Spoke with Metric Mechanic

[L84THSKY]

I decided to call Metric Mechanic today to ask about potential upgrades/options. I was speaking for about a half hour with a guy name Jim. He is the one who did all the write ups on the M42/44 engine, and owns a 318i.

It proved to be a very interesting conversation. basically I was interested in a performance head, since I may have a problem with mine. He told me swapping a head alone won't help much. When we got into why I was considering a head swap, I explained what the dealer told me when I last brought my car there. If anyone remembers, they said I had high leakdown on two cylinders. The first thing Jim said was, "I wouldn't put much consideration into a leak down test". What is most important is a compression test. He said a compression test will tell you if your cylinders are good. No revelations about doing a compression, but here is what surprised me. Jim said too high compression in a cylinder is BAD! In fact, our cars should NOT have compression over 200psi. Of course we don't know how accurate the dealer measured mine, or for anyone who did this themselves.

His numbers were like this. With a 10.5:1 ratio, times 16 or maybe 16.5, the compression should be about 180psi. He claims that if the cylinders have high carbon buildup, that would increase pressure, and be a bad thing. So in reality, too low OR too high is bad.

As far as potential valve problems, if my compression isn't low, I don't have a valve problem. If your rings are bad, he says you will get dirty oil very fast. He said to examine two things to check your engine:

1. Change the oil, and run the car for about 500-1000 miles. Open the oil cap, and check the color of the oil. If it's golden brown, your rings are fine, if it's black, your rings are bad. He claims the carbon gets around bad rings, and fouls up the oil, turning it black.

2. He says if your rings are bad, your crank case vent tube would be putting out high pressure. Check the vent to see what the pressure is.

The one thing he kept emphasizing, was to check the oil color. He said if I have good compression, and my car feels fine, your should be fine. Just gotta do that oil check test, next oil change.

Any comments?

[DustenT]

Where did he get 16 or 16.5 from? Atmosphere is 14.7.

10.5:1 = 154.35:14.7

I got 180psi on all 4 cylinders, and I was told that was great for a car with 140k miles on it (and a supercharger). I think it's more important to have equal compression on all cylinders than high or low. Higher compression usually indicates a healthier motor in general.

[L84THSKY]

This brings up a couple of questions:

Is your 180psi accurate, and would it have been higher 100K miles ago. He did state that 1 atm= 14.7, but he mentioned something about another factor at play, and used 16 or 16.5.

I agree that we all thought higher compression is better, but he claims 200+ could be a problem. In the end, I just wanna do his test, and see if my oil looks good. My brother bought me a nice compressor for my birthday, but even injecting 120 psi is not as good a test for leak down, as a true compression test.

If anyone would like to chat with Jim, please do. Would love to have something in writing that clears all this up.

Originally Posted by DustenT

Where did he get 16 or 16.5 from? Atmosphere is 14.7.

10.5:1 = 154.35:14.7

I got 180psi on all 4 cylinders, and I was told that was great for a car with 140k miles on it (and a supercharger). I think it's more important to have equal compression on all cylinders than high or low. Higher compression usually indicates a healthier motor in general.

[tastade]

I was looking around some, and if you search google for: compression ratio vs pressure or for: compression ratio vs cranking pressure you will find some interesting stuff. Basically that if the valves were closed fully during compression and if there are no restrictions on the intake (like restrictor plates) you should have compression ratio about = compression pressure * air pressure (so 10.5 * 14.7 =~ 154 psi).

However, cranking pressure doesn't always equal that because car designers can adjust valve timing and throttle position with different cam profiles to make it more or less than that. By changing the valve profile they can change the amount of air that gets sucked in and subsequently compressed. So we can't say the cranking pressure will always equal compression ratio * atmospheric pressure (I think I may have started that here a while back).

The point to be made is that if your pressure is too high you will be running into risk of detonation, but that can't really happen except by forced induction, changing your cams, or changing the bore/stroke of the engine. The carbon buildups are a problem, but I don't believe they would change your compression. Carbon buildups cause detonation because they remain hot (like a coal) and act like a spark plug when more fuel/air is sucked in and ignite the mixture early.

here is one of the better pages I found: http://www.chevyhiperformance.com/te...e_compression/

From what I have read a leakdown test is a superior way to diagnose engine problems than a compression test, when done right! Both tests (leakdown and compression) should be used in conjunction for best results. http://www.xs11.com/tips/misc/misc3.shtml
http://www.aa1car.com/library/leakdown.htm

[mohaughn]

I've always heard 15-20% of the stock spec reading is where you want to be. I'm pretty sure the haynes/chilton manuals call for something like 180. So 200 is starting to be in the high range. 210 is probably a bit high. Also keep in mind that atmospheric conditions and altitude can affect the readings. So without having a very accurate reading that can take all of the factors into play it's my opinion that when doing a DIY compression test you are looking for consistency. But, I have also heard that very high readings can be caused by carbon buildup.

[L84THSKY]

So some agree with what Jim said, and some don't.

Just need to get that TDC tool and I'll have everything I need to do a leakdown test.

Originally Posted by mohaughn

But, I have also heard that very high readings can be caused by carbon buildup.

[mohaughn]

I'd trust Jim on this one. He has years of experience building high end motors.

Here is a good pelican parts article on the issue. They say the same thing that Jim said.

http://www.pelicanparts.com/techarti..._rebuild-2.htm

In all practicality, all of the cylinders should be very close to each other (within about 5-10 psi). On a newly assembled and run-in motor, compression numbers are usually within this range. As the engine ages and certain parts wear faster than others, one or more cylinders may experience a bit more wear than the others. This will definitely show up in the compression tests. Needless to say, if you have all of your cylinders in the 150 psi range, and one cylinder is down around 120 psi, that should give you cause for concern. The important thing is to remember is that you want to gather consistent readings across all of the cylinders, without focusing on the actual values. If a reading is significantly off, go back and test that cylinder again to make sure that the measurement was not caused by some sort of fluke, which is often the case.