Fordnatics List Archive

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Mail From: (email redacted)

I have a 1993 5.0 LX. I checked the compression on one of the cylinders
last week and got a reading of 125. Is this correct or should it be
higher??? The car hasnt been running as well as it used to, could I have
broken something?
Mike.

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Mail From: (email redacted) (Robert King)

>
> >What are the other cylinders' readings? 125 *seems* to be a little
> >low but if the other cylinders are within 10% of that, then I
> >wouldn't worry too much.
>
> >If you have two adjacent cylinders whose pressures are down, you
> >probably have a leak in the head gasket allowing air to leak from one
> .cylinder to the other.
>
> >You also should do a wet test of the cylinders to determine of the
> >rings or valve seats are worn. If the pressure goes up significantly
> >after putting a tablespoon or so of lightweight oil into the cylinder,
> >then the rings are leaking, otherwise, its probably the valve seats.
>
> >-- Robert King
>
>
> What should the compression be from the factory?
> Mike.
>

Beats me. I've never been able to get an answer on that one.

- -- Robert

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Mail From: (email redacted) (George Chang)

The latest installment of the Saga of the Compression Test reads
something like:

>> >You also should do a wet test of the cylinders to determine of the
>> >rings or valve seats are worn. If the pressure goes up significantly
>> >after putting a tablespoon or so of lightweight oil into the cylinder,
>> >then the rings are leaking, otherwise, its probably the valve seats.
>>

>> What should the compression be from the factory?
>>
>Beats me. I've never been able to get an answer on that one.


Since the guys who really work on such things say,"Beats me",
maybe I'll hazard a guess. If compression ratio really means PV=nRT,
then a 10:1 compression ratio should give an absolute pressure of 10
atmospheres, or about 150 psi. But the other end of the compression
gauge is already at 1 atmosphere. So the difference should be 9 atm,
or about 135 psi.
Guessing further, I'd speculate that for a compression ratio
of r, the theoretical pressure on a gauge would be (r-1)atm, or 15(r-1)
psi. And if someone measured his/her compression and got a higher
figure than I expected, I'd say,"Ha! You've got carbon build-up!"
If the figure was lower that I expected I'd say, "Ha! It's leaking!"
And after you showed me that neither was the case, I'd mutter under
my breath that someone else's cockeyed theory was wrong.
Does anyone have a real answer???

Best regards,
George

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Mail From: (email redacted) (Frank Marrone)

Your reasoning is very good but remember that
you do not have 100% pump efficiency so expect
a little lower value than you calculated. I
have found that throttle angle or air cleaner
presence can significantly effect the static
compression reading.

P.S. It makes me chuckle to think that the 1994
mustang styling is where most of Fords effort went
since this is probably the #1 reason why I wouldn't
buy one (i.e. I think it sucks). I am very impressed
with its performance though, so much so that I really
wish it was styled differently cause I might of actually
sprung for one. Different strokes...

Frank Marrone at (email redacted)

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Mail From: (email redacted) (Clifton Koch)


> Since the guys who really work on such things say,"Beats me",
> maybe I'll hazard a guess. If compression ratio really means PV=nRT,
> then a 10:1 compression ratio should give an absolute pressure of 10
> atmospheres, or about 150 psi. But the other end of the compression
> gauge is already at 1 atmosphere. So the difference should be 9 atm,
> or about 135 psi.
> Guessing further, I'd speculate that for a compression ratio
> of r, the theoretical pressure on a gauge would be (r-1)atm, or 15(r-1)
> psi. And if someone measured his/her compression and got a higher
> figure than I expected, I'd say,"Ha! You've got carbon build-up!"
> If the figure was lower that I expected I'd say, "Ha! It's leaking!"
> And after you showed me that neither was the case, I'd mutter under
> my breath that someone else's cockeyed theory was wrong.
> Does anyone have a real answer???

Works on paper, but doesn't seem to hold in the real world. Awhile back I was
changing the intake on my truck. After the intake was off, I decided to do a
compression test, and the cylinders all came out between 185 and 195. I checked
the tester with my compressor, and it was accurate. OH MY GOD, I though, this
thing must be full of carbon, so I pulled the heads. Not much carbon. This
engine had been rebuilt by a complete idiot in the past, so I was wondering if
the incorrect heads were on the engine and cc'ed them. I measured the bore and
stroke (and was not happy when I found the engine bored .040 over), calculated
out the compression ratio, and the compression came out around 8.8.

Do I have an explanation? Nope. I wouldn't think cranking speeds would be
enough to get any sort of scavenging, but maybe so. The engine definately had a
wide open intake path.

