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MG MGB Technical - Aluminum or CI Cylinder Head?
|It's 2012 and I'm sure the jury is in on the aluminum cylinder head. When last searched, the archives were pointing to the tuned CI head as still the better choice when compared to the out-of-the-box aluminum head. Has manufacturing improved since 2001 for the aluminum head? Or does one still need to shave a few thou off to compensate for lost HP due to the temperature loss? My last big-valve head was shopped by Seven Shop in 2005 and was great until a recently discovered crack between the valves in No. 4 cylinder. Should I buy a used CI head, have it checked out, then ported, or go for the aluminum head? Is there more than one aluminum head manufacturer?|
Check the MGB Technical Archives under "Rebuilt Head Crack Probability". Some good answers from Peter Burgess and others. Sounds like you had some good years out of the modified head. Your experience sort of confirms my suspicion that you can still get a crack after tuning a used CI head. But, as pointed out small cracks may be eliminated in the rebuild process extending the life of the head.
I hope we hear about experiences with recent aluminum heads. Likely no improvement.
Also, like you I have an 18V which uses the smaller chamber. From what I read the currently sold heads have the large chambers. So, you end up with smaller piston dishes or a bigger than desired head cut to get the compression up on our engines.
|This new thread leads into a question I have. Are the CI heads sold by Moss, Victoria, and others any good? |
I believe Peter Burgess also sells new CI heads but they may be a different source. I suspect the Moss ones come from India.
1. Are they lead free design? I suspect seat induction hardening at best.
2. Valve guide material? I suspect CI.
3. What is the combustion chamber volume?
Can one use standard MGB valve train parts when service is needed?
4. Can they be ported or is the core placement questionable?
You ask about the aluminum alloy cylinder heads currently available. These relatively expensive items shave about twenty pounds off of the weight of the engine and tend to run cooler under the high stresses of racing. Their rapid transference of heat also helps in preventing the development of “hot spots” which can cause preignition under the thermal conditions generated by heavy loads. Because the cast iron of the engine block and the aluminum alloy of the cylinder head have different expansion rates, the use of a high quality resin type cylinder head gasket should be considered to be mandatory. Whenever an engine exceeds its normal operating range and overheats, the elevated temperatures can cause extreme stress in the cylinder head that may result in a cylinder head gasket failure. This is especially true in the case of an aluminum alloy cylinder head mounted onto cast iron engine block because aluminum alloy has a coefficient of expansion that is about two to three times greater than that of cast iron. When combined with the added stresses induced by overheating, this difference in thermal expansion rates between an aluminum alloy cylinder head and that of a cast iron engine block can cause the cylinder head to warp. This, in turn, may lead to a loss of clamping force in critical areas and allow the cylinder head gasket to leak. In addition, since the thermal expansion rate of aluminum and cast iron are greatly different, and aluminum expands much more, it makes the role of the cylinder head studs very critical in maintaining correct clamping pressures on the cylinder head gasket. In view of the fact that aluminum alloy expands and contracts much more than cast iron during thermal cycling, stress on the cylinder head studs is correspondingly greater, shortening their effective lifespan and thus requiring their replacement every time that the engine is rebuilt. They also require that washer-like steel shims or steel valve spring collars be placed under the springs in order to protect the aluminum alloy material of the cylinder head from being galled by the springs. Be aware that aluminum alloy cylinder heads should be torqued only when cold, and to no more than 38 to 40 Ft-lbs.
Note that whenever you install spark plugs into an aluminum alloy cylinder head, you will want to be sure to smear some antisieze compound onto the threads of the spark plugs. This will prevent the corrosion of the aluminum alloy threads that results from the electrolytic interaction between the steel of the spark plug threads and the aluminum alloy threads of the cylinder head, which can cause them to seize in place, as well as make the spark plugs much easier to remove.
