Definitely, but I only have a head on the bench and not a block and cam.
Dyno Testing Bolt-ons On The 4.0
- Thread starter Jezza
- Start date
Definitely, but I only have a head on the bench and not a block and cam.
An easier way to test would be just to remove the OEM cooling fan and just use a box fan on the radiator for the dyno test. That would approximate driving with the SPAL fan off.
Ideally we’d want four tests:
1. Fan clutch disengaged (control test)
2. Fan clutch engaged (alternate control test)
3. No fan/clutch
4. No fan/clutch, 850W extra alternator load
I had planned on doing the first three tests already. I have a bracket that locks up the clutch. That's probably going to be a bit scary. I'm not sure I have a good way to apply 60+ amps to the system. Ideas?
1st choice:
Or hook up the electric fan and run it on high.
Or use a heating element to pull the load.
Or just estimate the load. 850W @ 60% alternator efficiency = 1.9 hp. Less than most dynos are capable of reliably measuring.
It's always cool to test and verify, but given the order of magnitude we're looking at I wouldn't work too hard on it.
The test with the loaded alternator probably is the least critical anyways. We can calculate it's estimated torque/power impact mathematically. The only real variable is the alternator marginal efficiency, which I've been guessing is around 50%.
850 watts = 1.14 HP, or about 2.28 HP at the crankshaft, assuming 50% alternator efficiency. Should correlate to engine drag of about 18 ft-lbs at idle, 6 ft-lbs at 2,000 RPM, or 2 ft-lbs at 5,200 RPM.
Even then, most of the time we are running it at less than 150 watts.
As you know, the fan doesn’t pull anywhere near 60A at the controller settings we use unless the override switch is on (which is more of an emergency feature).
Wouldn’t it be easier for Jezza to simulate the load at a lesser rating that also reflects amperage under normal function?
I’ll get some readings today since it’s 85* out and feels hotter in the sun. I’ll do so at the settings you suggested (160/200) and with the AC on since that will put more load on the fan (since the fan speeds up when the AC system raises the coolant temps).
Edit: just saw your most recent post after submitting this post.
The load was 9.3A with the AC on and the fan was turning a good amount. So my guess is simulating around a 10A load would be pretty accurate for testing.
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That would be an interesting test. Visually, I'd expect the smoother intake to be better, but who knows.
Realistically shouldn't matter all that much. The big effect will just be from removing the fan and clutch on the water pump.
Even just turning your headlights or fog lights on if you have halogens will approximate the draw of a fan on medium power.
Only thing is you will still need airflow through the radiator unless you're testing before the thermostat opens. A box fan strapped to the front is probably sufficient
Man, you must have some powerful headlights - my SPAL draws something like 40 amps at full blast…
Man, you must have some powerful headlights - my SPAL draws something like 40 amps at full blast…
he did say medium power...fan power increases with the cube of speed, and airflow increases linearly with speed. So your 40A fan would only pull 5A at 50% airflow.
Interesting. Good to know.
Man, you must have some powerful headlights - my SPAL draws something like 40 amps at full blast…
@freedom_in_4low got it correct.
The fan affinity laws state that if you double shaft RPM, you get double the flow, 4X the static pressure, and 8X the power consumption (and also 8X the air watts as well).
In my other thread, I actually did a test that shows this:
For this, simply compare the fan RPM to the amperage draw. Note that 1,380 RPM is about half the 2,727 RPM peak in this test, but the corresponding amperage for full speed is 7.7X (approximately 8X) the half speed amperage.
Note these tests are done with the engine off, so actual engine-on running amperages at these speeds are about 86% of the amperages seen here. The fan automatically compensates for voltage to keep constant power. It also can’t quite reach top speed (2,850 RPM) with the engine off.
If we’re looking to approximate 50% fan speed here, we need a load of about 107 watts, or very close to a pair of 55-watt halogen headlights.
The more I think about it though, this electrical load is still so small it probably will be lost in the noise for dyno testing. We’re talking 1/7 horsepower at the alternator and maybe 2/7 horsepower at the engine.
Edit: Another way we can approximate this power draw instead of headlights is the HVAC fan at high speed, which draws around 7.5 amps with the engine on, or around 108 watts.
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When you need more airflow, it is way easier to increase fan diameter rather than spin a fan faster.
At a given shaft speed, air flow is proportional to the cube of fan diameter (but only linear with shaft speed). Meanwhile, static pressure increases with the square of fan diameter, meaning that a doubling of fan diameter results in 8X the flow at 32X the power given a constant shaft speed.
If we want to compare efficiency, if we assume power is constant, then airflow is proportional to the diameter to the 4/3 power. Meaning if we double fan diameter, we also get 2.52X the airflow at the same power at 31.5% shaft speed, largely because static pressure is only 39.6% of what it was.
The fan affinity laws state that if you double shaft RPM, you get double the flow, 4X the static pressure, and 8X the power consumption (and also 8X the air watts as well).
In my other thread, I actually did a test that shows this:
For this, simply compare the fan RPM to the amperage draw. Note that 1,380 RPM is about half the 2,727 RPM peak in this test, but the corresponding amperage for full speed is 7.7X (approximately 8X) the half speed amperage.
That is completely new info for me, so I'm trying to wrap my mind around it. I think I just assumed a linear relationship, i.e. double the speed required double the power. As freedom stated, "fan power increases with the cube of speed", is that assuming a stationary fan only? How does that play with push vs pull fan? Would movement forward or reverse, like in a vehicle, change that power requirement accordingly? Thinking out loud here, as a vehicle moves forward air pressure increases in front of the radiator/fan, so the fan should require less power to move a given volume of air at speed X. I'm spiraling at 1/4 the power of my mind here.
That is completely new info for me, so I'm trying to wrap my mind around it. I think I just assumed a linear relationship, i.e. double the speed required double the power. As freedom stated, "fan power increases with the cube of speed", is that assuming a stationary fan only? How does that play with push vs pull fan? Would movement forward or reverse, like in a vehicle, change that power requirement accordingly? Thinking out loud here, as a vehicle moves forward air pressure increases in front of the radiator/fan, so the fan should require less power to move a given volume of air at speed X. I'm spiraling at 1/4 the power of my mind here.
To avoid clogging up this thread more than I already have, I’ll leave the Wikipedia link:
https://en.m.wikipedia.org/wiki/Affinity_laws#Fan_affinity_laws
Applies for many different kinds of fans, including push or pull and axial or centrifugal.
This generally assumes no ram air effect. It is possible to calculate this with the use of partial differential equations, which is well beyond the calculus I actually remember. In general, ram air will reduce fan power consumption, but ram air in itself actually has a similar energy penalty as per aerodynamics.
Doubling road speed quadruples static pressure against the front of the car, meaning that fuel consumption to go a particular speed (relative to distance traveled) increases 4X, and also meaning that the power required to overcome that wind resistance is 8X.
This is why you might get 16 mpg rolling 70 mph, but only get 8 mpg at 100 mph.
I knew vehicle speed/power requirements increased in that manner, I just never thought fan speed acted similarly. Very interesting stuff, even if it is over my head.
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