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compression testing

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Dave, I worked it out to be 79.56% of sea level pressure, using the published altitude for the airport at Tahoe, 6246' MSL.
I'm going to coast on this one.
It's fun stirring the pot, as it always leads to new avenues to ponder. And, I generally count the day as lost if I don't learn at least one thing new!
Keep 'em on the road.
Jeff
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So, if I test again here then drive to the coast and repeat the test I can then tell which of you is most correct. But which coast (isn't sea level higher in the Atlantic than the Pacific)?
Just kidding. I really do appreciate all the help. The forum is an education for me almost every day.
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I made compression readings a while back with a cold engine and the throttle open. However, after reading everyones thoughts on this I decided to try it with a warm engine today and throttle open and the readings changed quite a bit. I went from an average in the low 160's with a cold engine to an average in the low 150's with a warm engine. Not really sure what that means though I think 150ish is probably ok. mac
 
I'm facinated by the fact that a healthy Healey has compression readings around 165 lbs. If that is taken at face value; that means the compression ratio is 165/14.7 ~ 11.2:1 This is much higher than the published numbers. That air must get really warm when you "squish" it.

(I think the change is only about 12° F warmer can anyone confirm this?)
 
Dave, I was thinking the same thing though earlier posts on the topic talked about the benefit of a warm engine on the shape of the cylinder walls thus making for a better seal. My thinking is that the cold oil just makes for a little bit better seal. mac
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by ThomP:
I'm facinated by the fact that a healthy Healey has compression readings around 165 lbs. If that is taken at face value; that means the compression ratio is 165/14.7 ~ 11.2:1 This is much higher than the published numbers. That air must get really warm when you "squish" it.
<hr></blockquote>

Thom,
Your calculation is correct for "isothermal" compression which assumes that no heat is gained or lost during the compression cycle & if the actual compression ratio were the same as the rated ratio. - See below. The formula for "adiabatic" compression takes heat loss or gain into consideration. Eg. as the air is compressed it is heated by the compression & the final pressure is higher than would be predicted by isothermal compression. The variables are rather large & the difference between the two calculations can vary by a large factor in the final pressure. The pressure rise due to heating is an expotential function. (more heat = more pressure = more heat = more pressure). The exponent will vary from one with complete heat loss to about 1.4 with no heat loss.

Rated vs actual compression ratio ---------
To complicate things , at low rpm such as cranking speeds --
The actual engine compression ratio is never the design calculated ratio because in reality the piston cannot begin to compress anything until the intake valve gets closed. If the intake valve were to be completely closed at BDC, which by design it never is, the actual & rated compression ratios would be the same. At higher rpms & thus higher intake gas velocities, the late closing of the intake valve is offset by the gas inertia in the intake tract & the engine can approach or exceed it's design compression ratio.

For a hypothetical engine with a rated 9/1 compression ratio, Assume that the actual mechanical CR is 6/1 due to late intake valve closing. We can calculate that the pressure for isothermal compression would be 14.7 * 6 = 88 psi. With an adiabatic exponent of 1.14 this would give a compression pressure of 165 psi.

Whether heat is gained or lost during compression depends on several things such as how quickly the compression takes place (starter & engine cranking speed), temperature of the engine during the test (is the air in the cylinder hotter or cooler than the air compressed in it) , & a few other variables. The end result is usually that the measured compression pressure is a bit higher than would be calculated by isothermal compression.

The compression pressures given at "face value" are empirically determined from measurments & experience. There are just to many variables to make accurate calculations.
D
 
In this dicussion Leak Down Testing has been mentioned. I found the web site location given by Dave Russell extremely interesting and can not understand why it is not more well known (to me anyway!)A question: in the above discription a method of obtaining TDC is given whereby an access port is shown in the in the R/H front wheel arch which allows 'something'to be poked thro.to, I assume, locate on/in the belt pulley and rotate it to TDC,is this correct? My '63BJ7 does not have such a port,can I put one in?I hope so because I always have trouble achieving TDC.
 
Hi John,
I can understand the confusion. The article was written by John Stangl & applies to the "Diamond Star Motors" cars. The plymouth Laser, Eagle Talon, & Mitsubishi Eclipse cars.

They have transverse engines & the pic shows a long extension with socket going through the panel to engage the crankshaft nut.

The equivalent for your car would be a hole through the radiator or frame. About the closest you can come is a large socket with right angle breaker bar on the front nut.
D

[ 12-04-2003: Message edited by: Dave Russell ]</p>
 
I think in the future I'll pose my questions so that I get a simple "yes or no". You guys left me in the dust a long time ago. I did learn a lot but I don't think I'll be doing that leak down thing too soon. Thanks for your responses. I value all the help.
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