what is the deal with my 225 /six compression?

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66_dart_270

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i picked this car up about 10 days ago for $500 not running. after cleaning gas tank/lines out, new fuel pump, cap and rotor, wires, rebuilding holley 1920 it started right up. i then was told by a friend that a cylinder pressure test is a good way to check the health of the engine. i took the plugs out and come up with these readings.


cylinder 1-150psi/10.4bar
cylinder 2-132psi/9.3bar
cylinder 3-145psi/10.2bar
cylinder 4-140psi/9.6bar
cylinder 5-142psi/9.9bar
cylinder 6-130psi/9.0bar

1966 Dodge Dart 270 4 door.
i honestly am new to working on engines, what is the mathematical equation to figure out what these numbers mean?
 

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The numbers simply represents cranking pressure. Should be around 100 psi, so yours seems. At the same time the numbers are consistent so you should be ok.
 
Those are good numbers, I would expect those numbers will improve a little after driving it for 100 or so miles. Remember that you are suppose have the throttle open when you do a comp. test.
 
SGBARRACUDA said:
Those are good numbers, I would expect those numbers will improve a little after driving it for 100 or so miles. Remember that you are suppose have the throttle open when you do a comp. test.
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whoops, i did not know that. is this whole first test worth nothing then??
 
SGBARRACUDA said:
Those are good numbers, I would expect those numbers will improve a little after driving it for 100 or so miles. Remember that you are suppose have the throttle open when you do a comp. test.
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Exaaaactly! AND fully warmed up. A valve adjustment would even be better!
 
66_dart_270 said:
whoops, i did not know that. is this whole first test worth nothing then??
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No it is still a good test, and the numbers won't change that much. The reason for the throttle wide open is to allow each cylinder to fill to capacity before the compression stroke.
 
the car had been sitting for nearly 6 years before i got it.. there is some oil coming out of the head around the 5th and 6th cylinders, at least that is what it looks like.. a small amount of oil.. and then the valve covers seem to be covered with oil.. is this normal? are replacing these gaskets reasonably easy with the correct tools?
 
66_dart_270 said:
the car had been sitting for nearly 6 years before i got it.. there is some oil coming out of the head around the 5th and 6th cylinders, at least that is what it looks like.. a small amount of oil.. and then the valve covers seem to be covered with oil.. is this normal? are replacing these gaskets reasonably easy with the correct tools?
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Replacing the valve cover gasket is pretty straight forward. But the early slant 6's also used spark plug tubes that seal with an O-ring around each tube. The spark plug actually holds the tube in and makes the seal. You may need to replace the tube seals also.
 
SGBARRACUDA said:
Replacing the valve cover gasket is pretty straight forward. But the early slant 6's also used spark plug tubes that seal with an O-ring around each tube. The spark plug actually holds the tube in and makes the seal. You may need to replace the tube seals also.
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i saw that when i had the plugs out, never seen a set up like that before. i will look into buying new seals for around those tubes. Thank you so much for the welcome and useful info!
 
All the compression test tells you is if your rings are good and seling and if your valves are ok. Basically what pressure can be created in the chamber. Generally there will be a specification like say 150 psi and you don't want more than 20% variation cylinder to cylinder. Your numbers are good. The highest readings will be with the throttle open and engine hot.

The oil near the 5th and 6th cylinder, is it coming from the sparkplug locale?

The rocker arm cover is common to leak. Its easy to do just make sure among basic hand tools ypu have a scraper and some red scothbright to clean the gasket surface. While you are in here adjust your valves. Take a feeler gauge with te engine hot and running and slide it between the rocker tip and the valve stem. .010" intake and .020" exaust I believe. Watch out for the change in valve order. It will be exhaust, intake,exhaust,intake,exhaust, intake, intake, exhaust, intake, exhaust, intake exhaust. Might wanna throw a rag on the inner fenderwell, this job can get messy.

After the valves are adjusted check your compression again the way outlined and your numbers might come closer together.
 
And i forgot tell you, while running the feeler gauge, if the feeling isnt just past snug, you will adjust a bolt, if I remember correctly on the middle of the rocker arm. Tighten the bolt and the feeler gauge will be tighter, loosen to relax it.
 
