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Old 08-17-2019, 01:09 PM   #1
Chris Haynes
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Default High compression head question

I see ads for a 8 to 1 head. My question is how long will the Babbitted bearings survive?
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Old 08-17-2019, 01:29 PM   #2
J Franklin
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Default Re: High compression head question

If you are racing, who cares!? Is that head designed to be used with poured bearings?
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Old 08-18-2019, 04:17 PM   #3
Dodge
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Default Re: High compression head question

8 might be pushing it a bit. Go with one of the 6-1 Z heads they work great
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Old 08-20-2019, 01:24 PM   #4
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Default Re: High compression head question

I live in a hilly area at 5300 ft. elevation. Installed the Prus 8:1 head on my stock ’26 runabout last year. Best upgrade for the money. Like the ad says, “Flattens the hills”. A hill I had to previously drop down into low with I can now maintain 35 mph in high. No pinging and no concerns with babbit or crank (have EE crank). Over 1000 miles now.
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Old 08-25-2019, 12:03 PM   #5
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Default Re: High compression head question

The P head IS NOT 8:1 or close; just because his ads say so doesn't make it true. It's still a good head, as is the Z head.
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Old 08-26-2019, 08:29 AM   #6
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Default Re: High compression head question

With the necessary clearance for valves on the L-head designs, they would be hard pressed to get to 7.5:1. This is why folks came up with the OHV conversions back in the day and some of those still only had intake valves in the head.

Poured babbitt bearings can take as much rough housing as any but they are a lot harder to replace when they wear out.
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Old 08-31-2019, 11:18 AM   #7
AzBob
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Default Re: High compression head question

I would like to see the data that proves the Prus head does not deliver the advertised 8:1 compression. I was assured by Mr. Prus himself that it indeed delivers 8:1.
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Old 08-31-2019, 04:32 PM   #8
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Default Re: High compression head question

The math is quite simple. You need the volume of the combustion chamber with the piston at TDC. You need the volume of the combustion chamber + the cylinder's swept area with the piston at BDC. Add the two. Then divide the larger sum by the other.


Variables include type of piston and the bore size. Several capable folks have measured the cylinder head and done the math with results in the neighborhood of 6:1 ratio. If Mr. Prus says otherwise ask him to provide the math that he is using to come up with a different answer.


I personally have never seen one but hear nothing but good reviews of the product. If you think a high compression head is what you need then why not just buy one and try it?




Quote:
Originally Posted by AzBob View Post
I would like to see the data that proves the Prus head does not deliver the advertised 8:1 compression. I was assured by Mr. Prus himself that it indeed delivers 8:1.
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Old 08-31-2019, 07:19 PM   #9
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Default Re: High compression head question

It's difficult to realize that high a compression ratio on any of the flathead designs let alone the Model T types. The OHV head was the most common way back in the day.

If you shave the head, there still has to be valve clearance. If you have higher valve lift with a race cam, it just makes things worse. The amount of compression possible is finite.
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