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Strange Compression and Leakdown numbers

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#1
Arkymiata

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  • One of the members of our race team acquired a 2001 VVT Spec Miata. He has no engine history on the car. The car seems down on power. I suggested that he do a compression and leakdown test. In general his compression averaged 175 lb,. with a 4% deviation. The leakdown test only showed a small average leakdown of 2%. I believe the compressions of a VVT should be 200+, but shouldn't the leakdowns be a lot more due to the compression being so low. Has anyone seen this?


#2
Dennis_

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How accurate is your gauge? Put it on the dyno and see how down on power you are.. 



#3
Steve Scheifler

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I agree, compression & leakdown are at odds with each other. But there are more ways to get lower/worse than actual compression numbers than lower/better than actual leak-down. Chances are that’s not the issue. However, there are multiple reasons the VVT will fail and that definitely will change the power. As Dennis mentioned, if there’s an SM experienced dyno in the area go to them, if it’s VVT or something else significant that will be obvious. And even if it’s perfectly healthy it’s always a good idea to get some solid baseline runs recorded for future reference.
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#4
FTodaro

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The best way to evaluate any car is starting with a know bench mark. Its all relative after that. Compression gauges are notorious for miss information, but i have never had numbers above 190 on anything miata and who knows what the real number was.


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#5
Arkymiata

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The gauges are accurate as I use them on my car and they produce accurate results. I thought about it for a while and got to wondering if VVT head was mated with a NA8 block by the prior owner. The difference in Compression ratio of the NA8 pistons mated with the 2001 VVT Head I believe would explain the issue. The car is not in my garage I'll report back what I find. 



#6
Dave D.

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We have a VVT in our stable that just whistled 9.7 in Daytona and was in the 170's for compression. Made decent power,nothing stellar but ran well enough. Engine was originally built for 12hr endurance races, so builder went for reliability and left some on the table.  Many variables with compression numbers alone, cranking RPM, throttle open/closed, gauges vary make to make,etc...



#7
JNJ

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For compression and leak down numbers to mean anything they must be performed on a warm engine.  The rings and valves are sticky when they are cold and the piston to wall clearance is greater when cold allowing the piston to rock more affecting the ring seal.  There are many more things but….  Cold numbers are meaningless; don’t do it and expect anything you can count on.

 

First, compression numbers are testing the engines ability to pump, period.  The numbers are static at cranking speed but you can check compression while the engine is running.  It gives better results, you will get very low numbers at idle but you will see it jump when the throttle opens.  Like when a NB intake valve recedes, the idle number will be low, like 50 or 70 but it comes up at high RPM.  So the engine will misfire at idle buy fire, at reduced power, with some throttle.  The idle numbers are low because you are compressing a vacuum, this is why the plugs need to be removed and/or the throttle should be open. 

 

Leak down tested the pumps ability to hold pressure, period.  Take a 2% motor with good compression numbers and move the cam by one tooth.  The leak down will still be 2% but the compression numbers will be lower.  Cheaters please note that the Mazda Speed intake cam and altering the cam timing will have this same affect so your numbers will be lower than your legit counterparts that you are comparing to.  Heads at minimum spec will pump lower than taller heads of the same combustion volume due to the cam timing being later.  There are more things but I am sure you get it.



#8
Steve Scheifler

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I’ll have to disagree on leakdown being pointless cold. I do 95% of them at room temperature. I don’t have to hurry, conditions remain constant, numbers are representative and I can identify where excess leakage is going. We aren’t running forged pistons that expand significantly when hot or special rings; a good cylinder will still hold at 2% or less every time. If a low reading *might* be sticky rings a warm but cooling engine won’t necessarily change that, and if readings change after spinning a few times or popping the valves you have one more variable in the mix.

