MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Combustion chamber volume - CR calculation

As part of my current engine rebuild we decided to calculate the engine compression ratio. Interestingly we found up to a 4.8% variation in combustion chamber volumes (38.6, 39.7, 40.0 & 38.1 cc) due to valve height and chamber volume variations. Resulting compression ratios were 11.1, 10.8, 10.8 & 11.2

Attached are a couple of photos of the set-up we used for measuring the combustion chamber volumes (BTW that is not me in photo by a male model from MGCC-Victoria).
Other values calculated were,
Piston/bore annulus volume 0.8 cc (piston/bore gap measured using two opposed feeler gauges)
Top deck volume 2.8 cc (sealed the piston annulus with grease) measured with the burette and perspex cover similar to combustion chambers.
Head gasket - used Payen AK660 - 6.0 cc
Bore 83.39 Stroke 88.9 engine capacity 1942 cc.

Any comment on these variations or method?

Mike




Mike Ellsmore

All sounds OK Mike. I reckon the gasket vol is wrong. We have measured a lot of used 660 gaskets at 0.88 mm so I think you are just over a cc too much with your measurement.

The variation needs sorting by either a few chamber mods, machine a little off the firefaces ( don't go below 1 mm on the margins) and/or drop the seats up a little. We try for +/- 0.25 cc on a race head.

Peter
Peter Burgess Tuning

Thanks Peter,
My old Payen AK660 measured 1.1 mm thick using a vernier calliper. I then traced out a gasket cylinder hole on a piece of paper and superimposed a circle of best fit so had an equal area inside and out. This circle had a diameter of 83.3 dia just smaller than the actual cylinder bore. That's how I ended up with the 6 cc figure.
Mike

PS I notice the new Payen gasket is made in South Africa but old one was made in England.
Mike Ellsmore

Hi Mike

I think AE( T&N, Federal Mogul or whoever owns them now) do deals to have the stuff made in SA.

off topic, at long last the mgb book rewrite has been published.....you get a mention on page 5 :)

Peter
Peter Burgess Tuning

Thanks Peter,
Coincidentally I received an email yesterday from Veloce Publishing advising of the new edition - your royalties should start rolling in!
http://veloce.co.uk/newsletter/webpack/V4615.html
Mike
Mike Ellsmore

Peter, now you can finally retire, to a desert island in the South Pacific, living off your lucrative royalties. RAY
rjm RAY

We yanks have to wait until October to get a copy?!?! Now, that's cruelty!
Stephen Strange

Chance would be a fine thing, mind you, I am assured the music industry pays the lowest Royalties!

Peter
Peter Burgess Tuning

Mike when I rebuilt my supercharged engine and CCed the deck etc I measured my used Payen gasket at .041". I then sat the head on the block with the old gasket and measured the compression of the old gasket under light and full tension and got just on a thou" so even an old gasket sprung a little. The MG special tuning manual gives the OE gasket .023" and 3.208CC so I got .040" and 5.58CC. Pretty close Denis
Denis4

I find the most accurate way to measure final compression ratio is on an aseembled engine on a stand you can rotate to put the spark plug hole upright. Run it up to TDC and use a burette. The only adjustment you need to make is the volume of the spark plug tip in the chamber (the number for which escapes me, but Peter or someone else may recall, and it depends on the plug).

Very accurate
Bill Spohn

Mike,

Mine measured exactly the same: 1.1mm or .043" = 5.53 cc

Ray
Ray 1977mgb

Bill the old special tuning manual gives the spark plug volume as .2cc but as you say the new type plugs could vary a little. CCing the engine assembled is OK providing you seal he top ring and dont turn the the engine during assembly, sometimes easier said than done. Denis
Denis4

Depends whether the plug runs hot or cool, hot more cc cool less cc.
I have been checking my used gaskets, vary from 0.875mm to 1mm. New gasket Payen 1.1mm and BGA 1.6mm. I have not measured a gasket under compression yet.

Peter
Peter Burgess Tuning

Does it actually matter that much?

Equality of capacity is important certainly but then as long as the crank throws are equal then the only real variable between cylinders is the chamber size.

The theoretical CR is not achieved under running conditions - blow by and valve overlap will reduce the actual CR so need to be taken into account in specifying the setup.

Chris at Octarine Services

The problem is, with a 1mm gasket with an 1860 engine if dished pistons and block to piston around 12thou at TDC you would need about 39.6cc in the head, with a 1.5mm head gasket you would need to take the head vol down to 37.cc to get 9.75:1, goes to show you need to measure a few things to get the best.

Peter
Peter Burgess Tuning

This thread was discussed between 30/08/2013 and 19/09/2013

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.