When I rebuilt the B20B engine the original C camshaft was replaced with a sportier (+12 HP) new K cam.
The valve clearance for an original B20B engine is 0.50 - 0.55 mm, so that's what I used for the first 200 miles.or so. However the valve clearance for a K camshaft in it's original engine, the B20E from 1974, should be 0.40 - 0.45 mm.
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| K-camshaft Volvo B20 |
There's a lot of info on Internet about camshafts and they all say that the camshaft dictates the valve clearance.
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| Specifications Volvo K-cam |
It made me wonder what the impact of a bigger or smaller valve clearance would be upon engine performance.
Let's take the valve clearance 0.50 -0.55 mm (original C-cam) used for a K-cam which should be 0.40- 0.45 mm (in its original engine). So the clearance for the K-cam is 0.1 mm too big.
A bigger than advised clearance is bad for you engine as the rocker will "hammer" the valve harder than normal, especially when the lobes are steep, and the valve will hit the valve seat with a higher than desired speed.....
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| Valve clearance too big |
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| Valve lash too big animation |
A too big valve lash will also create a smaller overlap (time that both the inlet and outlet valve are open), shorten the duration and give less lift, so less power (because the greater the lift the greater the power at all rpm). Hmmm...., ok first some cam basics:
Cam basics
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| Camshaft Lobe basics |
Normally the lash ramp is less steep than the opening and closing ramp, which will result in a lower velocity (speed) of the valve. If your valve lash/clearance is to big the push rod will already have reached a high speed before it hammers the rocker, the rocker will smash the valve stem and the valve will smash the valve seat. Driving too long with a too big valve clearance will damage the engine for sure.
The duration (duration of the valve lift) is the time that a valve is open during a cycle. For the K-cam it's 277°. If you know the intake opening and closing moment you can also calculate the duration; opening 27° before TDC + closing after 70° BDC +180° (from TDC till BDC is always 180°) = 277°
You have to realise that normally the cam angle specifications are given in crankshaft degrees. One turn of the camshaft is equal to two turns of the crankshaft. So the 277°advertised duration is only 138.5° on the cam. To make it even more complex; A cam can have more duration numbers. Duration normally refers to the number of crankshaft degrees that the lift is
greater than a specified value, e.g. duration at 0.050 lift. So it's really important to know the specified value when talking about the duration of a cam.
The earlier mentioned 277° duration is based on 0.020 inch lift. When we take the angles at 0.050 inch lift, 0° and 42°, the duration will be 222°. So if you want to compare camshafts keep this in mind!
The K-cam has the same intake and exhaust duration, if the opening and closing time for intake and exhaust aren't the Intake- and Exhaust Duration will be different.
Overlap: The time between the the intake Valve opening point BTDC and the Exhaust Valve closing point ATDC, in crankshaft degrees.
Formula to calculate Overlap:
intake opens + Exhaust closes
K-cam: 27° + 27° = 54°
Lobe Centerlines: The intake centerline is the point of highest lift on the intake lobe. It is expressed in crankshaft degrees after top dead center (ATDC). Likewise the exhaust centerline is the point of highest lift on the exhaust lobe. It is expressed in crankshaft degrees before top dead center (BTDC). The cam centerline is the point halfway between the intake and exhaust centerlines.
Formula to calculate the Centerlines:
Exhaust Centerline: Exhaust duration/2 - Exhaust closes
K-Cam: 277°/2 - 27° = 111.5°
Intake Centerline : Intake duration/2 - Inlet opens
K-Cam: 277°/2 - 27° = 111.5°
LSA: The angle between the intake and exhaust camshaft lobe peaks in camshaft degrees.
Formula to calculate the Lobe Separation Angle (LSA):
(Centerline Exhaust + Centerline Inlet) /2
K-cam: (111.5° + 111.5°) / 2 = 111.5°
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| Valve events graph Volvo K-cam |
Back to the quote "the greater the lift the greater the power at all rpm", is this true? Yes, as long as the valve won't hit the piston and as long as the valve is big enough... let me try to explain the valve part:
Under ideal circumstances (perfect valve clearance), according to above given K-cam specifications, the max valve lift should be 10.67 mm. When the valve clearance is 0,1 mm bigger the max valve lift will be only 10.57 mm. so, will you loose power? The answer is it depends on the valve diameter.
