[Chrysler300] Thermodynamics of a Ram
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[Chrysler300] Thermodynamics of a Ram
- From: "Rich Barber" <c300@xxxxxxx>
- Date: Fri, 9 Nov 2012 14:15:28 -0800
Warning: There will be a quiz at the end of this mini-lecture. If you're
not interested in engine performance and the ram induction effect, tune out
now.
If the cam allowed some valve opening before TDC on the compression stroke,
the effective compression could be lower. That would be a real power
robber, methinks.
Simply speaking, a 4-stroke cycle engine acts as an air compressor while
being spun by the starter. The compression pressure is measured as the peak
pressure in the cylinder over a small number of strokes-as PSIG (Pounds per
square inch-gauge). The Shrader valve in the peak pressure indicator acts
as a check valve to hold the peak pressure on the gauge until released.
Ideally, the peak pressure can be estimated by the cylinder's compression
ratio. The laws of physics mandate use of absolute units when dealing with
the gas laws, therefore we must use pressures expressed in PSIA (pounds per
square inch-absolute). PSIA=PSIG plus atmospheric pressure. Differential
pressures are expressed as PSI.
Examples:
Sea Level Atmospheric Peak Peak
Elevation Pressure Compression Pressure Pressure
Feet PSIA Ratio :1 PSIA
PSIG Engine & Location
16 14.69 9.6 141.02
126 (My ram K engine at Newark, CA)
16 14.69 8.5 124.87
110 (My C-300 hemi at Newark, CA)
5,280 11.68 9.6 112.13
100 (My ram K engine on the first step of State Capitol Building in
Denver)
16 17.69 9.6 169.82
155 (My ram K engine at Newark with a 3 psi boost from ram induction)
So the ideal peak pressure depends on elevation and cylinder compression
ratio. Using the same pressure measuring device and techniques in an engine
at operating temperature and at similar cranking speeds should provide
valid cylinder-to-cylinder comparisons. Lower peak pressure indicates air
leakage-either/or past the rings, valves, gaskets, pig iron. Adjusting the
valves on a solid-tappet engine before the test makes sense-although having
them buried below 30" long ram induction intake manifolds makes that a
challenging job.
BTW, the ram effect is designed to increase the apparent atmospheric
pressure at the intake valve while it is open. The only reference I found
indicated the ram effect could amount to as much as 3 psi at resonant speed.
As our engines are limited by the amount of air they can inhale (free air
capacity), a 3 psi boost at the inlet could theoretically increase the
engine's power by 20% at sea level. Chrysler only claimed 8.3% (390 HP vs.
360 for a non-ram K). This suggests a 1.2 psi ram effect. Anyone heard or
measured anything different?
C-300K'ly
Rich Barber
Brentwood, CA (Elevation at my home here in the Bay Area is about 120')
Ps: Quiz: What is the theoretical peak compression pressure of a cylinder
of a C-300 hemi engine parked outside the state capitol building at Denver?
(Just before the park police tow the car and arrest the mechanic) Answer:
Scroll down to under Marshall's signature.
rb
From: Michael Moore [mailto:mmoore8425@xxxxxxx]
Sent: Friday, November 09, 2012 5:01 AM
To: mgoodknight@xxxxxxxx
Cc: c300@xxxxxxx; Chrysler300@xxxxxxxxxxxxxxx
Subject: Re: [Chrysler300] Compression pressure , 300H
Marshall,
I have been gradually getting my 300H, which I have owned since 1964, back
into a more drivable condition. Some of the major problems which kept me
from driving it much (brakes, steering) are resolved, but the engine issues,
while better, are still with me. In general, it has suffered from a lack
of power dating back to the engine rebuild I had done many years ago. The
rebuilder, a highly respected race engine builder, misunderstood California
(where I was moving to) requirements. He also knew high test was going
away. Among the mods he made was to lower the compression with a custom
Barnes cam, rejet the carbs with richer needles and install a smog valve.
The car had power, but ran so rich your eyes burned. The carbs were rebuilt
and rejetted to stock needles and it began to run better. I know it still
does not run as strongly though as it did even in the 90's.
I obtained a correct 300H camshaft from a member and have a new set of
lifters. My plan is to replace the camshaft with the stock 300H camshaft.
Recently though, I have been wondering if the rebuilder may have installed
lower compression pistons. If so, now would be the time to replace the
pistons also.
I also don't fully appreciate how the custom camshaft lowered the
compression ratio other than he adjusted the overlap.
Thanks,
Mike Moore
Morgan Hill, California
300H
On Nov 9, 2012, at 3:44 AM, mgoodknight@xxxxxxxx wrote:
Thanks for your research Rich,
That's a good write-up on compression testing. One might conclude from that
reading that too much emphasis is often put on the results of compression
testing and that proper (valid) compression testing is not often
accomplished.
Much can be learned about the condition of an engine from studying the
results of a valid test, but inaccurate diagnostics are more commonly the
main outcome.
My recommendation is to avoid such a test unless you have strong evidence of
a problem that can be easily identified by a test. If the engine runs well
and performs properly, don't do the testing. You might see results that
will cause you undue worries.
