Leak down question
 

Hi,
What is more reliable source of judging an engine's life, a leak down or compression check? I did a compression check last year and everything looked good according to my old mechanic. I've recently moved and found another mechanic to do work on my car. I was having some backfire issues and loss of power in the upper rev zones. I took it in, as it was time for the annual checkup, and planned on having my webers rebuilt (again, it was time). The mechanic is a very nice guy and really seems to know what he's talking about. He's a Pcar guy that races them on the side. Anyhow, he did a leak down test and found some issues w/ the engine. The break down of the cylinders:

1: 6%
2: 9%
3: 7%
4: 17%
5: 90% !!!!!
6: 12%

I'm thinking it's time for a top-end job. However, when I mentioned this, the mechanic said I may as well have the case cracked open to check the rest of the engine and do a complete rebuild as it would be approximately another 4 hours worth of work. Your thoughts?

Thanks,
HB

If it were my car, then I would pick up the phone and call several people on the board that do this for a living - Henry Schmidt, Steve@Rennsport, John Walker and others on the west coast. There are a few on the east cost too. In the end, you may choose to use your local mechanic, but you will gain great insight by speaking with an expert or several experts.

You could run into some issues with an older 2.2 mag case that you would want to make sure were properly addressed. Plus, a bunch of potential updates. More education will help you make a informed and wise decision, not to mention reduce your risk and make you feel like you made the right investment. It WILL be worth your investment of time.

I assume he could tell you where the leak was from? intake, exhaust or other? and that the valves were all adjusted correct? 90% is pretty much an open cylinder!

Yeah the engine would be missing and you'd smell raw gas. Time to re-do the test.

Knowing the leakdown % on a cylinder isn't much help unless you know WHERE the leak is occurring.

But with those numbers, a top end overhaul appears to be needed regardless. Only 4 additional hours to do the case, crank, bearings and rods? Really??? What about subbed-out machine work on those items? Doesn't sound right to me.

The mechanic should have been able to tell you alot more based on those results.

Take it for a spirited drive and warm it up. Re-run test on #5, and check results again. This time listen to where it is leaking, out the intake or exhaust (valves) or breather (rings)
How many miles on this engine?

Thanks gents.

I was kinda shocked to hear 90% to be honest. I can't recall what the compression results were when I had it checked last year, but I do remember #5 being low.

Some more info. Prior to taking it in, I was having some trouble w/ engine backfiring through the exhaust. I tried to adjust the webers using a unisync. I got the carbs mostly balanced using the sync, but once I dropped the idle back down to 800 rpm, the backfiring would return after driving for about 10minutes. I had to increase the idle to ~1200-1400rpm to get it to run smoothly. Now that I think about it, the throttle response was kinda slow. Anyhow, I had planned on having the webers rebuilt and ordered 2 kits, as I was thinking originally, this was the problem. Now, I'm thinking of all kinds of potential issues, but I was thinking at worst, a burnt valve. According to Wayne's book, 90% would mean there's a hole in the cylinder!! I'm at a loss to explain why the car seems to pull fine after I made the carb adjustments if there is a 90% loss. It doesn't feel like it's running on 5 cylinders.

I'll ask him to recheck the tests especially on#5. I'll also ask him to tell me if he listened to where the leak may be coming from. Thanks again gents. These are questions I didn't think to ask.

It's late and I apologize for my scattered thoughts.

HB

Could be 90% with an open/bent valve. That would also help explain some backfiring. I would believe you would notice the performance degradation. If the mech. did not run the test right then it would be low as the piston moved down the bore.

4 hours? That sounds right for splitting the case and putting it back together without counting on cleaning the case and any machine work. Most shops will talk only about the time they are working on the engine.

I agree with recheck advice, however the low compression you remember on cylinder 5 from last year corresponds with the leak down test. If it is 90% the motor needs to come apart or it may come apart on its own .

It is premature to determine the extent of the work until you know what the problem is. My money is on a burnt exhaust valve . Who adjusted your valves last and how long ago? That said it l could be a broken ring/ring land - can you say mouse nest sitting on number 5 ?

IMHO if the leakdown is bad on retest the decision breaks down to this:

1. Tear down top and rebuild the bad head. Don't do it! With all the labor why only rebuild 1/6 of the top?

2. Do a full top end rebuild, measure everything, twice. Do if engine has <90 or 100 thousand miles.

3. Do a full teardown and rebuild, new bearings etc. if engine has many miles, poor maintenance, or a PO with unknown habits.

