Camshaft

choices

With friendly permission of Doug Jackson

We wish we could bring you information on every camshaft, but, because there are so many grinds available it would be a momentous task to actually check each individual cam (for comparision reasons) using a standardized checking method (0.050" lifter rise) to establish valve timing sequences and valve open duration numbers. However, I have concluded that many MGB owners who are rebuilding engines will purchase a camshaft that is unsuitable for their particular driving needs and, most importantly, unsuitable for the appropriate engine operating RPM range. So, before you make this very important decision, you should be aware of several factors. In the following article we are going to concentrate on camshaft profiles that are suitable to street and street performance driving conditions. Please note: camshafts for racing applications are not covered in this article.

Trying to strike a balance among fuel consumption, performance, and acceptable exhaust emission levels is very difficult, therefore, some compromises have to be made.

Let us take a look at the MGB/MGA cylinder head with its siamesed intake port design. By virtue of its shared design, the intake flow dynamics result in #1&4 cylinders trying to "rob" their adjacent cylinders i.e. #2&3 of their incoming mixture charge. This is kept well under control by retaining a mild duration (214 deg @ 0.050" lifter rise) single pattern camshaft, together with the H4 or HS4/HIF4 SU carburetor(s) and intake manifold assembly. Incorporated in the manifold's design is a balance tube, whose primary job is to balance out cylinder pulses. As mentioned above, installing higher duration camshafts will always have a tendancy to increase the "robbing", however, the balance tube will continue to offset this abnormality.

Installing side draught carburetors, such as the Weber DCOE or Mikuni 40/44PHH, especially with the long intake manifold, is a different set of circumstances. Because these carburetors have no balance tubes incorporated in their intake manifolds to take care of the cylinder pulses, "robbing" is compounded. With this type of carburetor setup the more camshaft duration you add into the camshaft design, the greater these effects become. Consequently, the cylinder balance power will not be equal for all four cylinders.

Valve Lift

Higher valve lifts on the stock cylinder head can be achieved by increasing camshaft lobe lifts or rocker arm ratios, or a combination of both. However, keep in mind that it makes absolutely no sense to lift the intake, or to some degree the exhaust valve, beyond a point where the CFM flow rate increase decreases proportionally to decimal inches of valve lift. This max CFM flow rate will occur at approximately 0.375" (stock MGB cyl/head 1963-71 and 1975-80) and 0.400" (stock MGB cyl/head 1972-74 1/2). Further cylinder head work can be carried out to increase these CFM flow figures (a good reference source for this is Peter Burgess book " How To Power Tune MGB 4-Cylinder Engines For Road & Track ").

Roller Rockers

We have already pointed out the benefits of using roller style rocker arms in technical article MGB16 " Rocker Arms ". When the customer's budget allows, we install 1.55:1 ratio rockers on all our engine rebuilds.

Camshaft Lobe Lifts

We recommend that lobe lifts be kept in the region of 0.290" maximum or less because high lobe lifts come at the expense of reduced lobe heel diameters and extreme pressures at the lobe nose/lifter interface.

Camshaft Intake Duration (Street and Street Performance)

Due to the "robbing effects" that occur within the siamesed intake ports (1/2 & 4/3) we recommend that the intake duration be no more than 225 degrees @ 0.050" lifter rise for any given carb/intake manifold combination.

Camshaft Exhaust Duration (Street)

Increasing idle vacuum (HG) and reducing exhaust emission levels will reduce valve overlap and promote good gas mileage. We recommend that the exhaust duration be in the same region as the intake duration above.

Camshaft Exhaust Duration (Street Performance)

For street performance, we recommend that ehaust duration be greater than the intake duration, around 235 degrees (again measured at 0.050" lifter rise).

Intake/Exhaust Valve Peak Lift

The OEM single pattern camshaft specifications are as follows:
Duration 214 degrees @ 0.050" lifter rise.
Lobe lift 0.264".
Lobe centers 107.5 deg.
Exhaust valve peak lift 105 deg BTDC.
Intake valve peak lift 110 deg ATDC.

Intake valve peak lift should ideally take place at maximum piston velocity (76 degrees ATDC). However, due to 4 stroke engine design constraints this is not practical. Therefore, you should advance the intake valve peak lift over that of the OEM 110 deg ATDC figure. Regarding the OEM camshaft phasing the camshaft lobe centers = 107.5 deg. (105 deg. Ex peak BTDC + 110 deg. in peak ATDC = 215 crankshaft degrees, however, divided by 2 = 107.5 camshaft deg.). If the OEM camshaft phasing are for example 107.5 deg. BTDC (exhaust valve peak lift) and 107.5 deg. ATDC (intake valve peak lift), then both valves will open an equal amount when a paticular cylinder is at TDC during the overlap (exhaust/induction) period. This arrargement is commonly referred to as "spit-overlap". From this we can conclude that the OEM 110 deg. ATDC (intake valve peak lift) is actually -2.5 deg. retarded (107.5 - 110).