Marshall Goodknight, engine failure analyst
(Answer: 88 psig-for about a 20% loss in potential power. The C-300
becomes a C-240)
Please note: message attached
From: "Rich Barber" <c300@xxxxxxx>
To: "'Michael Moore'" <mmoore8425@xxxxxxx>, "'300'"
<Chrysler300@xxxxxxxxxxxxxxx>
Subject: RE: [Chrysler300] Compression pressure , 300H
Date: Thu, 8 Nov 2012 22:47:28 -0800
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From: "Rich Barber" <c300@xxxxxxx>
Subject: RE: [Chrysler300] Compression pressure , 300H
Date: November 8, 2012 10:47:28 PM PST
To: "'Michael Moore'" <mmoore8425@xxxxxxx>, "'300'"
<Chrysler300@xxxxxxxxxxxxxxx>
Individual compression pressures on a long non-operated, 85,000 mile, cold,
ram K block ranged from 100 psi to 140 psi. I found this interesting text
on analyzing compression pressures. According to the 75% criterion, the
engine failed, although a properly operated and warmed up engine with
properly adjusted solid tappets would have probably tightened the range and
marginally passed.
C300K'ly,
Rich Barber
COMPRESSION TESTING
Note: although this is written primarily for 4cycle gas engine testing,
2cycle gas and diesel follow pretty much the same sequence of procedures.
Also, many diesel engine companies do not publish specific compression
readings so the values, in the chart, at the end of this section will be too
low for use with a diesel, you'll have to do the calculation.
1. Be sure the crankcase is at the proper level and the battery is properly
charged. Operate the engine for a minimum of 30 minutes at 1200 rpm, make
sure that the choke has opened, and the engine is at normal operating
temperature. Putting a load on the engine for at least 10 minutes, can
increase the accuracy of your test. Turn the ignition switch off; then
remove all the spark plugs (injectors on the diesel). If the engine has
closed cooling, (green anti-freeze) relieve pressure in the cooling system.
(carefully)
2. Set the throttle plates and choke plate in the wide open position. On 4
cycle gas engines: Try not to pump raw fuel into the intake with the
accelerator pump.
3. Install compression gauge in No. 1 cylinder.
4. Install an auxiliary starter switch in the starting circuit. Ground the
coil wire on gas engines. Next, using the auxiliary starter switch, crank
the engine (with the ignition switch off) at least five compression strokes
and record the highest reading.
Count the approximate number of compression strokes required to obtain the
highest reading.
5. Repeat the test on each cylinder as was required to obtain the highest
reading on the No. 1 cylinder.
If the readings degrade from front to rear, re-check the battery and
possibly re-run the test starting at cylinder #1. In this way you can have a
set of two tests to compare an average.
TEST CONCLUSION
2 Cycle Engines:
Your test results need to be within 15 psi range to be considered an engine
worth using. For example; a 3 cylinder with readings of 130, 135, and 120
would be considered a good engine. Some might argue that there should be no
more than 5psi difference between high and low cylinders. But, obviously,
the closer the readings are to each other, the better.
4 Cycle Engines:
The indicated compression pressures are considered normal if the lowest
reading cylinder is within 75% of the highest. Refer to the following
example and chart.
Seventy-five percent of 140, the highest cylinder reading is 105. Therefore,
cylinder No. 7 being less than 75% of cylinder No. 3 indicates an improperly
seated valve or worn or broken piston rings.
If one, or more, cylinders read low, squirt approximately one (1) tablespoon
of engine oil on top of the pistons in the low reading cylinders. Repeat
compression pressure check on these cylinders.
1. If compression improves considerably, the piston rings are at fault.
2. If compression does not improve, valves are sticking or seating poorly.
3. If two adjacent cylinders indicate low compression pressures and
squirting oil on the pistons does not increase the compression, the cause
may be a cylinder head gasket leak between the cylinders. Engine oil and/or
coolant in the cylinders could result from this problem.
Note: An engine compression test is easy to do but, your results need to be
tempered with the knowledge of testing conditions that reduce the accuracy
of your test. As the battery gets weaker, starter gets hotter, and the
engine cools down, your results from any one cylinder will change. It is
important to do the test a quickly as possible, with as little cranking as
needed to complete the test.
Circumstances arise where a more accurate leak down test should be utilized
before you start tearing the engine down. Readings that are at the extremes
of the chart, (meaning over-all very high or very low readings), may
indicate conditions inside the cylinders that a compression test won't
depict properly.
You can do the math yourself but, the following quick reference chart should
be used when checking cylinder compression pressures. ( to avoid
mathematical errors) The chart (below) has been calculated so that the
lowest reading number is 75% of the highest reading.
Special note: Although the "actual" compression readings are not all that
important, keep in mind that very few engines will run with a compression
under 100 psi. However, there are some engines that will test with readings
in the 90 psi range on a regular basis. If you're not sure if you have one
of these engines, contact your servicing dealer before you get excited.
Although 75% range is tolerable for an older motor, keep in mind that most
modern engines are designed at much closer tolerances. Also, on a V8 engine,
if 7 cylinders test at 150psi and only one tests at 113psi, this is not a
good sign...
-----Original Message-----
From: Chrysler300@xxxxxxxxxxxxxxx <mailto:Chrysler300%40yahoogroups.com>
[mailto:Chrysler300@xxxxxxxxxxxxxxx <mailto:Chrysler300%40yahoogroups.com> ]
On
Behalf Of Michael Moore
Sent: Thursday, November 08, 2012 11:13 AM
To: 300
Subject: Re: [Chrysler300] Compression pressure , 300H
Sorry, that would be engine compression.
Mike Moore
On Nov 8, 2012, at 11:00 AM, Michael Moore wrote:
Does anyone have available the pressure I should read for a stock 300H when
cranking (with all plugs out) ?
Thanks, Mike Moore
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