Moparts

340 Compression test

Posted By: mopowers

340 Compression test - 06/08/14 03:18 AM

I just did a cold compression test on my 340 and got the following numbers: Should I be concerned about #8 being so low?

Also, these numbers seem high to me. Will this thing be able to run (and live) on 91?

Intake is off and cam is MP 280/.474 purple shaft.

1: 205
2: 197
3: 200
4: 182
5: 185
6: 182
7: 190
8: 170
Posted By: Challenger 1

Re: 340 Compression test - 06/08/14 03:28 AM

Quote:

I just did a cold compression test on my 340 and got the following numbers: Should I be concerned about #8 being so low?

Also, these numbers seem high to me. Will this thing be able to run (and live) on 91?

Intake is off and cam is MP 280/.474 purple shaft.

1: 205
2: 197
3: 200
4: 182
5: 185
6: 182
7: 190
8: 170



No I would not be concerned about it, just as long as it stays that way. Least you know about it know and can keep an eye on that plug for any changes.

I built my 340 a few years ago and CC the head and measured the piston .029" above the deck and came up with 10.4-1 static compression and my motor has consistently cranked 165-170 on all cylinders. It runs fine on 91 octane straight gas when I'm at home with the car and I have run it on plenty of ethanol 92-4.

I like your higher readings, it should run good. How much octane is it going to need? I have no idea, but I would start hi.

Just got back from cruising my 340 challenger with the wife and dog. We go house hunting and rumble down these residential streets and everyone looks.

I am running that same cam, I like it.
Posted By: RapidRobert

Re: 340 Compression test - 06/08/14 04:23 AM

I'd be doubtfull on running 91 especially as you dont want to have to compromise the curve. Yes #8 is substantially lower (17%) than the highest one. As said I'd start with a partial % tank of race gas & work downward. Keep in mind you need a fast curve to take advantage of your build & for that plus your posted SCR you're gonna need good fuel. To adj the curve the way I do it (staying just under the pinging point) you need the fuel type/% of race gas to be finalized
Posted By: mopowers

Re: 340 Compression test - 06/08/14 09:00 PM

Thanks for the input guys. The motor is fresh and was built before I bought the car. It was running on 91 before I did the compression test, albeit not that well. I'm in the middle of an intake swap and figured I'd degree the cam to get a better idea of where everything is. Hopefully I can get everything back on the road with what I've got and running on CA pump gas.

Thanks again.
Posted By: autoxcuda

Re: 340 Compression test - 06/08/14 09:23 PM

Quote:

Thanks for the input guys. The motor is fresh and was built before I bought the car. It was running on 91 before I did the compression test, albeit not that well. I'm in the middle of an intake swap and figured I'd degree the cam to get a better idea of where everything is. Hopefully I can get everything back on the road with what I've got and running on CA pump gas.

Thanks again.



What's "not well"? Was is pinging, hesitating, not smooth idle, bogging....?

Did the previous owner say it had that MP cam in it. Receipts on motor work?

Why the intake change?

big question is how much actuall measured compression ratio you have.
Posted By: mopowers

Re: 340 Compression test - 06/09/14 03:45 PM

Quote:

Quote:

Thanks for the input guys. The motor is fresh and was built before I bought the car. It was running on 91 before I did the compression test, albeit not that well. I'm in the middle of an intake swap and figured I'd degree the cam to get a better idea of where everything is. Hopefully I can get everything back on the road with what I've got and running on CA pump gas.

Thanks again.



What's "not well"? Was is pinging, hesitating, not smooth idle, bogging....?

Did the previous owner say it had that MP cam in it. Receipts on motor work?

Why the intake change?

big question is how much actuall measured compression ratio you have.



It wasn't pinging before, I just never could get the tune right. I couldn't get it to idle very well in gear. It dropped 300 rpms when put in gear. I had the timing set at 22 initial and 35 total.

Previous owner told me he had installed the MP cam. I verified the cam with a dial indicator and degree wheel.

I'm swapping intakes because I have an RPM air gap and the it previously had a torker 340 intake on it.

