Compression Test Results

[Bob Bidonde]

My engine is a Model B with a 0.060" over cylinder bores & a Snyder's 6:1 cylinder head. Compression test results: #1 140 PSIG; #2 134 PSIG; #3 135 PSIG; #4 142 PSIG. I live on Long Island which is at sea level. Average cylinder pressure is 138 PSIG. 138/14.7 = 9.4 that indicates the actual compression ratio of the engine is much higher than 6:1.

I am thinking that the high compression ratio is the added effect of the actual combustion chambers in my engine, so the advertised cylinder head compression rating is solely based on the volume of the head's combustion chamber.

What do you experts say?

 

[WHN]

You have to get over 1,000 feet above sea level for that to really start effecting your compression ratio. 1,000 feet is .971, it really drops at 2,000 feet to around .94.

How do you think the high compression is effecting your engines lower level?

Enjoy.

 

[nkaminar]

An interesting test would be to actually measure the volume at TDC. Take one spark plug out and fill the chamber to the bottom of the spark plug hole with oil. Measure the amount of oil you put in and do this with the piston at the top of the compression stroke. After this turn the engine over by hand to expel as much oil as possible and then start the engine outdoors.

 

[Synchro909]

It's fairly well known that the stated compression ratio of these after market high compression heads is higher than claimed but I'm surprised it is that much over.
What is the maximum ignition advance you are running?

 

[Bob Bidonde]

Synchro909, if you all recall, I posted about my turmoil trying to regain the full travel of my spark lever quadrant with the 6:1 head because of violent spark knocking with trhe lever more than 2 notches down. My thought was to significantly reduce the spark advance by tweaking the ignition timing. However, many disagreed with my theory, so for the time being I stopped my experimentation. However, instead of tweaking the timing using the distributor cam, I am going to do it by bringing the notch in the camshaft gear into alignment with cover hole (TDC), then going past the cover hole (piston goes past TDC retarding the spark) until I get full travel of the spark lever without serious knocking.

 

[alexiskai]

Synchro909, if you all recall, I posted about my turmoil trying to regain the full travel of my spark lever quadrant with the 6:1 head because of violent spark knocking with trhe lever more than 2 notches down. My thought was to significantly reduce the spark advance by tweaking the ignition timing.

I believe you when you describe the knock behavior you're getting, but it really seems like you're treating a symptom rather than the disease. Two notches is 10-14° advance. I've never heard of an A or B engine with mild speed parts having that small a timing window. Severely retarding your initial timing won't solve the problem, it'll just mean you won't be aware of the problem anymore – at least until whatever's causing it puts another rod through your engine block.

 

[Jim Brierley]

My engine is a Model B with a 0.060" over cylinder bores & a Snyder's 6:1 cylinder head. Compression test results: #1 140 PSIG; #2 134 PSIG; #3 135 PSIG; #4 142 PSIG. I live on Long Island which is at sea level. Average cylinder pressure is 138 PSIG. 138/14.7 = 9.4 that indicates the actual compression ratio of the engine is much higher than 6:1.

I am thinking that the high compression ratio is the added effect of the actual combustion chambers in my engine, so the advertised cylinder head compression rating is solely based on the volume of the head's combustion chamber.

What do you experts say?

You are right, and 9.4:1 is rather high, and will require less spark advance than a stocker. It should give you good power and fuel mileage. Do not let it ping!

 

[Johnbeere]

Synchro909, if you all recall, I posted about my turmoil trying to regain the full travel of my spark lever quadrant with the 6:1 head because of violent spark knocking with trhe lever more than 2 notches down. My thought was to significantly reduce the spark advance by tweaking the ignition timing. However, many disagreed with my theory, so for the time being I stopped my experimentation. However, instead of tweaking the timing using the distributor cam, I am going to do it by bringing the notch in the camshaft gear into alignment with cover hole (TDC), then going past the cover hole (piston goes past TDC retarding the spark) until I get full travel of the spark lever without serious knocking.

Just wondering - do you know if you've got an A or B timing cover? The B covers have the timing pin in a different, more advanced position. I've got a 6:1 head on my car and the only time I've ever heard it pinging was when I was pulling a hill in 3rd OD at 55 mph w/ the spark lever any more than halfway down the quadrant. My car is timed such that one notch down on the advance lever fires at top dead center.

 

[johnneilson]

Hmmm
The overbore on the cylinders will change the CR but only a tenth or two not from 6 to 9
Timing cover can mislead TDC setting of distributor so could attribute to lever settings
Another timing issue that can lead to higher cylinder pressure is the cam timing
Here again the pressure will increase a moderate amount not 40 or 50 psi

The head needs the chamber measured for volume to get accurate CR
While the head is off check cam timing and TDC reference for distributor

Also check the pressure gauge used for the test they can be misleading

J

 

[AzBob]

My engine is a Model B with a 0.060" over cylinder bores & a Snyder's 6:1 cylinder head. Compression test results: #1 140 PSIG; #2 134 PSIG; #3 135 PSIG; #4 142 PSIG. I live on Long Island which is at sea level. Average cylinder pressure is 138 PSIG. 138/14.7 = 9.4 that indicates the actual compression ratio of the engine is much higher than 6:1.

I am thinking that the high compression ratio is the added effect of the actual combustion chambers in my engine, so the advertised cylinder head compression rating is solely based on the volume of the head's combustion chamber.

What do you experts say?

For comparison, my B engine with .060" bore and Snyder's 6:1 head averages 105 psig across all cylinders. I am at 5300 ft elevation. Max advance set @ 28° BTDC. I have no pinging issues.

