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Old 01-22-2012, 10:07 PM   #1
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Default Engine compression and altitude.

Hey guys. So I'm about to pull my motor to fix a few horrible oil leaks. While I'm in there the plan is just to clean it up, replace all the main seals, front main, rear main, valve cover, water pump/timing belt, oil pan, CAS, etc. I also figured before I do all this work I wanted to see how healthy this motor is, and if its weak I could justify a rebuild. Now its plenty quick, doesn't smoke, and generally runs badass. Power wise on 10psi tuned by myself with a fairly conservative timing map I pulled on my buddies bolt on and tuned 2007 wrx with 270whp 350ft lb by about 3-4 car lengths by 110 or so. Just to give you an idea.

The motor has 170k on it now, about 40k miles on new rings, 10-15k boosted, but stock bottom end. So I went ahead and ran a compression test on this motor before pulling it and this is what I got from front to back.

Cylinder | 1 | 2 | 3 | 4 |
Dry PSI | 123 | 122 | 125 | 125 |
Wet PSI | 133 | 130 | 132 | 130 |

These results were recorded with all spark plugs removed, a well charged battery, injectors and coil unplugged, and at WOT. The wet results were the same, but i added a cap of oil to each cylinder before testing.

At first I was pretty upset but the numbers were so consistent i wasn't sure what to think, then I remembered. Duh altitude. Altitude will change your compression numbers - of course. The same reason a NA car is down 25% horsepower at this altitude, the compression will read low.

My shop where i ran the comp test is at around 7,500 ft elevation, so i start researching on Google i found quite a few people reference this chart for correcting your compression test results for altitude.

Elevation Multiply by:
1000' ----- 1.029
2000' ----- 1.060
3000' ----- 1.093
4000' ----- 1.126
5000' ----- 1.160
6000' ----- 1.196
7000' - ---- 1.233
8000' ----- 1.272

Using this I can calculate my numbers out, i used the number 1.2500 and calculated.

Cylinder | 1 | 2 | 3 | 4 |
Dry PSI | 154 | 153 | 156 | 156 |
Wet PSI | 166 | 163 | 165 | 163 |

Much better. Still not really ideal, but acceptable I think. But what do you guys think? It's a cheap harbor freight gauge, but it tested out within 5 psi of another cheap harbor freight gauge i have on my air compressor. I really don't know what that says for its accuracy though.

So the question is, should i slap this motor back together with some new seals, tune for 15psi on my greddy and go on with life? That's what I'm thinking about doing right now, then when it finally goes out for good I'll take that time to swap a 1.8 and build a turbo kit for it. Maybe I'm over thinking this.
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Old 01-22-2012, 10:23 PM   #2
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What compression ratio is your engine?

I live in Michigan and at 112k before the first rebuild I was seeing 185 on all 4.
I got 200psi on all four after a schitty hone, new rings and a valve job.

150's is pretty worn out. Have the block checked and if cylinders are still in spec, give it a hone and rings, and get a 3 angle on the head.

Last edited by flounder; 01-25-2012 at 11:05 PM.
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Old 01-23-2012, 12:06 PM   #3
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What compression rato is your engine?

I live in Michigan and at 112k before the first rebuild I was seeing 185 on all 4.
I got 200psi on all four after a schitty hone, new rings and a valve job.

150's is pretty worn out. Have the block checked and if cylinders are still in spec, give it a hone and rings, and get a 3 angle on the head.
Stock as can be 1.6l that came with a manual trans, so 9.5:1.

I should probably just do a leakdown test. But that requires me buying a tester, blehhhh. I may see if VatoZone will rent me out one.

The numbers are so consistent, I may just fix the leaks and boost it until it pops, hopefully lasting me this summer. Then I can 1.8 swap with some upgraded rods, bigger turbo, and the works.
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Old 01-25-2012, 12:31 PM   #4
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Nobody local here rents out leakdown testers.

So.. I think I'm just gonna fix these leaks, drop this motor back in the car, and boost it until it blows the ---- up/starts spewing oil out the breathers.

I'm also gonna DD it all summer. Don't worry though I have a few backup cars.

I will keep you guys posted.
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Old 09-24-2012, 03:47 PM   #5
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Funny thing, I just pulled a 1.8 out of a 99 NB. before I pulled the motor I did a compression test using the same gauge. The numbers were almost identical to my 1.6 results, 120's across the board.. not a big change when tested wet.

Either my gauge is bunk, or both engines have degraded in compression equally across cylinders. I'm banking on my HF compression gauge being off.

Anyways I'm going to drop the 99 motor in as-is, drive it, then build it down the road. I drove the car before i pulled the motor and the difference in low end torque is HUGE between my 1990 1.6 motor and this 99 1.8. I'm so excited for the swap and new turbo setup!
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Old 09-24-2012, 03:52 PM   #6
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You have the HF comp tester, just turn it into a leak down tester.
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Old 09-24-2012, 04:29 PM   #7
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You have the HF comp tester, just turn it into a leak down tester.
Details? I've seen people build leakdown testers before. Anyways it doesn't really matter to me at the moment, motor ran excellent with no smoke before i pulled it. Good enough for me
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Old 09-24-2012, 04:33 PM   #8
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I believe there is a write up somewhere on how to do it.
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