Cliff

- -----------------------------------------------------------------------------
Cliff Koch
Motorola Cellular Infrastructure Division
(email redacted)

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Mail From: (email redacted) (George Chang)

>> Since the guys who really work on such things say,"Beats me",
>> maybe I'll hazard a guess. If compression ratio really means PV=nRT,
>> then a 10:1 compression ratio should give an absolute pressure of 10
>> atmospheres, or about 150 psi. But the other end of the compression
>> gauge is already at 1 atmosphere. So the difference should be 9 atm,
>> or about 135 psi.
>> .............(deleted)...............................

> Works on paper, but doesn't seem to hold in the real world. Awhile back I
>was
>changing the intake on my truck. After the intake was off, I decided to do a
>compression test, and the cylinders all came out between 185 and 195. I
>checked
>the tester with my compressor, and it was accurate. OH MY GOD, I though, this
>thing must be full of carbon, so I pulled the heads. Not much carbon. This
>engine had been rebuilt by a complete idiot in the past, so I was wondering if
>the incorrect heads were on the engine and cc'ed them. I measured the bore and
>stroke (and was not happy when I found the engine bored .040 over), calculated
>out the compression ratio, and the compression came out around 8.8.
>
> Do I have an explanation? Nope. I wouldn't think cranking speeds would be
>enough to get any sort of scavenging, but maybe so. The engine definately had
>a
>wide open intake path.
>
>Cliff
>
>-----------------------------------------------------------------------------

Cliff,
I think you're right. A cranking engine is not an isothermal
piston on a piece of paper. That PV=nRT stuff must be someone else's
cockeyed theory!
For those who understand such things, I'd bet that this is
nearly an ADIABATIC compression. Unfortunately all I ever learned
about such things is how to spell the word so I could get partial
credit on exams.
And I think that everyone who said,"Beats me" is right, also.
For any engine that's built to operate at more than a few hundred rpm,
there's going to be valve overlap, or whatever all that camshaft email
is talking about.
Finally, other Fordnatics point out that cranking speed and
even choke and throttle positions affect things. At this stage, I'll
just imitate the people in the know, and say, "Beats me!"

George

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Mail From: (email redacted) (Eugene Chu)

In regards to trying to determine compression pressure from the
compression ratio using the ideal gas law (PV=RT), remember that this
freshman chemistry exercise is only applicable to "ideal gasses": those
gasses that obey the ideal gas law! (How's that for a convoluted
description?) Even for the simplest experiments, we had to use Van Der
Wahl's equation, which is a high order equation to deal with effects
that stray too far from "ideal", such has high compression/temperature.

Now, as someone pointed out, a static compression check can give
differing results depending on whether the throttle was opened or
closed. You will get higher readings when open than when closed since
the opened throttle allows the cylinder to fill with atmospheric
pressure, and the closed throttle will generate vaccuum on the intake
stroke, filling the cylinder with something less than atmospheric
pressure. If you can monitor a vaccuum guage while doing the test with
the throttle closed, you might get better predictions of expected static
compression pressure by taking into account for the vaccuum.

eyc

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Mail From: (email redacted)

George was saying ..

"... Since the guys who really work on such things say,"Beats me",
maybe I'll hazard a guess. If compression ratio really means PV=nRT,
then a 10:1 compression ratio should give an absolute pressure of 10
atmospheres, or about 150 psi. But the other end of the compression
gauge is already at 1 atmosphere. So the difference should be 9 atm,
or about 135 psi. ..."

To which I say: Well, not really. Compression ratio is actually the measure
of the difference between the cylinder air volume at bottom-dead-center and
the volume measured at top-dead-center. In other words, for an 8:1
compression ratio, the volume of air/fuel measured
at top dead center would be 1/8 the volume of the air/fuel measured at bottom
dead center.

The actual measure of "cylinder absolute pressures" is going to change
drastically, depending upon where in the world you happen to be. The only
place you should expect to get the afore mentioned 150psi cylinder pressure
would be at sea level, and then only if the motor were in top shape. At
higher altitudes, the number will be lower, since true atmospheric pressure
is lower.

But also, as someone else brought up, there has never been an engine made
anywhere that has 100% volumetric efficieny. Things like a carb or
throttlebody that are too small, or other things like air filters, and
throttle plates obstruct the flow as well, further decreasing efficiency.
These little blips need to be factored into it as well...

I think I read somewhere that in place like death valley, the atmospheric
pressure is a little *greater* than at sea level. In that case, the
measurement would be a little higher...


George went on to say that...

"... Guessing further, I'd speculate that for a compression ratio
of r, the theoretical pressure on a gauge would be (r-1)atm, or 15(r-1)
psi. And if someone measured his/her compression and got a higher
figure than I expected, I'd say,"Ha! You've got carbon build-up!"
If the figure was lower that I expected I'd say, "Ha! It's leaking!"
And after you showed me that neither was the case, I'd mutter under
my breath that someone else's cockeyed theory was wrong.
Does anyone have a real answer???

Best regards,
George..."

Ummm, I don't know where you got the (r-1)atm, or 15(r-1)psi stuff (did you
do something with the PV=nRT equation? If so, I don't think it cranked
through too well).

