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Old 10-21-2020, 11:10 PM
oethel's Avatar
oethel oethel is offline
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Location: west ky
Posts: 100
Default Hat boost versus intake boost

I know there is boost drop across a innercooler and possibly the bends in boost side piping and I suspect there is some drop between the hat and below the carb.Is the drop below the carb always the same or does it go up or down proportionately depending on boost?

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Old 10-22-2020, 05:12 PM
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Tom Vaught Tom Vaught is offline
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Normal Naturally aspirated carbs were tested at certified (from Holley) at 1.5 inches of mercury or 20.4 Inches of water. If you bump the test pressure up to 28 inches of water you get a higher reading for cfm but you are just playing a game with the carb test pressure. I bring this up because your question refers to carb delta P pressure (above and below the carb.

Normally when you go to boost you increase the density of the air (more molecules of oxygen) therefore you can add more fuel assuming the same assumed airflow and get more power.

So the assumption is the difference across the carb is 1.5" of mercury naturally aspirated, or boosted.

So now I will give you an example based on a Steve Morris dyno test on a 505 chevy engine.
This engine was inter-cooled and had a good intake and 850 cfm carb.

The 505 engine made 1700 hp thru a 850 cfm #4781 type carburetor.
This was at a recorded Boost level which I do not remember. Somewhere in the low 20s on boost pressure if I remember correctly. This was a 505 chevy engine in an American Motors vehicle.

So at some point the Intake boost pressure vs the Hat boost pressure was increasing.
The engine made a bit more horsepower but the delta P across the intercooler and hat boost pressure had increased to a difference of 3 psi vs the intake manifold. This was now saying that the carb could not pass the required CFM/mass flow that the turbos were trying to move thru the system.

Steve Morris later installed a larger higher flow capacity intercooler and a larger Dominator carb and in that testing. One of CSUs blow thru carbs as well as several other boosted carb experts carbs were making over 1900 hp and in some cases over 2000 HP.
Different engine, different carb, bit just comparing the delta P across the intercooler/bonnet to the intake manifold.

So the answer is YES to your question. It depends on the engine size, the engine rpm,
and the carburetor/intake mounted on that engine. as well as the induction plumbing (intercooler, pipes, carb, intake).

Tom V.

This info is based on actual testing on Steve Morris dyno, not assumptions or my buddy said this.

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