Technical Information (MGB 12)

British Automotive's Big-Bore Conversion 1924cc

These forged alloy pistons (83mm) are custom manufactured by J.E. Pistons and are of a flat top configuration ready for piston dish (cc) machining and require a piston skirt to bore clearance of .002". NOTE: these pistons are designed for use with any conrod configuration.

The following is applicable to 5 main bearing engines only:

It is quite possible that your cyl/block has original factory cylinder liners installed (under no circumstances rebore these liners). These liners were installed to rectify cyl/block casting imperfections. Should this be the case, it will not be possible to bore the cyl/block to accept 83mm pistons, however, it is possible to use 83.5mm pistons P/N MGB1948cc (use 83mm boring procedure below), or alternatively, resleeve the cyl/block and bore to 010"/020"/030"/040" or 060", in preference, use piston P/N MGB1868cc (060"). If your cyl/block does not have liners installed, then careful boring of the cylinders should be undertaken to within .005" of the finished bore size. At this dimension, it can be determined whether or not the cyl/bore can be successfully honed to give the desired finish (please note, always start at #4 cyl/bore as this seems to be the troublesome area). In the event that your cyl/block cannot be successfully rebored to accommodate these pistons, it is unlikely that you will be successful in going to the next available bore size (83.5mm). Either proceed with liner installation and rebore or provide another cyl/block and start all over again!

Advantages of British Automotive's 83mm Pistons

• No cyl/block redecking required to facilitate piston compression height.
  
• No cyl/head modifications required for GCR work.
  
• After initial rebore, cyl/bore can be readily rebored to oversize (83.5mm).
  
• Pistons can be used with any type of conrod.
  
• No cyl/head gasket overhang into cyl/bore.
  
• Piston crown design can accommodate machining for the desired GCR.

While there are numerous big-bore modification kits available, there are usually some disadvantages associated with their fitment, some of which are listed below:

• Pistons can only be used with certain conrods.
  
• Piston crown thickness does not allow for any piston dish cc machining.
  
• Excessive redecking of cyl/block required due to the shorter piston compression height.
  
• To achieve the correct GCR, the cyl/head needs to be reworked to the correct capacity.
  

Listed below are the necessary clearance volume(s) (CV) required for a particular engine size to achieve the recommended 9:1 GCR:

Effective Compression Ratio (ECR)

This is more important than GCR. For ECR relates directly to the actual inlet valve closing (IVC) position relative to top dead center (TDC) and expressed as after bottom dead center (ABDC). To find this ECR, you must first install a valve timing degree wheel to determine exactly when IVC occurs. For convenient measuring purposes, I prefer to use .001" before actual IVC (under actual running conditions, this IVC position will be slightly different due to valvetrain component flexibility). Calculating the ECR can be done by: 1 - plotting a graph of piston movement to crankshaft angle, 2 - by applying the appropriate trigonometrically formula, or 3 - the use of a PC program covering this subject. Thanks to Dimitri Elgin of Elgin Racing Cams, Redwood City, Ca., for the following program information which is applicable to British Automotive's 1924cc Big-Bore MGB engine:

ECR Applications

• 6.0:1 ECR - low horsepower engine
• 6.5:1 ECR - fair engine
• 7.0:1 ECR - street gas 92 octane
• 7.5:1 ECR - good tuning required, street gas 92 octane
• Over 7.5:1 ECR - requires 104 octane boost
• Over 8.0:1 ECR - requires racing gas

  From the above information, it is easy to understand that we can optimize the resulting ECR by establishing earlier IVC. A comparison between 1 and 2 clearly shows these optimum and non-optimum aspects of IVC.

  So, if you are contemplating rebuilding your engine or already in the engine rebuilding process, you may want to review your camshaft choice. My recommendations are as follows: for street, performance street and rallying - use 9:1 GCR, choose a camshaft with around 220-225 duration, and with an IVC of between 35-37 deg. ABDC (both measured at .050" lifter rise). This should put IVC at approximately 60 deg. ABDC (+ or - 3 deg.) when measured at the intake valve. This puts us in the area of 7.0-7.5:1 ECR.

  CAMSHAFT CHOICE

  Increasing the air & fuel consumption results in more horsepower. There are several ways to accomplish this, one being the installation of a "hotter" camshaft. There is always a tendency to think "bigger is better" when choosing a replacement camshaft, however this can be counter-productive because it upsets the balance between the camshaft's breathing characteristics and the actual breathing requirements of the engine at certain engine Rpm’s.

  Let's take a look at two individual dynamometer tests that were performed "some time ago" on British Automotive's 1924cc engines (you will see this "adage" used several times during your browse through the web site's technical information portion). What we are going to show you is that you should always use a camshaft suitable for your particular engine operating RPM range, and by doing so you will see how different combinations of carbs, manifolds and GCR's can influence maximum BHP and TORQUE results.

  The following dyno test (March 1991) was conducted only up to 4500 RPM; unfortunately, this was due to a communication problem with the dyno shop. I have copied the original dyno test results and estimated (based upon known facts) the final BHP & TORQUE figures.

  Test 1

  - WEBER 45DCOE Carb (long intake runner manifold & exhaust header)
  - Camshaft duration 222 deg. @ .050" cam lobe lift, GCR 9:1

  RPM	BHP	TORQUE lb./ft
  3000	76.5	133.9
  3500	91.1	136.75
  4000	102.1	133.9
  4500	109.1	127.2
  5000	112.2 est.120.1 est.
  5500	112.2 est.107.3 est.

  Test 2

  - MIKUNI 44PHH Carb (short intake runner manifold & exhaust header)
  - Camshaft duration 235 deg. @ .050" cam lobe lift, GCR 10:1.
  - The cylinder head was identical to the one used in the above test.

  RPM	BHP	TORQUE lb./ft
  3000	70.7	123.7
  3500	86.2	129.3
  4000	99.7	131
  4500	110.3	128.7
  5000	118.8	124.8
  5500	124.9	119.2
  6000	128.6	112.6
  6500	126.9	102.5

  Test 3

  - MIKUNI 44PHH Carb (short intake runner manifold & exhaust header)
  - Camshaft duration 222 deg. @ .050" cam lobe lift, GCR 9:1.
  - The cylinder head was identical to the used in the previous tests.

  RPM	BHP	TORQUE lb./ft
  3000	63.9	120.7
  3500	83.4	125.2
  4000	95.8	125.7
  4500	105.9	123.5
  5000	110.7	116.2
  5500	113.5	108.4
  6000	111.7	97.8

  Let's make comparison checks and compare dyno results from Test 1 and Test 2. It can be seen that the WEBER 45DCOE BHP & TORQUE readings were superior until we reached 4500 RPM. From here the MIKUNI 45PHH set-up showed worthwhile gains in both BHP & TORQUE over that of the WEBER 45DCOE.

  ---