I spent many hours this week building a mathematical model of what happens in the cylinder when the exhaust valve opens. This is an effort to figure out how supercharger, NA, and turbo cams should look with respect to each other.

I used Excel and a sort-of finite element method. Basically, I had already built another [extemely complicated] spreadsheet that modelled piston motion and cylinder pressure as a function of stroke, rod length, and peak firing pressure. This first spreadsheet gave me some rough numbers to plug in for cylinder pressure @ Exhaust Valve Open (EVO). These numbers were in the 60 - 100 psi range. Anyhow, if you have exhaust port flow at 28" H2O, you can figure out exhaust port flow at 100 psi (Actual Flow = Actual dP * (100/1.1)^.5). You might be surprised to know that your exhaust port in actual use is flowing 400+ cfm at .200" lift. Apply a few thermo formulas and viola!

So, if you know the valve opening rates (use duration at .006, .050, and .200, then interpolate anything in the middle), exhaust port flow, and beginning pressure, you can begin to model the "blow-down" period. You might also be surprised to know that it takes 110 - 130 degrees to relieve the cylinder pressure.

So what do you do with this information? Well, the goal is to make the most power possible. When you open the exhaust valve during the power stroke, you are relieving pressure that could be used to do work. Conversely, when the piston is rising on the exhaust stroke, you want cylinder pressure to be as low as possible. So, if you minimize the power lost on the power stroke, and minimize the pressure during the exhaust stroke, then you make the most power at the crank. You basically have 2 triangles you're trying to minimize the area under. Again, I'm not looking for absolute numbers since decades of testing NA engines yields good baselines to start from. I'm just trying to figure out the relationship between the 3 types of induction and how the cam should be adjusted.

RESULTS:

I was surprised to find that a supercharged motor requires the exact same EVO as NA. There is more mass in the cylinder to relieve, but (a) there is more pressure, or motive force, helping it out of the cylinder and (b) there is more time to complete the task since SC motors will peak at a lower rpm than high hp NA. Then again, a few SC guys have made obscene power with typically NA cams.

The very interesting part was the turbo cam. I used 100 psi cylinder pressure at EVO and varied the backpressure from 10 psi to 60 psi. What I found was that there is a break point at about 30 psi exhaust backpressure. With anything less than 30 psi, the EVO had to be sooner. With anything more than 30 psi, EVO had to be later. This is verified by the turbo Buick crowd, who have been saying that if you have alot of backpressure, you use reverse split and vice versa.

Of course, in a model such as this, you have to make alot of assumptions. I tried to model typical high hp engines. The NA motor makes 600 hp at 7000 rpm, the SC motor makes 700 hp at 6000 rpm, and the Turbo motor makes 900 hp at 6000 rpm.

Mike

 

So how do you go about measuring backpressure?

 

I've done it in the past using an O2 sensor bung and getting the proper adapters to measure it with a boost gauge. I'm going to install a 1/8" pipe bung in my log header to get it now, but I think my 20 psi boost gauge won't go high enough so I have to find another gauge.