Opel GT Forum banner

1 - 20 of 22 Posts

·
Member
Joined
·
1,175 Posts
Discussion Starter · #1 ·
What cylinder compression should I be looking for in a 1972, 1.9 litre, Opel GT?
 

·
OPEL-LESS!!!
Joined
·
2,116 Posts
OGTS says 120 minimum, and anything less is a worn out engine. i and many others i've talked to only have 90-100 on our lower compression engines, such as you should have, and they run great. depends on who you talk to i guess, but if it runs good, why worry?
 

·
Super Moderator
Joined
·
5,918 Posts
Moderator Note: Moved to the "Engine Mechanical" Forum. Please use the GT forum for questions not covered by the Technical Forums. Thanks!
 

·
Registered
Joined
·
2,733 Posts
Make sure you are doing it right! That means have all the spark plugs out, wire the throttle wide open, unplug the coil, and boost the battery. Any other way might cause differences from cylinder to cylinder.
Keep in mind that the actual compression reading isn't as important as the difference (in percentage) in the highest reading from the lowest reading.
 

·
Old Opeler
Joined
·
5,564 Posts
Compression and Leak Down Rate

A more sophisticated way of checking engine condition is to use a "Leak Down Rate" test. This requires special equipment found at well equiped motor shops but lets you know the condition of the rings; valves and even gaskets. The cylinder is pressurised with air and the speed with which the pressure drops is measured for each cylinder. The rule of thumb is 1 to 2 % is a good motor 3 to 5 % is worn and any more is getting terminal. A good operator will be able to tell the difference between faulty rings and valves leaking etc.
The leak down rate can also depend upon if the cylinder has been oiled or done dry and also if the motor is hot or cold. But more considtency from cylinder to cylinder is available than from a compression test due to the varriables that Jeff mentions.
 

·
Member
Joined
·
1,175 Posts
Discussion Starter · #7 · (Edited)
Did a quick test yesterday.
3 of the 4 cylinders gave the same value. About 110 psi.
#1 was a bit higher. The end of the tester was worn and probably wasn't
creating a good seal. I suspect that if retested, the compression might be fine.
All spark plugs were out and the battery was boosted with a charger that increases charge rate when starting the car.

The dwell was adjusted as part of a tuneup before the compression was tested.

After the dwell was adjusted and the idle rpm set the engine runs very smooth and "quiet".

When I bought this car, it apparently had old gas and had a horrible exhaust smell. I drained out the bad gas with a model airplane fuel pump with a hand crank. Seems to have gotten most of the gas out. Replaced that with high octane fuel. The exhaust smell is gone and it seems fine now, like a new car.

I have yet to adjust the Weber carb. and think it's running rich. I do notice a bit of carbon out of the tailpipe. No signs at all of oil burning.
 

·
Super Moderator
Joined
·
5,298 Posts
Weber 32/36 "tuning"

BDD said:
Did a quick test yesterday.
3 of the 4 cylinders gave the same value. About 110 psi.
#1 was a bit higher. The end of the tester was worn and probably wasn't
creating a good seal. I suspect that if retested, the compression might be fine.
All spark plugs were out and the battery was boosted with a charger that increases charge rate when starting the car.

The dwell was adjusted as part of a tuneup before the compression was tested.

After the dwell was adjusted and the idle rpm set the engine runs very smooth and "quiet".

When I bought this car, it apparently had old gas and had a horrible exhaust smell. I drained out the bad gas with a model airplane fuel pump with a hand crank. Seems to have gotten most of the gas out. Replaced that with high octane fuel. The exhaust smell is gone and it seems fine now, like a new car.

I have yet to adjust the Weber carb. and think it's running rich. I do notice a bit of carbon out of the tailpipe. No signs at all of oil burning.
Carb was probably just slapped on right out of the box and never "fine-tuned", i.e. rejetted for your particular engine mods, if any, or even for "stock" engine. And DON'T just buy a rejet kit, most are wrong for Opels.

Here's my recommended jetting from another post some time ago . . . dead on for most 1.9 engines, even slightly modified. ;) Ask others here!
 

·
Member
Joined
·
1,219 Posts
The easiest way to get the gas out of a GT gas tank is to just remove the gas line under the tank. Unlike a lot of cars, virtually all of the gas will drain out.
 

·
Incurable Opelitus
Joined
·
203 Posts
I just bought a compression tester from Sears (Actron model cp7827, $24.99). As soon as I got home, I had to try it out, using the info in my FSM, and some additional info I got from this site. The results were as follows:

Cylinder 1
Dry Test 142psi
Wet Test 139psi
Cylinder 2
Dry Test 115psi
Wet Test 123psi
Cylinder 3
Dry Test 128psi
Wet Test 133psi
Cylinder 4
Dry Test 127psi
Wet Test 133psi

This test is for the 1.9l in my '73 GT, and I have never taken off the head, so I don't know the condition of the cylinder walls, valves, valve seats, or the type of pistons I have. So, I am wondering if any of you have any opinions on this data? When I first saw the dry test data for Cylinder #1, I thought, "Woohoo, maybe I have the high compression pistons!". Then, Cylinder #2 brought me back to reality. Luckily, Cylinder #2 is 80+% of #1, so according to the FSM, this is acceptable. The wet tests are only 2% (Cylinder #1) to 7% (Cylinder #2) higher than the dry tests, so I don't think I have any ring problems. But I am wondering whether anyone can tell me if they think the results of this test is an indication of high compression pistons? I tried to look into the spark plug holes to see if there are flat top pistons, but I couldn't see anything (I didn't think it would work). Or do you think the readings are still far enough apart that I need to think about looking at the valve seats or guides?
Randy
 

