Maybe this has been hashed to death, so excuse my ignorance. . . .
On something like a '26 Coupe, a relatively heavy car, would a high compression cylinder head make a significant difference? And where would the difference be felt --- on low rpm hill climbing grunt work, or more high speed cruising?
Also, I see two compression ratios available, an 8:1 and the Z-head. Any preferences? Or any reason not to go for the highest ratio of 8:1?
Huge difference you will love it.
With a high compression head you'll find more power at all rpms.
The head advertised in the catalogs as 8:1 is the P or Prus head. It's a quality product that's actually about 5:1 in compression, the 8:1 figure is a misunderstanding / exaggeration.
The Z head has also about 5:1 compression, but the combustion chamber shape isn't as effective as in the Prus head, so the top power at 1600 rpm would be about 2 hp less with a Z than with a P head, though at lower rpms where you'll spend most of the driving there won't be any measurable difference.
The slightly higher price of the Prus head is reflected in a machined squish surface in the combustion chambers, reducing the risk for pistons in conflict with the head and the need for grinding as some Z head customers have had to do.
The Pruss head is not 8:1 in my opinion. That being said, it is the one I like best.
To get the best performance I will recommend in no particular order;
High compression head
280 or similar cam
A high volume intake
A Stromberg OF carb
But before anything;
Make sure you have a properly performing ignition!!!
A stock T ignition PROPERLY tuned coils and a good properly centred timer WITH a decent working stock magneto will do the job. Of course there are alternatives but I won't go there right now
I put a Z head on my pretty much stock 26 Coupe and really like it. Main boost is in ability to pull hills. The car seems happier at all RPM ranges. Stock motor except for Stipe 280 cam, a Stromberg OF carb and E-Timer.
I have friends using the 8:1 Prus head that love it but they also have modified the crank, carb, cam, exhaust and ignition.
Have a completely rebuilt engine in my '26 (SCAT crank, new pistons, babbit, 280 Stipe). Originally I had a Z head on it, then went with a Prus head. I like the Prus unit better. I don't notice that much difference in the compression when crank starting it, but there's a difference in driving it. Prus head seems to have better power and a little more snap when accelerating. If your engine can handle it, either unit give you more usable horsepower than a stock head.
There are some excellent discussions about torque, Horse Power, cams, heads, etc. at the Tulsa Oklahoma web site.
Power & Torque see: http://mtfctulsa.com/Tech/power_and_torque.htm
That includes Z head and stock head information.
As a physics professor you may want to look at and possibly even use some of their information see: http://mtfctulsa.com/downloads/Physics101.pdf
Several heads and setups dyno-tested at: http://mtfctulsa.com/Tech/DynoSummary.htm
http://mtfctulsa.com/Tech/heads.htm has some great information and photos. But you need to click on the tabs on the left to get to the other pages with the data and photos of the heads. Lots of photos at: http://mtfctulsa.com/Tech/head_photos.htm
I don't think the Prus head was out when they were doing the testing -- it was not mentioned. But the Z head and Prus head are compared in the thread at: http://www.mtfca.com/discus/messages/257047/324600.html?1354029640 .
Hap l9l5 cut off
Not that there's any comparison, but as Jay Leno says, "I live in a hilly area."
A Z head made the car able torquey enough to shift at 11 mpg and chug up my fairly steep street while slowly accelerating to 15 mph.
Also, New coils and some commutator cleaning also eliminated an annoying spark hit and miss that previously impeded acceleration under load.