Topic: Compression test numbers, yours vs Honda book value.

[TwoTired]

So you did a compression check and the numbers were lower than book value.  Does it need an overhaul?
First question you should ask, is “Does my test equipment lie to me?”
Even if you assume the gauge is accurate (Hey, it’s brand new!), the means of connection can alter the readings.

We’ll make two examples A 350 Four and a 750 Four.
The 350 is actually 347cc displacement, making each piston move 86.75cc.  It has 9.3 compression ratio, making the combustion chamber volume 9.327cc.

My compression tester, purchased at the local auto parts store (see pic), has a long, fat hose on it.  The volume of this hose I measured as 11.5cc.  There is also the volume of the gauge itself.  But, I chickened out pouring alcohol in that for a volume measurement.  I also didn’t account for any rubber hose expansion incurred from higher pressures within the hose, increasing its volume capacity.

Anyway, the volume of test apparatus adds to the chamber volume during compression, because its volume has to be compressed to deflect the gauge needle.  So, instead of a 9.237cc combustion chamber, you’d have a 20.737cc total chamber with my compression tester, which effectively lowers the cylinder’s compression ratio from 9:1 to 4.1:1.

What of the 750?  736cc/4 is 184 cc piston displacement per cylinder.  Many (not all) 750s are 9:1 compression ratio, leaving a combustion chamber volume of 20.44cc.  Adding a test apparatus volume of 11.5cc yields 31.95cc total volume, which changes the actual CR to 5.76:1. 

Astute readers will note that the larger the displacement of the engine, the less effect test equipment apparatus volume has on the compression ratio, and the measurement numbers.  If you bought your tester at an AUTO parts house, like I did.  They expect your 350 to be cubic INCHES not cc.  The 350 in the auto world, is 5700cc.  Or, 16 times larger in displacement.  (Only 7 times of you have 750.) …Now you know.

So, why are Honda published numbers so much higher?
Looking in the Honda 500/550 shop manual, there is picture of someone doing a compression test.  The apparatus has a very long, thin, rigid metal tube between the gauge head and the spark plug hole.  Clearly this small tube apparatus was used so as NOT to add very much volume to the combustion chamber, which will result in a much closer representation of actual chamber pressures.   I don’t have such a gauge.  Therefore, I don’t know what the actual volume is added by the Honda test apparatus.

So, unless you have a test gauge like Honda or one that adds little volume to the chambers, your test result numbers will be lower than book values.

The numbers can still be meaningful even if they are lower.  All cylinders should still be within 10% of each other.  And, not significantly higher after adding a few drops of oil to the cylinder for a wet/dry comparison.  Don’t add too much.  Oil is not compressible, you just want enough to wet the piston rings to aid their seal onto the cylinder walls.  Good rings won’t be affected by adding oil.  Leaky rings will be.   
Note, adding oil volume increases the compression ratio by removing compressible space.  A few drops is plenty for small displacement engines.  If your combustion chamber is 9cc, adding a teaspoon of oil, lowers the compressible volume 5cc!  So, be careful that your noted rise in measured pressure is not due to too much added oil volume, rather than sealing off leaky rings.

[lone*X]

Excellent write up.  I have always known of the lower compression readings from the increased volume of the hose/gauge but I did not realize it could be so great on a small engine.  Like you, I have always been more concerned with the equality of the readings across all cylinders.  I do have one question on adding a few drops of oil to the cylinders.  On the 350/400/500/550's that have vertical cylinders this would work fine.  But what about the 750, with the cylinders at a forward angle.  Seems like a few drops of oil in the plug holes would only seal the forward edge of the rings?  Would using an aerosol oil be a better option for getting good dispersal?  That is what I have used in auto engines for years and it seems to work well.

[TwoTired]

Thanks, for the compliment. 

Aerosol oil should be fine.  Paying attention to the volume of course.  A thicker oil gives a better seal in this application.  But, I've always felt that the drops of oil under pressure in the cylinder atomized and distributed fairly well.  Further, any leakage at the rings would be an exit path for pressure and the oil, and therefore attract it's fair share for an effected seal.

V8 auto engines also have their cylinders canted.  So did the slant six.  VW engines were flat fours.  Never had an issue with getting oil to rings for sealing during a comp test.  Just a squirt from an oil can did the trick for diagnosis.

I've never done a comparison test between aerosol and oil drops, though.

Cheers,

[StrongPerf]

I thought the check valve in the hose made up for the volume... As the engine cranks each cycle builds up the pressure until it levels off at a certian pressure. Maybe it still matters...

I have made the mistake of using the check valve used in a car tire. The internal spring was much stiffer than the origional check valve spring. It gave an awfuly low reading. When I found my light tension origional valve and put it back in, the readings were back up to par.

