compression test question

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Did you set the valve lash yet? This made a tremendous difference in mine, idle smoothed out nicely. Read Slantsixdan's links on valve lash adjustment...posted many times
 
I have a 1975 Lull rough terrain forklift with a 225 Chrysler industrial gas slant 6 engine (!) The engine smokes badly (blue smoke) and burns oil. It does not smoke at start up; it only starts smoking after running for ten minutes or so. The compression readings are 120-125-120-135-145-120. From what I've read, these are pretty healthy numbers and are (just) within the +/- 25psi rule. I'm guessing that the problem is worn valve stem seals. My question is, with these compression readings, should I not even bother with a full valve job, and just replace the stem seals without removing the head? The engine runs like a champ otherwise. Thanking you in advance for input.
 
LullCamp said:
I have a 1975 Lull rough terrain forklift with a 225 Chrysler industrial gas slant 6 engine (!) The engine smokes badly (blue smoke) and burns oil. It does not smoke at start up; it only starts smoking after running for ten minutes or so. The compression readings are 120-125-120-135-145-120. From what I've read, these are pretty healthy numbers and are (just) within the +/- 25psi rule. I'm guessing that the problem is worn valve stem seals. My question is, with these compression readings, should I not even bother with a full valve job, and just replace the stem seals without removing the head? The engine runs like a champ otherwise. Thanking you in advance for input.
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I would have a full valve job done and check the guides.
 
I would try changing the valve seals before springing for a full valve job. As old as that engine is, the valve seals are probably hard and brittle.

You can change valve seals with the heads on the engine. You just need to rig up a way to put compressed air into each cylinder. I was able to connect my compression tester fitting to a compressed air coupler. The compressed air holds the valves closed so they don't fall down into the engine while you remove the spring and replace the seal.

While you have compressed air in each cylinder, check for air leaking out the carb or tailpipe which would indicate bad intake or exhaust valves.
 
I prefer to shove some rope into a cylinder and bring the piston up to hold the valves closed rather than use air.

I would change the seals too before doing anything just to see what happens. The cast a pittance. But the stem to guide clearance may well be way out at this point.

Also, be aware that if the oil rings or 2nd rings are badly shot, then the oil past the rings may make the compression look better than it really is. In that is the case, general performance and other symptoms are the indicators you have to depend on. And a leak down test.
 
Good cylinder pressure numbers for almost any engine would be 165 to 185, and all pretty even. Too much more than 185, and it gets hard to tune on pumpgas.
Poor numbers are subjective. It is my opinion that 125 and less is poor. It doesnt mean that the engine is junk, just that it is way below it's potential. A single cylinder engine will start on just 30psi, and power up a grain auger all summer running just 80psi. Your lawnmower probably has not got 100 psi.
So, if your slanty happens to be 125, but all the cylinders are more or less even then it will be a runner. Just don't be thinking of hopping it up.
Yours on the other hand...........doesn't look good. I'm surprised that it actually pulls 80mph.
Now as to the test;
The test does not need to be done at WOT if you don't have a starting issue; it can be done with the throttle on the fast idle cam; It just takes more compression cycles to reach it's maximum pressure.Also keep in mind that any cylinder you are testing that is not at WOT will be drawing fuel out of the low-speed circuit. At WOT, there may not be enough "vacuum" to activate any circuits.
The reason I do them at WOT is because I also keep track of how many shots it takes to reach its max pressure, and how much pressure it builds on its first shot, or two.An engine that makes 70% or more of it's peak,will be a good starting engine (assuming the tuner gets on it). An engine that reaches it's max on 5 or 6 cycles will be a good strong runner. If your engine takes 10 or more cycles with the carb at WOT, I don't care how big a number it posts, something is wrong.
As to the method;
the engine needs to be warm, and the valves properly lashed. The starting system needs to be up to snuff,the plugs all out, and the battery fully charged and ready to work hard.The carb needs to be working right so that the cylinders have not been washed of their oil.
With numbers like yours, I would be doing a cylinder leakage test next.
 
