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 - 1950cc Re-bore

Is re-boring to 1950 a good idea.
Alex

Alex, check the archives. Just type in 1950. You'll find lots of info.
Derek Nicholson

I've heard that the 1868cc rebore is superior to the 1950 modification as the walls between the cylinders are not as thin. I have a 1950 engine remanufactured by Engine Machining Services Co. UK. (i believe that they have gone out of business). This is the second engine as the first suffered catastrophic failure when it started up as cylinder #4 broke open into the water jacket. The replacement engine (717 Kent cam, modified unleaded yead and HS6 carbs) has performed flawlessly providing power throughout the entire rev range up towards 6k rpm.

rn
RN Lipow

I did it and if you would like to read about my expierences go to http://shadetreemg.com/1950engine.htm
In retrospect, I would not do it again, way too much work and investment for the gains
gerry masterman

Alex-
I'm in the process of building a Big Bore engine, so here's my thoughts on the matter- The 1950cc kits do produce more low-RPM torque when used in combination with stock heads, camshafts and modified stock carburetors, but without spending the money required for professional headwork with oversize intake valves such as in Peter Burgess' Fast Road Plus specification, a Big-Bore header and exhaust system, and 1 3/4" SU carburetors plus a special intake manifold, the potential of the increased displacement just can't be fulfilled. Most 1950cc kits use +.040 oversize 83.57mm domed Lotus TC pistons to produce an additional 9 cubic inches of displacement more than stock (8.2%). Unfortunately, the domes of the Lotus pistons interfere with flow and combustion characteristics. If the bore is increased radically, then the squish area increases and flame propagation becomes a problem, especially if domed pistons are used. Let's face it: A domed piston design and the Weslake kidney-shaped combustion chamber design aren't exactly in harmony with each other. Domed pistons present enough problems in a hemispherical combustion chamber, but in a Weslake kidney-shaped combustion chamber they're bad news. The best pistons to use for a Big Bore engine are JE flat-topped pistons, but, being forged pistons, they have a greater expansion/contraction coefficient than cast pistons due to their lower silicone content (silicone doesn't like being forged), so they have to be fitted with greater cold running clearances which can accelerate wear somewhat. They're also heavier, so the balance factors have to modified and the engine will vibrate a bit more due to the greater weight pumping up and down inside the engine. Of course, the extra weight could be compensated for by using Carrillo forged chrome-moly alloy connecting rods, but they're expensive ($$$$$)!
The combustion chamber volume is relatively smaller in relation to the cylinder volume on a Big Bore engine than on a 1868cc engine, so the pressure rise within it is faster than on the smaller bore 1868cc engine, resulting in the greater tendency of the Big Bore engine to detonate. In addition, the larger squish area of the Big Bore engine can create too much turbulence for flame propagation to be smooth and even, inhibiting flame propagation in the areas near the roof of the combustion chamber. Due to the positional relationship between the circular cylinder and the kidney-shaped combustion chamber, the increased squish area increases the velocity of the turbulence in the direction of the spark plug, thus guaranteeing that the turbulence around the valves will be at the lowest in that location due to the direction of the moving fuel/air charge being biased toward the spark plug. The position of the spark plug also plays a big part in the detonation problem. The flame travels outwards towards the lobes of the kidney-shaped combustion chamber, creating a pressure wave. As the pressure wave at the border of the combusting fuel/air charge advances, the unburned fuel/air charge in front of it is compressed against the roof of the combustion chamber. When the pressure wave arrives in the vicinity of the hot exhaust valve last, its velocity and pressure is at its greatest just as the remaining volume available for unburned fuel is decreasing at its fastest rate. Because the area around the exhaust valve is the hottest region of the combustion chamber, its conditions are best for producing preignition and detonation, and arrival of the pressure wave triggers the event. While opening up the combustion chamber to decrease the squish area will alleviate these problems, the increase in combustion chamber area can increase the likelihood of preignition at the expense of a lower compression ratio which in turn will prevent the potential of the engine from being attained. Obviously, it is difficult to reach a happy medium, so the distance between the piston crown and the head is critical to producing the correct amount of squish turbulence. It would seem that the most practical clearance is .012". This will create a problem when using a high-lift camshaft as it will be necessary to relieve the deck of the block to a depth greater than that of the piston/head clearance. The edge of the compression ring may be directly exposed to the heat of combustion, leading in turn to premature ring and possibly even piston land failure.
These problems could be minimized by using less spark advance, a lower compression ratio, and a mild camshaft such as the Piper BP270, but this solution will in turn result in the engine reaching its peak output at a less-than-optimum RPM. Due to the increased displacement, higher port velocities occur at lower RPM, resulting in a flatter power curve which reaches its peak at substantially lower RPM. What is really needed is a Piper BP285 camshaft in order for the engine to reach its power output potential and keep the power peak where it should be in order to retain the standard gearbox ratios and a compression ratio of 9.5:1 to keep the power output at a worthwhile level. So, as you can see, there’s still a problem to be solved: Find a way to use a Piper BP285 camshaft and still enable the engine to run reliably using the 93 Octane Oxygenated fuel. Fortunately, there is a solution: JE offers the service of custom-machining their pistons to order, thus the piston can be made with a dished crown, and that will accomplish the combustion chamber shape needed to decrease the tendency toward preignition and the desired compression ratio can be attained by milling the deck of the block to the appropriate height. Of course, that automatically implies that the pushrods will have to be shortened, but Crane Camshafts offers that service too, so that isn't a problem.
No matter what you do, the ignition timing and the carburetion have to be scrupulously maintained or you'll have problems with a fully developed Big Bore B Series engine. Due to the variances in cylinder wall thickness that are the result of a less-than-optimum casting process, it's necessary to torque the block to a reinforcing plate prior to overboring to prevent the finished bore from being distorted. Fitting of the Big Bore pistons requires boring the cylinders out so far that the side-thrust loading of the piston against the cylinder wall can cause the bore to distort, the consequent loss of compression becoming a headache. Sonic testing of the block to determine cylinder wall thickness prior to boring becomes a necessity at this point. Another downside is that the fitting of oversize pistons can't be done as the cylinder walls will be too thin. However, both of these drawbacks can be overcome by offset boring of the block and fitting oversize sleeves with adequate wall thickness. This involves offset-boring cylinders #1 and #3 to the front, and #2 and #4 to the rear in order to maintain sufficient clearance between the cylinders and prevent the blowing of head gaskets and the development of 'hot spots" which can cause cylinder distortion. Sleeves have the additional advantage of being made of spun cast iron which is of better quality than the 'block-type' cast iron. If the sleeves are shrink-fitted and silver-soldered into place the heat distribution should be as good as that of a normal cylinder of equal wall thickness, although the ultimate rigidity at the cylinder/block interface would theoretically be less. A higher-pressure oil pump can assist in protecting the bearings from the additional pounding of the increased power output. Of course, this implies that the engine would have to be built as oil-tight as possible, but all of these have been done before, so dealing with these issues would hardly involve blazing new trails into uncharted territory. All this, of course, is not to mention the problems of the excessive heat that would be produced with such an uprated power output, which in turn will require modifications to the cooling system. For anything other than use on a racetrack, a fully developed 1950cc Big Bore engine is likely to prove to be financially impractical. A compromise displacement of 1900cc-1926cc is probably the practical limit for a fully-developed street engine. Personally, I'm going for 1900cc, which will leave me with one more overbore before the cylinder walls get dangerously thin.
Steve S.