I have no idea what the measured compression ratio is. The previous owner told me it's a stock rebuild, but I have nothing to go on and don't really want to pop the heads off to measure.
Posted By: RapidRobert

Re: 340 Compression test - 06/09/14 04:30 PM

Actually your cranking compression tells the story. Challenger1 has a measured 10.4/165-170 psi and you're way higher than that on the psi plus I'm assuming no quench so I would say no way on pump gas. As you know pinging is extremely damaging & must be avoided at all costs.
Posted By: mopowers

Re: 340 Compression test - 06/09/14 05:01 PM

Quote:

Actually your cranking compression tells the story. Challenger1 has a measured 10.4/165-170 psi and you're way higher than that on the psi plus I'm assuming no quench so I would say no way on pump gas. As you know pinging is extremely damaging & must be avoided at all costs.



I get what you're saying, but why on earth was this engine able to run on 91 without pinging then? There were no signs of detonation that I could see. The plugs look fine, albeit a bit on the rich side.
Posted By: MoparforLife

Re: 340 Compression test - 06/09/14 07:13 PM

Quote:

Quote:

Actually your cranking compression tells the story. Challenger1 has a measured 10.4/165-170 psi and you're way higher than that on the psi plus I'm assuming no quench so I would say no way on pump gas. As you know pinging is extremely damaging & must be avoided at all costs.



I get what you're saying, but why on earth was this engine able to run on 91 without pinging then? There were no signs of detonation that I could see. The plugs look fine, albeit a bit on the rich side.

Nothing like that is a given. There are too many variables that will
can effect the end result.
Posted By: Challenger 1

Re: 340 Compression test - 06/09/14 07:25 PM

Quote:

Quote:

Quote:

Actually your cranking compression tells the story. Challenger1 has a measured 10.4/165-170 psi and you're way higher than that on the psi plus I'm assuming no quench so I would say no way on pump gas. As you know pinging is extremely damaging & must be avoided at all costs.



I get what you're saying, but why on earth was this engine able to run on 91 without pinging then? There were no signs of detonation that I could see. The plugs look fine, albeit a bit on the rich side.

Nothing like that is a given. There are too many variables that will
can effect the end result.




Cam timing can affect cranking psi with the same compression.
Posted By: mopowers

Re: 340 Compression test - 06/09/14 07:58 PM

It'll be interesting to see where the cam's installed at when I get a chance to degree it.

I'm just curious why with cranking psi #'s so high, it ran and didn't detonate on 91. I always thought cranking compression was the biggest predictor for detonation.
Posted By: 360view

Re: 340 Compression test - 06/09/14 08:06 PM

Some basics that should be considered:

1. The cylinder pressures generally decline from front to back. If the engine was a rebuild under the previous owner a less careful machine shop may have milled the block deck out of level with the centerline of the crank. Alternately, the machine shop may have milled the cylinder head surfaces without first checking to see if the finished cc's of the combustion chambers would be as equal as practical.

2. Is the gauge on the compression tester accurate?
If it were mine i would rig up some adapters and check it against "known good" gauges on a stable pressure source like a portable air tank.

3. Borescopes have become remarkable good and relatively inexpensive. The Harbor Freight 9mm diameter head size one made by Autel can slip through a 14mm sparkplug hole and show you whether abnormally high compression readings are due to carbon deposits.

http://www.amazon.com/Autel-AULMV20155-M...gital+borescope

4. The 170 psi reading low cylinder should routinely have a second test after adding two tablespoon sized squirts of 50 weight engine oil to the high side of piston gap in the cylinder and turning over the engine by hand a time or two.
If pressure increases with this "wet test" more than 20 psi there is something wrong with the rings or maybe bore finish or size in that cylinder.
Posted By: mopowers

Re: 340 Compression test - 06/09/14 08:46 PM

Quote:

Some basics that should be considered:

1. The cylinder pressures generally decline from front to back. If the engine was a rebuild under the previous owner a less careful machine shop may have milled the block deck out of level with the centerline of the crank. Alternately, the machine shop may have milled the cylinder head surfaces without first checking to see if the finished cc's of the combustion chambers would be as equal as practical.

2. Is the gauge on the compression tester accurate?
If it were mine i would rig up some adapters and check it against "known good" gauges on a stable pressure source like a portable air tank.