gpreston57 said:
It has a miss, once in a while you hear a slight pop at the tailpipe, could it be a burnt valve, or maybe I need to redo the valve lash adjustment?
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A once-in-a-while pop in the pipe is not a bad valve.
It is more likely fresh air getting into the exhaust system somewhere, accompanied by an over-rich low-speed circuit,and/or a faulty ignition system on at least one cylinder.And possibly late timing. These three (or four) often conspire this way. The rich carb,and/or unfired charge,and/or late timing, push raw unburned fuel into the pipe,where it mixes with the fresh air. When the mixture is just right, still burning fuel from any late-firing cylinder, will ignite that ready-to-burn mixture. This scoots the gasses down the pipe, and you hear the pop as it exits.
This is not the only way a pop is created, just the most likely.
A burned valve will pop every single time the associated plug fires. That cylinder is sending compressed burning gasses into the pipe on every combustion cycle.
A sticking-in-the-guide exhaust valve will do the same, but only when it sticks.But stems don't usually unstick themselves. Once they're stuck, they burn the valve, and the cylinder goes down, and when one of six goes on vacation, you know it right away!
There are other less common reasons for occasional popping.
 
Rockerdude said:
Is it just me, or do I kill a lot of threads?
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I wonder the same, sometimes.
Sometimes I am the last poster, and never hear from the OP again as to how,or if, his issue was ever resolved. But I think it happens to others as well. If I was giving out bad advice, I would hope that someone would challenge me.It's just another way of learning. I like to think I have a thick enough skin, a small enough ego, and a lot of potential to learn more stuff. I'm here to help. If I'm not helping in a particular thread, I would hope someone would say so.
Mostly I speak from experience, but occasionally I voice an opinion.It's hard to argue against either. But If I and someone else have different experiences as regarding a particular issue, it doesn't mean one of us is wrong. But opinions are mostly based on experiences and learned things. If I learned something wrong, well, then my opinion would be wrong too. I am not too proud to unlearn something, and change my opinion(s). I've done it here on FABO a time or two. Probably will again.........
 
My guage reads a bit low. Even on my freshly built yamaha xs650 engines I get 100 or so. LLeak down passes with flying colors on the guage. My slant has a slight rough idle and I could only get 100 psi or so on the guage. But with a leak down it was more than acceptable. So could be the reman. Holley 1920 on it. It's not real bad for roughness so I just kind of ignore it for now. Also have black plug on 6. Clean for half of the porcelain but black for the other half to the threads. Car doesn't seem to use oil. No smoke while dad follows me...float height is good. Kind of a mystery I haven't figured out yet that may also be related to my slight idle quirk.

So....I would leak down test anyways.
 
cdnEHbody said:
My guage reads a bit low. Even on my freshly built yamaha xs650 engines I get 100 or so. LLeak down passes with flying colors on the guage. My slant has a slight rough idle and I could only get 100 psi or so on the guage. But with a leak down it was more than acceptable. So could be the reman. Holley 1920 on it. It's not real bad for roughness so I just kind of ignore it for now. Also have black plug on 6. Clean for half of the porcelain but black for the other half to the threads. Car doesn't seem to use oil. No smoke while dad follows me...float height is good. Kind of a mystery I haven't figured out yet that may also be related to my slight idle quirk.

So....I would leak down test anyways.
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So what do you consider acceptable?
I'll tell you my acceptable;on my guage. zero to 4% maximum.
You say; What! Hey I don't make stuff up. This is straight off the pages of Smokey Yunichs Power Secrets. A brilliant mechanic and racer of yesteryear.
At 8% that cylinder is toast. At 6% she is no longer performing anywhere near her potential.And bear in mind that a leakdown test is a snapshot of leakage at TDC only. From a few thou down,the cylinder could be split wide open, or the wrist-pin could have left a deep gash in the wall,and the gauge would still read A-OK. But at least you would know that the valves are closed. Likewise, the guides could be wore right out, or not even in there, but at least you would know that the valves are closed. The LD test is a snapshot. It's up to the tech to figure out the rest of the story.

EDIT
When I say toast, or rather when Smokey says toast, it doesn't mean the engine is junk; it just means the guy in the next lane with the exact same car but less cylinder pressure leakage, Is gonna out-run you. Since the engine converts pressure to performance, or fuel to heat energy, and the % of energy that actually reaches the rear wheels is so pitiful, cylinder leakage is the front line. With about 1/3 of the heat going into the cooling system, and another 1/3 going into the exhaust, that leaves just 1/3 for motorvating the car. Then you subtract another 15 to 20% for powertrain losses, Man!.there is not much left over. Who can afford cylinder leakage?
 