Only #1 is a poor choice, in that you invest money to have the engine removed and at a minimum half of the top end removed to only get 1/6 of the engine fixed up. Seems a poor use of the money and asking for further issues. Any time the top end is to be torn down it should be stripped down all the way and all parts inspected and measured to prevent the same labor needing to happen 3 months later.

you're not doing a leakdown on a regular engine

911 leak down #'s may seem bad but really aren't

take it to a 911 guy who knows the leak down game

Hi,
Leakdown tested again. #5 is still very poor. I'm having him drop the engine and take a closer look. Once that's done, I'm going over to see for myself the issues. If I have to do a rebuild and nothing is too bad(pistons, cylinders, etc), the cost is going to be about 6k including machine work(out of garage machine shop). While I'm not made of money, I'd rather go ahead and have things fixed the first time instead of dealing w/ another engine problem down the road.

I'll keep yall informed.

HB

If you are going to do a rebuild on the top make sure you check your headstuds to be sure they are steel, if not you might as well replace those at the same time and save yourself a broken stud later. How is the bottom end, you are spending all that time getting the top opened up, might be a good time to split the case and check the main bearing, IMS bearing, chain rails, rods, crank, also replace the rod bolts if you plan on tracking the car. I am going through the same thing right now, mine is an 86 coupe. There is a lot of "while you're this far" things to do. 4 hours to to open the crank and fix anything seems very unrealistic if done right, may want a second opinion

Did he confirm if it was leaking past a valve or rings/cylinder?

I measured 45% leakdown when I did my engine, tore it down and found this. . . .

http://forums.pelicanparts.com/uploa...1171291379.jpg

90% leakdown, I would expect a pencil-sized hole. Good luck, can't wait to see the photos!

Hi,
I'm at work right now, but the leak is from the exhaust valve... either bent or burnt. Once the tear down begins, I'll try to get some photos.

HB

Well, looking at those leakdown numbers I'd be inclined to pull all the heads and cylinders. Have all valves done, and rering the pistons. Though I'd have a bore gauge used to see if the cylinders are all within spec. And I'd have the ring grooves measured to see if they are still narrow enough.

I like Total Seal rings myself - they give great leakdown numbers.

Your mechanic should have noted where the leaking was for each of your cylinders. Valves should have 0 leakdown, or afwully darn close to it. Rings are where good engines leak some - it is inevitable. 12% past the rings isn't all that terrible, I think. As a wild guess, I'd say you have valve issues on the right bank, and the left bank leakage is just past the rings.

But this engine is a 1970, and it sounds like you don't have a service history, and you haven't given us the mileage on this engine either. That complicates things. However, if your oil pressure has been good, and examination of the chain wheels you can see with the chain housing covers off (which will happen as part of removing the heads) look decent, I could see doing valves and rings (assuming nothing worse shows up) and calling it good.

If you still have some of the oil drained from the engine, you could send a sample off for analysis. That might show if there was excessive bearing material in it. If so, now's the time to split the case and at least replace the bearings.

Even if you can avoid "while we are in here, let's do x x x"itis, stuff shows up. A cam lobe or two might look like it is starting to go. Rocker arm wear surfaces might look a bit funky, rocker shafts might look worn, rocker bushings might have too much play. And some would say replace the chains and chain sprockets no matter what. All this starts really adding up quickly. So you need to be stout of heart if you are going to replace only what is broken, so to speak, and trust that you will get another 50 or 100K miles out of the rest.

One thing you shouldn't have to worry about are the head studs. These were steel from the factory, and work fine on this engine. I'd leave them alone. It was the 2.7s and later which ran into head stud issues.

Thanks for the replies folks. I'm still out of town and won't be back to speak to my mechanic until next week.

Given some of the information I've read, lets suppose that there is some cylinder/piston damage. I've been thinking of trying to get more HP out of that engine. I've read that the only real practical way of doing this is to increase the size. I've got a couple of things already added to the car to help liven up the performance, ex. MSD ignition, headers, etc. Anyhow, if the cylinders/pistons are bad, what our your thoughts of buying 2.4 cylinders and pistons? What would be the potential problems with this decision? What other options should I consider if I've already decided to have the engine rebuilt?

Thanks,
Brent

You should probably have the valve clearance checked on that cylinder before dropping the engine. If the exhaust valve is way out of adjustment it can hang slightly open at top dead center. Probably not the source of your problem, but it would be one of the easier things to rule out.

Brent

The 2.2 and 2.4 cylinders are the same diameter! 84mm. Not much performance enhancement there.

Porsche used a bigger crank to up the power of the 2.4.

Your T motor is the bottom of the food chain, so to speak. Anything you do is going to cost money. For instance, bringing it up to 2.2S spec would involve S Ps and Cs, increasing the port size on the heads, and finding the S fuel injection or a good set of Webers. On top of that, the 2.2 case is not the case anyone would choose for a long lasting street hot rod.

You really ought to purchase Wayne Dempsey's book on 911 engine rebuilding. He is Pelican Parts, and kindly hosts this forum.

And/or purchase Bruce Anderson's classic Porsche 911 Performance Handbook.

Do some bed (or other) side reading there, then you can aske more focused questions.