as easy as an inaccurate guage.
If you have an excessive buildup of carbon, your sparkplugs would
indicate this.
Chris
leep
Don`t forget, carbonized cylinders raises the compression to some extent. LEE
Earl Shimaoka
Ibrahim x2593) wrote:
> I too have done a compression test on my car ('92 sentra se-r,2 liter 4 cyl).
> The first three were about spec (180 psi) and the fourth one 170 psi.
> Compression ratio 9.5 : 1.
> My question is, if the fourth cylinder was at 180 when the car was new,
> is 10 psi enough to notice difference in power? My car does lack the
> throttle response it used to have, particularly in fifth gear. Is this
> a cause?
The cylinder pressures generated during the cranking test don't
necessarily tell you the entire story. The pressure may be at spec,
but it is measuring the peak pressure at a single point in time. You
could still lose pressure due to blow-by (the rings). The only way
to determine this is to have a leak-down test done on your engine.
This test pressurizes the cylinder and measures the amount of pressure
drop over a certain period of time. Leak-down numbers in the range of
7-10 percent is good. The higher the percentage, the less power your
engine will make because some of the pressure is simply blowing by the
rings.
If your problem is too much blow-by (and it does sound like it is),
you have 2 options for correcting the problem - neither is cheap.
If the cylinders are in good shape (i.e. not worn much), you might
be able to get away with a very slight overbore and replace the
rings with slightly oversized ones to match the new bore. Otherwise,
you'll have to overbore the cylinders more and replace the pistons
and rings.
Earl
TTILMM
shimao...@tandem.com (Earl Shimaoka) wrote:
A difference of 10 psi on one cylender is virtually nothing for a dry
compression test. A variance of 20 from highest to lowest is when you
should start worrying, and thats still not really that bad. Its certainly
possible that he has a problem, however, from just the one test he told us
of, you'd have to be a car hypochondriac to suggest that he needs to root
out the cause of a 10psi diff.
Hairy One Kenobi
>Your computation of 9.5 CR x 14.7 psi doesn't necessarily apply here. My
>L69 5.0L V8 (electronically controlled carb) has a cranking pressure of
>around 140 psi. It was around 130 psi before I had the cam changed to a
>"better" one. It has a 9.3 to 1 CR. In a previous car I owned which had
>a 350 V8 with 8.7 to 1 CR, it had a cranking pressure of 160 psi.
> 1) "smaller" intake ports in the cylinder head. Smaller ports means
> higher intake velocity at lower RPM, resulting in higher volumetric
> efficiency. The higher velocity creates a ramming effect for the
> intake charge in the runners from the air box through the manifold and
> the ports in the head.
>and
> 2) intake runners and manifold ports/runners are "smaller" too to help
> create the high intake charge velocity.
>Yes, there are probably several ways to get rid of it. The best way I know
>how isn't the easiest for you though. It involves adding water to the
>intake charge. This is easier to do on a carburated engine with a water
>injection system which is primarily designed for use in lowering
>combustion temperature
>to combat knocking. Water in the combustion turns into steam (?) and helps to
>remove the carbon deposits. Another way to do it is to "blow it out". A good
>way of doing this is to find a hill and make a couple of full throttle runs up
>the hill for say 1/8 of a mile. It won't get everything but it will remove a
>bunch.
Oh dear. Let's run through them in order: firstly, a compression tester tests
compression (hence the name). It does this be registering the maximum pressure
generated during a number of cycles. It is an implicit assumption that the
valves are closed at some point in the cycle, and that the compression will be
higher when that valves are closed than when they are hot ..
Over-registering is usually a factor of environmental conditions: the figures in
the manual are at a standard temperature and pressure. The thing that you should
be looking at is the _difference_ between the figures.
As for the size of the inlet ports, have you ever considered the effect of
choking? Port velocity isn't everything, you know.
A sporty cam profile doesn't generally close the valves more than a standard
profile.
The best way of decoking an engine is generally to remove the head and clean it.
"Magic" fuel additives such as Redex have their role, but there's only one way
to be sure ..
I take it from your reply that you are an engineering student with a
theoretical, but not practical, knowledgebase.
--
Ian the Petrolhead.
Earl Shimaoka
One Kenobi) wrote:
> Oh dear. Let's run through them in order: firstly, a compression tester tests
> compression (hence the name). It does this be registering the maximum pressure
The compression tester tells you the cylinder pressure but it does not always
directly reflect the compression ratio (regardless of whether or not you ad-
just the pressure for heat generated during compression). The induction sys-
tem can vary in efficiency quite a bit depending on whether you "design" things
for generating its peak torque level at low or high rpm.
