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Old 07-12-2014, 02:33 PM   #1
cbazq271
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Default low compression on a new motor???

Hi, I just built my new motor over the winter. It only has 500 miles on it and I decided to do a compression check on it , just to see how its doing. I checked all 4, after I let the car warm up a little. Just idling in the driveway for few minutes. I got 130 psi on all 4, with a really nice snap-on gauge. Is this normal for a motor with this low miles, because the service book says it should be 160 - 190 psi. The motor has never run bad or made any funny sounds. Both water temp and oil temps never to high. This is my motor build and everything was bought new and has 500 miles on it.

New oem short block
New oem heads
Manley 8.5-1 drop-in pistons
Brian Crower stage 2 cams
Brian Crower complete valve train
Then the basic stage two stuff, exhaust, intake, header

I just want to know if this is normal for a low miles motor and will the compression increase with a little more time on the motor. Thanks
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Old 07-12-2014, 02:43 PM   #2
Juice66
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Its a new motor!!!! 130 isnt bad. Beat the piss out of it, then repeat but only if tune is good, of course. Beat the piss out of my new motor starting around 40 miles and my compression is through the roof. Do not do the easy break in. Not sure what manley uses for ring material but normally you have less than 100 miles to seat rings
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Old 07-13-2014, 04:40 PM   #3
ResidentR
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The important thing is that the compression is the same in all cylinders. Drive on!
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Old 07-14-2014, 01:05 PM   #4
danger1138
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Mine is at 121 psi on all 4 cyclinders. Performed it in march on a somewhat warm engine. Currently at 51k on built motor.
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Old 07-14-2014, 01:35 PM   #5
sexyyrex
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Quote:
Originally Posted by ResidentR View Post
The important thing is that the compression is the same in all cylinders. Drive on!
this.
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Old 08-26-2014, 04:14 PM   #6
SpongeBobSquarePants
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I just did this on my built motor with Manley pistons and got 135,135,135,145
Leakdown was 5%,5%,5%,1%
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Old 08-26-2014, 04:52 PM   #7
BeastianSTI
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Quote:
Originally Posted by cbazq271 View Post
Hi, I just built my new motor over the winter. It only has 500 miles on it and I decided to do a compression check on it , just to see how its doing. I checked all 4, after I let the car warm up a little. Just idling in the driveway for few minutes. I got 130 psi on all 4, with a really nice snap-on gauge. Is this normal for a motor with this low miles, because the service book says it should be 160 - 190 psi. The motor has never run bad or made any funny sounds. Both water temp and oil temps never to high. This is my motor build and everything was bought new and has 500 miles on it.

New oem short block
New oem heads
Manley 8.5-1 drop-in pistons
Brian Crower stage 2 cams
Brian Crower complete valve train
Then the basic stage two stuff, exhaust, intake, header

I just want to know if this is normal for a low miles motor and will the compression increase with a little more time on the motor. Thanks
Did you push the throttle to the floor while cranking it over? Also, it's even more important to make sure that the engine is warm with a built motor due to the thermal expansive properties of a 2618 slug.
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Old 08-26-2014, 08:24 PM   #8
v_khotsky
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I will agree with "Juice66" about break in procedure.
Mine was something about 140 with JE 8.2:1 pistons


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Old 08-28-2014, 12:35 AM   #9
mrsmithz9
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Keep in mind some test gauges read low, I had my 100 mile block tested at 130 across the board, then with a different gauge 160.

Your #'s look fine though, consistency is what matters.
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Old 09-08-2014, 08:19 PM   #10
BeastianSTI
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Mine @ 300 moles... After sitting for just under 2 hours right in at about 138 across all cylinders. Cp pistons btw


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Old 09-08-2014, 08:33 PM   #11
SpongeBobSquarePants
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Wow beastin that's a lot of moles

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Old 09-08-2014, 09:30 PM   #12
2milehi
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cbazq271, what elevation are you at?


This is not exact but it hopefully explains roughly what is going on.

Say you have a 9:1 compression motor at sea level (14.7 psi) your rough compression would be 9.0 * 14.7 = 132.

Now you are at Denver (12.0 psi) with 9:1 compression motor your rough compression would be 9.0*12.0 = 108.

There is some other non unity exponential stuff going on with compressing a gas, but I am not going to go that deep into it.
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Old 09-08-2014, 11:51 PM   #13
Steve-HMS
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Your compression numbers look fine, I wouldn't worry at all with those numbers on a low compression turbo motor. The book is wrong on those numbers, the only time I have ever seen 160psi on a turbo motor that was less than 9:1 was on a 150k high miles motor that had a huge amount of carbon buildup (and that car had detonation issues), I generally see the numbers you got on a healthy turbo motor.
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