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Understanding my cam

How to Degree a Cam

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Hi Guys,

I have recieved the new cam for my engine build and I wanted to try to understand it more while I await the machine work currently underway on the block... I found a calculator of the net but the figures it is asking for are not really what has been written on the cam card... I was wondering if someone would like to help me in understanding what is written on the card when compared to what the calculator is asking for?

The Cam Card is below.

The calculator is here - https://mgispeedware.com/camshaft-calculator/

I have worked out roughly where my cam sits when looking at "Off the shelf" options, somewhere between the BP285 (Fast Road) and the BP300 (Rally) which can be found here - https://www.pipercams.co.uk/Camshafts-Car/Make-BMC/Engine-B-SERIES/

Thanks heaps,


Attached Files

The problem with making a comparison is that every value is different in those spec's - one will be from initial valve opening to actual closing and the other from points where it is already well off the seat, one refers to valve lift and the other to camshaft lobe lift, etc. The rocker ratio of the OEM B series is supposed to be 1: 1.426* - every 0.100 at the camshaft is ~0.1426 at the valve so that camshaft 'should' be around 0.4235" - but some camshaft manufacturers measure height from the base circle, and some from when the valve leaves the seat at spec' clearance - the first because it looks 'better', but the latter ismore accurate, so that lift could vary, either way, by ~0.020 either way, depending on how it's measured.

From the tag, both inlet and exhaust are a nominal 293 degrees duration, with it advanced 3.5 degrees from the usual 'split' where the lift is an equal number of degrees BTC and ABC, respectively. That would suggest a bias towards lower rpm, as higher is usually biased towards retarded (if I remember it correctly - otherwise it's the other way round), so some experimentaion with the timing may be advantageous (if you have the valve to piston clearance) as just a few degrees can make a big difference...

It seems to be between a fast and ultimate road camshaft, with a probable working rpm range of 2-7k rpm - biased towards the top end with a suitably reworked head, induction and exhaust system. In many ways, this engine is just like a bigger A series, so most of the tuning methods used for that should work with some variation for the different dimensions required.

*There are higher ratio rockers available from several sources, if you car to do a quick search, but in the meantime, you may find this of interest. Remember, while the rockers effectively increase lift, rather than duration as such, and 'should increase breathing at higher rpm with little low rpm loss, they put a much greater load on the components - especially the camshaft-follower interface - and need much more care in setting up, as that thread emphasises.

The thread was also started by a kiwi, so you may have a chance to make some good contacts over here, to expand your knowledge base :-)

Thanks Gord, greatly appreciated. I have emailed my cam guy and asked for the full spec sheet when they just grab from their software.

Things are getting serious now, we booted up the flow bench earlier today :) I am so happy I have found a head guy who knows what he is doing... he was able to show me the results of previous heads which were exactly the same as mine, so I have high but realistic hopes :)

Attached Files
  • 863114C4-4A30-4B75-BB44-43CD211BB6F8.jpeg
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Oops, missed posting the link to the thread in question - https://www.mgexp.com/forum/mgb-and-gt-forum.1/b-series-high-ratio-rocker-set-up.3543628/

Yella Terra (sp?) actually do high lift roller rockers for the B series, as do other suppliers, but they're Aussie. http://www.hi-flow.com/HP012bVT.html

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