Dyno Comparison - Z head / Prus head

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Model T Ford Forum: Forum 2012: Dyno Comparison - Z head / Prus head
Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Graham on Monday, November 26, 2012 - 09:53 pm:

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.

application/pdf
Lyle_PANDZHEAD_DYNO_0712_12.pdf (34.6 k)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dave Huson, Berthoud, Co. on Monday, November 26, 2012 - 10:01 pm:

Tom Graham:

Can you get a low Prus Head ?

picture


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Gruber- Spanaway, Wash. on Monday, November 26, 2012 - 10:25 pm:

I have a P head on my '24 Tudor and it works good.
I'm sure there's only one version.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Harold Schwendeman - Sumner,WA on Monday, November 26, 2012 - 11:13 pm:

I wouldn't spread that around too much Bob,......could give you a really embarrassing nikname!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ricks - Surf City on Monday, November 26, 2012 - 11:41 pm:

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


Top of pagePrevious messageNext messageBottom of page Link to this message  By Rob Heyen - Nebraska on Tuesday, November 27, 2012 - 12:13 am:


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dean Yoder on Tuesday, November 27, 2012 - 04:03 am:

Tom.
Thanks for the report!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Tuesday, November 27, 2012 - 04:48 am:

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:
Prus combustion chambers

while the Z head doesn't concentrate the combustion as tight around the valves and the spark plug:
Z head inside
Maybe that's the secret?

(Pictures from earler threads, not mine)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ricks - Surf City on Tuesday, November 27, 2012 - 04:58 am:

Results of tests I've seen on Model A head show increased compression increases HP at all rpm. I wonder, too.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ken Kopsky, Lytle TX on Tuesday, November 27, 2012 - 06:21 am:

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. :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Steve Blancard on Tuesday, November 27, 2012 - 07:02 am:

This is very interesting. It would be nice to see a third column in the table showing a stock head for comparison also.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Tuesday, November 27, 2012 - 08:56 am:

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.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Tuesday, November 27, 2012 - 09:19 am:

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.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Tuesday, November 27, 2012 - 10:20 am:

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)


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