I know rubber hose flex matters. I can feel the hose pulse in my hand at each compression stroke. I'm sure a hard line would show a higher reading.

Anyway, good to recognize that our tests aren't always that same as Honda did it!

[TwoTired]

I thought the check valve in the hose made up for the volume... As the engine cranks each cycle builds up the pressure until it levels off at a certian pressure. Maybe it still matters...

It matters where on the tester the check valve is located.  If located AT the cylinder tap point, then eventually the air compressed in the gauge apparatus will read the peak in the chamber.  Other wise, the volume between the spark plug hole and the check valve is decompressed each exhaust cycle and re-pressurized during the compression stroke.  That volume is added to the combustion chamber volume and effects the compression ratio.

Cheers,

[City Boy]

Very helpfull info Two Tired.I use a Snap On tester with check valve in end of hose fitting that threads into plug hole.

[void909]

Thank you for that post twotired. I had a buddy of mine come buy who works on motorcycles for a living. He did a compression test on my non running bike and said my compression was very low. We didnt put any oil in the plug holes (I had never heard of that until after) and the exhaust was off the bike. Would either of these cause a low reading?
 

[TwoTired]

Thank you for that post twotired. I had a buddy of mine come buy who works on motorcycles for a living. He did a compression test on my non running bike and said my compression was very low. We didnt put any oil in the plug holes (I had never heard of that until after) and the exhaust was off the bike. Would either of these cause a low reading?
 

The oil is the wet test used to diagnose leaky rings.  Ideally, you want to test compression at the engine's operating temperature without the oil (then with some oil, if the dry test readings are low).  No exhaust doesn't make much difference in a comp test, as long as the outlets aren't sealed.  But, if the comp tester was intended for larger displacement engines like cars or trucks, then the low reading could be an artifact of that particular test apparatus.

You also need to let the engine intake air easily, like with the throttle wide open and the choke butterflies wide open, too. (That's choke OFF!)

Cheers,

[Soos]

Ok, so you got me confused now.....But TT you can do that so easily to me

(steps off the short bus)


So anyone with more brains and info than I have....

What would my CR be on a 650 thats bored to 61mm reading  between 190 and 210lbs of pressure?
(depending on the tester, my father-in law''s units show from 190-195, the one my nmeighbor has reads 210, and it's fairly new)
Stock book value says 170'ish lbs of pressure I believe.



l8r

[TwoTired]

The CR is the ratio of the swept piston area to the combustion chamber volume.

Swept piston area is a function of the bore and stroke of the motor.

The chamber volume is a function of the piston top shape and the head recess shape/volume, when the piston is at TDC.

There are other things that can effect the chamber pressures made in a 4 stroke motor, such as the cam grind and overlap of the intake and exhaust valve opening.

There are simply too many variables to calculate the CR from a pressure reading.  Especially on an uncalibrated pressure gauge of unknown test apparatus volume.

Cheers,

P.S. It's not my intention to confuse, but rather elucidate.

[dusterdude]

also keep in mind,cranking pressure is affected by the overlap on the camshaft,so if you`re running a non-stock cam that could whack your numbers too

[KeithTurk]

I simply keep data ... if a camber goes bad it's easy to see

K

[kos]

Compresson guages  don;t tell you much. Perhaps how close the cyliders are to each over, and that is about it.


Get your self a leak down tester...now you are gathering usable data. When cylinder is under pressure you get the leak down number and listening at carb side you can hear if air is coming out which means you have a leaking intake valve. Do same for exhaust (pipe off) air leak there, ex valve leak.

Listen to crank breather and hear air....you have piston ring seal problem...now that is usable data.

KOS

[tbpmusic]

TwoTired -

I think it's a great and studied examination of the subject.
So good that I reprinted it over at another forum (didn't claim that I wrote it) - unbelievably, that bunch of guys were adamant that it was crap, completely discounting the irrefutable, implacable logic of the math.

I doubt if I'll visit that group anymore, despite being a long-time member. I won't mention the forum's name, but it rhymes with 450.
What a bunch of know-it-alls. How can you argue with math???

If I ever get to the point where I absolutely refuse to consider another angle on a subject, I hope Spiderman jumps out of the rafters and slaps me upside the head.....

bill2

[SteveD CB500F]

Thanks for ressurecting this thread - I must have missed it first time round.

I've put a copy in the (Engine) FAQ

Steve

[medic09]

Great post, TT!

On a slight tangent, it helped me reappreciate the importance of simple physics in so many things we do.  When we ventilate a patient on a mechanical device, the length of the tubing (dead space) and the compliance of the tubing can be factors. The same for any filters or humidifiers we add in the middle, and where we put them. The size of the patient (adult vs. baby) can change how much effect/importance that dead space has; like in the difference between large and small cylinders.

It really is amazing and interesting how the same principles govern so much of the world around us. 

