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Old 02-06-2019, 08:05 AM   #1
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Boost explained by a dummy...

My experience using boost has been on a personal basis and I'd like to discuss what I have found. And to be clear on my biases I fall into the supercharger camp, with my main complaint with turbos being spooling time, heat and exhuast restriction. We all know atomspheric pressure at sea level is around 14.7psi. I'll call it 15psi keeping it simple. I see vacuum as a absence of atomospheric pressure measured by how hard the atomosphere is trying to fill the void. On a Throttle body the throttle blade acts as a quick acting valve allowing metered air into the cylinders with appropriate amounts of fuel to allow the engine to run. Again vacuum being created by the piston pumping motion creating a void in which the atomosphere is attempting to fill. Measured as a vacuum. (Psi). Boost is air pressure greater than atmospheric 15 psi. With a SC on a cog belt or directly driven turns a given amount in relationship with engine rpms. So lets call idle 600rpm and max at 6000rpm and draw a linear line from idle to max rpm . At idle we see solid atomospheric pressure of 15psi. As the rpm increases so does boost to lets say 15psi of boost at 6000rpm. Keeping in mind boost is the air pressure above what atomosphere is providing. Meaning the engine at 6000rpm has an additional atomosphere of pressure trying to fill it's cylinders. With the boost gauge reading 15psi. This is a large amount of air flow as the engine alone with just one atomsphere of pressure moves something between 650-1000cfm of air. With 15psi of boost (2x atmospheres) it could possibly be flowing twice that amount. Theoretically making hp of a engine twice it's actual physical size. I found on my engine from a idle going wot that just over 2500rpm my sc is being spun fast enough to overcome the engine airflow requirements and starts building boost in direct relationship with rpm to where there is 15psi at 6000rpm. All of this is of course with a closed charge tube connected to throttle body. Now here is where the BOV on a sc is needed. Obviously there is a lot of air being moved at high velicities and when the throttle blade is slammed shut all this air has momentum and kicks back on the SC trying escape somewhere. The intake manifold senses a hugh vacuum opening the appropiate size BOV. I use two 50mm bov's both for volume and two for redundancy. At idle with a bov removed, I can put my hand over the hole and a slight pressure is felt. Blocking the hole and unblocking the hole doesn't even change idle speed. The engine easily compensates and only senses that atomsphere is slightly higher making cylinder fill more efficient even at low rpm in preboost conditions. Boost is a great power adder and there are no reasons to not use all of its potenial.
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Old 02-06-2019, 10:50 AM   #2
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Yep pretty much sums it up. I am shooting for the 15-18 psi of boost 2.2 atmospheres here very soon.
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Old 02-06-2019, 11:45 AM   #3
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Boost is awesome. It can make a small block have power like a big block engine.
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Old 02-06-2019, 01:05 PM   #4
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And with a reasonbly linear boost curve, transition is consistantly very manageable. At cruise ([email protected]) I suspect my charge tube is a few psi above normal atomsphere enhacing fill efficiency. Manners are such that I can leave a light in 2nd gear spinning tires without boost or going wot. I don't believe turbo's have that kind of control.
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Old 02-06-2019, 05:09 PM   #5
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To each is his own...
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Old 02-07-2019, 03:57 AM   #6
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Quote:
Originally Posted by Ghost03hemi View Post
Boost is awesome. It can make a small block have power like a big block engine.

And make a big block run like a Big block with boost.
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Old 02-07-2019, 05:55 AM   #7
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In theory if you run two atomspheres a 400ci engine becomes a 800ci engine for comparison. It's all about the amount of fuel you can burn.. If the engine could simply be enveloped in two atomospheres without struggling with the boost curve life would be easy with very manageable power. IMO, direct drive or cog belt comes as close as you can get having a linear boost curve. A side note: cog drive gets a lot of bad press being said they are difficult to maintain. I am still using the original belt I installed when I first built my cog drive. I bought two at the time thinking I might eat one or both. The spare is still in its box. These are cheap belts I bought just for testing purposes. But apparently plenty strong. The problem I think some people have with cog drives is they install them too tight which is unnecessary.
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Old 02-07-2019, 05:59 AM   #8
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yes cog does not get tight just remove the slack maybe a touch of tension and you golden. timing belts are the same way.
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