355 Compression results

If you want to get crazy you can initiate the compression for each cylinder while at top dead center. Then each piston will be in the same spot for each cylinder at the beginning of the test.

I respectfully disagree wholeheartedly with your earlier comment about leak down tests because the comment is incomplete and somewhat misleading. Every test can be manipulated to generate spurious results. When performed correctly, the leak down test does not lie. Period and end of story.

Any qualified mechanic will tell you that. If you want to split hairs and say no test is perfect, fine. But the leak-down is a great diagnostic test for an engine. Naturally, incorporated into the idea of a leak-down test being done by a qualified mechanic is the principle of performing multiple trials for each cylinder to ensure no spurious results due to a valve not seating, etc. No one said give it the old college try and bet your life on the results of the first trial. The mechanic must be familiar with the car in question and be able to interpret the results correctly.

But to your point generally about the variables within a test's framework that can generate results which I call spurious, coupling the leak-down test together with the compression test is a good strategy precisely because the more data, the better in the case of 355 engine health diagnosis. This isn't data overload. It's data collection to determine the big picture.

There have been countless posts here by Rifledriver, tbakowsky, fatbillybob, and countless other outstanding mechanics on this board. Threads on this topic exist since the beginning of F-Chat.

Some reading material:
http://www.ferrarichat.com/forum/search.php?searchid=10106613&pp=50
http://www.ferrarichat.com/forum/search.php?searchid=10106638

 

Hey Dave,

Leak rates related to the cylinder head are the integrity of the valve seal and related to:

- The valve sealing grind
- The valve seat grind
- The concentricity of the valve to the seat
- The valve guide clearance or wear
- Damage to any other those components
- The build up of carbon

So, in a static situation, that seal integrity won't always be consistent. That's in part why a compression test is valuable do to the dynamic nature.

So, to "split hairs", a LD should be done on the same cylinder several times after rotating the engine over a few times to see if the results repeat.



And, somewhat related, here is a video I did to demonstrate the axial movement with a normal, .001" valve stem to valve guide fit. Of course, when the valve closes, the seat / valve angle will center the two within reason.

[ame]http://www.youtube.com/watch?v=2f77G_iRUjM[/ame]

 

Thank you for the explanation, as others will benefit from it. These things I know already, and I alluded to them in my remarks.

 

As I told you before, too many variables.

 

So what are you trying to say?

 

I'm lazy, and I have forgotten how to code in FORTRAN. Revise that. I don't have a FROTRAN compiler.

 

LG = life is good

 

But you know I couldn't take that lying down, and the 24 hrs of Daytona is a little boring at the moment. So, here are some results with the following assumptions:

1) Swept volume = .4 L

2) Combustion volume = 0.04 L

3) Gage + tube volume = 0.008 L

Oops, a Lambo just caught fire in the pits.

4) Target pressure is 200 psi. That is, 200 psi is the pressure the cylinder would reach w/o the compression tester attached.

5) perfectly sealed cylinder, no leakage and no losses across the check valve, etc.

6) first crank is full compression stroke.

7) Atmospheric pressure is 15 psia.

Crank 1 P = 163.99 psi

Crank 2 P = 193.787 psi

Crank 3 P = 198.797 psi

Crank 4 P = 199.6 psi

Crank 5 P = 199.76 psi

That's as far as I got before I fell asleep.

Now, this is like the old physics joke,

"Can you calculate how a bird flies?"

"Sure. Let's start by assuming a bird is a sphere."

But you get the idea.

The many crank pressure can be approximated as

P = 215 x (1 - exp(1.359555 x N)) - 15

where N (>5) is the number of cranks.

Thus after 10 cranks, P = 199.999 psi

OK Dave?

 

Maybe

Unless I'm missing something (highly likely ) you are back calculating....

My 2 direct questions are (for clarity)

1) Can we (well, can YOU) determine via calculations, the maxium pressure (PSI) the cylinder can ever reach, regardless of strokes (I'm certain a formula exists for this, I just don't know it)

2) How many cranks would it take to get there?