[TheHun]

My compression hose is little over 1.5 foot with 3/16 ID hose...but i guess as long as the numbers are the same across the cylinders, I should be fine.

[6pkrunner]

Gonna be the fly in the ointment here. Sorry. While your calculation for swept area (displacement) is fine...

pi/4 x bore x bore x stroke x number of cylinders for total displacement

or

p/4 x bore x bore x stroke = one cylinder displacement.

3.14159265/4 x 61 x 61 x 63 = 184.11539ccs

However this is just piston swept area (displacement).

Compression ratio is total cylinder volume divided by compressed volume.
For total volume one needs to add to that swept area compressed head gasket volume, combustion chamber volume, deck volume (deck height), piston dome volume (any piston dome minus valve reliefs) and divide by all those minus the swept area.

For the sake of simplicity lets assume a zero deck height with a flat top piston and a compressed head gasket volume of 2ccs and a chamber volume of 21 ccs.

Therefore 184 + 2 + 21 / 2 +21 = 207/23 = 9.0:1 static compression.

If anyone has the actual CB750 values for compressed head gasket volume, true deck height and total piston dome volume - plug and play.

As Dusterdude stated for dynamic compression cam overlap will come into play as compression will be bled off. Old hot rodders trick. Also will bad ring seal, poor valves and/or seats, etc.

[TwoTired]

OK let's use your formula, 6pkrunner.

Therefore 184 + 2 + 21 / 2 +21 = 207/23 = 9.0:1 static compression.

But add 11.5 cc test apparatus volume to both the sides of the division.

(184 + 2 + 21 + 11.5)/ (11.5 + 2 + 21) = 218.5/34.5 = 6.33:1 compression ratio with the compression test apparatus added

My simplified (and flawed) calculation showed 5.76:1 for the 750, an error of 0.57:1.
Does this still not demonstrate how you will get lower compression readings with an apparatus that adds more volume than what Honda used for the published value?

Should the FAQ be changed to get the fly out of the ointment?

Cheers,

[SteveD CB500F]

My brain hurts.

Do you want me to change the FAQ or not?

 

[6pkrunner]

**EDIT** I'm sorry I woefully misread your last post. You are asking how you got a much lesser value than what you should have.

Two things - engine wear, and primarily "mass production". Even the almighty Japanese must bow to production tolerances. The advertised static compression ratio is a factory derived number for all things being equal under ideal circumstances. While I have not done actual compression checks measured by actually cc'ing  and mic'ing bores and checking deck heights on SOHC4s, I have done it many, many times to Mopar engines. The advertised compression ratio for a 1966-1970 440 Mopar is 10:1. Not one that I have measured has left the factory anywhere near that. The worst was a kerosene ranged 8.2:1 - and this for a hi-po 1969 440. Deck heights and chamber volumes are the greatest areas of variance. A 906 hi-po head is supposed to be in the 73-76 cc range and if you get one less than 80-86 you're doing good. The 915 closed chamber head from 1966/1967 is closer but still in the 78 cc range. The head gasket volume were closer, but that is a much smaller area to affect compression. Castings change as the molds they are cast from wear, and even crank and rod production varies. Compression height of pistons during batch runs vary. Get 4 pistons that are identical for shape and weight and that's doing good. The factory manual call out what the specifications should be - but never are. Just the facts of life.
The closest to a 10:1 compression ratio for a 440 I got was 9.6:1. This is where blueprinting an engine comes in. Bring it to factory specs and they work a world of difference better.
But measuring compression via gauge has to take into account engine condition, leakdown and such. Static (advertised) compression is measured and really has no bearing on what the real world may find. Ignore the following - unless you're into it for giggles and chits.

Again sorry for misreading your prior post.

Here's the misread reply.....

But you are adding volume. Total volume divided by compressed volume will vary by any changes one makes. Its a dynamic ratio (relationship) not static. Keep all the items the same and add just a domed piston. Lessens total volume and lessens compressed volume - the compression ratio goes up. Add total to the total (11.5 ccs) and add to the total compressed volume and compression will go down. Ratios are affected any changing of the variables

Base  207ccs/23ccs = 9.0:1

Add your 11.5 = 218.5/34.5 = 6.33

lets instead add a 7cc domed piston to the mix

200 (gotta subtract as the dome lessens both total and compressed volume) / 16 = 12.5 :1

Your 11.5 ccs is only a 5.26% change to the total volume (207-218.5) but those 11.5 ccs make a 33% change in compressed volume (23 - 34.5) - so you're gonna get those results.

The percentage of total volume is less affected than the percentage of the compressed volume and therefore the compression ratio changes. Lessen the total volume and lessen the compressed volume by the same amount and the ratio will increase. This is where domed pistons came into play. You are doing the opposite, so naturally the compression will go down. One cannot change both volumes and expect ratios to remain the same. Its not a constant 11.5ccs, but the percentage of those 11.5ccs on total volume affected by the percentage change of compressed affected.

