I had 2 new Prus heads and one Z head and a seasoned Model t engine in the shop at the same time. It was the perfect opportunity to put the engine on the Dyno and compare the two heads.
The engine was an older rebuild with 2 or 3 thousand miles, a Model A crankshaft, distributor ignition,straight thru carb, late model t intake manifold and stock exhaust manifold.
The following graph shows the superior performance of the new Prus head at the higher rpm's and somewhat less horsepower at the very low rpm's.
Lyle_PANDZHEAD_DYNO_0712_12.pdf (34.6 k) |
Tom Graham:
Can you get a low Prus Head ?
I have a P head on my '24 Tudor and it works good.
I'm sure there's only one version.
I wouldn't spread that around too much Bob,......could give you a really embarrassing nikname!
My Dad, who bought a 1923 Roadster new, had a favorite Model T joke:
"Hey, did you know Jones bought a new car?"
"No, what kind?"
"I don't remember for sure, but it starts with P."
"Oh, that has to be a Ford. All other cars start with gasoline."
rdr
Tom.
Thanks for the report!
Thanks for testing
The compression is about equal - even slightly lower in the Prus head, so where does the extra 2 hp in the high end come from?
If we look at the shape of the combustion chambers we see that the Prus head follows the Ricardo principle fully, the squish area covers most of the pistons:
while the Z head doesn't concentrate the combustion as tight around the valves and the spark plug:
Maybe that's the secret?
(Pictures from earler threads, not mine)
Results of tests I've seen on Model A head show increased compression increases HP at all rpm. I wonder, too.
The Prus head is a more "modern" and proven design for the flathead. When I saw the picture above I thought I was looking at 4-cyl in-line Briggs & Stratton head.
This is very interesting. It would be nice to see a third column in the table showing a stock head for comparison also.
The Prus head looks like it has a smaller combustion chamber. Probably has a higher compression ratio than the Z head. The Z head is able to keep up with the Prus head at normal driving speeds in terms of torque. When you get up to crankshaft breaking speeds the Prus head is making more power.
I would like to know the combustion chamber volume of each. If my theory is true the Z head should make more power than the Prus head if it could be milled enough to have the same compression ratio.
OK just confirmed my suspicions. The Z head is advertised as 6:1 compression ratio. The Prus head is advertised as 8:1 compression ratio. Therefore the design of the combustion chamber in the Z head is superior, being more efficient at lower speeds at filling the cylinder and allowing the spent gasses to exit.
The Prus head would likely have a significant benefit in a slight redesign to copy Ralph Zajicek's original design.
The Prus head 8:1 advertising is misleading. Actual compression is about 5:1 for the Prus head according to my measurement of the combustion chambers. The Z heads actual compression ratio is about 5.3:1 according to Larry Young's calculations at the Tulsa Model T club site: http://mtfctulsa.com/Tech/head_design.htm
Here is the most recent forum discussion regarding the compression ratio: http://www.mtfca.com/discus/messages/257047/315273.html?1349381684
This dyno result confirms Larry Young's earlier results that combustion chamber shape is at least as important as compression ratio. Here's more from the Tulsa club: http://mtfctulsa.com/Tech/head_dyno_testing.htm
"A 5.7:1 compression ratio would theoretically produce a 25 percent power increase, and yet the Z head and Riley head produced a 42 percent power increase. The greater than theoretical power from these heads is evidently due to their improved combustion chamber design."
(the 5.7:1 compression ratio for the Z head comes from 0.060 overbore and a Model A crank on the test engine)