Steve,
Thank-you for taking the time to explain in simple terms this complex subject. It's much appreciated.

Charlie.
Charles Goozee

I echo Charles comments, what an explanation, and what a dumbo it makes me feel. How little I know.

Steve says he is going for 1900cc, which is not a long way from the 1868cc RN is going for.

Would Steve, and anyone else for that matter, care to comment as to whether the 1868cc would be a reasonable compromise, given that I'm looking to pep up the engine, whilst still being able to tick over evenly.

Alex

Alex,
This is the spec I had my engine rebuilt to and I'm more than pleased with the result. It measured 85 bhp at the rear wheels on the rolling road.

Engine is +60 overbore, Stage 2 head 9.5 compression, Piper 270/2 cam, standard HIF4 SUs with AAA needles, and K&N air filters, free flow manifold and exhaust, and an Aldon performance dizzy.

Charlie.
Charles Goozee

I am converting my strangled gasper to twin SU's, and was intending to replace the LC pistons with the HC.

I was hoping to keep as much of the originality of the engine 18V, as possible.

A slightly better cam was one idea, and balancing was another, although I do not know if this would add anything to the party.

I too would like a frisky car, but not too frisky as to ruin tickover.



Jack

Alex-
Yes, the 1868 is a viable compromise.

Alex & Jack-
If you want a stronger camshaft without sacrificing a smooth tickover, use the Piper BP 270.

Jack-
Balancing is worthwhile because vibration is just power wasted.
Steve S.

Beware of using G type high comp pistons with an unmodified 18V head!
Terry

i have a 1950 engine in my B. It's torquey, revs fairly well, (all the rest of the set-up is standard)and generally the car goes very well.
However - it also drinks oil like nobodys business. About 200 miles to a (imperial) pint. I carry a 5 litre can around behind the passenger seat.
I don't think anything can be done about it. Its been discussed to death on this site in the path. The consensus is thin cylinder wall. I've been told forged pistons might help, but at £300 pound a set, I'll live with it.

The general consensus from the many people I've spoken to, is that 1950cc may not be ideal for a road engine.

Mike
Michael Watkins

hi alex
if you would like a drive in an 1950cc B give me a call you can have a go in my demo car.you will get the phone number of the web site
cheers
cameron
cameron

Many thanks,

Should anything be done to the rockers, if I go with the 1868cc.

If yes, please elaborate.
Jack

Alex- My 60 over, 9.50:1, Piper 285,flowed head, non-roller rocker, Aldon stage 2, HS4, Peco, K&N, seemed to out perform the cross-flow version of about the same spec in another car.Yes, it has a Burgess Fast Road head, and when that little miracle changed out the stock CI head, it was just amazing. The x-flow came on early with huge pull to 5500 RPM. The 285/Burgess head engine literally jumps up to 6500 rpm before I knew it was happening.Personally, I think SteveS, well thought out 1900 buys the best of both worlds. Did he happen to mention he's screwing a pair of Webby siders onto the flowed Derrington x-flow , like a cherry on top of the modest 1900?
Jack- Don't use rollers, even Doug Jackson's 1.55, with the 285 ( per Senior Burgess). As I recall, Cameron resleeves all his engines for the 1950 bore he is having phenominal success with. Does he offset or use thicker, spun sleeves? Finally has anybody ever offset ground the journals to get a mini stroke and a few more cc's? Anybody know anybody driving a 2100cc MGB?
vem myers

Vem: check out the 0-6 in 8 sec thread. Input on your time would be quite valuable. Thanks
Terry

Vem talks about a non-roller rocker, and says don't use rollers.

Please explain what this means.
Jack

This thread was discussed between 08/03/2003 and 17/03/2003

MG MGB Technical index

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