First 911 please help with compression/leak down numbers

[harv666]

Looking to buy my first 911, its an 83 911sc Euro/ROW. 129,000 miles. Theres a lot of good about this car, but then theres these compression and leak down numbers and I don't know how to read them properly so I'm reaching out for help from the community. The car really hasn't been driven in the last year. This was done today. Any help is much appreciated.

compression test
#1 150psi
#2 150psi
#3 135psi - 130psi op temp
#4 145psi
#5 145psi
#6 140psi

leak down test input pressure 90psi room temp

#1 40psi
#2 60psi
#3 50psi
#4 75psi
#5 74 psi
#6 75 psi

Operating temp
#1 80psi
#2 75 psi
#3 55psi

[ClickClickBoom]

Originally Posted by harv666

Looking to buy my first 911, its an 83 911sc Euro/ROW. 129,000 miles. Theres a lot of good about this car, but then theres these compression and leak down numbers and I don't know how to read them properly so I'm reaching out for help from the community. The car really hasn't been driven in the last year. This was done today. Any help is much appreciated.

compression test
#1 150psi
#2 150psi
#3 135psi - 130psi op temp
#4 145psi
#5 145psi
#6 140psi

leak down test input pressure 90psi room temp

#1 40psi
#2 60psi
#3 50psi
#4 75psi
#5 74 psi
#6 75 psi

Operating temp
#1 80psi
#2 75 psi
#3 55psi

All is worthless, unless you use it as a bargaining chip. The information provided is incomplete and incorrectly done.
Look up Textron Lycoming Service Instruction 1191A. Differential Compression Test is not an absolute diagnostic tool, in normal use it is a trend monitoring tool/process.According to those numbers, the engine needs a rebuild. Done correctly, it would likely show a healthy engine with normal wear. I have do more than a thousand Differential Compression Tests according to the Lycoming and Continental Service Letters/Bulletins. Want to know what Differential Compression Test results are on my '84 3.2? Don't know, because its almost impossible to do the test correctly so there is accurate results. If you like the car, check the plugs and inspect the body for damage. At 129K there is a chance that you may need valve guides, but my car at 145K burns less than 3/4 quart between 4000 mi oil changes. Lots of really bad info on the interwebz, buy it take the ride, it's a 35 year old car.

652 Oliver Street
Williamsport, PA 17701 U.S.A.
717/3236181
INSTRUCTION
SUBJECT:
MODELS AFFECTED:
TIME OF COMPLIANCE:
Cylinder Compression
All Textron Lycoming aircraft engines.
As required to determine cylinder compression.
TELyXcToRmOiNng SERVICE
DATE: September28,1998 ServiceInstructionNo.1191A (Supersedes Service Instruction No. 1191) Engineering Aspects are
The condition of the working parts in the combustion chamber of a cylinder can be determined by measuring the static leak rate of the cylinder as compared to the leak rate through an orifice of specified size. This is accom- plished by attaching a differential compression measuring device, which incorporates the orifice, to one spark plug hole of the cylinder while the piston is at top center of the compression stroke.
CAUTION
TAKE ALL NECESSARY PRECAUTIONS AGAINST ACCIDENTAL FIRING OR ROTA- TION OF THE ENGINE.
The piston is held at top dead center by firmly holding the propeller to prevent the engine from turning when air pressure is applied through the differential compression device to the combustion chamber.
CAUTION
USE GLOVES OR RAGS TO PROTECT THE HANDS WHILE HOLDING THE PROPEL- LERBLADE.ALSO, BEFOREATTACHINGTHECOMPRESSIONTESTER,CHECKTHE AIRSUPPLYREGULATORTOMAKESURETHEAIRPRESSURETOTHECYL INDER IS NOT EXCESSIVE. AIR PRESSURE IN THE CYLINDER CAN CAUSE THE PROPEL-
LER TO TURN; KEEP CLEAR OF THE PATHOF THE BLADES.
To assure that the piston rings are seated, the propeller is moved slightly back andforth with a rocking motion while air pressure is applied; thus providing a more accurate reading. Meanwhile, a second person adjusts the air supply pressure to 80 psi, indicated on the supply pressure gage of the differential compression device. Then, observation of the engine cylinder pressure gage will give an indication of the condition of the parts in the com-
bustion chamber of the cylinder.
NOTE
The orifice size of the differential compression measuring device is critical if consistent and meaningful cylinder analysis are to be obtained; the larger the orifice the less chance of detecting potential problems. Therefore, a specific orifice size that provides an acceptable leak rate has been selected for all Textron Lycoming engines; the instructions described herein are based on thisorificewhichis.040in.dia.(No.60drill)x.250inlo ng,withentranceangleof59/60 °.
At any time loss of power, increasing oil consumption, hard starting or other evidence of unexplained abnor- mal operation is encountered, a compression check of the cylinders is recommended with equipment and in the manner described above by personnel experienced with the equipment and with the type of engine to be checked. In practice the procedure is as follows:
GeneraAlviation ManufactureArsssociation
Page 1of2
FAA Approved

