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Compression Testing Issue

Rod Stewart

Rod Stewart

Active Member
Hi All;
So at home I have a basic old compression tester with a push in rubber tipped spigot, that has worked great for many years. Wanted to have one here in Arizona as well, so went to Harbor Freight and picked up a nifty kit with several adapters and an extension flex hose.
The bike is an '07 RK with 103" bigbore kit, SE 255 cams, stock 85 cc heads, flat top pistons, etc.
Allowing .009 for deck clearance, .045 for head gaskets, 1.5 cc for valve clearance, I figure 9.72 SCR and 9.27 CCR using the 255 cam intake closing angle of 25 deg. And this should give a CCP of around 191 psig. (184 psig corrected for altitude)
Initial testing of the bike produced 155/160 psig F/R readings using the 18" flex hose and 12 mm plug hole adapter supplied in the tester kit. I was puzzled as to why these readings seemed so low.
So today I reran the tests but this time using the short metal offset rubber-tipped adapter, which requires two hands to seal against the pressure. This time I got 185/190 psig F/R readings. Pretty much bang on what I expected all along. (Altitude here is about 1200' ASL)
In thinking about this I figure the difference must be the volume of air in the plug adapter and hose. This volume effectively becomes part of the total head "squish" volume because it compresses and expands with every compression stroke. The gauge release valve is located on the base of the gauge itself.
So my conclusion is that short test adapters give the most accurate compression results, and longer hose adapters should be avoided unless its the only option.
Anyone else out there noticed anything like this?
 
We use Snap On compression and leak down testers here at my shop. Both have a 10 inch adapter hose connecting the to the gauge that has an additional 27 inch hose, the gauge has the relief on it. As long as your cranking 5 revolutions, with a strong battery and a starter that is consistent in providing even RPM, I don't think the 18 inch hose will affect the reading as long as there is no leakage from it or the gauge. I would first test the accuracy of the Harbor Freight gauge, this can be done even with an air compressor by allowing it to reach full pressure, 110 lbs lets say, than plug the air hose into the quick disconnect and see how close it reads to the air compressor gauge, or compare the reading with a third quality tester.
I had a tech that use to buy his stuff from Harbor Freight and 1 was a compression tester, it was way off.
 
I would agree with Joel; the hose is a non issue. Test the accuracy of the HF tester as well as check the accuracy of the hand held tester gauge. I would not trust a compression tester that required hand held pressure to seal the cylinder. I use an OTC tester and found it to be 2psi off; not much of a variance when checking compression but makes more difference when leak down testing. Once you know you are working with an accurate tester, then you can compare actual results with calculated results. However, unless you have measured and verified all the data you enter into the calculator, you cannot be sure that the calculated CCP is correct. For instance, HD combustion chambers can vary from 85-86cc and I have measured some at 89cc. Nor have I seen seen any 96"103" pistons that were .009" down the hole; possible but just haven't seen it. True that the later 96" and 103" pistons are typically lower in the cylinder than in the 88" engines. The later pistons are typically .005"-.007 down in the hole compared to the 88" pistons which were typically down .003"-.005" or at least that has been my limited experience, but I digress.

Point being that unless your data inputs to the calculator are accurate, it will be impossible to make a direct comparison; close but not spot on. It has also been my experience that many times, actual CR does vary from calculated. Having said that, the variance won't be such that calculated is 190psi and actual is 150psi; that would surely indicate a problem.;)
 
Using the threaded adapter is the prefered method in today's world but there was nothing wrong with the hand held pressure gauge, it was used for many years by professional mechanics.

My dad who was a master mechanic both gas and diesel engines still has 1 in his box and i'd use it in a heartbeat.

Brings back old memories of me as a kid cranking over the engine while my dad took the readings.
 
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The difficulty of proper fit as to where plugs are located nowadays a non threaded gauge would make it difficult to properly seal gauge in plug hole.
 
Are you holding the throttle wide open during testing?
 
One thing I may not have made clear; it was the same HF gauge used in both tests. One test used the 18" flex hose; the other used the short angled adapter. The only difference in the two tests is the method of connecting the gauge to the spark plug hole.
The suggestion to test the gauge accuracy is a good one regardless. I have no means of testing it here in AZ but I will test it once we are back home against other gauges I have, in a couple of weeks.
But the fact remains: the readings were about 30 psig different and the only tangible explanation is the larger volume of air trapped in the 18" hose adapter compared to the shorter metal adapter.
What else could it be?

Jack Klarich said:
Are you holding the throttle wide open during testing?
Click to expand...
Check, Jack. Wide open in both cases, and pumped up at least 8 compression strokes each time.
This ain't my first rodeo doing compression testing, meaning no disrespect. :D
 
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