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Author Topic: Compression - Leak Down To Power Loss
Jim Venable
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Sorry this is a bit long but occasionally we have to be to make engine questions clear. Sometime this season well have to freshen our motor based on the figures below. The answer to this question will not only assist me and perhaps others as well in knowing when were at the must do it now point. Hopefully this makes sense and has value. If not, do let me know.

Completed a leak down and compression test for the first time on a 1.6. Yeah, I know with the leak down the compression check isnt as needed. But, for us old timers a trip now and then down amnesia lane watching the gage jump brings back glory day memories of high compression builds and the stories that go along with each one. Anyway, each test was performed twice one hour apart with the motor at operating temperature.

Compression: 175 across the board dry.

Leak: 1 - 22 2 - 10 3 - 22 4 - 10

The only audible hiss was through the oil breather cap opening for one and three. Obviously the head needs a bit of work and maybe rings depending on how loose one likes to turn the bottom end.

Using 100% as a baseline lets say the ideal compression is 185 with the leak at 5. The above numbers indicate the compression is down 9.5%. 1.75 / 185 = 9.5%. The average leak for all four is 11%. So the average leak is just under ten percent at 9.75%.

Certainly I am aware the amount of power loss can vary depending on how bad a particular area is deficient as one could have good head readings and poor ring readings. Therefore I ask the following question based on an average to get a general feeling how much one may be down and to the point of rebuild.

So heres the question. If a motor was to dyno at 115 at 100% capacity and later in the season dyno with the above figures would the 9.75% average loss equate to an equal amount of horsepower loss meaning 115 is now 104 rounded? While this appears to make sense in ones thinking Im not sure it does in reality. Perhaps some of the builders on here have run such test who might share their expertise such as, do we lose more power if the rings or head is bad or

Thanks all,
Jim

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Jim:
At the risk of sounding like an idiot (or like a smarty ass giving an idiot lesson), what was the air pressure you were applying, and what was the leak rate by cylinder?

As for your base question, I'll defer to the numerous engineers lurking hereabouts to give you a definitive answer, but the general answer is "no". A X% loss does not equal a X% HP loss. As I understand it, the proportionality of HP loss to the leakdown percentage, shifts as the leakdown rate worsens, increasing disproportionately as the leakdown rate gets worse.
Rick

--------------------
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Jim Venable
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Rick,

The pressure was 100.
Looking at my post I see the leak numbers are a bit jumbled. Do keep in mind this is the first ever leak test I've performed so if I'm answering incorrectly do let me know. The leak rate figures are.

1 = 22%
2 = 10%
3 + 22%
4 = 10%

Jim

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Jim:
Those numbers are (unfortunately) indicative of a new engine in your future. Assuming you're just getting into racing WTW and it runs okay (and doesn't use oil), I'd run it this season and plan on a rebuild - or a new motor this winter.
Rick

--------------------
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Keith in WA Verified Driver Made Donation to Website
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Before giving up on the motor, I'd ask two questions:
1) Are you using a leakdown tester where you're confident it's accurate. If it's Harbor Freight, I'd start with a better tool before I bought a better motor.

2) Are you sure the engine is at TDC? The timing marks on these cars can be way off. A few people have had a few degrees off. Mine is a whopping 9* off for some unknown reason.

--------------------
Keith Novak
(Will work for tires)

Jim Venable
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Rick,
Pretty much knew the motor had a limp. Still runs fairly decent. Motor will have to wait until around mid-season as we're completing a new build after the car disagreeing with a wall last year at season's end.

Keith,
Not a Harbor Freight but rather a ProForm. Definitely at top dead center as I used a dial for one test and the that looks like TDC eyeball method for accuracy should the need arise at the track for the second test.

My original post stated the compression was down 9.5% when it's actually 5.4% so the overall numbers are off some. Still, no matter what the little bugger is in need of medication for it's head.

So to all I ask, does a 5.4% compression loss with no visible exhaust sound when performing the leak down test warrant re-ringing as well?

Jim

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quote:
Originally posted by Jim Venable:
Rick,
Pretty much knew the motor had a limp. Still runs fairly decent. Motor will have to wait until around mid-season as we're completing a new build after the car disagreeing with a wall last year at season's end.

Keith,
Not a Harbor Freight but rather a ProForm. Definitely at top dead center as I used a dial for one test and the that looks like TDC eyeball method for accuracy should the need arise at the track for the second test.

My original post stated the compression was down 9.5% when it's actually 5.4% so the overall numbers are off some. Still, no matter what the little bugger is in need of medication for it's head.

So to all I ask, does a 5.4% compression loss with no visible exhaust sound when performing the leak down test warrant re-ringing as well?

Jim

Simply, yes.

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Jim Venable
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Simply, yes.

Ouch! Not what I was hoping to hear but understand.

For future reference and keeping in mind we are not competing at the national level, does anyone know what is an acceptable percentage loss or compression number before having to rebuild.

Jim

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I've been told greater than 3% is time for a refresh. And I was not able to get a leakdown number at anything other than TDC. Off just a little bit and the sucker will spin itself around.

--------------------
Rich Wiese

Spec Wrecker Ford

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2) Are you sure the engine is at TDC? The timing marks on these cars can be way off. A few people have had a few degrees off. Mine is a whopping 9* off for some unknown reason. [/QB][/QUOTE]

Kind of a highjack but how'd you figure that one out?

--------------------
Matthew F. Davis
Texas Region SCCA

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I found it while my head was off. I turned the pistons to TDC by eye and then to the marks and the marks were nowhere close. Then I made a couple of WTF phone calls. [Smile]

With the head on, you can pull the #1 spark plug and put a screwdriver or a socket extension down the hole so it's resting on the top of the piston and turn the crank until you find the high point. The last few degrees off TDC, your high tech measurement rod doesn't move much but if you rock the crank about 20* +/- either way you can zero in on TDC. (Thanks Ken!...Sorry if I interupted dinner.)

To say you're rocking the crank back and forth an equal ammount, you can take a scale (accurate ruler) and make some marks on the pulley the same distance away on each side from where you think TDC is to use as reference points.

Once I found TDC, I noted where it was carefully in my shop manual compared to the timing marks, and also put a little nick in the pully and the flange on the oil pan (very bottom of the pulley) with a Dremell tool cutting wheel so I can check if things changed. The cutting wheel could nick both parts at the same time so I know if I'm lined up if my harmonic damper is gradually twisting or something.

--------------------
Keith Novak
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TORacing
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Since no one has asked...did you perform the test(s) at operating temps? This is a crucial detail as you need the pistons/rings to expand and seal properly. Some say you also should do it statically, moving the crank just a bit to ensure of your TDC...of course if you move it too far...well the air pressure will take over. Tommy O

Jim Venable
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Yes, each test was performed at operating temperature. Did the rocking back and forth while being careful not to rock too far so as not to seat the rings incorrectly. The bottom line is the angels are calling this one home so we'll run it a few races before saying our good-byes.

To everyone, thanks for the education.

Jim

   

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