·
Registered
Joined
·
1,494 Posts
Inconclusive

An excellent low-compression motor, can register up to around 150psi on a compression test (even higher when brand new). You do need to view the top of the piston, to know for sure what type of pistons you have, as there are variables involved (rings, valve adjustment, crank speed, gauge accuracy, head warpage, gasket condition, etc.) than can affect compression test results.

The change in wet/dry readings for #1, questions the validity of the results. Maybe the car battery was cranking slower the second time around?

The #2 cylinder results, may indicate a valve setting that is slightly too tight. You could back off the rocker nuts 1/8 turn (45 degrees) each, and recheck the #2 compression results.
 

·
Incurable Opelitus
Joined
·
203 Posts
D,
Thanks for the input. One thing I haven't done is adjust the valves. Maybe that will even out the test readings. It's possible the battery was slowing down a little, but it sounded plenty strong thoughout the test. I wondered about that odd Cylinder #1 reading, too.
Randy
 

·
Member 1000 Post Club
Joined
·
1,520 Posts
I like to do the compression test with a battery charger hooked up, seems to keep the cranking rpms more consistant...
 

·
Incurable Opelitus
Joined
·
203 Posts
Thanks for the idea, Paul. Although, I don't know what effect cranking the engine will have on the charger. Maybe it would draw too many amps and burn it up?
By the way, has anyone been able to see their pistons w/o removing the head? The flashlight I used just couldn't get enough light in there (or maybe it's just impossible to see the piston top thru that little hole?). Maybe I could snake a fiber optic flashlight in the spark plug hole for more light? I wish I had a borescope....hmmm, I work at a place that makes communication and military grade lasers. Maybe I could make one from some fiber optic cable? Maybe I'll just have to wait until I need a rebuild, or when I put in a "combo" cam.
Randy
 

·
Registered
Joined
·
6,344 Posts
Thanks for the idea, Paul. Although, I don't know what effect cranking the engine will have on the charger. Maybe it would draw too many amps and burn it up?
Randy
Randy, almost all of the quality chargers, even the ones with the ability to crank over an engine with a bad battery, have a thermal shutoff switch internally. So you "shouldn't" smoke your charger. If you do see some magic smoke coming out of it, it's probably toast by then.
 

·
Registered
Joined
·
2,733 Posts
Again, the compression test has to be done properly or it's causing you false alarm. Back to this thread's post number five.
A battery charger keeps the battery's voltage from declining steadily as you test each cylinder. You crank the engine over for about ten seconds max per cylinder. Since you have the spark plugs out you're only making compression on the cylinder you are testing, as you only test one at a time. Since you have the throttle wired open the cylinders are getting as much air as possible to compress, not being restricted by a closed throttle plate. The ignition should be disabled so you don't have loose sparks looking for a path, simple way to do this is remove the coil wire from the distributor and ground it. Important, ground it good, give that forty thousand volts a real easy path to take so it isn't trying to go through where it shouldn't, which could cause damage to things like the rotor, cap, and wires.
Regardless of the charger being used, it is only being maxed out while you're cranking for that five to ten second period, only a really tiny one would be overworked in that time to cause it to shut down. If this is a problem have a second battery hooked up via jumper cables, the vehicle jumping it would be idling which is keeping the voltage up.
This is all really simple stuff, and quite effective but only when you do it right and then analyze the results clearly. Basically it tells you if your engine is in pretty good shape internally or not. When we're surveying a boat that is perhaps being traded in, this compression test is a very important factor in determining what we'll give for it!
 

·
Opelizer
Joined
·
146 Posts
Been reading all these posts. I hate to admit it, but I still have no clue how to do this. I catch the references to the settings but would not know what I am looking for to do it. My engine is also running rich and the gas mileage sucks. I've know just enough to know the carb needs some sort of adjustment but don't know what to do with it. I am not a mechanic and would really like to know if there is any kind of documentation that would help a novice understnad what do? I'm not afraid to give it a try, just what steps to take to do it or understand it. Any help with simplified directions would be appreciated.

Rick
 

·
Registered
Joined
·
1,494 Posts
Consider the "Search" function

On this site, it's best to use the "search" function on the front page, to find the answers your are looking for. They're there, but you do have to invest some effort to get them.

A ready-made source, is at:

http://www.opelclub.com/TuneUpPart1aJune2006.pdf

http://www.opelclub.com/TuneUpPart1bJune2006.pdf

These are also accessed via this page:

Engine

(But you already knew that, because you saw the links on a prior thread about tune-ups, after searching for them, right?)
 

·
TonyB
Joined
·
87 Posts
Does attitude effect the compression reading ?

Hello,

Does attitude effect the compression reading ? I live in Colorado about 5100 feet attitude. Just wondering. :confused:

Thanks Tony
 
1 - 20 of 22 Posts
Top