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MG MGB Technical - Sport grind camshaft worthwhile?

I just have my B's engine block back from the machine shop.

The engine will be a stock rebuild, however I wondered whether a sport grind camshaft by itself would be worthwhile or even advisable.

Thanks! David

PS I love the built-in spell check for things like this that Firefox has...
David Steverson

There is little to gain from a "sports" cam by its self. The OE MGB cam is pretty good to start with and a slightly wilder cam will only lose power at low revs with little or no increase at higher revs unless combined with cylinder head improvments.

Denis
Denis4

David-
If it were my engine, I would not spend any money on a non-Original Equipment camshaft except as a final modification performed in order to compliment the headwork, and then only if I was not satisfied with the results of the sum total of the previous modifications. Changing the camshaft before doing the headwork is putting the cart before the horse. The specifications of the Original Equipment pre-1975 factory camshaft are hard to improve upon for general duty use, producing a smooth idle and ample low-end and midrange power. Also, realize that changing the camshaft to one with lift that is more radical and / or more duration will increase wear on the tappets, camshaft lobes, valve guides, and valve stems by means of the increased side thrust loads. Should you simply wish to relocate the existing power curve in order to suit your driving style, you might consider retarding or advancing the timing of the Original Equipment specification camshaft a very few degrees (4 degrees maximum, beyond that point the gains are increasingly small while the losses become increasingly excessive) in order to respectively move the power up or down the scale as much as 400 RPM. This is the approach taken by the engineers at the MG factory when they introduced the 18V version of the B Series engine, so such a modification does not involve going into uncharted territory. If you move the power downward, with professional headwork you should still have at least as much power at high engine speeds as an Original Equipment specification engine, and vice-versa. Be advised that you will need to change the fuel-metering needles in your carburetors if you choose to pursue this course.

As Denis4 pointed out, a switch to a higher performance camshaft will only result in a trade-off that will never fulfill its potential without modifications to the cylinder head. In fact, you can retain the Original Equipment camshaft and get considerable improvements in power output by using a properly-reworked cylinder head. For this I can recommend Peter Burgess, whose cylinder heads I use on my cars. He has a website at http://www.mgcars.org.uk/peterburgess/

Econotune specification that adds his own custom intake valve guides that have been tapered (bulleted), plus the combustion chambers and valve throats are all modified to enhance the flow of the fuel / air charge and smooth combustion. Neither the valve nor port sizes are increased, thus the resulting high port and valve seat velocities produce a broad spread of very useable power from idle to a maximum power output peak at around 4,800 RPM. This results in an increase in power output of approximately 30% at 3,000 RPM, and maximum power is increased by approximately 18% at 4,800 RPM.

Peter Burges also has a Fast Road specification in which the cylinder head is fully reworked prior to the lead-free fuel compatible valve seats and Peter’s custom tapered (bulleted) manganese silicone bronze valve guides being fitted. The intake and exhaust ports are modified to enhance the flow of the fuel / air charge without increasing the port sizes to any great extent. This keeps the port velocities high and aids in the production of low-end torque. The increase in power output from idle with a gain of approximately 25% at 3,000 RPM, as well as a maximum increase of power output of approximately 30% at 5,200 RPM when equipped with an Original Equipment camshaft and less restrictive K&N air filters. Beyond that point, the power will fall off much more gradually than with a Original Equipment specification cylinder head, so you can say good-bye to that frustrating “after-that-the-engine-seemed-to-run-into-a-wall” experience. If you add a less restrictive Peco exhaust system, it will extend the peak further (to about 5,500 RPM) with yet more power which afterward will decline much less precipitously afterwards. The Fast Road cylinder head also takes beautifully to a Piper BP270 camshaft, the combination sacrificing a little power down very low in the powerband where you rarely go anyway (below 2,000 RPM) and singing merrily all the way to 6,000 RPM. As you can see, the Fast Road cylinder head should be considered to be the jumping-off point when it comes to a quest for really serious power. It is the essential foundation that everything else is built on. To do it last is putting the horse behind the cart. This specification of cylinder head performs well with an Original Equipment camshaft, and shows even more impressive gains with either the Piper BP270 or the Piper BP285 camshaft. While the cylinder head works extremely well with the standard twin 1½” SU’s, it will also show worthwhile gains at high engine speeds with either modified 1½” SU’s or twin 1¾” SU’s. The Piper BP285 camshaft is recommended to compliment this increase in carburetion.
Stephen Strange

Stay away from my bank account, Stephan! :)
Curtis Walker

All,
Thanks for the advice, I will stay with my stock camshaft.

