Roger Williams contacted us in early 2007 about the possibly of a Flowspeed contribution to the upcoming revision of his book “How to Improve MGB, MGC & MGBv8” (ISBN 978-1-84584-187-4). He hoped we might illustrate what differences in power output various stock MGB cylinder heads provide when bolted to a standard MGB engine. In addition, he was hopeful we might also have information regarding the improvements our ported heads deliver. To everyone’s benefit, we already had such testing planned for later in the year, and at his request, we decided to move testing preparations forward to make our contribution to his book.

     Roger wanted us to test all the different cylinder head combinations on the same engine in as close to one session as we could. In order to carry out Rogers' request, there were two challenges which immediately had to be overcome. The first, was to acquire a large number of factory and modified cylinder heads for testing; the other was to build an engine which might best represent the average MGB engine currently on the street.

      One might think that being in the cylinder head business, finding good core candidates would be easy. Unfortunately that really isn't the case, and two of the cylinder heads we used in our testing were externally cracked heads that were otherwise OK for the short term use we had in mind... although these would not have been reliable for long term use in an actual vehicle.

      When it came to alloy cylinder heads, we have to thank our very willing friends and customers who supplied these items in order for us to complete this extensive comparison project. The generously loaned cylinder heads from Basil Adams, Jimmy Hilton and Greg Fast were invaluable, and we could not have done our testing without them. Many thanks to those individuals!

The Test Engine:

      With the required cylinder heads at our disposal, we began building an engine we felt might best constitute the average street-going MGB engine in use throughout the world. An 18V block was bored .040" over. The original crankshaft was ground .010" under on both mains and rod journals before being zero balanced. This means the crankshaft is first balanced without any other components attached. Once the crank is in balance, the damper is added and it too is balanced, then the flywheel, etc. This ensures that if a new flywheel or damper is installed later, the balance of the engine will not be upset. The original 18V connecting rods were resized to ensure proper bearing crush, and then balanced on both the big and little ends to equal weight.

      The .040" over AE pistons (shallow dish type) were also balanced before being final assembled to the connecting rods. We used Grant top and oil rings supplied by British Parts Northwest, while keeping the original AE second rings as supplied with the pistons. The top-ring gaps were set at .012" and the second-ring gaps at .018" with the intent of maximizing top ring sealing and performance. We also like a plateau cylinder-wall finish, which we achieved by using a coarse stone for initial sizing, followed by three passes using a very fine stone before finishing with Sunnen’s ultra-finish brush hones. This gives a very smooth outer surface, but still retains oil via the coarseness of the valleys in the cylinder wall. We find that rings seat almost instantly with this method and extended break-in periods are a thing of the past.

      To complete the bottom end, we used new main, rod and cam bearings before disassembling the new County brand oil pump to check for proper clearances and any obstructions within the passages. We found the clearances in check, but the passages needed some attention with the die-grinder in order for reasonable oil flow to be obtained. We used an early pattern stock camshaft (215@.050" on a 107.5 lobe separation angle) and timed it to a 105 degree inlet full- lift center line via an offset cam-key. We also used a new double-row timing set and tensioner. Our choice in lifters was the lightweight 18v style units which were mated to a set of stock 18V push rods for the duration of the test.

      We chose a stock, rebuilt rocker-shaft assembly for all our testing, while carburetion came from a set of HIF-4 carburetors, modified for use with early HS-4 non-biased needles. We used stock MG velocity-stacks for all the tests in question. Exhaust gas was handled by the short-style "3 into 1" exhaust -header (Moss number 459-011) which we modified for installation of exhaust gas temperature probes. Fuel pressure was regulated by a Holley regulator set at 3.5 PSI. Water temperature was set to 180 degrees F and controlled by the dyno itself. Fuel was Shell 92-octane premium unleaded.

      We chose a locked-out Lucas electronic pick-up distributor, and the MSD ignition system available on the dyno. This ensured cylinder timing from cylinder to cylinder would be as identical as possible, and that the firing of the spark plugs would be consistent for the duration of the test. We used NGK BP-6ES spark plugs for all our testing, with gaps set at .032". Timing was optimized for each cylinder head combination and ranged from 32° up to 36°. We found most heads worked best with 34-35° of timing.

      This description of our test engine might make it first appear somewhat exotic. But, it was really nothing more than a very standard street engine with some extra attention to detail given during buildup. It's important when building an engine for a test such as this to make the engine as reliable as possible, yet unfavoring any given cylinder head. It would defeat our purpose, after all, to build a highly modified engine that might favor higher performance heads, but not those which were stock configuration.