If the intake valve peak lift was 105 deg ATDC then we would be +2.5 deg advanced (107.5 - 105). The intake valve peak lift is always referred to "advanced", "split" or "retarded" in respect to the camshaft lobe center angle. All camshaft regrinders will indicate this posistion in their camshaft specification literature.

Let us continue to use the OEM camshaft as our example and do some camshaft phasing experimentation:

I have decided to install an offset keyway to the camshaft to attain the following: 103 deg ATDC (intake valve peak lift), 112 BTDC (exhaust valve peak lift) having now achieved a 7 deg. shift over the original OEM intake 110 deg. ATDC position and a 7 deg. shift from the OEM Exhaust 105 deg. BTDC position. This 7 deg. shift for the intake and exhaust valve posistions work to the advantage of the intake CFM flow rate (remember max piston velocity) and to the disadvantage of the exhaust CFM flow rate.

The critical area in the exhaust CFM flow rate is the flow at low exhaust valve lift heights (i.e., approaching TDC) and whic we compromised by closing the exhaust valve earlier. To calculate how much we have restricted the exhaust flow by this early valve closing we need to know how far the exhaust valve was originally open at 105 deg. BTDC and compare this to the new exhaust valve open height at the same 105 deg. BTDC. We know it will be less, but how much less? If we had the cylinder head CFM flow bench tested prior to assembling our engine, we could actually compare the exhaust valve CFM flow rates for these two exhaust valve open heights.

Another issue often overlooked during the engine rebuilds is we invariably increase the engine bore size (reboring). This will in effect increase the engine swept volume. Therefore, we are theorectically trying to flow a greater volume of exhaust gases, when compared with the original bore size, through a reduced area. It would behoove someone about to embark on an engine rebuild to do a little homework. Based on this homework you can make sound decisions as to proper components, for example type of camshaft, high ratio roller rockers, exhaust valve and sizing/porting.

Camshaft Recommendations (Street)

Part# 6625075-15 single pattern (Elgin Camshafts stock OEM grind) 217 deg.
duration @ 0.050" lifter rise.
0.262" lobe lift.
107.5 deg. lobe centers.
Intake valve peak lift 103 deg. (minimum) ATDC.
Exhaust valve peak lift 112 deg. BTDC.
Cam timing @ 0.050".
Intake 5.5 deg. BTDC & 31.5 deg. ABDC.
Exhaust 40.5 deg BBDC & 5.5 deg. BTDC.
Overlap 0 deg.
Valve clearance Int/Ex .013" hot. Use with 1.55:1 roller rockers, GCR 9:1,
LCB exhaust header with free flow system.
Good street grind with plenty of torque.

We recently rebuilt a MGB street engine (+0.020") 9:1 GCR with the above camshaft, A stock cyl/head 68-71, with 3 angle valve job (Intake 1.565" Exhaust 1.344"). Listed below are the components (in addition to the camshaft) that made this particular MGB a real pleasure to drive.

Compression Ratio 9:1
*Roller Rockers 1.55:1 Part # 1.55:1/KIT
*LCB Exhaust Header Part # EXL18
Free-Flow 2 inch exhaust system (Glasspack F18 front. F15 rear)
*Maniflow intake maniflod Part # SUB4-2
1.5" HS4 carbs with #6 metering needles * Part # AUD1005(2) Fixed *K&N Air
filters Part # 222-950(2)
*Velocity Stacks Part # 222-970(2)

*Total package for these items $980.57

Part# 671018X289 single pattern (Elgin Camshafts) 222 deg duration @
0.050" lifter rise.
0.289" lobe lift. 110 deg. lobe centers.
Intake valve peak lift 105 deg. (minimum) ATDC.
Exhaust valve peak lift 115 deg. BTDC.
Cam timing @ 0.050".
Intake 6 deg BTDC & 36 deg. ABDC.
Exhaust 46 deg BBDC & 4 deg. BTDC.
Overlap 2 deg. Valve clearance Int/Ex 0.016" hot.

Use with 1.55:1 roller rockers, GCR 9:1, LCB exhaust header with 2" free flow system. Requires exhaust CFM flow increase. Exceptional torque.

