British-Cars.org

Welcome to the DMR Site for British 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 MG Y Type - What is the correct compression?

Upon renewing the spark plugs, I noticed that there were considerable deposits of carbon which would eventually require that I decoke the engine.

I checked the compression and got uniform readings of 140 pounds on each cylinder. This seems a little high. Would anybody please know what would be the normal reading I should get?

My concern is that there is so much carbon deposits that the combustion chamber is reduced, elevating the compression. Higher than normal compression can accelerate bearings & crankshaft wear.

Thank you.

Gilles Bachand

Gilles,

140 psi sounds pretty good to me. I can't find any actual data for compression pressures on XPAG engines, but from my experience with other classic car engines, anything in the range 120 psi to 160 psi is fine. Obviously it depends on various things such as compression ratio ( our Y engines are fairly low at approximately 7.3:1 ), piston ring & bore wear, how well the valves seal etc., but I would be quite relaxed about a uniform 140 psi.
Perhaps more important than the actual pressure readings is the fact that they don't vary too much between cylinders, ideally they should all be within say 10% to 15% of each other, so your uniform readings are a good sign.
If it helps, I'll be happy to check mine next time I run the engine - let me know.

Bill.
Bill Bennett

Thank you Bill. In the mean time, I found some data for the XPAG which corroborates you and says that from 100 to 140 psi is OK. If you mill the head, you can go up to 175 psi, which corresponds to around 9.5/1 compression ratio. Higher than that could be harmfull in the long run.

A friend of mine has a 1931 Ford. He milled down the head in order to get higher compression and more horsepower. He achieved those goals but ultimately ruined his crankshaft, engine bearings and connectings rods because none of these were originally designed to withstand the added knock and stress.

My carbon buildup is evidently caused by a too rich mixture. In my research, I discovered the existence of a wonderful little British invention which will help me tremendously in achieving the correct mixture: the Colortune! What a great & simple gadget. How come I never heard of this before? Here is a "copy & paste" of an info-mercial on the Colortune:

«The Colortune is a sparkplug that takes the guesswork out of setting mixture screws. The theory behind the Colortune is quite simple. Mixtures with differing fuel/air ratios will burn with different colors. You already know this, if you've ever adjusted a gas burner, Bunsen burner, welding torch, whatever. Rich mixtures burn yellow or orange, whereas mixtures with the correct fuel/air ratio will burn a bright blue. Wouldn't it be nice if you could peek inside the combustion chamber of a running engine to look at the flame color as you tweak the mixture screws? With the Colortune, you can. The Colortune is a sparkplug with a transparent window that lets you observe the flame color in a running engine. With the engine running, turn the mixture screw out until you obtain a yellow flame (rich). Then, turn the mixture screw in until the flame just turns blue. This is the correct mixture screw setting for best performance. The kit contains the 14mm Colortune plug itself, a "viewerscope" with an adjustable mirror that makes it easier to see the plug from weird angles, an extension HT lead, a copper sealing washer, a cleaning brush, and cleaning solution.»

Here is a little movie of how it looks in action: http://www.youtube.com/watch?v=NpZ5EW_Kupw
Gilles Bachand

"Rufus" the YA has 150 psi in each cylinder - engine history is unknown, so I'm happy with those readings. It runs fine on Premium Unleaded at about 3 degrees BTDC, so I suspect the compression is higher than standard - Have to measure the cylinder head thickness to confirm.

I havn't had a chance to compression check "Meg" the YT yet, but as a freshly rebuilt engine, I'd hope for around 140 each cylinder.

As Bill said, even readings are important, not highest or lowest. I would only worry about engine wear when the readings get below 90psi.
Tony Slattery

Gilles, use the colortune (I'm a big fan, too) and after that try some careful use of a cleaning additive. But most importantly; drive it for a long distance! It did wonders for my Y.
Willem van der Veer

This thread was discussed between 22/06/2007 and 24/06/2007

MG MG Y Type index

This thread is from the archives. Join this live forum now