DC Power Engineering Blog


The “Truth” behind Denso Hairpin High Output Alternators

We have been getting an increasing number of calls about how “X” competitor is offering a “Denso Six-Phase Hairpin” while selling it for a significantly cheaper price. However, what many people fail to know the truth due to the “Smoke and Mirrors” tactic that our competitors use. The units that our competitors are advertising happen to be the small case Denso Hairpins while DC Power sell the large case Denso units. DC Power always recommends to our customers the large case units over the small case units for one goal, idle output.

The Large Case Hairpin is 138mm OD while the Small Case is only 129mm OD.

The Large Case Hairpin is 138mm OD while the Small Case is only 129mm OD.

As seen above, the small case Denso alternators are based on the 129mm OD stator, which range from 100 to 180 amps (cold rated) that are found in most vehicles today such as Toyota, Honda, Ford, General Motors, etc. Not only is the stator outer diameter smaller, the gauge wire used isn’t near as thick as the larger 138mm stator as well. The small case Hairpins are great alternators and that’s why DC Power sells them as our HP Series. However, when we are getting an increasing number of calls saying that a competitor is selling a “270-amp hairpin”, claiming the same power curve as our large case hairpin. Since the two are nothing like one another this is where we as a company have to draw the line and put an end to this nonsense.

DC Power began selling the small case hairpins back in 2006, which is exactly why we know what their strengths and weaknesses are. First, how does a small case hairpin that is rated to produce 180 amps (cold) reach as high as 320 amps?  Simple, modify the stator to increase the top end output of the alternator. However, this has the major drawback of efficiency loss, which is increased heat and loss of low RPM output. Here are two computerized power curves of the small case Denso from our D&V Electronics alternator test machine, which shows the differences when the stator is modified for higher output.

270 Amp Large Case (Stock Stator)

This test was taken at Ambient Air Temperature of 72F

This test was taken at Ambient Air Temperature of 72F

270 Amp Small Case (Modified Stator)

This test was taken at Ambient Air Temperature of 75F

This test was taken at Ambient Air Temperature of 75F

Newer vehicles are under strict regulations to increase fuel economy and reduce emissions. This is done decreasing the engine idle speed to 550-650 RPM on most of the newer vehicles while decreasing the crank pulley size. Most cars such as Toyota and Subaru run a 5.33” (135mm) crank pulley while the majority of our competitors are advertising their power at 800 engine RPM on a 5.75” (146mm) crank pulley. However, what many fail to realize is that as little as 50 engine RPM can be disastrous to alternator output and be frustrating to sit in traffic on a hot summer day with your voltage dropping to 10 or 11 volts with the air conditioner barely keeping the vehicle at a comfortable temperature.

Looking closely at the power curve you will see that anything under 2500 alternator RPM is essentially significant loss in idle output. This happens because unfortunately, you must pick idle output or top end output; you can’t have your cake and eat it too. The small case hairpin alternators were designed to have the perfect balance of a low turn on speed for the new vehicle regulations and as much as 150 amps of output when they’re at operating temperature in the vehicle at 1,800 engine RPM. To translate engine RPM to alternator RPM, take the crank pulley diameter of the vehicle, divide it by alternator pulley diameter to get the ratio of how many alternator revolutions happen per a single engine revolution. Next, we take the ratio found in the last equation and multiply it by the desired engine RPM when at full operating temperature at idle to get the alternators actual RPM to match the curve above. Now, lets look at two of the most popular vehicle configurations with detailed results: 5.75” and 5.33” crank pulleys.

5.75" Crank Pulley

By using the 5.75” crank pulley as a starting point, we can add the desired alternator pulley size into the equation to get the engine RPM to alternator RPM ratio. Based on this information, we can calculate the output of the Small Case 270 amp in correlation to our vehicle’s RPM below:

5.75" Crank Pulley / 1.8” Alternator Pulley Size = 3.195 Alternator RPM to 1 Engine RPM

196 Amps = (3000 Alternator RPM / 3.195 = 938 Engine RPM)

141 Amps = (2500 Alternator RPM / 3.195 = 782 Engine RPM)

99 Amps = (2250 Alternator RPM / 3.195 = 704 Engine RPM)

39 Amps = (2000 Alternator RPM / 3.195 = 626 Engine RPM)

No Output = (1900 Alternator RPM / 3.195 = 595 Engine RPM)

Based on the equations above, that would mean that if you wanted 190+ at “IDLE” (like many of our competitors are advertising) your vehicle would have to idle at close to 1000 engine RPM on a 5.75” crank pulley which is not realistic and far off the numbers that other companies are advertising. Here is a graph of the most common pulley sizes that various companies offer to see how big of a difference the size of the alternator pulley matters:

270HP 5.75" Crank Output

As we look at the graph above, we can see that the difference between pulley sizes can have a significant impact depending on your vehicles idle speed which is exactly why our top competitors sell 1.7" and 1.8" pulleys to compensate for the poor idle performance.