125-135 psi is NORMAL for a stock 225 /6 motor in very good condition. I got 130 psi +/- on all cylinders with a solid, fresh rebuild with fresh bores, pistons, and moly top rings, the head shaved a bit, and a very mild cam. The compression ratio is just not there for higher numbers in a stockish /6. And yes, the throttle being open vs closed makes about a 5 psi difference on a /6.

Acceptable leak down %? I gotta laugh about this one. 4 cylinder piston aircraft engines are allowed far,far more leakage than is being cited here when cold! Something like 30%!
 
AJ/FormS said:
So what do you consider acceptable?
I'll tell you my acceptable;on my guage. zero to 4% maximum.
You say; What! Hey I don't make stuff up. This is straight off the pages of Smokey Yunichs Power Secrets. A brilliant mechanic and racer of yesteryear.
At 8% that cylinder is toast. At 6% she is no longer performing anywhere near her potential.And bear in mind that a leakdown test is a snapshot of leakage at TDC only. From a few thou down,the cylinder could be split wide open, or the wrist-pin could have left a deep gash in the wall,and the gauge would still read A-OK. But at least you would know that the valves are closed. Likewise, the guides could be wore right out, or not even in there, but at least you would know that the valves are closed. The LD test is a snapshot. It's up to the tech to figure out the rest of the story.
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the yam definetly passed with low percentage on a fresh rebuildand lapped valves. The slant, probably around 6% or so. I was decently hhappy with it being a 45 year old engine. I did stem seals with my retired chrysler mechanic father. We noted no real play in the stems...but that's not a true tell obviously. But I did figure what you said could be happening. Either bore problem further down or sticky oil control ring. Who knows. Car runs darn good as it is and has plenty go highway legs so I haven't felt the need to pull the head yet. There was no alarming differences between cylinders when I had each one pressurized, so I never tested any other ones with the guage as it stands. One day...the head will come off. And I left it at that for meow.

30% !? **** an old bike engine I had had once cylinder leaking like that and it was a cold exhaust pipe !
 
Well, I didn't say the 30% was good..... just what was used as a lower limit. And with a cold air cooled engine, I can't really relate that to a warmed up A/C engine or to a normal water cooled auto engine. I just threw that out for some interest......
 
nm9stheham said:
125-135 psi is NORMAL for a stock 225 /6 motor in very good condition. I got 130 psi +/- on all cylinders with a solid, fresh rebuild with fresh bores, pistons, and moly top rings, the head shaved a bit, and a very mild cam. The compression ratio is just not there for higher numbers in a stockish /6. And yes, the throttle being open vs closed makes about a 5 psi difference on a /6.

Acceptable leak down %? I gotta laugh about this one. 4 cylinder piston aircraft engines are allowed far,far more leakage than is being cited here when cold! Something like 30%!
Click to expand...

Air cooled?That would be a whole nuther story.
But,even if liquid, those engines have much different operating parameters.
Our land engines have it pretty easy. They operate over a very narrow temp,and barometric pressure, window. The timing and fueling can be optimized over that window fairly easily. I've never worked on aircraft, but I have heard that they operate over a very narrow rpm band, but with widely varying air pressure and density.Plus IIRC they run a completely different fuel formulation. I have to wonder how that figures into their design.
 
You are right on the narrow RPM band.... 1800-2600 for pretty much all conditions for smaller 4 and 6 cylinder flat air-cooled engines (320 to 540 ci). And a relatively narrow band of HP ranges too and low HP to C.I. ratio, like 1/2.
 
Woah - I wonder now after reading this thread if my newly-built /6 has too much compression. I decked the block 0.050", and the heads 0.030", aiming for about 9.5:1 compression. Pistons are +0.030", too.

All six cylinders are within 3.5% of each other, with an average of 194 psi. Wet. Throttle tied wide-open. High-torque starter from a late-model Dodge V8 motor. All spark plugs removed for the testing.

It's only been driven for 10 miles, and I know the stock distributor is having problems that I suspect are tied to the centripetal weights seeming to have worn springs. Even with the adjustable bracket at the bottom of the distributor bottomed-out in its slot, I couldn't get base timing at idle to be less than about 8 degrees BTDC. Was aiming to get 0 or 2 degrees (with vaccuum line disconnected).

And it's got fairly bad 'clatter' / pre-ignition at wide-open throttle. With 89 octane fuel only, so far.

Wondering now if I did the math wrong and ended up with a 10.5:1 motor??

I have a rebuilt distributor I'm installing in a few days. Be curious to see if it helps.
 
GPreston.... Sounds like it is time for a freshen up of that little motor.

