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MG MGB Technical - Engine capacity

Hi, I don’t know how to work this out . After having the engine bored out to 60 though oversize I wondered what the cubic capacity of the engine would be. Trev
Trevor Harvey

OK I love this sort of thing! Standard MGB engine has a bore of 80.26mm and a stroke of 88.9mm. Your overbore is plus 60 thousandsth of an inch, or 0.06 inch which converts to 1.52mm, so the bore of your engine is now 80.26 + 1.52 = 81.78 mm

The swept volume of a single cylinder is the circular area of the cylinder multiplied by the stroke of the piston. Circular area is PI x radius squared.

Radius = 81.78 / 2 = 40.89 Squared = 1671.99

Multiplied by PI (3.14159) = 5252.7 sq mm

Multiplied by the stroke = 466,965.66 sq mm or 466.97 sq cm (cc)

Multiplied by 4 (four cylinders) = 1868 cc

Bingo!

Mike Howlett

Can't fault the maths but doesn't capacity include the combustion chamber capacity plus a bit for the gasket? Depending on the head can be 38-45cc (X4)
Michael Beswick

Mike's got it
Michael, You're on combustion chamber capacity, which is calculated to work out compression ratio---whereas engine size is swept volume as calculated by Mike
William Revit

Thanks Willy. I knew I was right. I never would say I'm good at maths, but that sort of calculation is fun. My younger son went on to get a Cambridge University degree in pure maths. He must have got the ability from somewhere. Neither my wife nor I would claim that level of skill.
Mike Howlett

I did these sums when I had a new head from Peter Burgess and new rings in what turned out to be 1933cc overbore pistons. for compression ratio you can treat the head gasket as a simple geometric cylinder. Its not a major contributor so you can neglect a small error in it. I bought a plastic pippete greased the cylinder wall then got a piston at TDC and filled the space above it. The cylinder head vendor will have a good number for head gasket volume and once they know the volume to the top of the block and your measured, or the machine shops declared bore they have all they need to set compression ratio.
I make your extra swept volume less than 4% so your existing head has a good chance of being fine.
Stan Best

Here's one: would the 1868cc engine make any more noticeable power over the standard capacity engine assuming both capacity engines being in good fettle and all other things equal?

Peter Allen

30 th. oversize you can hardly/just notice
60 th. over (1868cc)makes a difference ----yes you would notice it stronger driving the car,specially low to mid range-
willy
William Revit

Thanks, Willy. Low to mid range is what I want. I've just refreshed a spare engine (new rings, big ends etc) that happens to be +60 thou with good bores. Used optimum standard bits ie head with larger inlet valves, double row timing chain etc, plus will use a standard cam and fixed needle HS4s. My current engine is a worn +40 thou, with a Kent 718 cam, which is OK once motoring, but a bit flat off the line. Tried HS6s in the past with another standard spec engine, but for my type of driving seemed to lack oomph low down.
Peter Allen

Thanks Mike, that’s just what I wanted to know, It certainly has made a big improvement to the power of the engine but of course that’s compared to it in it’s worn condition. Trev
Trevor Harvey

The increase in torque at low RPM was very noticeable, and welcome in my car. It went a long way to filling the big gap between 2nd and 3rd in the 3 synchro gearbox. Swept volume and BMEP are the only things that affect torque so if you do nothing to help breathing but just overbore it gives you extra torque at all RPM.
Stan Best

This thread was discussed between 04/02/2019 and 11/02/2019

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