a technical discussion, engine efficiency Options

[r_towle]

so, I have been thinking (not always a good thing)
If an engine is basically an air pump, and the objective is to make it as close to 100 percent efficient, why is the exhaust valve so much smaller than the intake valve?

Forget emmisions, forget fuel delivery, just want to understand why this is so...it is so on most engines...

Rich

[lapuwali]

The cam in the stock engine was chosen more for emissions reasons than power reasons, so it's unnaturally restricted.

There are two problems with using D-Jet with a wild cam. One is that wild cams set up substantial intake pulses in the intake tract, which cause the manifold pressure to fluctuate wildly at small throttle settings. You'd have a very lumpy idle and very poor drivability at or near cruise. You can fix this to some extent by using a mechanical damper (some people use a fuel filter, others use a small restrictor in the vacuum line), at the cost of response. The other problem is the MPS isn't actually measuring airflow. It's estimating airflow based on manifold pressure, air temp, and engine speed.

Any engine (or air pump) does not pump with 100% efficiency. When the intake valves close, the charge pressure will be some percentage of one atmosphere (the volumetric efficiency, or VE). The actual amount will vary based on how far the throttle is open, the air pressure outside, the engine speed, the cam timing, the valve sizes, and the efficiency of the ports. The VE curve will be pretty much fixed for a given engine, so if you know some of the outside parameters and the VE curve, you can estimate airflow, and thus how much fuel you'll need.

Putting in a wilder cam, or cleaning up the ports, or installing bigger valves, or making the engine bigger, will change the VE curve. So, while you still know manifold pressure and engine speed, the VE curve in the D-Jet ECU is no longer correct, so the right amount of fuel isn't delivered. If you can change the VE curve programmed into the ECU, you can adjust this for the new engine.

Note that L-Jet doesn't have this problem, because it measures airflow directly. Thus, it's possible to install a hotter cam in an L-Jet engine and it will work. There's still a potential limit here, as the intake pulsations can confuse the L-Jet airflow meter, so very wild cams can't be used with L-Jet. You can install L-Jet on a bigger engine and it should work just fine. The limit here is the size and airflow capacity of the airflow meter itself.