Ideal compression for a 1.8t?

[lookaught]

I searched and did not find a thread specific to this, so I thought I'd start one for the benefit of both myself and future AZ searches.

What is considered "ideal" compression for a 1.8t? Ive seen numbers from 170 to 205psi.

I tested my car this weekend and got (90,300 miles):

1 - 190
2 - 185
3 - 190
4 - 190

The numbers are a bit rough as my compression tester is Equus brand and isnt as precise at some of the higher end tools. I was more just interested to see if any cylinder was really straying from the rest.

My research while STFA seems to show a trend of cylinder #2 being the typical one to show lower compression. Im not sure why, but most cars seem to have matching numbers for 1, 3, & 4 and lower numbers for #2.

I havent done a leakdown yet, I need to buy a tester first. Based on the compression test alone it appears my car is in pretty good shape for the miles, which is good, considering a GTRS kit will be added in the spring to celebrate 100,000 miles.

There was very little fuel in my last oil analysis so I don't think that blowby is a problem.

Any comments?

[dougyfresh]

Compression numbers look good for the 1.8T and that mileage. Did you do a compression check a few times? I typically run through the cylinders twice to compare my results. I think mine were 170-180 with 120k on the old motor. Leakdown was another story...

Here's come compression info: http://www.audigeeks.com/forums/index.php?topic=2769.0

[lookaught]

That link is perfect, very nice writeup. I did check each cylinder twice. The first test on cylinder #2 the o-ring leaked a bit (which I heard) and it read only 165psi, which caused a slight heart palpitation before I rechecked it two more times and got 185 each time.

[A4ringedONE8T]

That was on how many cranks? A solid motor should hit 160-170+ by the 3rd crank. As Doug said, check leakdown as well. You can commonly have good compression but have horrible leakdown results. We just rebuilt a 1.8T here at the shop, had perfect compression but had 85% leakdown @ 10psi on cyl 3 & 4

[absolutegtr]

I tested my compression at 100K....I am at 105K now

Just did the compression test. Seems like GREAT results.

Motor has 99,515 miles on it.

195, 195, 195, 200 respectively

[dougyfresh]

That was on how many cranks? A solid motor should hit 160-170+ by the 3rd crank. As Doug said, check leakdown as well. You can commonly have good compression but have horrible leakdown results. We just rebuilt a 1.8T here at the shop, had perfect compression but had 85% leakdown @ 10psi on cyl 3 & 4

Sounds like my old motor. You sure that car doesn't have my old motor in it? haha. I doubt it seeing as Clint has the block and the pistons & rods are in my basement and the head is on a car out in San Francisco.



I typically crank the motor until the needle stops moving. It usually results in more than 3 cranks though.

[A4ringedONE8T]

Sounds like my old motor. You sure that car doesn't have my old motor in it? haha. I doubt it seeing as Clint has the block and the pistons & rods are in my basement and the head is on a car out in San Francisco.

I typically crank the motor until the needle stops moving. It usually results in more than 3 cranks though.

haha I know, the motor was overheating like crazy as well, I wonder why?

I used to crank until the needle stopped as well, Adrian pointed out to me that even a motor on its way out can eventually hit the target numbers and more could be told by how qucikly pressure built with just a few cranks. May or may not be the truth but definitely what I go by now!

[lookaught]

That was on how many cranks? A solid motor should hit 160-170+ by the 3rd crank.

Probably seven cranks until it was clearly topped out at max compression. It was getting up quickly and the last 3-4 cranks just moved the needle up the final 10psi or so.

I plan to have it leakdown tested, Im just debating over buying my own tester for $85 or paying my indie mechanic to test it for about the same. The leakdown test is a bit more involved and subjective if something is wrong so I'm kinda leaning toward having the mechanic do it so he can apply 30 years of experience.

[A4ringedONE8T]

Probably seven cranks until it was clearly topped out at max compression. It was getting up quickly and the last 3-4 cranks just moved the needle up the final 10psi or so.

I plan to have it leakdown tested, Im just debating over buying my own tester for $85 or paying my indie mechanic to test it for about the same. The leakdown test is a bit more involved and subjective if something is wrong so I'm kinda leaning toward having the mechanic do it so he can apply 30 years of experience.

However you have us here to help you who have experienced all that crap as well FYI, Habor Freight sells some cheape leakdown testers which actually work pretty well. We have one here at the shop as a back up and its actually decent for the money. IIRC less than 50 bucks

[dougyfresh]

I have the Harbor Freight leak-down tester but I couldn't really get good results on it. Most likely was not using it correctly and didn't have the time to dedicate to figure it out 100%.

In the end, I wound up paying an indie place (Jon.... European Car, the place I told you about) to do the leak-down test for me. Also went to a few other places to get independent results for comparision.

[A4ringedONE8T]

I have the Harbor Freight leak-down tester but I couldn't really get good results on it. Most likely was not using it correctly and didn't have the time to dedicate to figure it out 100%.

In the end, I wound up paying an indie place (Jon.... European Car, the place I told you about) to do the leak-down test for me. Also went to a few other places to get independent results for comparision.

