April 16, 1999. This is the first time I have put my bus on the dyno. For those not familiar with the dyno, it is technically a chassis dynamometer, which is a device that measures the horsepower and torque output of your vehicle's engine at the rear wheels. It is extremely useful for tuning your engine to get the most performance possible from it. Below this graph I will explain what all of the lines and numbers mean.
The curves nearest the top are the engine torque. They correspond to the numbers on the right verticle axis of the graph. Torque is the amount of twisting force that your engine produces per revolution. This is a simplification, but it should be good enough to get the point across. The three curves represent three different runs. The first one (the light blue line, dynorun.003) was made just as the bus drove in. The second one (red line, dynorun.004) was made with the air filter removed. The third one (black line, dynorun.005) was made with the carburator covers removed. Peak torque was on the second run and was 85.3 lb/ft.
HORSEPOWERThe curves nearer the bottom of the graph are horsepower. They correspond to the numbers on the left vertical axis of the graph. Horsepower can be thought of as torque multiplied by RPM. Again, that is a simplification but it makes it easier to understand. The dark blue line is the first run (dynorun.003), the red line is the second fun (dynorun.004) and the green line is the third run (dynorun.005). Peak horsepower was on the second run and was 56.4hp.
DISCUSSIONFirst of all, the numbers seem a little low because they are REAR WHEEL numbers. Car manufacturers and shop manuals state their horsepower and torque figures at the flywheel, meaning that things like driveline losses and losses from power accessories are not factored in. Typically people assume 10-15% drivetrain loss, which turns my 56hp into about 65 flywheel hp. That's a little closer to what I would expect a 2-liter with stock carbs to put out.
So you're wondering what happened to dynorun.001 and 002? 001 was made with timing set to 0 degrees. 002 was made with timing set to 10BTDC. Both of these settings produces less horsepower and torque than dynorun.003 where the timing was set to 7 degrees BTDC. I left the timing there for the remaining runs.
So you're wondering how much horsepwer your air filter costs you? The answer is Nothing under 3200rpm and up to 2hp at 3700 and above. Well, that's the answer for ME, might be different for you but now at least you have an idea.
So you're wondering why the third line (dynorun.005) is so much lower and jagged looking? At first, I thought this was due to a lean condition created by removing the covers from the carbs. I put in richer jets and went back to the dyno to try removing the covers again and the result was the same, which tells me it's not a lean condition but an airflow-pattern problem. As a result I will be leaving the covers on the carbs for the forseeable future. WHAT DOES IT ALL MEAN?
I'm still digesting it. I am pleased that torque is so flat between 1800 and 3100 rpm, and so substantial at lower revs. I am not too pleased that it falls off so quickly after 3100rpm. My normal freeway cruise RPM is somewhere around 4000 rpm which is well past my torque peak. Cruising at torque peak RPM should produce the best mileage, however it's not realistic to think that my bus could cruise at 3100rpm - horsepower is too low and so is the speed at that rpm. I need to increase torque and gear the bus way up, or simply move the torque peak up nearer to 4000 rpm. I am probably going to take the latter approach first.
WHERE AM I GOING FROM HERE?I am installing new (MSHP rebuilt) heads which will increase compression and decrease intake valve size. MSHP is going to unshroud the combustion chambers and set them to 47cc's for me which should bump my compression up considerably and increase torque across the rpm range. I am going to find a '74-'75 tranny which will increase my final drive ratio from 5.375 to 4.86. When all of this is installed I will bring the bus back to the dyno and get some new numbers. It should be interesting!
Chad Frost built a very interesting Aerodynamics chart plotting horsepower against speed, with my dyno results thrown in. The point where the lines cross indicates that my bus should top out at just under 70mph. In real life I can go a bit faster (75-ish) but this chart is still very interesting and useful. Check it out!
UPDATE: After installing the MSHP heads, returning to the stock venturis with rich (137.5) jets, peak horsepower is up to 59.9 and torque is up to 90 lb/ft. A 7% gain, which was just barely enough to be felt. I still need more - much more. Next step is to replace the tall '73 manifolds with the shorter '72 ones, which should improve the top end, and returning to a 132.5 main jet. If that doesn't help enough, I am considering aftermarket carbs and a different camshaft. The cam will have to wait for the next engine, since replacing it would require a complete engine teardown. I'm considering the Web Cam 107i or 91 grind (http://www.webcaminc.com). The 107i looks very similar to the stock Porsche 914 grind.