3. Borescopes have become remarkable good and relatively inexpensive. The Harbor Freight 9mm diameter head size one made by Autel can slip through a 14mm sparkplug hole and show you whether abnormally high compression readings are due to carbon deposits.

http://www.amazon.com/Autel-AULMV20155-M...gital+borescope

4. The 170 psi reading low cylinder should routinely have a second test after adding two tablespoon sized squirts of 50 weight engine oil to the high side of piston gap in the cylinder and turning over the engine by hand a time or two.
If pressure increases with this "wet test" more than 20 psi there is something wrong with the rings or maybe bore finish or size in that cylinder.



Thanks for the input

To answer one of your questions, The compression gauge is an old Proto. I tested it with an air blow gun and my compressor. They were equal.

I haven't done a wet test on #8, but that sounds like a good idea.
Posted By: MoparforLife

Re: 340 Compression test - 06/09/14 10:45 PM

Also if the engine is fresh, give it some time to fully seat the rings. They say that they seat relatively quick now days but there are a lot of variables here too. Including type of rings used. Give it a little time. If it runs strong don't worry about it.
Posted By: maxwedge1

Re: 340 Compression test - 06/10/14 01:17 AM

I have similar readings 180-190 psi with x heads, 10.4:1.
a bit larger cam, 108 lsa and need minimum 100 octane to keep it from rattling.
Posted By: Challenger 1

Re: 340 Compression test - 06/10/14 01:33 AM

Quote:

I have similar readings 180-190 psi with x heads, 10.4:1.
a bit larger cam, 108 lsa and need minimum 100 octane to keep it from rattling.



Good point, my heads are Edlebrock aluminum.
Posted By: Crazy68Dart

Re: 340 Compression test - 06/10/14 01:37 AM

How exactly did you perform the compression test?
Posted By: RapidRobert

Re: 340 Compression test - 06/10/14 02:03 AM

Oh OK I see what you are saying. it did not ping but it was just not "right". A person could dribble some water in the carb at a fast idle to loosen up any carbon then take it out for a good blast to expell the carbon which "may" give a false high reading (clean the plugs afterward) and I loosen the plugs on any compression test several turns then tighten em back up then start it & gun the throttle several times to expell potential loose carbon particles coming off of the threads which could lodge under a valve and give a false low reading then pull the plugs & continue on with the compression test. Kinda picky but that's me. Actually I would forget about the SCR/cranking psi for now and button it up with the new intake then concentrate on fixing (in this thread) what ain't "right" on the tune then move on to the fuel needed and the ign curve but your initial sounds healthy and the total ain't outrageous & not sure when it comes in at (springs) but we gotta address/fix the "problem". But you do have iron heads with no quench and those cranking psi numbers are very high & I hear that gas out on the left coast is the worst. But If it ain't pinging then it ain't pinging & that's the bottom line. Stay on it & keep us posted
Posted By: 360view

Re: 340 Compression test - 06/10/14 02:00 PM

Now that NASCAR has limited static compression ratio,
the tech inspectors are said to be using a
"whistler" device
to make quick no tear down checks of static compression ratio.

http://www.katechengines.com/motor-sports/services/whistler/

has anyone on moparts used one of these?

Posted By: 360view

Re: 340 Compression test - 06/10/14 02:14 PM

Explanation by Jon Jeffords of Hartford WI:

the way it was explained to me,
the Katech whistler is basically an acoustic measuring device that functions much like when you blow air over the top of a beer bottle.

Oversimplified, as the whistler blows air over the the spark plug hole while the piston is at TDC, it measures the resulting natural frequency of the air volume resonating in the combustion chamber. Knowing the air temperature, the Whistler calculates combustion chamber volume at TDC. Knowing the engine's displacement (bore & stroke) and number of cylinders, the Whistler can then calculate compression ratio.
Posted By: mopowers

Re: 340 Compression test - 06/10/14 03:55 PM

Quote:

Explanation by Jon Jeffords of Hartford WI:

the way it was explained to me,
the Katech whistler is basically an acoustic measuring device that functions much like when you blow air over the top of a beer bottle.

