Technical Information (MGB 20)
Last Modified - 03/05/01
I wish I could bring you information on
every camshaft out there, but unfortunately, there are so many grinds available
it would be a momentous task to actually check each individual cam (for
comparison reasons) using a standardized checking method (.050" lifter rise) to
establish valve timing sequences and valve open duration numbers. However, I
have concluded that many MGB owners, who are in the engine rebuilding process,
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, which should be helpful and guide you in the right direction.
In the following, we are going to concentrate on camshaft profiles that are
suitable to street and street performance driving conditions. Camshafts, for
racing applications, are not covered in this article.
Trying to strike a balance between 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 this 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 was all kept
nicely 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, who's primary job is to balance out cylinder pulses.
Installing higher duration camshafts will always have a tendency to increase
the "robbing" effect that we have mentioned above, however, this balance
tube continues to offset this abnormality.
Installing side draught carburetors, such as Weber DCOE or
Mikuni 40/44PHH, especially with the long intake manifold, is a different set
of circumstances altogether, with no balance tube incorporated in the intake
manifold design to take care of the cylinder pulses, we compound this "robbing" problem. With this type carburetor setup the more camshaft
duration you add into the camshaft design, the greater these effects become
and, consequently, the cylinder balance power will not be equal for all four
Camshaft Lobe Lifts
Camshaft Intake Duration (Street and Street
Camshaft Exhaust Duration (Street)
Camshaft Exhaust Duration (Street Performance)
Intake/Exhaust Valve peak lift
Ideally, intake valve peak lift should take place at maximum
piston velocity (76 degrees ATDC) unfortunately, due to engine 4 stroke design
constraints this is not practical, However, we should advance the intake valve
peak lift over that of the OEM 110 deg ATDC figure.
Now, If the OEM camshaft phasing had been, let us say 107.5
deg BTDC (exhaust valve peak lift) and 107.5 deg ATDC (intake valve peak lift)
then both valves would be open an equal amount when a particular cylinder is at
TDC during the overlap (exhaust/induction) period, this 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 position in their camshaft specification
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 degree 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 degree shift for the intake and
exhaust valve position work to the advantage of the intake CFM flow rate
(remember max piston velocity) and to the disadvantage of the exhaust CFM flow
The critical area in the exhaust CFM flow rate is the flow
at low exhaust valve lift heights (i.e. approaching TDC) and we have now
compromised this by closing the exhaust valve earlier. To figure out how much
we have restricted the exhaust flow CFM by this early valve closing we would
have 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 point often overlooked during the engine rebuild
process, is, we invariably increase the engine bore size (reboring) which in
effect increases the engine swept volume. So, theoretically, we are trying to
flow a greater volume of exhaust gases, when compared with the original bore
size, thru a reduced area.
So, do a little homework and make decisions, before
attempting to rebuild your engine, as to component selection, such as camshaft
choice, high ratio roller rockers, exhaust valve sizing/porting etc, etc.
Camshaft Recommendation (Street)
We recently rebuilt a MGB street engine (+.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"). Over and above the camshaft, I would like to take the
opportunity to list the components that made this particular MGB a real
pleasure to drive.
Compression Ratio 9:1
Part 671018X289 single pattern (Elgin Camshafts) 222 deg
duration @ 050" lifter rise. .289" lobe lift.
Use with 1.55:1 roller rockers, GCR 9:1, LCB exhaust header
with 2" free flow system. Requires exhaust CFM flow increase. Exceptional
Further information from Elgin Camshafts has indicated that,
due to the 671018X289 camshaft lobe ramp design, we can actually run valve
clearances as low as .012" on both the intake and the exhaust. This has the
advantage of being able to increase the intake and exhaust valve open duration.
As a rule of thumb, every .001" decrease in valve clearance will result in
approximately 2 degrees of valve open duration.
With this change comes increased valve overlap, with
possibly lower engine idle vacuum readings and decreased gas mileage.
The above would allow the enthusiast the ability to tailor
his driving needs for the occasional autocross etc.
Camshaft Recommendation (Street Performance)
Our earlier recommendation Moss part 222-270 dual pattern
(Crane part 342-0010) did not perform as forecasted. So, subsequently we no
longer install this camshaft.
We were unaware at the time that the camshaft lobe material
required a lifter with a minimum hardness Rockwell RC scale of 60. Iskenderian
was the only company making such a lifter (part SP-002). Unfortunately, they
have been discontinued for many years. Consequently, using this camshaft with
lifters less than 60RC will result in lifter failure.
Also, this camshaft requires a valve spring seat of 90# and
172# at full valve lift. Any increases in seat or full lift #s will result in
possible rapid lifter failure. The maximum operating engine 5200RPM must be
strictly adhered to.
I am going to leave the following original text information
in place just for reference purposes only.
Although to date (05/31/98) 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 the intake profile to be almost identical 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) @ .050" lifter rise. .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 .050". Intake
6 deg BTDC & 36 ABDC. Exhaust 51 deg BBDC & 1 deg ATDC. Overlap 7 deg.
Valve clearance (hot) Int .014" Ex .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.
Numerous other camshaft profiles that are available will probably work just as well as the ones I have outlined above, Unfortunately, we are unable to give any information on their characteristics, but we wouldn't be surprised in the possible similarity of their profiles, when related to the 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. If you read technical article MGB16 "Rocker Arms" you will understand why there is no way you can use this number to establish this valve lift. In all my years of engine rebuilding I have never been able to confirm this theoretical figure of 1.426:1.
One final note, Always degree your camshaft directly at the
camshaft do not use any references in regards to valve clearances to establish
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
Firstly, a camshaft lifter must be allowed to rotate to
eliminate rapid wear on the lifter face and subsequently 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, this centerline contact should be offset, and in
conjunction with the lifter radius and lobe taper design will promote rotation
of the lifter.
Back to the problem with the premature failure of the tall
We have not ruled out the fact that lifter hardness plays an
important part in lifter failure. Here at British Automotive 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 imagined these 2 lifters finding their way into 2 separate
engine rebuilds. That is a hell of a lot of work to redo for the price of $1
the price for the hardness test. So, 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
(see price list) which also incorporate a small bleed hole in the lower portion
of the lifter, which allows for better camshaft lobe lubrication and allows
accumulated oil within the lifter to drain out.