The Cylinder Heads:

Stock 12H-1326:

     With the test engine built, we went about preparing the various cylinder heads. Our fist cylinder head was a 12H-1326 casting that utilized exactly the valve-job shown in figures D2-2 and D2-3 on pages 20 and 21 of the book. It should be noted the specific angles and widths of these valve jobs have been carefully developed to require no hand-blending or other modification work. Anyone can have their original small valve style cylinder-head machined to these specifications, and expect to achieve the airflow numbers seen in the chart below (as well as produce the basic power figures shown in the book). This should be looked upon as a big advantage to anyone who wishes to keep an otherwise stock cylinder head intact, but still would like to get the best from it. What some find interesting is, 50% or more of the improvement we effect in our modified cylinder heads comes from seat profiles like these! Chamber volume for this cylinder head was 41cc.

Modified 12H-1326:

     For our modified 12H-1326, we increased inlet valve size to 1.625" and ported the cylinder head as per our standard "Fast Road" work seen elsewhere on our web site. The chamber volume was 41cc as per the stock cylinder head.

Stock 12H-2923:

     The next cylinder head in question was a stock 12H-2923 "Big Valve" factory version. This cylinder head had very good port coring and very little wear. It was cracked externally above the number two spark plug, but this did not affect our tests (as had the cracks in the modified cylinder head below).This cylinder head utilized the valve seat specifications seen in figures D2-2 and D2-4 on pages 20 and 21 of the book and flowed extremely well. Not all big valve castings are cast alike however, and we'd like to stress that while the inlet valve job D2-4 will help any of them, it may not produce the good results seen here. with a less accurate castings. One should, however, expect a healthy increase in performance over a more run-of-the-mill valve seat job, as the power figures in the book will attest. This cylinder head had 39cc combustion chambers.

Modified 12H-2923:

     Rounding out our iron cylinder head comparison was a modified 2923. Our original cylinder head for this session failed and we were forced to substitute results from a different 2923 cylinder head. Note that this cylinder head still retained the exact same valve job as the one above, and as this accounts for much of the improvement over a stock version, the increase in both flow and power of the modified version is not terribly astounding. This type of head really favors higher lift cams and hi-ratio rocker assemblies to make full use of it's increased high lift flow, but we were not testing those items here.

Aftermarket Cylinder Heads:

Stock Alloy 5-port:

     The first aftermarket cylinder head to be tested was an aluminum 5-port (stock replacement style) cylinder head by Pierce Manifolds. As stated in the book, the greatest advantage of these cylinder heads is in their lead-free compatibility, light weight, and the reduced likelihood of cracks developing in normal use. We do not consider them a performance item right out of the box however. This was evidenced both in the airflow numbers seen below, and the power figures seen in chart D2-1 on page 20 of the book. Chamber volume was 38.5cc.

Modified Alloy 5-port:

     After modifying the cylinder head as per our normal practice (including larger 1.625" inlet valves) we obtained a 26CFM increase in peak airflow, as well as the most increase in power for all of the 5-port cylinder heads tested. Chart D2-1 on page 20 of the book will illustrate these improvements. The modified chamber volume ended up 39cc, which we deemed close enough to the previous state for reasonable comparisons to be made.

Stock Alloy 7-port "MSX":

     The next aftermarket head tested was the Pierce Manifolds "MSX" 7-port cross-flow cylinder head. These cylinder heads are patterned after the original HRG Derrington version of the 1960's and incorporate individual inlet ports for better airflow potential. In our testing however, these cylinder heads do not live up to their potential in a box-stock configuration. All airflow numbers are based on the bare cylinder head only, with port entry radius attached. In spite of their generous size, the exhaust ports disappointed, having worse performance than the smaller 5-port counterpart offered by the same manufacturer. The chamber volume was a minimal 36cc, which gave the highest compression ratio of all the cylinder heads tested.

Modified Alloy 7-port "MSX":

     Our modified example of the above cylinder head was an extensively reworked item sporting larger 1.625" inlet valves and many hours of delicate port work. We felt well rewarded for our efforts as can be seen both from the large gain in airflow potential, as well as the dramatic increase in power on the dyno. Again the flow results are of the bare cylinder head, with an inlet radius attached. At a generous 42cc's, this cylinder head achieved significant increases in performance with the largest combustion chamber size and subsequently, lowest compression ratio of any cylinder head tested.

End Notes and Final Comments:

     To our great advantage, a solid week of sunny weather made for uncanny repeatability and comparisons. After spending the first day running-in the engine and making sure everything was working as expected, we spent the next four days swapping cylinder heads and making dyno runs. Both carburetor and ignition settings were tuned to yield the best performance for each cylinder head, with the median of three runs being chosen for publishing (more information can be seen in photo captions at right).

     With all said and done, we hope our contribution to Roger's book will stand as a reference for many years. It is our sincere belief that no more honest or unbiased test could have been made, and that's exactly what Roger requested.