Re: Difference in manifold pressure versus boost
[Re: Cab_Burge]
#1048172
08/06/11 03:57 PM
08/06/11 03:57 PM
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Joined: Jan 2003
Posts: 28,067
Irving, TX
feets
Senior Management
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Senior Management
Joined: Jan 2003
Posts: 28,067
Irving, TX
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Don't think of boost making power. You'll get horribly confused and want to smash your head against the wall. In a way, high boost is a bad thing. You see, boost is a resistance to air flow. Blowers, turbos, and all that are cramming more air into the engine than it can handle. When that happens, the air stacks up in the intake and pressure goes up. When pressure goes up, heat goes up. Heat is a bad thing. A really efficient engine will take more air than a less efficient engine. Example: Engine A is a 360 with stock heads, fctory cam, and stock manifolds. Engine B is a 360 with a killer set of heads, blower specific cam, and excellent exhaust. Both engines have the same blower with the same pulley ratio. Engine A will make 450 hp at 10 psi. Engine B will make 500 hp at 6 psi. Engine A can't handle the air flow. It barfs up all the air. The air stacks up in the intake as boost because the blower is shoving it in but the engine can't use it. Think of it as being constipated. Air temperature goes up. Efficiency goes down. Engine B has the hardware to actually use the air you're feeding it. That means air moves through it easily and there's less air backing up and making pressure (boost). Since engine B is doing such a good job you can change the pullies and start shoving more air into it. Change the pullies until it makes the same 10 psi as engine A and you'll have a lot more horsepower. The air will still get all clogged up in the intake and temperature will climb but the better engine is making more power. There ya go. Clear as mud. 
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Difference in manifold pressure versus boost
[Re: Cab_Burge]
#1048173
08/06/11 04:00 PM
08/06/11 04:00 PM
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Joined: Jan 2003
Posts: 28,067
Irving, TX
feets
Senior Management
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Senior Management
Joined: Jan 2003
Posts: 28,067
Irving, TX
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Crap! I totally misread your question. Since I typed up all that garbage I'll leave it for the lurkers to read.
Boost is read as PSI. Pounds per square inch.
Inches of pressure usually refers to inches of mercury which is a much older way of measuring things.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Difference in manifold pressure versus boost
[Re: feets]
#1048175
08/06/11 10:10 PM
08/06/11 10:10 PM
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Joined: Aug 2003
Posts: 43,116
Bend,OR USA
Cab_Burge
OP
I Win
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OP
I Win
Joined: Aug 2003
Posts: 43,116
Bend,OR USA
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Quote:
Crap! I totally misread your question. Since I typed up all that garbage I'll leave it for the lurkers to read.
Boost is read as PSI. Pounds per square inch.
Inches of pressure usually refers to inches of mercury which is a much older way of measuring things.
My N/A airplane had a manifold pressure gauge in it, it is the same as a barometer with the engine shut off, it will read the actual outside barometeric pressure in inches, IE at mean sea level in instrument standard conditions(29.92 at 59 F at MSL))it will read 29.92, at 5000 ft with the same barometric pressure it would read five inches less due to the loss of pressure at the higher elevation, one inch per 1000 Ft in standard conditions  My question is is there any colilation(SP?) between manifold pressure and pounds of boost, IE is 6 lbs of boost equal to or close to 36 inches of manifold pressure? N/A airplane motors with a manifold pressure gauge will read at or close to zero at idle, the more you open the throttle the more manifold prssure you will see, at wide open throttle on take off it will read the outside barometric pressure  The supercharged or turbocharged airpalne motors will have above the outside B.P. at take off power IE above 31.00 inches all the way up to or above 44 inches of manifold pressure depending on the throttle position and RPMs Canton, 
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Difference in manifold pressure versus boost
[Re: Cab_Burge]
#1048180
08/07/11 09:20 AM
08/07/11 09:20 AM
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Joined: Jan 2003
Posts: 8,163
Plymouth, MI
Blusmbl
master
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master
Joined: Jan 2003
Posts: 8,163
Plymouth, MI
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Quote:
Thanks, that helps  Another member has PM me a link to a chart but I couldn't get it to display earlier  I hope it will soon so I can print it for future use 
You're welcome! You already had it mostly right (knowing the difference between absolute and gauge pressures, thanks to your airplane experience), just needed the conversion formula.
This program is very good, if you're interested and want to install it on your computer:
http://joshmadison.com/convert-for-windows/
BTW, my first post wasn't that accurate. 6 pounds of boost at sea level is 20.7 psi. But, 6 pounds of boost in Denver is only 18.1 psi, since atmospheric pressure is lower.
'18 Ford Raptor, random motorcycles, 1968 Plymouth Fury III - 11.37 @ 118
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