As for doing compression tests on a running engine, perhaps there’s some scenario where that makes sense but it’s not something I’ve ever needed to do.
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#9
JNJ

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Steve, feel free to disagree but it would not be wise to pull down an engine based on cold leak down numbers just as it would for cold dyno numbers.  before we go into the weeds, we are suppose to be helping someone with a question/concern and he has 2% numbers and 170 ish compression so if his numbers are good, they are good.  BTW, there is no spec for the orifice between the two pressure gauges.  Usually both gauges are at 100 PSI with no leak (but can be any pressure) and the difference comes from the choked flow of the orifice.  The flow is tied to the leak down numbers and is somewhat ambiguous and arbitrary.  The using 100 PSI and 100 PSI for the no leak condition will give you the percent in pressure drop of the choke from the orifice.  If your leak down gauge has a big orifice, it will give you good numbers and a small one will give the same engine bad ones.  BTW orifice sizes are usually 0.040" (actually 1.0 mm or 0.03937") and that is too big for a 3" bore.  it is better suited to a 5" bore but there is no standard.  Also, the hose length matter to some degree too.  I could write a lot more about his but you get the drift.  It would be better to make your own or al lease reduce the orifice size.  That would be unless you want all the numbers to be good.



#10
Steve Scheifler

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No, I really don’t get the drift. First, this old topic suddenly got revived in the middle of a similar one and so my comments are somewhat jumbled between the two because I hadn’t noticed. Regardless, I never recommended a teardown based on cold leakdown numbers alone, and in fact recommended ways to further diagnose. If all the numbers are nearly the same and low, and the person testing is experienced in identifying where it goes and how much, then done, the cold test was fast, simple and definitive. None of your other factors come into play for that. My question about PSI test pressure was simply because some old testers used a lower pressure which often didn’t expand even good rings, 100 psi is generally adequate for our purposes. When, as in one of these threads, some cylinders are dramatically different than others then again none of those other factors about orifice size etc. are particularly relevant, it’s the difference that needs to be explained not the absolute values. Then it’s time to identify where the air is going and consider additional testing steps to verify your results and identify the cause. A hot test is one such option but it won’t necessarily look any different or tell you anything new. It may, or it may not. The overwhelming majority of the time I have what I need in a matter of minutes without burning my fingers or rushing to test while still hot. If you have better success doing it differently that’s fine, but blanket statements I read in one of these about cold numbers being worthless are patently false. I think that was my only real objection.
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#11
JNJ

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Steve,  I didn't say they were worthless, that was someone else.  I did say, however; I wouldn't judge a cylinder bad by a cold number.  You pretty much agreed with me so there is no issue.  I have a lot of experience with cylinder bore finish, rings and many other very specific engine information.  I don't want to get into a spat with someone that has 2,700 posts as I don't have the time and energy for that.  I hesitate to post for this very reason, however, I was trying to share some knowledge her and help someone.  I am not looking for any business as I am done working and don't need the money.  I would suggest that you run your leak down gauge at 100 PSI and use an orifice of 0.040" or less.  Some aircraft gauges are 0.060 for over 5" bore and will call 3" bore good when it isn't.  the 0.040" orifice is better suited to a Chevy V8 with 4" plus bores.  

 

I worked on a project that the cylinder was barrel shaped, nominal at the top and bottom but big in the middle, due to the casting splitting around the liner.  Leak down is usually performed at TDC.  With the cams removed and a fixture to lock the ring gear, you can check at intervals.  as for getting low numbers with the right orifice, the block surface finish (Rpk, Rk,Rvk, see attachment) The hone angle matters too. 

 

This is from a very strong engine that I built for a friend, you will notice I cut the specs off.  This was hot honed with a plate, diamond hones on a CK 30, it has bore correcting capabilities.  Note the the waveyness profile is in uin so it is very straight.  When I mentioned checking the engine hot, I know haw much the cylinder expands at temp.  The has a 3X affect on ring end gap.  This engine got very good lead down numbers with a very small orifice, hot that is.  Too bad, I wasn't allowed to post the image, PM me and I will send it.






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