SN95 Short Block Suggestions?

FastDriver said:

It would only make that with boost, but the answer is absolutely. In fact, I think an explorer motor with cheap blower or turbo is probably just about the most cost effective way of maxing out the stock block.



Like me, you're an analytical guy. We like to think about textbook physics and to understand why things work the way they do. Many of the guys that build and race cars, even in racing applications, build based on experience. In the practical execution of just about anything in the real world, experience trumps theory. However, the inferences they draw from success can be misleading. The subject you just brought up is an example. They are right in a way, but their explanation is not satisfactory to an analytical mind. There are 3 reasons why on a given motor, a given boost may not produce the same amount of power: efficiency, parasitic loss, other changes on the entire system in blown vs. turbo applications.

Efficiency: Boost vs. the amount of oxygen in an intake charge
Regarding boost and not considering the source, 10 psi IS 10 psi, period. It's just an amount of pressure, which is essentially the force that molecules of air are exerting on each other. The force comes from molecules of air bouncing off of each other and against the outside of their "container." It's just the kinetic energy of all of the molecules in the container. You can increase the total kinetic energy in the container by either increasing the number of molecules or by increasing the temperature of them. The temperature, after all, is just the average intermolecular kinetic energy. In this given container, the intake tract or a cylinder if you prefer, we know the volume and we know the pressure, and so the final peice of the puzzle we need to determine the amount of air molecules and the corresponding amount of O2, is the temperature.

With turbos, you can get a pretty good idea of this temperature or the amount of air by looking at the compressor map. The compressor map tells you how much kinetic energy put into the wheel will be converted into actually compressing air instead of creating other unwanted forms of energy heat/noise/etc... To read one of these things, you have to know a little about your motor. You have to know at what RPM your motor will operate, and you also have to know the volumetric efficiency of the motor at that RPM. Based on those variables (RPM, VE, & displacement), you can calculate the volume of air a motor will move. Volume alone won't tell you the amount of oxygen, though. To get that, we need to know the pressure and temperature, too. Since we can directly control the pressure with a wastegate or pullies, the final variable is temperature. So, using this volume and boost, we refer to the compressor map to determine the efficiency. The lower the efficiency the higher the amount of unwanted heat produced. This heat finds its way into the air and into whatever is cooling the turbo. So, it's intuitive that the lower the efficiency of the compressor, the higher the temperature, the lower the amount of air/O2, and thus the lower the gross power produced. I'll define gross power in this case to be before any parasitic loss.

Parasitic loss
This is a little more complicated than people think. It's more than just the difference between robbing mechanical energy from the drivetrain instead of "wasted energy from the exhaust." It's also about the amount of energy needed to turn these wheels, screws, roots fans. We talked about how a wheel with lower efficiency results in higher temperatures at a given boost, but there is also something else going on here. Where is the energy for this higher temperature coming from? The lower the efficiency, the more energy needed to produce the same amount of boost + a higher temperature, which is kind of a double whammy because the boosted air with a higher temp from the less efficient wheel has less oxygen density. It's a triple whammy when you consider that the higher temperatures require less timing advance to avoid detonation. It's a quadruple whammy when you also consider that the energy from the blower isn't "wasted exhaust energy." Ok, enough with the whammies.

A centrifugal blower should actually require less energy to turn, because there is less rotating mass, and because it's directly linked to the kinetic energy from the crankshaft, but unfortunately the energy is coming directly from the power output of the motor instead of exhaust energy. Turbines are an added source of loss in a turbocharged system. Actually efficiency maps also exist for turbines, too. However, we don't look at them very often because too many of us consider it "wasted energy" anyway. It's a mistake though. The result of increased parasitic loss with a turbo is increased backpressure and increased exhaust temps. Backpressure definitely impacts power & valvetrain durability, and is too often overlooked.

Impact to system design: blower vs. turbo
The routing and blockage of the exhaust causes problems blower motors don't have to deal with. Additional backpressure and obstruction in the exhaust cancels out the advantage of exhaust tuning (exhaust scavenging/helmholtz wave resonance tuning). Secondary effects on the system of running additional back pressure associated with a turbo include reducing overlap of the camshaft to avoid reversion. This could also theoretically have a negative impact on the tune, though I'd be talking out of my ass if I tried to explain the tuning effects. The reduced overlap is actually nice for guys with street cars, because reducing overlap has the added benefit of increasing driveability.

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mikestang63 said:

damn...... too early for that man

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ry94stang50 said:

Now that's the type of explanation I was looking for....wow....I have put thought into the rest of the car in fact the suspensions done just have to swap out balljoints and maybe a new steering setup as this one has seen betterdays. I ordered new rotors and calipers I forget the size off the top of my head I do know there an upgrade from the stockers. Ill need new tires soon. And the fuel system Im figuring in with the motor. I've been trying to get a plan together since before I bought the car. I know what my build is going to take. After seeing woodys website I'm back on board with the 347 stroker with a decent hci setup. I was going the trick flow route but I think I'm going to piece something together as I want a better more aggressive cam nothing crazy

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