And Stephen Thank you for such a concise answer!
Dave
David Steverson

Concise!? Don't you mean comprehensive? :o)
PaulH Solihull

Curtis-
Power = Money.
Stephen Strange

David-
If you do not want the extra expense of professional porting, remove your old valve guides, and then spray the ports with machinist’s bluing so that you will be able to see what you are actually removing. Use a Dremel tool with a #80 grit, and then a #100 grit flap sander in order to gently smooth the existing port contours, removing the typical turbulence-inducing lumps and bumps that are a result of the casting process. Do not be surprised if progress on the exhaust ports takes longer than it does on the intake ports. Because the exhaust ports only see hot exhaust temperature gas, whereas the intake ports are cooled by the incoming fuel / air charge, they will have become carbon-case-hardened. A mirror finish can be advantageous in reducing future carbon build-up in the exhaust ports. However, a mirror finish on the intake ports and the combustion chambers is not only unnecessary, but is actually undesirable because it will eliminate border turbulence along these surfaces, thus leading to fuel condensation and a consequent loss of power. Be sure to carefully blend the port to the valve seat in order to remove any steps. Do not yield to the temptation to knife-edge the port divider thinking that such a modification will improve the flow of the fuel / air charge. The opposite will be the result. It is a common misconception that the port divider exists to channel the flow of the incoming fuel / air charge into two streams that each flow to its own intake valve. However, most of the time only one intake valve is open, thus no such channeling is occurring. A knife-edge at the nose of the port divider would actually serve to inhibit the transfer flow of any residual fuel / air charge carried by inertia into the port of the closed valve on into the port of the open valve. Instead, a smooth radius at the nose of the divider will actually improve its transfer flow performance.

Have installed at equal depth lead-free fuel compatible three-angle valve seats and three-angle 214N alloy Austenitic stainless steel valves with chrome plated stems and stellite tips (do not panic, they may sound exotic, but they are easily obtained and not very expensive), tapered (bulleted) valve guides, a set of the highly superior Fel-Pro Teflon-lined valve stem seals on the intake valves, 6” diameter X 3¼” deep K&N air filters, 1½” SU carburetors with richer fuel-metering needles, and a 1¾” Peco exhaust system. This will get you started with a relatively small investment and you will be both surprised and impressed at the improvement. The richer fuel-metering needles are needed as the Original Equipment airfilter housings restrict airflow enough to create a pressure drop downstream of the air filters. This pressure drop results in a pressure differential between the atmosphere above the fuel jet and the ambient pressure inside of the float bowl that causes more fuel to flow through the main fuel jet. The greater vacuum inside of the vacuum chamber (dashpot) also results in the vacuum piston rising to a higher level, thus the fuel-metering needle will be at a higher, richer stage. When the restrictive airfilter housings are eliminated, you will have a smaller pressure differential and thus less fuel flowing from the fuel jet to mix with the increased airflow, hence the need to introduce more fuel into the carburetor in order to maintain the proper fuel / air ratio.
Stephen Strange

"It is a common misconception that the port divider exists to channel the flow of the incoming fuel / air charge into two streams that each flow to its own intake valve."

The two ports have to join somewhere and the divider is just the point where they join! A blunt edge will generate undesirable turbulence, a sharp edge will not.

I doubt there is much if any flow reversion round the divider, whilst you are right in that flow will stop 25 times a second at 3000 rpm the inertia of the mixture in the main port is likely to ensure that nothing much can get back from the closed port.
Chris at Octarine Services

Chris-
I must disagree. Air doesn't like to flow around sharp angles, and thus flow-nhibiting turbulence is created. That's why a three-angle valve flows better than a single-angle one does. A smoother, curved surface on the nose of the port divider is more helpful.