Engines with large intake ports, manifold runners, etc. will have very low
intake charge velocities and will have lower volumetric efficiency at low
rpm. These engines may see cranking cylinder pressures down around 120-130
psi even though they may have a CR over 10:1. In this case, the volumetric
efficiency is way down, maybe around 70 percent. With the Chevy TPI V8,
it is designed with relatively small runners and ports to generate high
velocities at low rpm to produce its peak power at lower rpm. At cranking
speed, it may already have a volumetric efficiency in the 80+ percent area.
At peak torque, it is in the neighborhood of 95 percent.
> generated during a number of cycles. It is an implicit assumption that the
> valves are closed at some point in the cycle, and that the compression will be
> higher when that valves are closed than when they are hot ..
> Over-registering is usually a factor of environmental conditions: the
figures in
> the manual are at a standard temperature and pressure. The thing that
you should
> be looking at is the _difference_ between the figures.
>
> As for the size of the inlet ports, have you ever considered the effect of
> choking? Port velocity isn't everything, you know.
You cut out an important fact that I mentioned in my post that the 305 TPI
V8 quits making power at around 4500 rpm. If it had bigger intake runners
(and throttle body), manifold ports, and intake ports, it wouldn't be choked
at that low an RPM. However, it wouldn't make as much low end power. It's
torque curve would move farther up the RPM scale and would make the car less
responsive in street driving at lower RPMs.
Port velocity is everything because it directly affects the engine's ability
to fill the cylinder. The higher the velocity, the more momentum the column
of air/fuel will have which can result in a ramming effect. It is never seen
on street engines, but race engines using tunnel ram manifolds have been
measured to have more than 100 percent volumetric efficiency at very high
RPM even though it is *normally* aspirated. Given atmospheric pressure and no
restriction in the induction system, how would you account for the volumetric
efficiency above 100 percent? The only answer is the intake charge momentum
pulling in additional a/f. No restriction in the induction system is a pipe
dream because there are always going to be bends along the way to the cylinder
that will have a slowing effect on the intake charge. Therefore, the fact
that a normally aspirated engine can see > 100% VE is a testament to the
wonders of intake charge velocity and designing a tuned induction system.
> A sporty cam profile doesn't generally close the valves more than a standard
> profile.
Say what? I don't understand what you're trying to say with "close the valves
more than a standard profile".
> The best way of decoking an engine is generally to remove the head and
clean it.
> "Magic" fuel additives such as Redex have their role, but there's only one way
> to be sure ..
>
> I take it from your reply that you are an engineering student with a
> theoretical, but not practical, knowledgebase.
> --
> Ian the Petrolhead.
No, I'm not an engineering student ... never have been, never will be.
As for the knowledge base being "not practical", what knowledge I have comes
from GM engineers that have contributed to various books and magazine
articles I have read and books/articles by David Vizard, who has been build-
ing and testing engines for many years and has written several books that
talk about this subject. I've learned a lot from David Vizard because of
the practical advice he gives based on empirical knowledge.
Earl
Earl Shimaoka
tti...@primenet.com (TTILMM) wrote:
> > The cylinder pressures generated during the cranking test don't
> > necessarily tell you the entire story. The pressure may be at spec,
> > but it is measuring the peak pressure at a single point in time. You
> > could still lose pressure due to blow-by (the rings). The only way
> > to determine this is to have a leak-down test done on your engine.
> > This test pressurizes the cylinder and measures the amount of pressure
> > drop over a certain period of time. Leak-down numbers in the range of
> > 7-10 percent is good. The higher the percentage, the less power your
> > engine will make because some of the pressure is simply blowing by the
> > rings.
> >
> > ... deleted some stuff
>
> A difference of 10 psi on one cylender is virtually nothing for a dry
> compression test. A variance of 20 from highest to lowest is when you
> should start worrying, and thats still not really that bad. Its certainly
> possible that he has a problem, however, from just the one test he told us
> of, you'd have to be a car hypochondriac to suggest that he needs to root
> out the cause of a 10psi diff.
Please reread my first paragraph. I suggested that he have a leak-down test
performed on his engine. That is a different kind of test than a compression
test.
The compression test may not tell you that you are leaking pressure. Only
the leak-down test will tell you that with certainty. I had a 350 V8 that
exhibited the very problem I am talking about. It just wasn't generating
much power (compared to earlier life). When I had it tested, the compression
test showed that all cylinders were +/- 5 psi of spec. However, the leak-
down test showed most cylinders in the 15-20% range. Some of the pressure
from combustion was simply blowing by the rings, resulting in less power.
The leak-down test is totally independent of what the spec'ed cylinder
pressure should be. It measures the loss of cylinder pressure over time
based on whatever pressure you use for the test.
Earl