Austin 850 performance conversion

[abri]

So, it's been a long time since I've done an update on my 850 performance conversion build.

Last time I left it at some measurements that I took. These went into a compression ratio calculator and the aim was to up the compression ratio slightly, from 8.3 to 9:1. These calculators are very handy but unfortunately I can't seem to paste a screenshot of mine in here.

Anyway, to take a step back, I started with a 12G202 cylinder head which would have come standard on the SA 998 cc engines as well as on the 997 Cooper. This is not the best head to use for this sort of project because there is a much better flowing one with less shrouded valves called the 12G295, which came a bit later and was fitted to the 998 Cooper. The 202 heads are obviously much easier to find and command less financial layout. I happened to have one that I bought a few years ago for a very good price, so that's what I used. When I bought the head it looked recently redone, so I was under the impression it would just need to be skimmed. Incidentally the 295 head has large chambers so that would also need significant skimming to up the CR for use on an 850.

So, how much can you skim off a cylinder head before ending up hitting a water jacket and having an expensive paperweight? I asked some advice. A good place to start is to measure the thickness of the head. Obviously this will give you an idea of how much (if any) has been skimmed off already. Secondly, measure the depth down the oil holes from the top of the head. Take the biggest of the oil hole depth readings and subtract that off the head thickness. What you are left with is an estimate/guide of how much "meat" is on the head that can hypothetically be removed (this assumes the spot that you measured is the lowest hollow point in the casting). Vizard recommends adding a certain "reserve" to this, to be safe from over heating (I think).

In my case the numbers stacked up like this:
Head thickness - 2.746" (this means the head had been given a 4thou lick. Standard would have been 2.750)
Depth down oil hole - 2.585"
Meat below galleries = 2.746 - 2.585 = 0.161

Subtract from this Vizard's reserve of 0.080 and the max left to skim is 0.081"

The CR calculator told me that in order to get a CR of 9:1, I had to have chamber volume of 20.8cc. Seeing as you're starting with 26.1cc on a standard 12G202 head, that is quite a large reduction in the chamber volume, especially since the chambers are quite narrow (ie. you have to skim more to reduce the volume).

Here is a picture of the combustion chamber on the 202 head, quite narrow and deep.


So I clamped the head in a vice, making sure that is was perfectly level, turned a spark plug into one of the combustion chambers, poured 20.8cc of engine cleaner into a burette and slowly drained it out into the chamber. 

It ended up looking like this


I then attempted to measure the depth from the face of the head to the fluid level and found it was pretty much spot on 0.080"



I repeated this on another chamber and got a very similar result, so I was happy that I could get the 0.080" skimmed off and end up very close to the 9:1 aim.

I took the head in for machining but was soon told that the valve seats had been cut badly and would need to be redone and matched to the valves. This unfortunately ended up recessing the valves somewhat and throwing off my meticulous calculations and measurements. Long story short, when I got the head back, 80 thou had been machined off as requested but the chamber volumes were 22.5 cc instead of 20.8, so this meant I had a CR of just above 8.5. This is not ideal but seeing as the car is just a daily driver I guess it will be ok.

More in a bit....