cylinder presure

So what does cranking compression tell you? Two things mainly----1) it gives you an idea on the minimum octane to prevent knock, and 2) helps you find any dead or uneven compression cylinders.

Compression ratio is a two-edged sword, meaning that a super high compression ratio is not a guarantee of big power if the rest of the engine isn't matched up with it. Take a look at NASCAR restrictor plate engines---these guys are nearly pulling full manifold vacuum at WOT, so they run very high comp ratios to make up for the poor cylinder filling. On a normal track, they run less ratio since the restriction is gone, and the cylinder filling is optimized.

So if you have poor flowing heads, an early IVC, and a small carb on your tractor, your cylinder filling will not be optimal, so a higher comp ratio and higher octane fuel to give yourself some knock margin at low piston speeds is a good recipe. However, a set of good flowing heads/manifold/carb will deliver more HP/torque than any increase in compression. Enjoy.

Let me explain a little further. Like you said, if you take the same engine and just raise compression, there will be gains on the table. One of David Vizard's SB chev books had a nice table showing the percentage gains in HP for each increase in comp ratio. Going from 8:1 to 10:1 is a 4.9% increase---going from 8:1 to 14:1 is a 9.8% increase in power. And this would be at the same RPM. Going from 5.5 to 10 on your AC is a huge jump and no doubt responsible for an increase in your power. Compression is an easy way to gain power on these old engines, especially if your RPM limited. But, if you can spin faster than stock, better flowing heads will be more of an advantage than compression. I've built a few 88 Olivers that do just that. So, if you dynoed your stock AC at 540RPM for example, and then the ported heads at the same RPM, you probably would see little gain (like you did). But if you can spin it faster than that, the power has the potential to be higher than the power from the compression change. It's just plain difficult to build good HP at low RPM's, and high flowing heads may actually give you less torque at those low revs. So increasing compression is a good way to increase performance at the same RPM. Good topic.

AC WD's which have open combustion chambers my not be suitable for high compessions and possibley the JD 2cyl engines due to detonation problems.

300 is not uncommon for a well built waukesha engine. More compression=more heat=more expansion=more torque.

Google (online engine calc carb size) for a quick guess at carb size cfm requirements. use 70 % vol eff for most tractor engines.

A high vacuume at WOT while pulling indicates a restriction in the induction system.