Oversimplified, as the whistler blows air over the the spark plug hole while the piston is at TDC, it measures the resulting natural frequency of the air volume resonating in the combustion chamber. Knowing the air temperature, the Whistler calculates combustion chamber volume at TDC. Knowing the engine's displacement (bore & stroke) and number of cylinders, the Whistler can then calculate compression ratio.



Pardon the ignorance, but what would number of cylinders have to do with calculating compression ratio?
Posted By: mopowers

Re: 340 Compression test - 06/10/14 03:56 PM

Quote:

Also if the engine is fresh, give it some time to fully seat the rings. They say that they seat relatively quick now days but there are a lot of variables here too. Including type of rings used. Give it a little time. If it runs strong don't worry about it.



Thanks for the input. Ultimately, it's gonna go back together and run. I'll see how it runs before I make any changes.
Posted By: mopowers

Re: 340 Compression test - 06/10/14 03:59 PM

Quote:

How exactly did you perform the compression test?



The intake is already off. I first pulled the plugs. The battery was dying, so I actually had to jump it with the battery in my running truck so it had enough juice to do the test. I'm wondering if jumping it off a battery from a running truck may have given false high readings???

I spun each cylinder over for 5 pumps until the gauge quit going up, then took my reading.
Posted By: mopowers

Re: 340 Compression test - 06/10/14 04:03 PM

Quote:

Oh OK I see what you are saying. it did not ping but it was just not "right". A person could dribble some water in the carb at a fast idle to loosen up any carbon then take it out for a good blast to expell the carbon which "may" give a false high reading (clean the plugs afterward) and I loosen the plugs on any compression test several turns then tighten em back up then start it & gun the throttle several times to expell potential loose carbon particles coming off of the threads which could lodge under a valve and give a false low reading then pull the plugs & continue on with the compression test. Kinda picky but that's me. Actually I would forget about the SCR/cranking psi for now and button it up with the new intake then concentrate on fixing (in this thread) what ain't "right" on the tune then move on to the fuel needed and the ign curve but your initial sounds healthy and the total ain't outrageous & not sure when it comes in at (springs) but we gotta address/fix the "problem". But you do have iron heads with no quench and those cranking psi numbers are very high & I hear that gas out on the left coast is the worst. But If it ain't pinging then it ain't pinging & that's the bottom line. Stay on it & keep us posted



Thanks Rob. As always, I appreciate your help. I'll definitely keep you posted.
Posted By: 360view

Re: 340 Compression test - 06/10/14 04:34 PM

Quote:

The battery was dying, so I actually had to jump it with the battery in my running truck so it had enough juice to do the test. I'm wondering if jumping it off a battery from a running truck may have given false high readings???

I spun each cylinder over for 5 pumps until the gauge quit going up, then took my reading.



Were the higher reading cylinders done first, perhaps with fresher batteries and higher voltage to the starter?
Posted By: 360view

Re: 340 Compression test - 06/10/14 04:38 PM

I wondered about that "number of cylinders" input too.

Perhaps they need it to convert total engine cubic inches to per cylinder cubic inches, but i would think bore and stroke would give them that.

?
Posted By: 360view

Re: 340 Compression test - 06/10/14 04:48 PM

The $2300 Katech Whistler looks like a 1980s microprocessor with a microphone and frequency analyzer soundcard.

Todays smartphones or tablets have the computer chip, microphone and soundcard.

Someone who wants a home business
could write an Android/iOS app,
and supply the low volume air regulator
and whistling sparkplug screw in adapter.
Such an updated package kit could sell for less than $100

I wonder if there is a US Patent Number for the Whistler?
Posted By: RapidRobert

Re: 340 Compression test - 06/10/14 04:57 PM

# of cyls ain't needed for CR only for CID such as when you do the formula: bore x bore x stroke x .7854 (1/4 of Pi & dont ask me why) x # of cyls.
Posted By: Paul_Fancsali

Re: 340 Compression test - 06/10/14 10:01 PM

If it doesn't smoke oil foul plugs quit worrying and drive it That comp is fine by the time you hit that cylinder the battery was probably on it way down.
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