Yes, reversion can occur. The intertia of the fuel / air mixture in the runner of the port piles up against the valve and then a sonic pulse "echos" backward. The reason that the design of the Original Equipment SU intake manifold was given such a large volume crossover tube is that these sonic pulses could dissipate inside of it when using the shallow "pancake" air filter housings of the period. That's why the Maniflow intake manifold with its small-diameter crossover tube functions best with 3 1/2" deep aircleaners. The sonic pulses can dissipate inside of the deeper airfilter housings.
Stephen Strange

When I rebuilt my engine I didn't do much to it so I stuck with the stock cam. I basically did what Stephen suggested and using a Dremel cleaned up the head a little and I also equalised the chamber volumes as much as I could. Other than that I had all the engine parts balanced, fitted HC instead of the original LC pistons and installed K&N filters.

Of course doing this one engine (first one!) by myself in this way and not knowing how it ran before I pulled it apart means I have no clue if any of this actually helped.

I was just thrilled it runs!

Simon
Simon Jansen

Simon-
With balancing, equalized combustion chamber volumes, plus the smoothed-out ports I would expect it to run smoothly, plus put out a bit more power. If you did a three-angle grind on both the valves and the seat, plus tapered valve guides, then you should have a reasonably capable daily driver.

David-
I guess that I should have described another do-it-yourself modification that's well worth doing: equalizing the volumes of the combustion chambers. Aside from matching the weights of the reciprocating components and independent dynamic balancing of both the crankshaft and the flywheel, perhaps one of the best ways to create a smooth engine is to equalize the compression and thus the power impulses occurring inside of each cylinder. Once the throws of the crankshaft have been indexed, and the lengths of its throws and the lengths of the connecting rods have been properly matched, this can be accomplished by making sure that the combustion chambers are of equal volume so that the compression ratio inside of each of the cylinders will be the same. After the cylinder head has been skimmed flat, the volume of each combustion chamber can be measured by using a clear piece of sheet plastic with a small hole drilled in it . Simply put a bead of chilled grease around the edge of a combustion chamber and press the plastic down onto it so that the grease forms a seal. Using a syringe or an eyedropper with a scale of measurement on it, carefully fill each combustion chamber with light oil, keeping a record of how much is necessary to fill each one. A useful pictorial description of this procedure can be found at http://www.mintylamb.co.uk/?page=measurecc.htm .

Next, use a Dremel tool to gently remove small amounts of metal from the smallest combustion chamber. Work slowly, making repeated checks. For small-bore engines (+.040 or smaller), the walls of the combustion chamber should be kept perpendicular to its roof in order to ensure the best squish (quench) characteristics. The roof of the combustion chamber should be flush with the valve seat and reasonably flat in order to ensure the best airflow characteristics. Finishing can be done with a sanding disc, care being taken to not undercut or groove the base of the wall where it adjoins the roof. This juncture should have a generous radius in order to both permit smooth airflow and to discourage the formation of carbon deposits that can lead to preignition. Because the roof of the combustion chamber is very thin, having coolant passages above it, remove no more material than is absolutely necessary in order to achieve your goal. Do not polish either the walls or the roof of the combustion chamber in an attempt to discourage carbon buildup as this will lead to condensation of the fuel / air charge both as it enters the cylinder and as it is being compressed. A glass-beaded finish will produce sufficient border turbulence to do nicely in terms of discouraging not only this problem, but that of the development of surface cracks as well.

Equalizing the volumes of the combustion chambers has a long-term benefit as well. When performing a compression test, there should be no more than a ten percent variation between the compression figures of the cylinder with the highest compression and the cylinder with the lowest compression. By eliminating the variable of differing combustion chamber volumes, the compression figures can be considered to be a more reliable indicator of the state of the affairs that relate to compression conditions.
Stephen Strange

This thread was discussed between 22/08/2010 and 24/08/2010

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