Heads with S\C?
Safety Car
Joined: Dec 2004
Posts: 4,477
Likes: 2
From: Boca Raton, FL
St. Jude Donor '05-'06-'07-'08-'09-'10-'11-'12-'13-'14
Safety Car
Joined: Jul 2006
Posts: 4,647
Likes: 1
From: Objects in your mirror are losing , Long Island, NY
St. Jude Donor '08
Ok, let me tackle this from a different way. If you change heads only to better flowing ones, you will lose boost pressure with no other changes (including pulley diameter) because you have less resistance to flow (same output/flow from the SC going through larger ports=less resistance). You'll experience a gain in flow (in CFM), but even if flow stayed the same, you gain in two ways. First, with the supercharger putting out less pressure, it takes less HP from the crank to drive it. The HP to drive the SC is given by the equation HP= PSI*CFM*C where PSI is the boost pressure, CFM is the flow, and "C" is a constant. Second, with lower boost pressure, the air is heated less (lower temperature) giving a denser charge/more HP. You can always put a smaller pulley on to increase the CFM output of the SC and HP of the engine to take advantage of the extra flow of the heads...then you'll see boost pressure increase (more flow through a given port=more resistance), but again, it's just a measure of resistance to flow. Boost pressure alone is not an indication of flow to the cylinder or how much HP an engine will make, but give me the flow alone to a cylinder, and I can make a reasonable calculation of how much HP that engine is going to make.
This is a perfect analysis of of boost versus flow. I used to own a 93 Mustang drag cobra with a 302 built short block with ported canfield heads cam and the works. I had the procharger P600b blower with the smallest pulley showing 15 psi of boost under full throttle with this motor. I changed the motor to a 331 stroker built short block, with the same combo (heads, blower, slightly different cam) and generated 100 hp increase to the rear wheels. However, the psi dropped to 11 with lower water and oil tempratures. This proves that higher boost levels doesn't always indicate efficiency, it may well indicate inefficiency of an engine combo and lack of air flow given the parameters of an existing engine.
Last edited by welcome2try; Jun 13, 2007 at 09:02 PM.
Melting Slicks
Joined: Nov 2006
Posts: 2,463
Likes: 2
From: Santa Barbara, CA
Ok, let me tackle this from a different way. If you change heads only to better flowing ones, you will lose boost pressure with no other changes (including pulley diameter) because you have less resistance to flow (same output/flow from the SC going through larger ports=less resistance). You'll experience a gain in flow (in CFM), but even if flow stayed the same, you gain in two ways. First, with the supercharger putting out less pressure, it takes less HP from the crank to drive it. The HP to drive the SC is given by the equation HP= PSI*CFM*C where PSI is the boost pressure, CFM is the flow, and "C" is a constant. Second, with lower boost pressure, the air is heated less (lower temperature) giving a denser charge/more HP. You can always put a smaller pulley on to increase the CFM output of the SC and HP of the engine to take advantage of the extra flow of the heads...then you'll see boost pressure increase (more flow through a given port=more resistance), but again, it's just a measure of resistance to flow. Boost pressure alone is not an indication of flow to the cylinder or how much HP an engine will make, but give me the flow alone to a cylinder, and I can make a reasonable calculation of how much HP that engine is going to make.
Do these exact same principles applies when you have restrictive exhaust? Say stock headers on back? Do you need a retune if you do only the heads?
