When it comes to FI (forced induction), it still mostly boils down to the pressure differential at the throat or more importantly the back of the valve. When you generate boost and thus create a relatively high pressure region, you have a greater pressure differential between the back of the valve and the inside of the combustion chamber. There are basically three ways that FI increases power.
One, during valve overlap the scavenging process still works the same as N/A, but with greater force. Let’s assume we are at TDC with both valves closed and the spark plug just fired a brief moment ago. Now as the piston approaches BDC the exhaust valve begins to open to relieve the high pressure within the cylinder as a result of combustion and the end gas that is still continuing to burn. Gasoline does not explode like some think, it burns slowly. Compression just exacerbates this controlled burn to such a magnitude that it is mistakenly perceived as an explosion. Anyway, as the piston starts to come back up on the exhaust stroke, the intake valve is opened about 50°-75° BTDC on a street/strip motor depending on the cam profile. As soon as the intake valve is lifted the exhaust valve is still partially open and the hot expanding and still burning exhaust gases are rushing out of the exhaust valve very quickly. This provides a relative low pressure zone (pressure differential) at the top of the cylinder and actually helps to draw more of an a/f mixture into the cylinder by way of the intake valve. This is valve overlap. The more powerful the combustion process, the greater the ability of the scavenging process of overlap. Kinda like a vicious cycle but good. When you “stuff” more air into the cylinder with boost, you obviously have a greater combustion process and therefore a greater ability for scavenging during overlap.
Two, when you generate boost and apply greater pressure to the back of the valve, you are compressing the air and the suspended fuel that is waiting behind the intake valve. This creates a relatively high pressure region behind the intake valve. Continuing on from above, once the intake valve opens, this increased pressure differential allows the compressed air/fuel to flow into the cylinder with much more force and therefore “stuffs” the cylinder with more air/fuel.
Three, like mentioned above boost compresses the air and suspended fuel behind the intake valve. Whenever you compress a gas and a vapor
suspension within a given and fixed volume (port volume) you increase the available mass of oxygen and fuel within the port (venturi). So when the intake valve opens, not only is it being rushed in with a greater force, but it is also a much denser mixture and therefore gets more oxygen and fuel into the cylinder.
To optimize horsepower in an internal combustion motor, you want to have your overall ignition timing set to such that the resulting "normal" combustion process makes peak cylinder pressure at around 14°-15° ATDC, 16°-17° for an all out drag race motor. Obviously, you don't want peak cylinder pressure to occur before TDC. That excess heat and pressure would apply abnormally excess pressure on the piston during the compression stroke and place undue stress on the wrist pin, rod, top ring land, and can severely scuff the skirt. All not good.
When cylinder heads are prepped specifically for FI applications, there are some differences from that of a N/A head. For instance more attention is placed on making sure that you do not have excessive tumble or vortices. On FI heads you have to also work them so that you all but eliminate boundary layer shear. You hear lots of people toss around the term “Port and Polish”. You never want to polish the intake runner, but back in the day on carbureted motors, engine builders would polish the exhaust port on the head. This was simply done to help eliminate carbon buildup inside the exhaust port. On EFI motors, polishing the exhaust port is a complete waste of time and money and is actually rarely done if at all. It has turned into more of a marketing gimmick much like blueprinting. The reason you don’t want to polish the intake runner is because you would disturb the boundary layer and create shear. This would disturb the laminar flow and would result in increased Reynold’s numbers/ratios. The little ridges that are left inside the intake runner after CNC porting are purposely machined. It keeps the fuel in
suspension and promotes flow/velocity within the port.
That’s about the best I can explain boost and timing without going off the deep end.
Stan pretty much summarized flow testing. Normally it is done with an acrylic cylinder fixture at a depression of 28.0” of H20. The length and bore of the bore fixture will affect the observed flow. Just the same, the dimensions of the exhaust pipe used will affect the exhaust flow. There are lots of different types of flow benches out there, but the SuperFlow products are widely recognized to be very accurate and professional. For example you have the SF-600 which can test up to 600 CFM or 240 HP, more than enough for most street/strip motors. Then you have the SF-1020 which can test up to 1200 CFM or 240 HP for the more radical race engines.
Sorry for the book.
. The whole corner of the shop was green, including me. We had no idea what we were doing at the time, so I had just talked to a few friends of mine in Las Vegas that had done this before with their own setup and they gave me a few pointers. At the time, we did not know about the white Dykem that was available to contrast the green or red dyes used to look at under the black light, so we were allowing way too much dye to be pulled through and well, we made a big mess.
