High compression heads and carb jet

Bill Goddard

Bill Goddard

Well-known member
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
 
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.
 
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.
 
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.
 
Raising the compression ratio helps to increase a more complete burn of the fuel within the combustion cycle which produces more work or cycle efficiency. The goal in achieving a more complete combustion cycle is to raise the compression chamber temperature and atomize the fuel as fine as possible. Doing this helps to improve a more complete burn for a given volume of fuel. Just adding more fuel doesn't improve engine performance/efficiency. Todays turbocharged engines are designed to raise the combustion temperature and atomize the fuel for improved perfomance, fuel economy, and lower emmissions.
 
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