Unfortunately, the poor thermal efficiency of aluminum alloy forces the use of an increased compression ratio of about one point in order to produce the same amount of power. This higher compression ratio, coupled with the obsolete kidney-shaped combustion chambers in turn creates a problem with preignition when running on the gasoline available at a gas station unless frequent and considerable attention is paid to the maintenance of precisely correct ignition and carburetor settings. This is due to the fact that the power stroke takes place at the same speed as the compression stroke. Put crudely, an “Octane” rating is merely an indication of the fuel’s resistance to combustion. This means that it can be compressed to a higher ratio within the same timespan without the greater heat that is generated by compression resulting in the initiation of combustion (preignition). The more rapid heat transfer capability of aluminum alloy allows a faster rise in pressure without preignition occurring because some of the heat resulting from compression is conducted out of the combustion chamber by the more rapid heat conductivity factor of the aluminum alloy. The caloric value of the fuel remains the same, so power output at a given compression ratio is greater with a cast iron cylinder head than with an aluminum alloy cylinder head due to cast iron’s reduced heat loss through the roof of the combustion chamber. This is why it is necessary to boost the compression ratio by about one point in order to attain the same power output when substituting an aluminum alloy cylinder head for a cast iron cylinder head. That is, a Geometric Compression Ratio (GCR) of 9:1 when using a cast iron cylinder head must be increased to 10:1 when using an aluminum alloy cylinder head, but the needed octane rating of the fuel can remain the same. A beneficial side effect of this increase in compression ratio will be a minor improvement in fuel economy. In addition, careful attention to the coolant system is a necessity as an aluminum alloy cylinder head has a severe tendency to warp should it overheat. If this overheating occurs, the temper of the alloy will have been ruined and thus the torque settings of the nuts of the cylinder head studs will not hold, the cylinder head having become just so much scrap metal. You do not get something for nothing!
Aluminum alloy cylinder heads come in two basic types: an aluminum alloy version of the Original Equipment cast iron cylinder head, and a seven-port crossflow cylinder head design. Unless you are replacing a cracked cylinder head or building an engine that employs a hot camshaft such as the Piper BP285 and requires the use of high compression pistons, there is little practical advantage to the extra expense of using the aluminum alloy five-port design in a street application. At present, there are two versions of this design. One is the American-made cylinder head offered by Pierce through multiple aftermarket suppliers such as Brit Tek, Moss Motors, and Victoria British. The other is the UK-made George Edney cylinder head that was marketed exclusively by Brown & Gammons, the latter having a superior port design that has significantly more tuning potential.
Always good input. I managed to get almost 30K miles on the head after a Sevenshop rebuild. The Pierce aluminum head would require porting anyway (see 'before' photos on Sean Brown's website) and that cost is on top of the Pierce cost of ~$800. There are plenty of used 2923 heads in town for ~$100. I'll likely go that route. Already have a new ARP stud set and Cometic gasket waiting.
|C.T. Well, Steve basically nails it again. I've done em all. Ran an MSX alloy cross flow, which Peter flowed for me 10 years ago, and have 2 others, both CI heads, flowed by Peter: one on the Mauve MGB with a P285, Harlan rollers, goes like the wind and pulls smoothly like a really good dog. In OD on an empty freeway, I get 110 MPH and it's still straining to go when I chicken out and let up. The car is just a big smile. Peter did the SC head for me also about 6 or so years ago and it achieved a RWHP of 123 on the SAE dyno 4 years ago ( see upload).|
Mike Pierce is just down the road in Gilroy, and we have maybe 4 of the alloy heads on smog and non smog MGBs. In all cases the heads were ported, flowed, and smoothed extensively, then shaved up to 70k in one case to get the TE effect back.
Perhaps the best bang for your buck : pick up a crack tested larger valve CI head, press in hardened exhaust seats before a 3 angle cut and Brnz/Mgnz guides, then have Peter or Sean flow the bejeezusgawds out of it. You'll be into it a smidge over 1-large, and really improve the smileability and drive.
FWIW however, the alloy x-flow, is without a doubt, the strongest runner overall and that includes the sc car. With technical input from Steve and Doug Jackson, then complete outfitting, flow, and shaving by Peter, the engine is amazing, spinning to 7k and pulling like a freight train all the way. And this with the SU, not the DCOE!
This thread was discussed between 11/02/2012 and 23/02/2012
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