Daily Archives: January 27, 2019

This has been bothering me

And keeping me awake at night.

Remember that dyno test video I linked to last week? The one that had 490 Ft-lbs of torque but only 405 HP? I keep going back to the graph that could be seen near the end and wondering why the power graph just went flat above 5000 RPM. A flat power graph usually indicates maximum airflow has been reached and no matter how high you twist the engine T*RPM/5252 just isn’t going to get any better because torque is falling as fast or faster than RPM is rising. You just can’t burn any more fuel because you can’t get any more air in the engine. And what has been keeping me awake has been where is the restriction? The possible sources are the manifold, the heads, and the camshaft profile not allowing the other two to work above a certain RPM. If it’s the cam or the heads then that means just a different choice of parts during the expensive part of the build, but if it’s the manifold I might be able to crutch around the problem with gearing.

Because that torque number is freaking ridiculous. This is a normally aspirated build that had higher torque numbers than some supercharged or turbo builds, meaning the ram effect was taking the engine to >100% Volumetric Efficiency (VE) at torque peak. And looking at the torque graph in the video the peak torque was on the second torque peak, there was another peak at much lower RPM that was almost as high. For a NA engine that would be a torque number more natural for a much larger engine, the engine was taking in way more than its displacement without the use of mechanical assistance. Now all I need to do is figure out how to keep that up well past torque peak or I’m going to be stuck with a power band that is only about 1000 RPM wide, the space between peak torque and peak power. I’m still talking about installing a crazy cam profile, probably stupid high lift, to keep the airflow up well past the torque peak, and adjusting the gearing to put the power higher in the vehicle speed range, which would be fantastic for fuel economy. I mean the gears in the transmission are set in the factory, so the only thing I can change would be the final drive ratio to a higher (lower numerical) ratio. And because I don’t have room for a clutch pedal with a SBC that means if I set the final drive to hit redline in first at 40-60 MPH the cruise RPM would be ~4.6:1 lower than that. Which would mean very low cruise RPM and pretty good fuel economy.

Now I need to get ready for game.