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post #1 of 14 (permalink) Old 05-31-2016, 06:12 AM Thread Starter
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Spec for compression

When you run a compression test on a healthy 451 engine, what would the readings be? 150 pounds? 160? I haven't been able to find the answer anywhere on the internet. Surely the specs for a new healthy engine could be found somehwere.
Here on the forum a search of compression test gives a whole bunch of scenarios with readings ranging from 125 to 200 lbs for reading that people got when testing their engines, so I haven't a clue as to what it should be. DCO
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post #2 of 14 (permalink) Old 05-31-2016, 08:55 AM
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alldatadiy.com says....



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post #3 of 14 (permalink) Old 05-31-2016, 09:06 AM
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That 15 bar converts to 217 psi, which sounds high to me....
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post #4 of 14 (permalink) Old 05-31-2016, 09:33 AM
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Originally Posted by jwight View Post
That 15 bar converts to 217 psi, which sounds high to me....
That sounds a little high to me as well, but remember, it's a 10:1 compression engine. At the lower limit it's 159 psi.

I know on my Dodge turbo motors we like to see 150-160 psi, and they are only 8.4:1 compression, so 217 isn't really out of the question...
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post #5 of 14 (permalink) Old 05-31-2016, 01:12 PM Thread Starter
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I wanted to get to the bottom of it so I bit the bullet and called my "not so local" Smart center and they connected me with their head Smart mechanic and I asked him the same question. He quoted me exactly what JWIGHT said, but he too said that seemed a little high.
So now I'm gonna spill the beans......Before I tore my 451 down to get the valves and guides replaced my commpression readings from cyl 1 to 3 were 155-150-105. So I had the head done with all new guides, valves and seals. I assembled it and it seemed to run okay. I haven't driven it much yet. Now just for the heck of it I thought I'm gonna pull out those plugs and see what the compression readings are now. So I did and I got 120-120-120. I was flabbergasted. Now I re-tested again with another compression gauge just to be sure and I got 120-125-130. So since they are pretty consistent I would "assume" the valves are right and adjusted properly. So with 36,000 miles and the cylinder look great with no scarring and there's no smoking and no noticable blow by of any kind. So that's why I asked because I figured 150-160 was par for the course, but after reserching it here nobody quotes a compression test reading of 217 pounds. Now my original 155-150-105 was on a hot engine and my latest test was on a cold one,but I doubt hot to cold would cause 30 pounds difference. So I'm disappointed and anxious and don't know what to do next. If I drive it like this and the readings are indeed acurate the power is bound to be lower. And secondly I don't like running something that's not right. So maybe I'll just go and buy a new compression gauge and test it warm just for my peace of mind.
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post #6 of 14 (permalink) Old 05-31-2016, 01:24 PM
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Leakdown test?

Before the head was replaced on our 2011 the technician found cylinders 1 and 3 leaking 5%, but cylinder 2 leaking 75% - hence the P0302 code. Might want to check for leaks as well.
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post #7 of 14 (permalink) Old 05-31-2016, 01:58 PM
 
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Just my 2 cents if the valve seats were reground this could create a compression lose, and it has been my experience that a warm engine does relate to higher compression.Smart should advise you what is proper procedure to checking compression.
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post #8 of 14 (permalink) Old 05-31-2016, 02:47 PM
 
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What about carbon deposits within the combustion chamber? If there were deposits when you tore the engine down, this would cause the reading to be higher. I wouldn't worry about the readings that you recorded. If the car runs well, drive it and enjoy it.
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post #9 of 14 (permalink) Old 06-01-2016, 12:05 PM
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If you just put the engine back together then I would not get too wound up about the compression readings. I would drive the car awhile and see how it runs. After a few hundreds of miles to get all the new parts familiar with each other, then test the compression with the engine warm and the throttle fully open.
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post #10 of 14 (permalink) Old 06-01-2016, 01:02 PM Thread Starter
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Thank you Rusted Wrench. I was waiting to hear from you. Smart Bob said the same thing. I'll take your advice and finish buttoning it up and drive and see how it goes. The underneath is so clean and shiney that I hate like hell to hit that first mudhole or drive it on a rainy day. You might read in the paper that I hit myself in the head with my crutches......twice. Thanks everybody !
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