[ClickClickBoom]

Originally Posted by harv666

Looking to buy my first 911, its an 83 911sc Euro/ROW. 129,000 miles. Theres a lot of good about this car, but then theres these compression and leak down numbers and I don't know how to read them properly so I'm reaching out for help from the community. The car really hasn't been driven in the last year. This was done today. Any help is much appreciated.

compression test
#1 150psi
#2 150psi
#3 135psi - 130psi op temp
#4 145psi
#5 145psi
#6 140psi

leak down test input pressure 90psi room temp

#1 40psi
#2 60psi
#3 50psi
#4 75psi
#5 74 psi
#6 75 psi

Operating temp
#1 80psi
#2 75 psi
#3 55psi

All is worthless, unless you use it as a bargaining chip. The information provided is incomplete and incorrectly done.
Look up Textron Lycoming Service Instruction 1191A. Differential Compression Test is not an absolute diagnostic tool, in normal use it is a trend monitoring tool/process.According to those numbers, the engine needs a rebuild. Done correctly, it would likely show a healthy engine with normal wear. I have do more than a thousand Differential Compression Tests according to the Lycoming and Continental Service Letters/Bulletins. Want to know what Differential Compression Test results are on my '84 3.2? Don't know, because its almost impossible to do the test correctly so there is accurate results. If you like the car, check the plugs and inspect the body for damage. At 129K there is a chance that you may need valve guides, but my car at 145K burns less than 3/4 quart between 4000 mi oil changes. Lots of really bad info on the interwebz, buy it, take the ride, it's a 35 year old car.

[Glenfield]

There you go, Harv666. Clear as mud?

Even if compression / leakdown tests were an absolute test, mechanics can and do get them wrong. Additionally, the numbers can be skewed by carbon deposits if an engine hasn’t been run at operating temps and hard. That leaves the punter in a precarious position. Buying based on a test that can spit out false negatives (engine is fine when it ain’t) and walking when it can just as easily spit out false positives (engine is a swine’s ear when it’s really a seal purse). I got on the wrong side of one such test and expensive rebuild later, alls well in the world again.

If there’s a lot to like about the car, you have a data point that helps you and hurts the seller. “Cars got poor compression and needs a top end rebuild”. See if you can get enough off the price to insulate you should you actually need to do a top end. Also see if you can test the theory a little more: does it smoke excessively and what color or is it down on power? How much oil does it consume / 1000.

Or just walk as most people might and wait for a test that shows good compression (and hope that it’s in fact the case).

[Travis Neff]

For the leakdown test you want the engine at operating temp so that the normal thermal expansion of the engine has taken place. They build the engines to have a certain ring gap when cold (larger gap) as they will tighten up as things expand with heat. Also only remove 1 plug at a time and test each cylinder. What should have been noted is where the air was escaping from, intake, exhaust or rings.

Open up the oil tank cap and if you hear air there - rings
listen, feel and smell at the intake - leak there and that is your intake valves
listen, feel and smell the exhaust - leak there and that is your exhaust valves.

If your valves are adjusted too tight it will skew your results.

Compression will tell you how well the sum of all things (rings, intake and exhaust valves) as a whole are working together. It does not pinpoint a problem area as well as a leakdown test will.

[Reiver]

I have an '83 row and it is a sweet six with more HP/T than the US models. I would investigate the leak down issues with a Porsche indy shop with a great rep.
The 930-10 ROW you have has a compression ration of 9.8-1 which is higher than the US models and can be a bit detonation prone if run too lean...especially with 91 octane that we have.
I've had no issues as I set my AFR a tad rich and I make sure I run it with the factory cooler plug recommendation (it had the US model hotter plugs in it when I bought it)....and I have a Carrera oil rad in place to keep temps down (I live in the Phx area).
Any/all of those issues can lead to detonation and that can break rings.

I'd also check your WUR for the proper specs.....that is what controls your AFR as you have no lambda...it can work / run fine at idle but lean out under a wot event...again... detonation issues.

Not trying to scare you off, it is a great six, just giving you some information.

[Bill Douglas]

Also check the head studs.

[87maniac]

I wonder, is pulling the plugs and 'reading' the color and deposits
a more accurate indication (than compression) of cylinder-to-cylinder condition?

Highly recommend running the hell out of it before compression test.
I was fooled by a cold engine test that showed low compression on 1 cylinder,
then 'all normal' upon a full warm up.

If compression is within 10-20% on all cylinders, and plugs all look alike (good),
is that a good indicator of a healthy engine, and if not, time for a leakdown ? ? ?

[Fairview]

Glenfield and Bill Douglas, I concur.