 

[Synchro909]

I thought about this thread last night and told myself to check it today.
I would do away with any form of manual advance and have a distributor put together by a specialist that gives a good curve and a limited advance.
On one of my engines, I run a 6:1 head and made a stop that I screwed to the side of the distributor. On that stop was a screw like the throttle stop on the carburettor. I set that to prevent the lever going past 22° advance. WHAT A PITA! When I moved off in first gear, I had to gradually move the lever down till I was ready to change into second gear, then return it to the retarded position as I changed gears, then advance again as I went up through the revs, then bring it back to the retard stop again as I changed into top gear and advance again as I built up speed. I simply ran out of hands so I decided to let a better distributor do the work for me. I had an ignition place near our clubrooms build a distributor for me with a max advance of 22° and to my surprise, when I picked it up, I found they had added a vacuum advance mechanism too.
Once I fitted that and got the base timing right, the car was transformed - and I could enjoy the ride. Unless I open the hood, no one knows what is under there. (except me!)

 

[Hitman]

Verify with another spark plug adapter, hose and tool/gauge before doing anything else

Who manufactured the one you have now and when was it made?

 

[Fullraceflathead]

I would borrow somebody else's compression testing gauge and see if you have similar readings.

 

[rotorwrench]

I think the basic initial compression ratios advertised for aftermarket heads is based on a standard bore. If a person knows the clearance volume of the cylinder head then it's relatively easy to calculate the actual CR based on current bore size. If the manufacture doesn't list the CC of the clearance volume then it will require a flat piece of plexiglass with a hole drilled to fill the chamber, a way to secure it to the head, and some liquid to fill the chamber. The amount would require a syringe to hold the liquid and get a basic CC volume measurement.

The clearance required for the valves to open pretty well limits the chamber size on a flathead. The lift of the cam and the dome height of the piston affects the size as well so there are a number of factors to consider for a minimum amount of clearance volume and still have safe and efficient operation.

 

[Bob Bidonde]

This is my primary compression test gauge which is built for aircraft & automotive use. It was made by Universal, but the hose & adapter are my additions. I have been using this gauge for many years & I know it is accurate.

 

[Bob Telep]

My engine is a Model B with a 0.060" over cylinder bores & a Snyder's 6:1 cylinder head. Compression test results: #1 140 PSIG; #2 134 PSIG; #3 135 PSIG; #4 142 PSIG. I live on Long Island which is at sea level. Average cylinder pressure is 138 PSIG. 138/14.7 = 9.4 that indicates the actual compression ratio of the engine is much higher than 6:1.

I am thinking that the high compression ratio is the added effect of the actual combustion chambers in my engine, so the advertised cylinder head compression rating is solely based on the volume of the head's combustion chamber.

What do you experts say?

Bob, calculating compression pressure by multiplying ambient pressure and compression ratio only does not take into account the temperature rise and heating of the air by compression. The actual pressure rise depends on how the rate of heat transfer of the air to the surrounding area of the engine. If there was no heat transfer, you would have what is called adiabatic compression and the theoretical pressure rise would be 14.7*(6^1.4) = 181 psi. If all heat was dissipated the pressure rise would be 14.7*(6^1)=88 psi. In practice, the exponent will be somewhere closer to 1.3, which would result in about 150 psi. Another variable is the timing of the intake valve closing, which reduces the static compression ratio. Thirdly, of course is cylinder leakage. Lastly, I assume the "6:1 head" is based on a standard bore, so increasing the bore will raise the static compression ratio.
In taking into account all these variables, I would venture to say your cranking compression pressures are very much "in the ballpark".

 

[nkaminar]

Bob Telep, Good call. Thanks.

 

[Hitman]

This is my primary compression test gauge which is built for aircraft & automotive use. It was made by Universal, but the hose & adapter are my additions. I have been using this gauge for many years & I know it is accurate.

It maybe accurate, but even you are questioning the validity of the results. Another gauge and test would confirm or further question.

 

[Diastole]

The compression ratio is the ratio of the volume of the combustion chamber (a), plus the volume of the cylinder (b), divided by the volume of the combustion chamber (a).
Or, (a + b)/a. This reduces to 1 + b/a.
If the original combustion ratio is 4.22, then 1 + b/a = 4.22, so b/a = 3.22.

Snyder's 6:1 head is listed to have a quench volume of 152 cc.
Your cylinder's are overbored 0.060". That increases the sweep volume (displacement) from 50.1 cu. in. to 51.69 cu.in. or 821 cc to 847 cc.

Assuming your gasket space volume is the same or the piston rises above the gasket, pop-up, this should not change. And, if your piston is flat (stock), no dome, then the quench volume stays the same.

Therefore, 0.060" over bore increases volume by about 1.5%. This changes the sweep volume (displacement) 3%.
displacement volume = pi x radius x radius x stroke length (4 1/4" in Henry's engine)

The 6.1 head decreases the combustion chamber by 41% but it is a much smaller volume than the cylinder volume.

so the calculation works out that your predicted CR should be 6.57:1 for an overbored engine by 0.060", stock piston with stock piston-pop-up, stock crank and rods, standard headgasket and appropriate torqued head nuts, standard valves (if your rebuilt valves ride high, they will consume some of the combustion chamber volume and the quench volume, subsequently the CR will go up because the combustion chamber is smaller), and standard plugs (the plugs take up some of the volume too by riding below the head.
The smaller plugs with the adapter increases the combustion chamber volume).

All these can affect the size of the combustion chamber the pistons are compressing the increased displacement volume into.

All the HC heads I have measured have CRs higher than stated, even in sleeved cylinders (returned to stock dimensions).
The Snyder's 6.1:1 head calculates to a 6.4:1 compression ratio if Henry, himself, were to place it on a new engine at the Rouge Plant.

I hope this helps.

 

[Dodge]

My engine has a verified 7:1 comp ratio with a .080 bore and it reads 140psi on the compression gauge.