Keep us posted
:)
 
69DartDave said:
Woah - I wonder now after reading this thread if my newly-built /6 has too much compression. I decked the block 0.050", and the heads 0.030", aiming for about 9.5:1 compression. Pistons are +0.030", too.

All six cylinders are within 3.5% of each other, with an average of 194 psi. Wet. Throttle tied wide-open. High-torque starter from a late-model Dodge V8 motor. All spark plugs removed for the testing.

It's only been driven for 10 miles, and I know the stock distributor is having problems that I suspect are tied to the centripetal weights seeming to have worn springs. Even with the adjustable bracket at the bottom of the distributor bottomed-out in its slot, I couldn't get base timing at idle to be less than about 8 degrees BTDC. Was aiming to get 0 or 2 degrees (with vaccuum line disconnected).

And it's got fairly bad 'clatter' / pre-ignition at wide-open throttle. With 89 octane fuel only, so far.

Wondering now if I did the math wrong and ended up with a 10.5:1 motor??

I have a rebuilt distributor I'm installing in a few days. Be curious to see if it helps.
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Yeah that's a lot! But the up-side is there's plenty of room for a much bigger cam, to reduce your Dcr,to help with the low-speed and part-throttle operation. Unfortunately, WOT will still be timing sensitive.And for sure, it will require a top-grade fuel.
For a streeter,I actually prefer an engine built this way.Mind you,not as drastic as you seem to have come up with; but around 160psi, or so. Once you get the tune into it, it will have plenty of low speed grunt,and make gangbusters mpg, unfortunately, the engine may not accept "full" timing under full load, so,being knock-limited,the horsepower numbers might not be there. No biggie to me, if it pulls harder under part throttle, and gets great mpgs. When I need my HP fix, I have a different car for that. And If you really wanna drag-race it, theres always race-fuel.
I built my slanty similarly, with plenty of compression, for to cruise with very small throttle opening, and saddled it with really tall gears, and a wide-ratio,loc-up tranny.After much tuning, it paid off. I have been driving that 1980 Volare almost continually since the early 1990s; going on 25 years.The body is done now, soI kinda think I will retire it soon.Powertrain is still going strong.
But yeah;pressure as high as yours is, or may be, difficult to tune for.On the other hand, a full wet test can be misleading.Especially with high compression design; 2 or 3 ccs of oil will change things dramatically;perhaps .3 to .5 of a compression point. And with just 4 or 5 compression cycles required to build peak pressure, the oil might not all have a chance to escape.I would redo the test after it has run a bit and burned off the oil.This time DRY.And be sure the valves are properly lashed.

FWIW A stock bore slanty has a swept volume of about 614cc. If the Scr is 10.5 then the Total chamber volume will need to be 64.7cc. If 3cc of oil does not escape, during the test, the total chamber volume will be just 61.7 and the actual Scr will be 10.96;nearly half a point, to .46 of a point
But if the Scr was 9.5, then the Total chamber volume would need to be 72.3cc. And again if 3cc of oil got stuck in there, the total chamber volume would effectively be reduced to 69.3cc, and the Scr would be increased to 9.87; this time just .37 of a point.
These are probably extreme examples, designed to make you aware of the presence of the oil in there. How this translates to cylinder pressure is anybodies guess at this point.
 
69DartDave said:
Woah - I wonder now after reading this thread if my newly-built /6 has too much compression. I decked the block 0.050", and the heads 0.030", aiming for about 9.5:1 compression. Pistons are +0.030", too.

All six cylinders are within 3.5% of each other, with an average of 194 psi. Wet. Throttle tied wide-open. High-torque starter from a late-model Dodge V8 motor. All spark plugs removed for the testing.

It's only been driven for 10 miles, and I know the stock distributor is having problems that I suspect are tied to the centripetal weights seeming to have worn springs. Even with the adjustable bracket at the bottom of the distributor bottomed-out in its slot, I couldn't get base timing at idle to be less than about 8 degrees BTDC. Was aiming to get 0 or 2 degrees (with vaccuum line disconnected).

And it's got fairly bad 'clatter' / pre-ignition at wide-open throttle. With 89 octane fuel only, so far.

Wondering now if I did the math wrong and ended up with a 10.5:1 motor??

I have a rebuilt distributor I'm installing in a few days. Be curious to see if it helps.
Click to expand...