5.33" Crank Size

Vehicles like Toyota and Subaru usually have 5.33” crank pulleys. When comparing this to a 5.75” crank pulley, the 5.33” crank being smaller will naturally slow down belt rotation speed on it’s own. However, when taking into account of engine idle speeds being decreased due to new regulations, it causes all of the accessories that are driven off the crank to rotate even slower as well. When both of those factors are taken into consideration, you have a drive system that can turn an alternator at 1,600 alternator RPM nominally with a 2.25” OEM size pulley. The way many of our competitors will offset this issue is to run a 1.7” or 1.8” alternator pulley, which can decrease the alternator turn on speed as much as 200 engine RPM. Unfortunately, DC Power doesn’t send out these “overdrive” pulleys because it adds potential risk factors including by not limited to:

  1. Excessive heat on the alternators pulley from the smaller surface area.
  2. Belt slip due to lack of belt wrap or not enough surface area to turn the alternator.
  3. Reduced lifespan of the alternators brushes and bearings.
  4. Catastrophic belt failures due to increased heat.
  5. Alternator failures at extreme high RPM speeds.

Point number 5 brings an excellent argument up regarding high RPM speeds. When you idle at 650 engine RPM in your vehicle but your redline is at 8,200 RPM your alternator would be spinning at around 24,000 alternator RPM with a 1.8” pulley versus approximately 18,900 alternator RPM. This can also cause big issues with vehicles that have fast gear shifting. When shifting from 8,000 engine RPM to 2,000 engine RPM on a small 1.8” solid aluminum pulley can cause your alternator to slip due to the lack of belt wrap and pulley surface area.

Now that we know main issues that this causes, lets do some more math to break down the performance curve of the alternator using a 1.8” pulley:

5.33” Crank Pulley / 1.8” Alternator Pulley = 2.96 Alternator RPM to 1 Engine RPM

196 Amps = (3000 Alternator RPM / 2.96 = 1013 Engine RPM)

141 Amps = (2500 Alternator RPM / 2.96 = 845 Engine RPM)

99 Amps = (2250 Alternator RPM / 2.96 = 760 Engine RPM)

39 Amps = (2000 Alternator RPM / 2.96 = 676 Engine RPM)

No Output = (1900 Alternator RPM / 2.96 = 641 Engine RPM)

Based on the equations above, if your Toyota or Subaru idles at less than 650 RPM, you will be essentially seeing no output at idle. For example, in my 2002 Toyota Camry with the 2.4L 2AZ-FE motor, ALLDATA specs my vehicle with an automatic transmission to idle between 610-710 RPM. However, if I had a manual transmission, it would be between 650-750 RPM (Ouch).

Many people fail to realize that when the alternator and vehicles engine begin to heat up, resistance will increase which will have the output of the alternator decrease. DC Power simulated this in house by running an endurance test on the alternator for an hour which essentially brings the alternator to do the most output it can do at 14V for an hour straight. After the endurance test is done cycling, it then measured a power curve from the machine. In the case where the alternator stator is modified, the 270 amp hairpin loses an average of 30% of output from 2,000 to 8,000 alternator RPM as seen below which also means that the alternator’s idle output just saw a substantial hit. For the individuals that have noticed that their alternator worked great for the first half hour of a show but then started to see a significant issue with idle output after an hour, this is the reason why.

Cold vs Hot 270 Amp Small Case Output


Consolidated Hairpin Curve

There is no doubt that there are a lot of high output alternator companies out there today. DC Power does their best to lead the industry in selling solutions that work without having to fix idle output issues with running extremely small pulleys. To give everyone an idea, I compiled a list of the most commonly sold small case units and made a graph comparing them to our large case units. Hopefully this will clear the air about our "competitors" selling the "same product" for a significantly less price when it's not even comparable by any standards in the real idle output world. Keep in mind that this graph is based on cold ratings and not real world ratings. When the small case hairpins are already this disappointing it will only get significantly worse when heat comes into play.

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