For a short cammed engine vacuum is much higher which tends to vaporize the fuel better, and add to that a stock heated intake manifold increases vaporization for an easily ignited fuel . Distribution cylinder to cylinder will also be good.
Higher compression of yesteryears reduces residual exhaust pollution.
Lastly a true dual plane ( fully divided ) intake has less cylinder to cylinder pollution at idle because all the cylinders are not connected to a common plenum.
An example. 350 LT1 1970 used about 240@.050” duration camshaft on at least 114 wide LSA with 11:1 compression . Fully divided dual plane intake manifold with crossover heat. Good vacuum and Good idle.
396/375 hp used about 240@.050” duration camshaft on at least 114 LSA with 11:1 compression. Fully divided dual plane intake manifold with crossover heat.
And these cams had slow ramps on solid lifters. Good vacuum and Good idle.
Now we install a high duration cam on a tighter 110 LSA with an unheated intake manifold and compression ratio is modest . The RPM AIR GAP intake manifold runs cooler and has a cut out ( not fully divided ). This results in less fuel vaporization and some exhaust pollution within the intake manifold due to the cutout .
Dilution of the incoming charge by exhaust residue left in the combustion chamber at the end of the exhaust stroke. In addition you have exhaust dilution due to exhaust flow reversion at the end of the exhaust stroke. Overlap and tighter LSA .
Lower manifold vacuum and cooler intake manifold temperature means less chance of good fuel vaporization for a smooth low rpm idle.
The Fix
Idle Mixture ratio needs to be richer and more air added , even with best idle timing .
I run a camshaft with 107 LSA and a single plane intake witch took a .037” IFR to keep it happy with 32 degrees idle timing.Statistics: Posted by rgalajda — Wed Sep 13, 2023 10:14 am
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