Mill the head - the total volume decreases and so does the compressed volume. however by milling the head the compression ratio increases as the percentage of compressed is more affected than the percentage of total volume.

23 ccs is 11.1% of 207ccs

34.5ccs is 15.78% of 218.5cc

The ratio cannot remain the same.

[TwoTired]

My brain hurts.

Do you want me to change the FAQ or not?

 

No, I don't *think* so.
From the pictures I see in the Honda manual, the compression test gauge used by Honda added very small volume to the chamber.
The automotive testers I see commonly available at local stores have larger attachment hoses, which, by visual observation must add more volume to the chamber being tested.

My assertion is that if you use Honda's gauge you will likely get the PSI numbers in the book when the engine is in good condition and fairly fresh.  But, if you don't note the volume differences between the apparatus Honda used vs. the one you purchased locally, you can't expect to get the numbers published by Honda from ANY bike, as the test apparatus affects max PSI readings.

If, I understand 6pkrunner's argument, he is saying that Honda's published numbers are for an ideal engine and that few, if any, still under our scrutiny would meet the published value, even if you used the Honda compression test apparatus,  as production tolerances and wear factors different head gasket thickness, etc., all add to reduce the ideal value.  He also agrees that the amount of volume added during the compression test, does effect the CR ratio and thus the peak pressure that can be created. (Let me know if I got this wrong, OK?)

The element I'm going to focus on (and perhaps disagree with 6pkrunner) is Honda published PSI value, which I don't believe is an ideal engine calculated value, but a maintenance check number intended for Honda's own service techs (in the field), to be used as a determination to do warranty repair.  Honda techs would be expected to use the same style apparatus that was used to make that maintenance manual's comparative PSI number.  If tested numbers were lower or higher than the book values, Honda would pay the service center to correct the discrepancy.  I expect our forum readers wish to determine actions based on the same criteria.  Check out page 18 of the CB500-550 HMM.  Or, page 174 of the CB750 HMM.

Readers in this forum often buy a local cylinder head pressure gauge and assume its readings are indisputable, then read Honda's manual and are alarmed at the disparity when they test their recently acquired bike's engine, without considering that the test apparatus may not give absolute information comparable to the published Honda Maintenance Manuals (HMM).  The FAQ post still addresses this error of assumption properly, I think.

I don't dispute 6pkrunner's posted formula(s).  However, it uses numbers (gasket cylinder volume for example) that are estimates.  My formula for pointing out test apparatus contribution to absolute PSI readings was also an estimate, but accurate enough to demonstrate the effect of test apparatus volume on the test.  If necessary, we might add these variables (as an unknown quantity) into the CR equation.    Is it necessary? 
Personally, and with my rather high volume test apparatus, I just confirm that all (warmed) cylinders are within 10% dry and then again wet with minimum (and the same amount of) oil added to each cylinder.  If it's out of whack, then I take it apart.

What part of the original post do you wish to change or have issue with, 6pkrunner?

Cheers,

[6pkrunner]

I was working on compression ratio viewpoint. You're working on cylinder pressure. The two are related, but more like cousins than siblings. Cylinder pressure is more real wold performance for checking variances between cylinders than an absolute. Cylinder pressure will check cylinder to cylinder and give you a general overall idea of how the engine is internally. General rule of thumb for pressure is 15% or so between the cylinders. This leeway right away lets you know there are variances in production tolerances and that within the same engine parts and wear is not absolute.
Cylinder pressure lets you know the amount of leakdown and/or lost compression you have in a real work working engine, not what the manufacturers designed.

[Michael pinfold]

HI all   I have been reading about the discrepancy in real verses apparent  compression ratios  due to the added volume of the sampling instrument   and im going to try this idea on my own tester ..  why not just fill the internal tubing of the   compression tester with a non compressible fluid  like   maybe engine oil  and thus eliminate the compressible  volume .perhaps one could pick a fluid that may be kinder to rubber  on the  valve  and tubes  im sure the gauge is metal inside  , soyabean oil ??  would that be safe on rubber ??  just a thought   regards  Mike .... CB350F  .. in   Rotorua  NZ

[TwoTired]

Probably would work in theory.  But, there's this thing called gravity.  I suppose you could test the motor upside down to keep the fluid inside the test apparatus, instead of it dribbling down into the cylinder.  Still, there the gauge calibration.  Just because the gauge has markings, doesn't necessarily mean the indication is real.  Compression testers are good for comparative measurement among cylinders to indicate equality.

You want to know better what's going on, get a leak down tester.  This will tell you WHERE there are leaks, rings, valves, gaskets, etc.

Cheers,