Good to know, we always verify bad results with the upper end tester as well as the cheape, since the car I was talking about was my responsibility, I personally used both not 2 weeks ago, and both showed nearly the same results. The Harbor Freight tester does commonly leave the adaptor screwed into the plug hole as everything with it is generic and uses adaptors everywhere

[lookaught]

In the end, I wound up paying an indie place (Jon.... European Car, the place I told you about) to do the leak-down test for me. Also went to a few other places to get independent results for comparision.

I trust the mechanic that I use 100%. He builds and maintains Porsche racecars in addition to regular maintenance on German cars. I actually just called him and scheduled an appointment for next Monday since I'll be off work. This is one test that I can't afford to mess up, not when the consequences could be a engine rebuilt that I do not care to undertake.

If it comes back all clean and clear = BT here I come
If it comes back bad... = no BT, drive car for a few years and get something different, maybe a B6 S4 avant seeing that the prices have fallen so far.

I'll let you all know how it goes.

[A4ringedONE8T]

Understandable, good luck and keep us posted!

[dougyfresh]

Sounds good. I concur with your approach, Jon. Keep us informed of how things go.

[mike-2ptzero]

That was on how many cranks? A solid motor should hit 160-170+ by the 3rd crank. As Doug said, check leakdown as well. You can commonly have good compression but have horrible leakdown results. We just rebuilt a 1.8T here at the shop, had perfect compression but had 85% leakdown @ 10psi on cyl 3 & 4

That is because high amounts of carbon build up can cause the compression test to read higher then it would if there was no carbon build up.

[J-jizzle]

What is leakdown?

[A4ringedONE8T]

What is leakdown?

Leakdown test is a setup that connects to a compressor air line and screws in the plugs holes. You set the cyl you are testing to TDC (or the combustion stroke) so the valves are all sealed. The leakdown tester has an air pressure guage, a regulator, and then a predetermined "leakdown" gauge with percentages on it. You start low and slowly increase the pressure regulator while watching the leakdown gauge. You want to typically use low pressure, 5-10psi but it never hurts to go up to what your max boost will be for a forced induction application. The higher pressure you go to, the more "leakdown" you will get but you want it to stay within reason of the pressure being put into the cylinder. For our motors, @ 10psi, you should be losing arouund 10% of that pressure being applied, as you reach 20-25psi it will go up to about 15%. As the rings or valve seats start to go bad or are bad, the leakdown percentage will go way up to 50% plus. As I said earlier in the thread, I have seen a running motor have 85% leakdown @ only 10psi which is pretty bad for the car to still run!

[J-jizzle]

^^^Ok thanks. I might have to get Compression and leakdown tests done just to see where my car is at with it having 81k

[lookaught]

Alright I just got back from my mechanic and the leak down test results are (now at 90,700 miles):

1 - 2.5%
2 - 4.5%
3 - 3%
4 - 2%

Needless to say Im quite pleased with the results *high five!* and the BT upgrade will continue as planned! The car was tested at 100psi inlet pressure, losing a few psi to the high 90's.

Woot!

EDIT: it was also really interesting that my mechanic George just put the car in second and bumped it forward and backward little by little to get each cylinder to TDC. I wouldn't have thought of that, so I guess you really learn something helpful when you watch a pro. The actual testing was pretty simple, and I'll probably buy a tester for the future. It was one hour of labor and that included some other miscellaneous diagnosis of ticking and rattles. The serp tensioner is going bad as I suspected, and possibly the idle roller below it. I'll get to those as soon as time and motivation allows, probably when I put on the FMIC.

[dougyfresh]

Awesome! that is really good news!

[lookaught]

Awesome! that is really good news!

Yeah Im really really pleased with the results... as it also means I'll be keeping my car for a long time and will likewise not have a car payment for a long time as well! The B6 S4's were really looking tempting....

[Spinnetti]

Leakdown test is a setup that connects to a compressor air line and screws in the plugs holes. You set the cyl you are testing to TDC (or the combustion stroke) so the valves are all sealed. The leakdown tester has an air pressure guage, a regulator, and then a predetermined "leakdown" gauge with percentages on it. You start low and slowly increase the pressure regulator while watching the leakdown gauge. You want to typically use low pressure, 5-10psi but it never hurts to go up to what your max boost will be for a forced induction application. The higher pressure you go to, the more "leakdown" you will get but you want it to stay within reason of the pressure being put into the cylinder. For our motors, @ 10psi, you should be losing arouund 10% of that pressure being applied, as you reach 20-25psi it will go up to about 15%. As the rings or valve seats start to go bad or are bad, the leakdown percentage will go way up to 50% plus. As I said earlier in the thread, I have seen a running motor have 85% leakdown @ only 10psi which is pretty bad for the car to still run!

10% leakdown!? Yikes, that sounds like a lot.. my racy car is around 2, but its not force fed either, and uses total seal rings..

[A4ringedONE8T]

10% leakdown!? Yikes, that sounds like a lot.. my racy car is around 2, but its not force fed either, and uses total seal rings..

Increased cylinder pressure causes more leakdown

[Maverick]

I need to do a leak down test now. I had my engine compression tested and the result is 150 on all five cylinders and 140 on the other.
Maybe one of my piston rings is not sealed and I have a lot of blow bys.
I will see.

[Hamotee]

Great info!