Actually it's possible to calculate the maximum effective valve lift. It isn't hard to imagine that the maximum flow can never be greater than the area of the valve. First let's calculate the valve area; For the B20B 1969 engine the inlet valve diameter is 42 mm and the valve stem diameter is ca 7.96 mm.
The valve area is 0.25 x Pi(ca. 3.14) x valve diam² --> 0.25 x 3.14 (Pi) x 42² = 1384.74 mm² MINUS area valve stem 0.25 x Pi x valve stem diam (7.96 mm)² = 49.74 mm² --> 1335 mm²
Keep this in mind for later.
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| Area and max lift |
When a valve opens there's a three dimensional area that becomes bigger the further the valve opens. The formula to calculate this area is A = 2 x Pi x r x h. (r = half the diameter of the valve, h = lift of the valve). When A is the same as the area of the valve it has reached the maximum flow, in other words the valve isn't obstructing the air flow anymore. Let's calculate A for a K-cam in combination with a 42 mm inlet valve --> A = 2 x 3.14(Pi) x 21(r) x 10.67(h) = 1407.16 mm². So the K-cam lift is more than enough for my B20 engine with 42 mm inlet valves (because A = bigger than the valve area).
The maximum effective valve lift for my B20 engine is 10.12 mm (calculate h when A = 1335 mm²), the K-cam in my B20 engine will lift the valves an extra 0.10 mm (10.67 -10.22 - 0.45 mm valve lash) but it won't create more power.
The original engine (B20E) has a 44 mm inlet valve diam , so the calculated Valve area is 1519.76 - 49.74 = 1470.02 mm². The calculated A = 2 x 3.14 x 22 x 10.67 = 1474.17 mm² The maximum effective lift is 10.64 mm (1470.02/2/3.14/22 =10.64 mm). So to unleash all the power, the max lift (10.67 mm), minus the valve clearance must be at least 10.64 mm. But because the valve clearance for the K-cam is 0.40 - 0.45 mm the maximum valve lift will be only 10.22 mm (10.67-0.45 mm lash).
Are you still there? I know...
Luckily there's a rule of thumb that you can use to estimate the maximum effective lift:
Max effective valve lift is around 24-25% of the valve disc diameter.
Another important rule of thumb is: The exhaust valve area should be 75% to 80% of the intake valve area; since the exhaust gasses are less dense and under more pressure, the exhaust valve can be smaller than the intake valve. Note that this is area of the valve and not diameter.
Let's take the 42 mm intake valve to calculate the diameter of the exhaust valve:
1384.74 mm² x 77% = 1066.25 mm² = Exhaust valve area.
10.66.25/0.25/3.14 = 1358.28 = exhaustdiameter².
Exhaust diameter = 36.85 mm
Of course I didn't want burned valves, this can happen when the valve lash is too small, especially when driving at high rpm. Valves, especially the exhaust valves need a certain amount of "contact time" with the valve seat in the cylinder head so they can get rid of the build up heat during the opening time of the valve (duration).
The 1974 Volvo B20E engine with the original K-cam has exactly the same exhaust valves and valve seat width (width of the valve seat edge that's making contact with the valve when closed), but the inlet valve disc diameter is 44 mm instead of the 42 mm in the 1969 B20B, this will make it a bit hard to compare.
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| Volvo B18 and B20 engine specifications |
But ...The B20B has a lower compression ratio (CR), 9.5:1 vs. 10.5:1 in the B20E.
The higher the CR, the higher the cylinder pressure, the higher the cylinder temperature. The higher the compression ratio, the colder the spark plug needs to be. A good rule of thumb is to go about one heat range colder for each full point in compression ratio increase. The term hot/cold is commonly used to describe whether a spark plug heats up easily (hot) or whether it provides resistance to heating up (cold).
The only thing is that Volvo advised a hotter spark for the early B20E than they did for the early B20B, which could indicate that the operating temperature of the B20E is a bit lower than the B20B.
But as the exhaust valves in the B20B and B20E are the same and this B20 engine has now hardened exhaust valve seats and a clean good working cooling system, I'm willing to take the risk, so I adjusted all the valve clearances to 0.45 mm and set the jets a bit richer. This resulted in a better driving 122S and a less noisy engine. I can still hear the tappets ticking but not as loud as before.
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| Volvo 122S |