However, your assumption is pretty good. Carbon buildup on the valves and
combustion chamber reduces the volume at TDC, and results in a higher
compression (by a few pounds at most, but not by a whole lot), and also
results in a greater cylinder temp, and possibly a ping here and there...

And definitely, if it was lower than correctly calculated (ie, taking into
account the actual cylinder volumes, temps, and altitudes, obstructions,
etc), you betcha - there's a leak somewhere...

Mark F. Mitcham
(email redacted)

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Mail From: (email redacted)

People have been talking!

">
> What should the compression be from the factory?
> Mike.
>

Beats me. I've never been able to get an answer on that one.

- -- Robert
"

Well, again, the factory is not going to be able to give you exact numbers
with regard to cylinder pressures, because the actual pressures will
be different, depending on where it is in the world that you drive
your car.

As far as cylinder compression checks go, as long as all the cylinders
are within a few percent of each other (ie, nothing more than a 10-15%
difference between cylinders), then the motor should be fine.

The actual wear on things like rings, the cylinder walls, and the
valve guides will introduce blowby, and over time will result in
what should be uniform decreases in cylinder pressures.

The trick is that if you are going to measure this kind of stuff, you
need to be consistant in the measurements. Make sure you account
for differences in temperature, as temperature can affect the sealing of
rings, valveguides, and overall pressure.

Mark f. Mitcham
(email redacted)

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Mail From: (email redacted) (Frank Marrone)


> But also, as someone else brought up, there has never been an engine made
> anywhere that has 100% volumetric efficieny. Things like a carb or
> throttlebody that are too small, or other things like air filters, and
> throttle plates obstruct the flow as well, further decreasing efficiency.
> These little blips need to be factored into it as well...
>

I have heard that (has been done/proven) normally
aspirated engines can have VEs' in excess of 100% at certain RPM. Of
course blown engines can exceed 100% VE.

Frank Marrone at (email redacted)

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Mail From: chucko (Chuck Fry)


[Frank Marrone said:]
I have heard that (has been done/proven) normally
aspirated engines can have VEs' in excess of 100% at certain RPM. Of
course blown engines can exceed 100% VE.

From: (email redacted)
Date: Sun, 24 Jul 94 19:33:09 EDT

Hmmm... I haven't seen anything like that; but I can speculate on two
things: first, this was a racing motor they were talking about, and second,
that >100% VE is obtained from WOT, zero restrictions, and its gotta have
something like individual injectors/throttle bodies on each
cylinder. The air would need to be going *very* straight, and the temp would
have to be just right.

Yes and no. It's fairly well known that >100% VE can be had by taking
advantage of resonant effects. These might include ram effect intakes
(e.g. "tunnel ram" and late model long-runner EFI manifolds) and tuned
180 degree headers. Standard 4-into-1 headers don't really succeed at
this because the exhaust pulses are too uneven to establish a resonance.

Might also be an effect of cam overlap; if the air had enough velocity
coming into the intake, it might be enough to exert a pressure of slightly
greater than atmosphere, thus loading the cylinder more than normal...

Close but not quite. The purpose of overlap is to use the slight vacuum
at the exhaust port to start pulling in the intake charge. After that,
inertia takes over, and the mixture just keeps coming...

Thats a really interesting thing. Anyone know about this?

I haven't yet come across a good reference, but I'll bet Dave Williams
has.
-- Chuck

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Mail From: (email redacted)

I think it is time I did all you late model guys a favor. I sense a great
void in your technical library. Now that you have spent so much time going
around race tracks, working on brakes, suspension, tires etc, it is time
your thoughts focus on the internal combustion engine that has served you
so well from the factory. To better understand what is involved I would
suggest two things:
1. Getting a list of books available from Steve Smith
Autosports, in particular Racing Engine Preparation
By Waddell Wilson. This book is almost 20 years old but
is extremley readable and covers more than just expensive
parts. BMEP, BSFC, VE (115% in my 289 vintage racer at
6200rpm, single Holley) and other engineering level
measurements and metrics are briefly covered.

2. Make friends with a local Dyno shop and pay a visit. Be
there for set-up (sometimes help is appreciated) and
look at the things they monitor. It is really exciting
to:
1. Hear a new motor come to life.
2. Watch the HP come up with break-in and tuning.
3. Seeing the difference small adjustments make.
4. Debunking myths with Dyno facts

I am sure there are a ton of newer books that can really give some
technical insight to motor testing and prep, but this is a good start. If
anyone is really interested, send me a note and I will list my technical
library of the last 30 years (although I haven't kept up in the last 10,
maybe Dave has some recent offerings).

Needless to say, the best info in a periodical used to be Circle Track and
it was marginal. All the rest were just junk food.

I would bet that a GN stock car engine by Mr. Yates has a VE of over 125%
somewhere in the 7800 RPM range. Dave, how close am I?

Tred