Is this a 225, 198, or 170 ci /6? I think I would try another compression gauge first; 194 seems awfully high for the mods you have done. The pistons are still in the hole a good bit with .050" off the block for a 225. If need be, you could put in a thicker head gasket. Which one did you use in the rebuild?

The ignition timing thing looks to me to be that the distributor is off a tooth, or you need to monkey with the timing adjustment on the bottom side of the distributor body. You should be able to set it anywhere you please.

I'd be putting in some 93 octane to see if the clatter goes away, and stick with that until you get things sorted out better.

What carb do you have? Do you know the year model of the carb? And under what conditions do you have the pinging?
 
nm9stheham said:
Is this a 225, 198, or 170 ci /6? I think I would try another compression gauge first; 194 seems awfully high for the mods you have done. The pistons are still in the hole a good bit with .050" off the block for a 225. If need be, you could put in a thicker head gasket. Which one did you use in the rebuild?

The ignition timing thing looks to me to be that the distributor is off a tooth, or you need to monkey with the timing adjustment on the bottom side of the distributor body. You should be able to set it anywhere you please.

I'd be putting in some 93 octane to see if the clatter goes away, and stick with
that until you get things sorted out better.

What carb do you have? Do you know the year model of the carb? And under what conditions do you have the pinging?
Click to expand...

He said: "pistons are +.030,too"

9 makes a good point about proving the gauge.
And I have to concur, cuz my 225 was built about the same but at Zero-deck(says the builder;I never checked), and mine was only pushing 160psi,dry, with a "HP" cam .No spec available.I had the machine-shop assemble this unit, and it's never been further apart than for checking the cam degree-in.It seemed lazy to me, but was right on.And I solved the problem with plenty of initial timing.

But really, the WOT clatter with such a small amount of timing, kindof says it all. Mine runs 87E10 with full timing.
 
Mine is a 225, and to be honest I didn't measure the thickness of the cylinder head gasket upon reassembly. I'd have to go look it up in the paperwork. I do recall that the pistons were still below the block deck, at TDC.

This high-compression motor has the Holley Avenger EFI kit, which gives you the option of having an HEI-style ignition system with the computer controlling the timing, or just using the stock distributor and keeping the old-school vacuum+centripetal weights -based timing (which I did). I did put in the Pertronix upgrade goodies, though, including their coil.

I tried fiddling with the bracket on the bottom underside of the distributor, after verifying that it was not installed one tooth off (I tried reinstalling it +1 tooth and -1 tooth from where I had it). And upon building the motor, I did formally verify that the harmonic balancer TDC marking was indeed accurate (right on the mark!).

Pinging happens at large throttle settings at mid to high rpms.

My compression gauge is a good 20 years old, and I'll borrow a new one when I put in the rebuilt distributor in a few days.

Hmmm, I wonder if 92 octane fuel + octane boost additive is in my near future. The car has great off-idle and part-throttle torque. Oh, and it's mildly cammed with an Erson RV10 profile on the intake and RV15 profile on the exhaust.
 
69DartDave said:
Woah - I wonder now after reading this thread if my newly-built /6 has too much compression. I decked the block 0.050", and the heads 0.030", aiming for about 9.5:1 compression. Pistons are +0.030", too.
Click to expand...

I'm in the process of putting my slant together and with 90 thou off the block, 10 thou off the head with my flat top pistons also 30 over, several sites estimated i would have about 9.4 to one with a composite head gasket and about 9.7 to one with a stock steel head gasket and DCR of about 8.1 or 8.3 depending again on the head gasket used. I would estimate yours ought to be less than 9.5 depending how far down in the hole the pistons are. Maybe the block and/or head has been previously cut?
 
Where is the cam timing set BTW? Too advanced would contribute to a higher measured CR and operating DCR, and that would also make the off-idle and part throttle response pretty snappy.

On the head gasket, was is a steel shim or a composite like a Felpro? The Felpro has a thicker head gasket than the old stock shim which was round .022" thick IIRC.

With the pinging being up in the mid to higher RPM' ranges, I would now suspect mainly the mechanical advance, in conjunction with the moderately advanced initial timing. That has been discussed in other threads as an issue with the /6: a lot of initial timing gives too much total with the stock mechanical. So, you are on the right track to get the initial adjustment right, and then work on the total.

IMHO, fer sure check the compression with another gauge. A 194 psi compression reading with a mild or stockish cam would make me think 10.5 to 11 SCR.... which seems awfully high with the amount of shaving you did and with a Felpro type gasket to boot.
 
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