/6 Matt said:
All the compression test tells you is if your rings are good and seling and if your valves are ok. Basically what pressure can be created in the chamber. Generally there will be a specification like say 150 psi and you don't want more than 20% variation cylinder to cylinder. Your numbers are good. The highest readings will be with the throttle open and engine hot.

The oil near the 5th and 6th cylinder, is it coming from the sparkplug locale?

The rocker arm cover is common to leak. Its easy to do just make sure among basic hand tools ypu have a scraper and some red scothbright to clean the gasket surface. While you are in here adjust your valves. Take a feeler gauge with te engine hot and running and slide it between the rocker tip and the valve stem. .010" intake and .020" exaust I believe. Watch out for the change in valve order. It will be exhaust, intake,exhaust,intake,exhaust, intake, intake, exhaust, intake, exhaust, intake exhaust. Might wanna throw a rag on the inner fenderwell, this job can get messy.

After the valves are adjusted check your compression again the way outlined and your numbers might come closer together.
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i am not certain if it is coming out of the plug tubes/seals or the head gasket. i need to do some more research of the valve adjustment as even though you have walked through every part.. i still dont fully understand how to do it lol. thanks for giving me these tips! my hands have been covered in oil this last week and i have to say i love it :)
 
I got $10 on the tubes. Never seen a head gasket leak oil. And if I wasn't on my phone I could easily get you links of where and what to read. Go to www.slantsix.org and choose articles then engine and read through the stickies in the engine section cause I believe dan has a full write up in there.

I tried my best to explain but I've only done it twice and it's been three years or more since last lol!
 
/6 Matt said:
I got $10 on the tubes. Never seen a head gasket leak oil. And if I wasn't on my phone I could easily get you links of where and what to read. Go to www.slantsix.org and choose articles then engine and read through the stickies in the engine section cause I believe dan has a full write up in there.

I tried my best to explain but I've only done it twice and it's been three years or more since last lol!
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believe me, you did great! im happy to hear that! thanks for the link!
 
No problem! And welcome to the forum!:D

We're always happy to help a newbie cause we all were newbs at one point.
 
Leaks from the spark plug tubes are so common, some wags here term them "drool heads". The seals are cheap and available (rockauto, ...). Later slant heads (1977+?) had a solid spark port. You could change to those someday, but you can't pull your lifters out without removing the heads. Changing the valve cover gasket is easy, especially for your early engine. Some people pull the cover twice a year to adjust valves (a bit anal). Don't over-tighten the bolts and hammer flat bulges from prior owners doing so.

Re what other engines had removable spark plug tubes - the infamous 429 Hemi. Thus you can call your slant a "Hemi 0.75".

Your compression readings are great. I have never measured >145 psi in a slant. The value depends on many things, like the camshaft installed. Besides all cylinders being uniform, another metric is that the pressure reaches the final value in 2-3 compressions. If it takes 7+ pulses to reach the final level, you might have bad valves. It is also easy to get bad readings - if your gage isn't well-sealed, the Shraeder valve at the bottom is not the special light one (white), if the volume of the threaded part isn't proper, ... To insure a good seal, best to remove the spark tubes before screwing in the gage.
 
Bill, I believe the peanut (non drool tube) head came put in '77 so I thinl you are correct. Also twice a year is extremely anal. Factory says do it every 55k miles.
 
Compression pressure should be within 10% of each other. Couple of yours are a little out of that range. Maybe valve adjustment as mentioned. Might be just normal wear. They are good numbers.
 
/6 Matt said:
And i forgot tell you, while running the feeler gauge, if the feeling isnt just past snug, you will adjust a bolt, if I remember correctly on the middle of the rocker arm. Tighten the bolt and the feeler gauge will be tighter, loosen to relax it.
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Not quite; somebody has been working on too many small block Chevy V8s...

The adjuster bolt is on the end of the rocker arm, just above the push rod tip.

It's a self-locking-type screw you'll be turning, with no jam nut to hold it; it has enough intrinsic friction not to move, once adjusted.

Hope this helps...


