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100-4 Head

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J

jandkellis

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Most owners will agree that the best way to improve performance of the 100-4 is to replace the head in order to align the ports for improved flow; is it as effective to machine the existing cast iron head? Has anyone done this and experienced a significant improvement?
Thanks,
Jim
 
I've not done this specifically to a 100-4 cylinder head, but I have modified cast iron heads for Triumphs and Austin Minis. In each case, the head was not the only modification, so its hard to tell how much benefit there was...

Also, the key is not just to better align the ports, but to remove material around the valve guides and in the intake "bowl" under the valve. Port matching will not yield much of a result on its own. Also important is combustion chamber shape, particularly around the valves. I will say the cast iron is significantly harder to work than aluminum, and therefore more time consuming, but its also more forgiving than aluminum. It will require a valve job after modification, so that's a good opportunity for some larger valves and a 3 angle hardened seat for unleaded. While you're at it, a cheap way to raise compression is to "mill" the head face to shrink the combustion chamber size. I once milled over an 1/8 inch off of a Mini head to raise compression and found good results.

In general, though I can't speak to the AH 100 head specifically, Cast iron British cylinder heads respond well to port and combustion chamber modification especially when combined with CR and a cam change.

* Don't attempt this though without knowing what you are doing-- it is easy to ruin a head with too much grinding. The David Vizard engine modification book is a good resource.
 
I have not read much about porting a 100 head and I did quite a bit of research when I rebuilt my motor about 7 years ago. The old long stroke four is not a revvy or peaky engine by nature, and you can't make it rev (and live a long time) without some expensive parts and work, so maybe porting has limited benefits at the lower rev ranges? Also the 100 head is prone to cracking, don't know if porting out would weaken any of the already suspect areas, but something to think about.

I don't think milling the head on a 100 is recommended, I seem to recall that maybe some of the water or oil passageways run at an angle and don't mate up well if you machine too much off. The engines respond well to high compression pistons (stock ratio is a low 7.5/1) a mild cam like the M spec unit, and lightening the flywheel, just because it is so heavy and it is easy to machine off quite a few pounds, I did this to my motor and it ran as sweet as stock but had a little added oomph.
 
Jim--

Over and above the considerations already expressed is that these old heads are prone to cracking. You may want to consider an AL head as sold by Denis Welch which is not only a better casting but has been been modified along the lines suggested. He makes several different configurations along with exhaust headers to match.
 
The only way to get more performance is to fit a hotter camshaft. They make a big difference by effectively pushing the power further up the rev range, which is a good reason for not doing it IMO. Making the head etc work perfectly mike make 5bhp difference on a good day and a cam 20, but with a poor tick over and more gear changing, not to mention a loss of charm.

However, if you're car hasn't been on a rolling road with an expert turn, you'll probably be amazed and at what be achieved with a bog standard car. Believe me it's money well spent.
 
In my experience, it is better to avoid milling or porting a cast iron 100 cylinder head. The face can warp if much is milled off and cracks often occur in inaccessible places in the ports.
Alwyn
 
Another side effect of having too much milled off the face is that it becomes difficult to adjust the tappets. With the original cylinder head on my car, I had to place spacers under the rocker posts so that tappet clearance could be achieved. I now have a cyl head from an Austin Atlantic fitted.
 
Have had mine done some time ago, larger valves, ported etc, however for longevity put in lower compression pistons and lighter flywheel
 
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