Yes, you read right.

I have been curious for awhile now what the effect of changing the rotating weight of certain parts would make on the WHP of an EVO.

In the quest for the ultimate daily driven Evolution I have continued to test parts on my personal RS.

The latest round of testing was to see the effect on the whp in changing to a set of lighter stock replacement rotors.

Below are the weights of rotors I have been weighing over the last month or so preparing to change the rotors on my car:

The stock front rotor weighs in at 21.35 pounds.

Project Mu front rotor weighs, 18 pounds.

Performance Friction front rotor weighs 17.7 pounds.

Stop Tech's larger 332 mm front rotor weighs 17.5 pounds. (this rotor is in their big brake kit)

Giro Disc front rotor weighs 17.4 pounds.

**Side notes. I did not get a chance to weigh the Baer rotors and couldn't find anyone willing to weigh one on an accurate scale for me. We used our UPS digital scale for all the weights. The Stop Tech rotor, even though larger than stock, is very light. I'd like to see them build a stock replacement rotor, I think it would be the lighest of all if they did.

The stock rear rotor weighs in at 14.6 pounds.

The Giro Disc weighs in at 11.1 pounds.

There are only two manufactures of standard sized replacement rotors with aluminum hats. Baer and Giro Disc. Again, I didn't get a chance to weigh the Baers.

Since I was looking for the lightest combination of parts I choose the Giro Discs. Very nice looking rotor and the weight was the lightest. Black hats so they look good on the car.

Total for the front and rear that I saved on the car was 7.9 pounds on the front and 7 pounds on the rear.

They say every 100 pounds of weight is equal to a tenth in the 1/4 mile. So I am a long way from dropping a .10 of a second from weight alone.

On the dyno it told a different story.

I didn't do any adjustments to the dyno or the tune. I made two dyno runs in 4th gear with the stock rotors. The runs were identical with no variance (less than 1-2 whp and ft lbs) This gave me a great baseline.

I unloaded the car from the dyno, swapped rotors and had it back on the dyno within 1 hour.

Below is the result of the testing. As you can see the rotors had a clear effect on the WHP of the car:



More to come.

 

Hey Dave was this with cross drilled or slotted ? Also what was the material of the rotors ? I would figure some of the lighter rotors might not be as reliable as the heavier ones unless the metal was different.

 

Interesting fact. I checked the datalogger on these two runs. Timing the car on the log from 5,000 rpm to 7,000 rpm the car was .06 seconds faster in 4th gear. That is a fairly major change if you think about it. No way that much weight savings is going to help you that much in acceleration if the weight was just removed from "body" weight.

More to come.

 

Interesting. I got the same weights on my stock rotors, but then my aftermarket slotted rotors were only a pound lighter unlike your 3, which are all 3+ lbs lighter. Interesting data point.

 

Alright, as you will notice from the graph above the PSI remained identical and so did the AFR's. This will make all the critics happy and say that I finally did a fair comparison of a part. For those that fit that profile here is another dyno chart.

This chart is also a back to back dyno pull but in 3rd gear. For the guys that wanted to critique the fact that the boost changed in the 02 housing test thread you will see that the boost went down in this test too. What you will also see is that AFR's stayed the same BUT the WHP still went up over 5780 rpm. The max boost was less on the test with the rotor change by .6 psi and the average was a full 1 psi off. I attribute this to probably going to WOT a little later after the rotor change. Interesting enough though even with an average boost level of 1 psi less the car still made more power up top.

Check out the graph:

 

keep in mind too tire weights could have a great affect too. the farther out it is from the center the more the weight matters. Find a nice light tire, you could be even more suprised. i wish I could remember the formula to calc this. 1 pound rotationl weight is equal to.......was it 4 or 5 pounds sprung weight? i dont recall.

 

Is this every 100 pounds of rotational mass? Or static mass? If I remember the formula correctly, adding one pound of rotational mass is the equivalent of adding ~4 lbs to the cabin, in terms of acceleration. And I'm not sure, but I think it becomes exponential when at higher speeds. Just something to look in to.

 

The rotors, Giro Disc's, weight savings is mainly becuase the center part (the hat) is made of aluminum. The stock rotors and most other replacement rotors are steel centers just like the outside where the brake pads clamp.

The Giro Discs are slotted, not drilled.

We do sell the Performance Friction, Stop Tech and Giro Disc rotors etc.

Total weight savings from this was 14.9 pounds, all rotating/unsprung weight.

 

The weight and the effect I think is all a guess. The general saying is 100 pounds of static weight is worth a tenth of a second in the 1/4 mile.

I have heard that every one pound of rotating weight is worth 10 pounds of static weight.

It's all a guess as a far as I am concerned and at best a general rule of thumb.

In this case seeing a .06 second drop from 5,000 rpm to 7,000 rpm from taking 14.9 pounds off is pretty impressive. IF the car gains that much in each gear it could be as much as .24 seconds over the course of the 1/4 mile. That would be huge! Even if it is only half that it would be .12 and that would be enough to drop my RS from it's 11.11 @ 124 mph best to a 10.99!! That is my goal.

I refuse to remove anything further from my car that would have an adverse effect on safety or cut anything. The car is light but I haven't cut anything or removed a single safety feature. (For those of you that consider the front and rear crash beams for US crash testing a safety feature, I am lieing to you) The air bags are both in and working, all the crash beams are in the doors, the original safety glass is all in etc.

 

Rotational mass is worth more.

I think Luke from Tire Rack said one tire that 5 lbs. of unsprung weight = 1 usable horse power = 0.10 1/4 mile time changes

there are a lot of things coming in to play here.

 

well that is very interesting, but not worth 1500 bnoes for as much as 2 tenths of a second, for me anyways. but still very good research.

 

Well, the point of the brakes is not to make more power - it's to have more stopping power. If in the pursuit of extreme stopping power, you also gain 5-6whp/wtq through the entire powerband, then that is quite a bonus.

 

I totally concur. Gravy is Gravy

 

Just so I get it straight, the rotors you used during this dyno run are the Giro discs? And you did all 4 corners correct?

Also, why again did the boost drop by 1psi?

 

Yes, all 4 with a savings of 3.95lbs per front rotor and 3.5lbs per rear rotor for a total of 14.9lbs rotational mass reduction.

He said he started the pull a little later on the 2nd run. Dynographs are affected by when you hit the gas (rpm).