And, twice a year is not over-doing it for valve adjustments on older engines, I think. You can't hurt an engine by adjusting its valve clearances, if you do it correctly. Good insurance... just my opinion.:sign7:
 

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Thanks for clarifying Bill, and believe it or not I haven't been working on Chevy motors lately I just couldn't remember lol!

I still think twice a year is overkill though. It doesn't hurt to do it too frequently but to me, it's a waste of rocker arm cover gaskets, money, time and lower back health. I'll agree to disagree there.
 
"Back when" I had the same cam, lifters, pushrods and 273 rockers on the 340 as I have right now on the "low buck" 318, that thing used to routinely see 6K and sometimes more in mud holes. Once a year turned out to be plenty. I ran the you-know-what out of that thing

So far as compression tests

On an engine you have acquired and don't know history I would ABSOLUTELY set the valve lash if they are adjustable.

I do NOT believe in this nonsense of arguing with a hot, running engine, it's completely not necessary. If you memorise the "EOIC" method, you can correctly adjust anything that has pushrod valves, from a 1 lung Briggs to a 4 -row aircraft radial.

EOIC means that you rotate the engine until

the EXAUST just starts to OPEN, and set the intake on that cylinder

Then rotate until the INTAKE has opened and is nearly CLOSED, and set the exhaust for that cylinder

On a V8, you can warm it up, pull one side, and go front to back, install that cover, re--warm if necessary, and pull and set the other side.

On a slant, simply look the situation over and get things prepared for a quick removal, warm it up, pull the cover, and set the valves.

I use the proper feeler(s), as well as feeler(s) about .002 larger. The correct feeler should slip right in, the .002 larger forced or not at all, IE a "no go" gauge.

Perform hot

Try to check each cylinder in the same manner, IE same number of compression "pulses" and be sure you crank enough that the gauge peaks.

Block the throttle open.

Any that are questionable, squirt some oil into the cylinder and recheck. This checks for poor ring seal. Be careful not to get too much oil, which will raise compression ratio!!!! and give you a false reading
 
67Dart273 said:
"Back when" I had the same cam, lifters, pushrods and 273 rockers on the 340 as I have right now on the "low buck" 318, that thing used to routinely see 6K and sometimes more in mud holes. Once a year turned out to be plenty. I ran the you-know-what out of that thing

So far as compression tests

On an engine you have acquired and don't know history I would ABSOLUTELY set the valve lash if they are adjustable.

I do NOT believe in this nonsense of arguing with a hot, running engine, it's completely not necessary. If you memorise the "EOIC" method, you can correctly adjust anything that has pushrod valves, from a 1 lung Briggs to a 4 -row aircraft radial.

EOIC means that you rotate the engine until

the EXAUST just starts to OPEN, and set the intake on that cylinder

Then rotate until the INTAKE has opened and is nearly CLOSED, and set the exhaust for that cylinder

On a V8, you can warm it up, pull one side, and go front to back, install that cover, re--warm if necessary, and pull and set the other side.

On a slant, simply look the situation over and get things prepared for a quick removal, warm it up, pull the cover, and set the valves.

I use the proper feeler(s), as well as feeler(s) about .002 larger. The correct feeler should slip right in, the .002 larger forced or not at all, IE a "no go" gauge.

Perform hot

Try to check each cylinder in the same manner, IE same number of compression "pulses" and be sure you crank enough that the gauge peaks.

Block the throttle open.

Any that are questionable, squirt some oil into the cylinder and recheck. This checks for poor ring seal. Be careful not to get too much oil, which will raise compression ratio!!!! and give you a false reading
Click to expand...

Our engine, a turbocharged 225 with a measured 9:1 compression ratio, has 165 psi on 4 cylinders and 160 on 2. Those were generated after five "hits." The lower-compression cylinders are not adjacent nor are they on the end cylinders. We have no idea why two would be lower...

The cam is a 210/210@ .050", solid lifter unit with .484" (nominal) lift and is ground with 115-degrees of lobe separation.

No material has been removed from either the block, the head sealing surface, nor the combustion chamber walls.

I would imagine that this is probably the sort of numbers a stock slant six would generate.
 
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