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MG MGB Technical - Skimming cylinder head

MGB lovers,
I have a 1978 californian MGB (lead-free low compression head) with the engine bored out to 1950cc and twin SUs fitted, but feels a little down on performance, especially at low revs. Would it be worthwhile skimming the head to increase the compression ratio and hence gain a little more power. If so, by how much? Does anyone have experience of doing this?

Thanks in advance for any advice.
Dean
Bilbao, Basque Country
Dean

Hi Dean...

Pinking (pinging) seems to be a common problem with standard engines when using the fuels that are commonly availible in the UK, and it is difficult to lower the compression ratio if you go too far with skimming.

It is also worth noting that the head may already have been skimmed... it might be worth going to the trouble of measuring your actual compression ratio before taking any more metal off.

Don
Don

It would be best if the motor could be C.C. to get the comp. ratio, BUT it is hard to do with the motor all together! The best comp. ratio with 89 oct is 9-1 with a stock cam, if you would like more power & can afford high test go up to 9.75. The number on the piston box might say 9.5-1, but it is NOT that when you C.C. it out, you will find it is .5 to 1 point LOWER than what thay say. On my motors that I rebuild we use the 8.8 comp. pistons & cut .060 to .90 (there are different size combustion chamber) to get the comp. up were the motor runs good with 89 oct. I do not know how good the gas is over there in spain But if every thing is right with the advance curve in the dist. & the timming goes up th 38 deg. after 3,000 R.P.M. & you get no ping with regular gas I would cut the head. Here in the U.S. we get 3 grades of gas at most gas stations 87, 89 & 92 sometimes there is 94 oct. but we have to look. If you want it to run it starts with compression. There is a motor that we work with & the factory said that the comp. ratios were 8.8 & 10.5 to 1 & when it was C.C.ed out the REAL comp. was 8.3 & 10 to 1 The H.P. was 185 & 205 BUT were realy 135 & 185
Glenn Towery

At what point does the oversize bore make the problem of the exhaust valve overlapping the block disappear? Does it clear at 1950cc? Will you have to cut deeper eyebrows into the block-top if you skim the head?
Marvin Deupree

With a 1950cc engine you should open out the combustion chamber around the intake valve to de-shroud the valve, and then the head should be skimmed to restore compression ratio to normal. This should give you more power (better flow) at high revs without making it too likely to knock at low revs.

fwiw.
--
Olly
Oliver Stephenson

Dean, I assume you work for Euskaltel. Good luck to the Euskaltel Team in the Tour de France!!

Tom
Tom Custer

Grassroots Motorsports magazine did an article/dyno tune, [February '03] on a stock 1978 MGB /engine.
They acheived 64 HP at the rear wheels, up from 59, with the use of a Paeco header, a free flowing exhaust, pertronix ignition, K&N air filters, with volocity stacks inside, and 35 degrees of total timing advance at full mechanical advance.
They already had twin HIF's on the car, stock pistons and bore size. Which getting the better manifolds and twin carbs. are already an improvement.

OK, so I've done all of the above, plus a 0.40" overbore, balancing, lightened rods and flywheel, ceramic coating of the Paeco header, and I use 1 1/2" HS4's. And had the crank/cam 'degreed'. No I haven't dynoed it yet, but it does have low end torque. and rev's easily to 6000 rpm.
Safety Fast
Dwight
Dwight McCullough

This thread was discussed between 03/06/2003 and 05/06/2003

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