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10-06-2016, 04:18 PM | #1 |
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Join Date: May 2010
Location: Shrewsbury,Pa
Posts: 513
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High compression heads and carb jet
Has anyone ever determined if jet sizes should be increased for Hi compression heads? If so, by how much for say a 5.5:1 head.
Bill G |
10-06-2016, 05:25 PM | #2 |
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Join Date: May 2010
Location: Windy City
Posts: 2,919
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Re: High compression heads and carb jet
There is no relationship between CR and stoichiometric fuel ratios. The increased flame front velocity from a higher compression ratio will somewhat affect the overall scavenge/breathing for a given physical configuration and valve (cam) spec. If anything, the ideal in a Model A going from 4.22 to 5.5 may require a slightly leaner mix, not richer.
You probably won't need to change anything if your car runs with properly colored plugs now. The nice thing about an A is you have a GAV to fiddle with a bit. If that doesn't find the sweet spot only then play with jet flow rates. |
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10-07-2016, 07:32 AM | #3 |
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Re: High compression heads and carb jet
What he said.
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10-07-2016, 02:00 PM | #4 |
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Location: Oregon
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Re: High compression heads and carb jet
The slightly higher compression Model-B engine came with a carb having a slightly leaner main jet. Model-A was a #19 main jet and Model-B was # 20.
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10-08-2016, 12:16 AM | #5 |
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Join Date: May 2010
Location: PASADENA, CA
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Re: High compression heads and carb jet
I just installed a 5.5 head on my otherwise stock good running Model A engine with an original Zenith Carb.
I did not have to do anything to the carb. Nice improvement in the performance. Chris W. |
10-08-2016, 08:06 AM | #6 |
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Location: Hebron, CT
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Re: High compression heads and carb jet
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