Z and P High Compression Heads

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ModelTWoods
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Z and P High Compression Heads

Post by ModelTWoods » Tue May 18, 2021 12:34 am

There have been many posts on this forum about Z and P high compression heads. In no post have I ever read where someone successfully milled even as much as a 32nd of an inch (.031) off of a Prus head. Does anyone know if this is possible, assuming the block has stock deck height?

Then on the subject of Z heads, occasionally, someone will post that their new, out-of-the-box Z head had piston interference problems, necessitating the removal of either material from the top of a piston or the head's combustion chamber over the piston. I don't remember if the owners having Z head interference problems stated that their block's deck height had been modified, or not. Then, along comes the Tulsa MTFCA club with their dyno tests of different heads. One of the heads that they tested was a Z head that had been milled .050. I'm scratching my head. How could one Z head have interference problems OUT of the BOX, and another Z head be able to be milled .050? Is the manufacturing quality control of Z heads so irregular that there is that much difference in different castings? I have one of each head and if I knew there was no risk involved, I'd have each one milled to the amounts that I've mentioned for each head. Any thoughts from you engine gurus?


Allan
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Re: Z and P High Compression Heads

Post by Allan » Tue May 18, 2021 6:35 am

In my experience, the Z heads are a little inconsistent in casting. I have experienced problems with clearance on cylinder no four on two heads. One needs to be careful when installing them, moving them around on the bolts before tensioning, checking for any interference. One had been welded to fix a casting fault. The correction was done well, but I feel such work should be declared at least, so the customer can make the purchase decision with full knowledge.

I have used just one Prus head, with no issues.

Allan from down under.


Joe Bell
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Re: Z and P High Compression Heads

Post by Joe Bell » Tue May 18, 2021 6:38 am

I have had Z heads that have needed to be addressed from pistons hitting the head, yes the block had been decked to clean it up as all should. If I had a choice it would be a high compression Iron head, they heat and cool at the same rate as the block so less head gasket issues. Just my thoughts on it!

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Henry K. Lee
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Re: Z and P High Compression Heads

Post by Henry K. Lee » Tue May 18, 2021 6:53 am

Same experiences here with Z heads and have a very early one on my 1925, no issues. I agree with Joe on the heat transfer, dead on. But if you can not find a good original, Kevin Prus head is the answer. Well balanced chambers, excellent castings, etc.

My $0.03 worth,

Hank


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Re: Z and P High Compression Heads

Post by speedytinc » Tue May 18, 2021 8:44 am

I have installed 2 z heads that had combustion chamber casting variations & talked to other members with similar problems. My first one the piston hit #1 chamber. with CCing, i found smaller front to back, like it was machined @ an angle. The 2nd one was smaller @ both ends & higher in the middle 2 Both heads required 3 combustion chambers to be machined to get even CC's. 5.5:1 compression (actual) Have also had serious corrosion issues, even with an anode.
Recent Pruis head install for a customer. Pruis head CC'd perfect between cyls. 5.6:1 just a tad above Z head. Very good quality head. Their claim of 8.25:1 aint so. I talked to KP & got their CC #'s. Confirmed my compression ratio finding. I believe their high compression claims do more to scare off potential customers. Until i was convinced by comparing CC #s with a bunch of known CC readings that the compression wasnt that high, i would not have bought one. I am a big fan now. Highly recommended to customers to warm up T performance.
KP claims a modern aluminum head sealing process that should limit corrosion issues.

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Re: Z and P High Compression Heads

Post by Hudson29 » Tue May 18, 2021 7:53 pm

Does the Prius head look like a stock head? One of the things I like about the Z head is that once painted to match the motor is detectable only by the Z cast into it up by the water outlet.

The '14 Touring could use some perking up and if it is still sluggish after switching out the carb & getting the electrics up to snuff it might be time to think about a head.

Paul
The man with a watch always knows what time it is, the man with two watches is never sure.


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ModelTWoods
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Re: Z and P High Compression Heads

Post by ModelTWoods » Tue May 18, 2021 9:27 pm

Hudson29 wrote:
Tue May 18, 2021 7:53 pm
Does the Prius head look like a stock head? One of the things I like about the Z head is that once painted to match the motor is detectable only by the Z cast into it up by the water outlet.

The '14 Touring could use some perking up and if it is still sluggish after switching out the carb & getting the electrics up to snuff it might be time to think about a head.

Paul
Yes, the Prus head looks just like a stock 1926 -1927 head. Most, if not all, except maybe early 1926 heads DID NOT have Ford script and Made IN U S A on them. So if using on a pre-26 engine, it is NOT going to look like your original. It will NOT have Ford script or Made In U S A on it.


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Re: Z and P High Compression Heads

Post by Allan » Wed May 19, 2021 5:29 am

With an application Devcon patches to the top of an alloy Prus head, I added MADE IN CANADA and the Ford script, so it does look absolutely stock on my roadster. It took a little time with a Dremel tool and trimming knife, but it worked out really well.

Allan from down under.


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ModelTWoods
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Re: Z and P High Compression Heads

Post by ModelTWoods » Wed May 19, 2021 12:12 pm

Allan, almost anything can be accomplished if the will and desire is there. Fortunately, I have a 26 and a 27, so the non script head is correct for them without modification.
Regarding my questions in my original post about milling either head, Mike Bender from the Tulsa club, cleared my questions for me.


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Re: Z and P High Compression Heads

Post by speedytinc » Wed May 19, 2021 12:55 pm

Regarding your original post. In my experience & friends, yes, the Z head castings chambers are all over the place & not consistently.
I found the Pruis CC'b all cyls the same.
Regarding milling more from either head. Use bits of modelers clay on top your motor's pistons without a head gasket. You will determine the actual amount that can be milled. Your piston height & block surfacing can vary from the other guy.
A head with 5.6:1 compression makes a T perform drastically better. Dosent need extra milling.
If you are looking for max compression for racing ok. I built a 14 with a milled low head & milled pop up pistons 95# compression 6.5:1
Thats as much compression as I would go on a stock crank.
You can go higher.

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