What I know for certain is the max pressure is not 200 PSI as I've gotten higher results when doing compression tests prior



And, let me know when the next Lambo burns

 

No back calculating anything. It starts with the air in the cylinder at 15psi. But it's a model and it is necessary to say something about the compression process. The relationship between volume and pressure is

P2/P1 = (V1/V2)^x

So what is x? If the process was isothermal x =1 and P2 = 150 psi. If the process is isentropic x = 1.4 and P2 = 415 psi. But it is neither. So what to use for X? I know V1 = .44L, V2 = 0.04 L, P1 = 15 and need a reasonable value for P2. Assuming P2 = 200 psi (215 psia) yields x = 1.11. If I used P2 = 215 psi, or 230 psia, x = 1.13.

The bottom line is that the exponent, x, represents the compression process. Changing its value will change the max pressure but it won't have much effect on how long it takes to get there. Think of it this way. Remember the distance example, each day go 1/2 the remaining distance. You never get there. Ok do it over but this time go 17/32 of the remaining distance. You still never get there. If I remember it correctly the distance left is (1 - y)^N where y is the fraction of distance you travel each day. So for y = .5, after 5 days you have 0.031255 of the total distance left. For y = 17/32 you have 0.0226 of the total distance left after 5 days.

But look, it's the best I can do. If you can give me a better approximation to the compression process let me know. I'll be waiting.

 

Typical engineer (like myself)......

Ask a guy "what time is it?" And he tells you how to build a watch. Almost like politician answering a question (almost)

 

I told you Dave, too many variables. If you don't want the watch you can go out in your garage, connect a compression tester to you engine and video the gage as you crank, like someone mentioned above. But I think I have demonstrated why 5 or 6 cranks is typically sufficient.


Frankly, as I before, I think most of these static an cranking tests have little impact on how the car actually runs unless the number are really bad. I'd like to see the results of a running compression test at idle and maybe with 1/2 a dozen throttle blips and compare cylinders under those conditions. After all, you don't drive at cranking RPM. never see anyone report that for a Ferrari.

 

John, yes, I understand there are too many variables. And I like the watch just fine

You and I have discussed this over the phone more times than we have fingers and toes....

I'd just like to have a baseline of a new motor that is as leak free as it will ever get. With that baseline, we would have all we need for comparison.

When I get my motor done (sometime within this decade, hopefully), I'd run the test to get the baseline.

 

Reread my last post. Sounded kind of hostile. Didn't meant too.

 

You need something in the system for a while not a spray

I use this type system with BG chemicals for 30+ years now never let me down

[ame]http://www.youtube.com/watch?v=J4DaQEs--vo[/ame]

 

I do not understand how anyone can do a major with cam timing any easier than with cam cog and cam shaft removal. When cams are out all the valves are closed. It literally takes about 1 minute per port to walnut shell and clean out any residue. A chemical cleaner is better than nothing but really like using mouthwash instead of brushing your teeth. I guarantee you there is cr@p in there.



...And for those who have not figured out how to take their cams out you can clean one lung at a time by finding TDC compression and going through your firing order just like doing a leakdown test but that will take you 5x as long.

 

FBB. What exactly can you clean like this? I'm confused.

 

Some on this thread have seen this but fit those that haven't. Maybe this will be helpful.

This is a post from a thread about a car I had a PPI done on and the compression was off in a couple cylinders. The tech took it for a 5 Mile "Italian tune up" and redid the tedt. You can see the before and after numbers. No cleaner was used.

http://www.ferrarichat.com/forum/141996965-post51.html

 

I believe I posted this in this thread......

With the use of BG 44K (Recommended by Tim Socal), I raised one cylinder from 180 to 200 PSI after a hard run. I don't recall the miles but it was substantial (100-200)

Would the hard run alone helped? I suspect yes. Washing a car with water and no soap will clean it, but the use of soap will yield better results.

 

Carl, the problem with this is that a majority of the buildup is on the valve seat and combustion chamber and you are not going to clean that with valves closed but I recall you doing this on the 550 to get the crap above the valve seat cleaned.