bcdavis

Another thing to consider:

If you feel that it is a problem, and decide to pull the engine, and rebuild it,
that will be a lot of cost, time, etc... So if you put the supercharger on as is,
the worst case scenario is that you have to pull the engine, and rebuild it.
So why not just put it on, and hope for the best? If you are lucky, nothing
will happen, and you will have a lot of fun. If something goes wrong, then
you are just back where you started, and in the future rebuild, you can
build it with boost in mind...

Sharks

bcdavis, I was thinking the same thing, but I will give it one more shot just for piece of mind. Either way I am going to install the SC. If I blow the engine I would rebuild it and drive the auto trans car in the meantime. I love the fact that I can have two 928's (maybe even three) for less than the cost of a new Mustang, and they are more fun to drive.

Like a wise man once told me ... all it takes is time and money .

Fogey1

I've used both and I'll never go back to the "push on" tester. I'm a firm believer in the "screw it in" type. A piece of crud would be worse with a push and hold.

A good one should have a quick connect on it so you can screw it in with just the hose, like starting a spark plug at the bottom of a hole, and then snap on the guage.

As for the numbers, I've done them cold and hot, open throttle and closed and I care more about evenness than absolutes. Lots of experts say #'s w/in 15% are OK. You miss that. :-(

139 sounds like about 9-1 to me, though I've never read a scientific test on what you see with a given CR, just speculation and guesstimate swags like mine. Anyway, not necessarily bad.

So ... DON"T PANIC!

SharkSkin

Worst that could happen is you score the block in the process of blowing the motor. Spinning a rod bearing wouldn't be pretty either. You could save some money by bringing the motor up to par initially, but sometimes you just have to say WTF...

FlyingDog

...and a couple drops on the pressure tester O-ring.

Ian928

You are sure your engine is not a Euro/Australian spec? In that case, the compression is much lower, spec is 9,3:1. Engine code for these engines are M44/45

Warren928

Sharks,
If your not getting smoke from the exhaust pipe after running it for 10 minutes or oily crap in your radiator reservior tank, I would guess the 240 hp "problem" would have something to do with a MAF, fuel delivery, etc. Not the engine compression.
Generally the engine will show signs of a problem like low compression in other ways.

Sharks

Update:
I added 20cc’s of Marvel Mystery Oil to each cylinder (I think it just trans fluid with a different scent), put everything back together and drove it for about 100 miles in the last couple day. I purchased a new “Professional” compression gauge for about $40 made by Actron and retested it today:

The good news:

I got 174 +/- 4 psi for all of the cylinders except #7.

The bad news:

Cylinder #7 came in at 120 psi.

I installed Andrews gauge again to check for accuracy and it read about 45 psi lower in the good cylinders. As some of you said, it is the variation, not the absolute numbers as gauges are different.

I will be pulling the valve coverers off to paint/powercoat them during the SC install and I will check valve clearances and timing (paying particular attention to cylinder #7). Thanks for the help guys.

jeff jackson

My guess is ...you need a valve job on that head. More than likely, #7 cylinder has "leakage" past the seats . Plug the Exhaust...then the intake...and retest #7 cylinder when warm...See if the numbers change. OR...just have a mechanic do a leakdown test. Either way...you'll get definitive answers.

FlyingDog

Euro S3s are M28.45/46. I wonder what the true compression is since often 10:1 S4s are really 9.3-9.4:1.

Sharks

Matt/Ian, thanks for reminding me.
My engine is stamped M28/43 so not a Euro as far as I can tell.

Sharks

Jeff, can you be more specific as to how I would plug the intake ... exhaust?
When you say plug the intakes (stuff with rags, ...) and rerun, then plug exhaust tailpipe and rerun?
Or both at the same time and rerun?

jeff jackson

A couple different ways to go about this...basically a "leakdown test"...would be the next logical step to determine if the low cylinder pressure in #7 cylinder is due to worn or damaged compression rings (or piston ringlands)...or as I suspect...valve seat/face leakage. And at the risk of taking the "easy way out" that would be my reccomendation. You can rent the Leakdown tester, at most Rental supply stores, and do a search for leakdown test procedures then easily do this test yourself, or pay a competent mechanic to do this test for you. There are also other more "primitive" ways to isolate the exact location of the pressure leakage in this cylinder...but for the most accurate and reliable results (and this is important, as the repairs you need will be determined by this tests results)...I suggest you rent the leakdown tester, and either do the test yourself, or have a friend familiar with the test help you do it. I'll leave it to others here to explain your options regarding capping the MAS sensor and exhaust and othertypes of tests that can be done at home....

Sharks

Thanks Jeff.

Looks like I will be holding off on the SC install until I run a leakdown test. A few more days to get a better idea of what is wrong in #7 is defiantly worth the time spent now so I know what to look for when I pull the valve covers.

I have a compressor and may have all the parts to build a leakdown tester already. Thanks for all the help guys.

Imo000

There is a simple way to test that cylinder to determine if the leak is at the rings or at the valves. Here's how you do it:

Do a regular compression test on that cylinder, just likek you did in the past, except this time pour in a little bit (half a tes spoon) of that oil into the cylinder just before your do the comp. test. If the compression goes up alot, you'll know that the leak is at the rings and not the valves. If the numbers hardly change then probably the a valve or valves are leaking.

In either case, you have two oprions. Install the SC and drive it untill this becomes a problem. The SC will compensate for the low compression, so the engine shuold run fine as long as you don't get way to much blow by. OR you will have to remove the head and have the problem solved. If you decided to go with the second option, then migh as well pull the other head off and do a complete refurbishing or rebuilding (refurishing is alot less $$$) of the top end.

I hope this helps!