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MG MGB Technical - cam shaft

I have a 79 Canadian MGB that was converted to RHD. I changed from the single ZS carby to twin SU's. A bloke told me that the cam in the single carby model is milder than the normal one used whit twin SU's
Is this true? If so what grind cam should I use
Thanks Jeff
jeff jorgensen

Jeff. There is complete information on this in the archives, but they are somewhat cumbersome to use. As I remember it, the basic profile of the chrome bumper and rubber bumper cams were the same/almost the same and the basic difference between the two was the timing. As I remember, the RB cam was four degrees retarded to provide slightly more power in the lower and mid rpm ranges at a slight expense of power in the upper rpm ranges.

I am running one 79B with an RB cam and one with a CB cam, so have some basis to compare. There is not a great deal of difference between the two under average driving conditions. At least, not that I notice.

Going to a different camshaft, one of the "improved" ones offers little benefit over the factory camshafts if the rest of the engine remains stock. Changing out of the cam can make a significant difference when done as a part of a comprehensive upgrade program.

Peter Burgess' book, "How to Power Tune the MGB 4 Cylinder Engine" has a lot of good information on how to modify the engine in a comprehensive manner and should be able to answer many questions that may come up over the years.

Les
Les Bengtson

Les Bengtson,
Thanks for your reply I will look up the archives
Jeff
jeff jorgensen

Les, later, single row timing chain and sprockets are ADVANCED 4 degrees which lowers the RPM power range by perhaps 400-600 RPM. All that is needed to retard the timing back to earlier CB engines is a set of sprockets and a chain. Offest keys can be used also, check Mini-mania, Moss also has them but they do not work at this time due to a quality control issue.
If any one is interested, contact me for roller lifter XPAG camshaft kits.
Len Fanelli

Oops http://www.7ent.com/detail.cfm?pageid=120, offset cam keys from seven enterprises, not Mina-mania.
Len Fanelli

Oops http://www.7ent.com/detail.cfm?pageid=120, offset cam keys from seven enterprises, not Mina-mania.
Len Fanelli

Len is correct. The later single row sprocket is 4 degrees advanced.
Ray 1977mgb

Jeff-
Your friend spoke of "the normal one used with twin SU's". Well, evidently your friend is unaware that there were two camshafts that were used on North American Market MGBs that had twin SU carburetors. The first was (BMC Part # 88G 303) and was used until August of 1971. It opened the intake valves at 16 degrees BTDC and closed them at 51 degrees BBDC. It opened the exhaust valves at 56 degrees ABDC and closed them at 21 degrees ATDC. Overlap was 37 degrees. Duration was 252 degrees.

In August of 1971 a new camshaft was introduced, the camshaft lobe profile design of which was slightly altered, the keyway of the camshaft driven sprocket (BMC Part # 12H 4200) was advanced another 2.25 degrees in order to increase midrange torque output. It opened the intake valves at 18.25 degrees BTDC and closed them at 53.25 degrees BBDC. It opened the exhaust valves at 58.25 degrees ABDC and closed them at 23.25 degrees ATDC. Overlap was increased by 4.5 degrees to 41.5 degrees. Duration was increased 256.5 degrees. It should be noted that due to the larger 1.625” diameter intake valves of the 12H 2923 cylinder head casting with which this valve timing was used, the power output at high engine speeds remained unaltered. In October of 1972 the camshaft drive chain and the drive and the driven sprockets were changed from a duplex (dual-row) camshaft drive chain system to a Simplex (single row) camshaft drive chain system on the North American Market 18V-672-Z-L and 18V-673-Z-L engines.

In December of 1974, Rubber Bumper cars for the North American Market with 18V-797-AE-L, 18V-798-AE-L, 18V-801AE, and 18V-802AE engines received a new camshaft (BMC Part # CAM 1156). It opened the intake valves at 8 degrees BTDC and closed them at 54 degrees BBDC. It opened the exhaust valves at 42 degrees ABDC and closed them at 18 degrees ATDC. This was a revised, shorter duration camshaft in which Overlap was reduced by 15.5 degrees to 26 degrees. Duration was 230 degrees for the intake valves and 252 degrees for the exhaust valves.

Starting during the month of June of 1977 cars with 18V-846-F-H and 18V-847-F-H engines for the UK Home and Export markets (not North America) received a new camshaft driven sprocket that further advanced the camshaft timing by another 1 degree. It opened the intake valves at 20 degrees BTDC and closed them at 55 degrees BBDC. It opened the exhaust valves at 52 degrees ABDC and closed them at 17 degrees ATDC. Overlap was increased by 11 degrees to 37 degrees.
Duration was 252 degrees for the intake valves and 252 degrees for the exhaust valves.
Steve S.

This thread was discussed between 31/12/2008 and 10/01/2009

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