Camshaft Recommendation (Street Performance)

Although we have not actually installed the below camshaft into a rebuilt engine, we did however took a close look at its profile and found that its intake profile is almost indentical to camshaft part # 671018X289 which is the camshaft we have been using for many years as an alternate camshaft for street applications. The exhaust profile also meets our current criteria for exhaust CFM flow.

Moss part# 222-270 dual pattern (Crane part# 342-0010):
222 deg. duration (intake) and 232 deg. (exhaust) @ 0.050" lifter rise.
0.280" intake lobe lift and .294" exhaust lobe lift.
110 deg. lobe centers.
Intake valve peak lift 105 deg. ATDC.
Exhaust valve peak lift 115 deg. BTDC.
Cam timing at 0.050".
Intake 6 deg BTDC & 36 ABDC.
Exhaust 51 deg BBDC & 1 deg. ATDC.
Overlap 7 deg. Valve clearance (hot) Int 0.014" Ex 0.016".

Use with GCR 9:1 and all components listed for previously mentioned camshaft part # 6625075-15 and 671018X289. No reason why this should not be an exceptionally good street performance grind.

Part# 7008-10 single pattern alternate camshaft that we have used for many years with good results:
224 deg. duration @ 0.050" lifter rise.
0.278" lobe lift.
108 deg. lobe centers.
Intake valve peak lift 103 deg. ATDC.
Exhaust valve peak lift 113 deg. BTDC.
Cam timing @ 0.050".
Intake 9 deg. BTDC & 35 deg. ABDC.
Exhaust 45 deg. BBDC & 1 deg. BTDC.
Overlap 8 deg. Valve clearance (hot) Int 0.010" Ex 0.012".

Also, suggested components as outlined previously.

Numerous other camshaft profiles that are available will probably work just as well as the ones I have outlined above. However, we are unable to give any information on their characteristics. Also, we would not be surprised if the profiles of some of the other camshafts were similar to those described above.

Many camshaft regrinders establish their valve lift specifications by multiplying the cam lobe lift by the OEM rocker arm ratio of 1.426:1. [...] In all our years of engine rebuilding we have never been able to confirm this theoretical figure of 1.426:1.

A final recommendation, always degree your camshaft directly at the camshaft; i.e., do not use any references in regards to valve clearances to establish phasing positions.

For camshaft profiles Part# 6625075-15, 671018X289 and 7008-10 contact Elgin Camshafts at 650 364 2187. Be sure to request that your camshaft be nitrided (heat treated). Camshaft Part# 222-270 can be purchased through British Automotive at the current Moss Motors catalog price sheet less 10%.

Camshaft Lifters

There were two types of lifters fitted to the MGB engine i.e., the long style with short pushrods (up to 18V engines) and the short style with long pushrods (18V engines). Because we have found a very high failure rate with the long style we recommend using the 18V style combination only.

A camshaft lifter must be allowed to rotate to eliminate rapid wear on the lifter face and the camshaft lobe nose. This is taken into consideration in the design of both components. The lifter face is radiused and the camshaft lobe nose has a slight taper machined to it. However, the main criteria is the actual contact centerline between the lifter and the lobe nose; i.e., the centerline contact should be offset. This in conjuction with the lifter radius and lobe taper design will promote rotation of the lifter.

(Returning to the premature failure of the tall type lifter) If you check the centerlines of the camshaft and the rocker arm adjusting ball (when viewed from the front) you will see that they are different, resulting in pushrod lean towards the valve. It follows then that there must be some side loads associated with this lean and this side load must be transmitted to the lifter. Consequently, pushrod lean on the longer lifter is greater than the shorted lifter. This can be understood by simply making a drawing. The longer lifter also has a greater surface contact area than the shorter lifter. Therefore, the longer lifter suffers from greater sideloads. This sideload pressure also increases with increased valve spring #s and increased valve lifts by way of higher camshaft lobe lifts and/or increased rocker arm ratios.

We have not ruled out the fact that lifter hardness plays a key role in lifter failure. Because of this threat, we have every single lifter hardness tested (Rockwell C scale). And, yes, some lifters do not pass this test. Recently, we had a batch of 50 lifters tested and 2 did not pass the test. Can you imagine these 2 lifters finding their way into 2 separate engine rebuilds? That would be a hell of a lot of work to redo for the price of $1 the price for the hardness test. Be sure to have your complete set of lifters checked and throw out any that don't register in the region of 55 on the Rockwell C scale. Alternatively, we can supply the set(8) Part# 2A13/HP which also incorporate a small bleed hole in the lower portion of the lifter, which allows for better camshaft lobe lubrication and allows accumalated oil within the lifter to drain out.

For more information concerning parts, prices etc. please contact Doug Jackson at British Automotives.


All pictures courtesy of Andreas Pichler.

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