These results are perfect for the buyer. They really are awful results for the seller and show an engine with serious needs. So if the buyer is able to discount the price enough to cover most of a top end rebuild, say $10,000 as a good estimate, and the rest of the car is nice, then go for it. Then drive the car for a good while and see if the numbers improve. If they don’t, the buyer has the savings to pay toward a rebuild to the buyer’s liking. And yes, check the head studs because if there are broken ones (likely) that will make it mandatory that the engine be pulled and the work done. There goes the $10,000.00.

[harv666]

Originally Posted by Reiver

I have an '83 row and it is a sweet six with more HP/T than the US models. I would investigate the leak down issues with a Porsche indy shop with a great rep.
The 930-10 ROW you have has a compression ration of 9.8-1 which is higher than the US models and can be a bit detonation prone if run too lean...especially with 91 octane that we have.
I've had no issues as I set my AFR a tad rich and I make sure I run it with the factory cooler plug recommendation (it had the US model hotter plugs in it when I bought it)....and I have a Carrera oil rad in place to keep temps down (I live in the Phx area).
Any/all of those issues can lead to detonation and that can break rings.

I'd also check your WUR for the proper specs.....that is what controls your AFR as you have no lambda...it can work / run fine at idle but lean out under a wot event...again... detonation issues.

Not trying to scare you off, it is a great six, just giving you some information.

Im not so educated yet on some of this can you explain some of these to me? Happy to call you anytime if thats easier?

[dad911]

Originally Posted by harv666

......
leak down test input pressure 90psi room temp

#1 40psi
#2 60psi
#3 50psi
#4 75psi
#5 74 psi
#6 75 psi
......

Mechanic doing the leakdown should have reported where the air was escaping. Intake, Exhaust, or case (vent)

[911obgyn]

That’s how I got my 911 cheap,did compression test Engine cold sat for a year or so, got bad #
Needs an engine rebuild, or just an Italian tuneup. Over 100k motor runs strong will need valve job

[Glenfield]

Detonation is when air / fuel doesn’t get ignited by the spark plug (and flame front it creates atop the piston), and instead “self-combusts” as a result of heat and pressure. So you have three components:
1. Ignition
2. Heat
3. Pressure

You’re looking at an engine with higher compression, so that’s#3 ticked off. When an engine runs lean, it runs hotter. So that’s #2. Not an inherent condition however and more of a reduced margin for error. Your engine was designed for Europe which has better gasoline i.e. higher octane and better ignition. So that’s #1. There’s another element related to #1 that’s to do with piston dome shape and location of spark plug but probably don’t need to go there. Only know that neither of those things help improve ignition.

The self-combustion in an uncontrolled manner creates explosive forces that can damage the engine. If you’re starting from scratch, you can design around it with lower compression pistons (9.5:1) or dual plugging (i.e. two spark plugs, improving ignition). If not, and you’re looking at a RoW 911SC just be careful to:
1. Keep to 93 octane or higher gas if you can
2. Vigilantly make sure you’re not running too lean - also a problem with CIS the way it meters fuel at WOT but probably too much
3. Make sure you keep engine cool - upgrade oil cooled etc.

To be honest, some of what you’re seeing in the compression numbers may be the result of the prior owner not doing any of the above. Low ball him and bank on a top end rebuild where you can also swap in a hotter cam (not too much duration). That will solve the compression issue and give you a kick-arse engine with the RoW CIS and larger ports.

[harv666]

Originally Posted by Glenfield

Detonation is when air / fuel doesn’t get ignited by the spark plug (and flame front it creates atop the piston), and instead “self-combusts” as a result of heat and pressure. So you have three components:
1. Ignition
2. Heat
3. Pressure

You’re looking at an engine with higher compression, so that’s#3 ticked off. When an engine runs lean, it runs hotter. So that’s #2. Not an inherent condition however and more of a reduced margin for error. Your engine was designed for Europe which has better gasoline i.e. higher octane and better ignition. So that’s #1. There’s another element related to #1 that’s to do with piston dome shape and location of spark plug but probably don’t need to go there. Only know that neither of those things help improve ignition.

The self-combustion in an uncontrolled manner creates explosive forces that can damage the engine. If you’re starting from scratch, you can design around it with lower compression pistons (9.5:1) or dual plugging (i.e. two spark plugs, improving ignition). If not, and you’re looking at a RoW 911SC just be careful to:
1. Keep to 93 octane or higher gas if you can
2. Vigilantly make sure you’re not running too lean - also a problem with CIS the way it meters fuel at WOT but probably too much
3. Make sure you keep engine cool - upgrade oil cooled etc.

To be honest, some of what you’re seeing in the compression numbers may be the result of the prior owner not doing any of the above. Low ball him and bank on a top end rebuild where you can also swap in a hotter cam (not too much duration). That will solve the compression issue and give you a kick-arse engine with the RoW CIS and larger ports.

Thank you so so much for that explanation.

[Glenfield]

Sorry, I meant not too much overlap on the cam. Not duration, although you don’t want much of that anyway. Keen to see how it shakes out but definitely do your diligence on that particular engine and negotiate hard.