VettesandZs

:beatdeadh orse::beat deadhorse:

J-Rod

I wanted to add some things in to address lionelhutz assertion about eddy current dyno'd vs inertial dynos. I spent quite a bit of time in Australia.

They use Dyno Dynamics eddy current based dynos. If you put big heavy wheels on a car and you go dyno it, its still going to dyno lower on an eddy current dyno. Its not an artifact of a inertial dyno.

If you increase gearing on a vehicle (numerically) in an Ls1 you will normally see a decrease in RWHP, but often times you will see an increase in RWTQ. This is not because the dyno is off. Its because you've changed the torque "seen" by the rear wheels.

By increasing the gear ratio from 3.42 to 4.10, you increase torque to the wheels by 19.9% in each gear. However, you do not necessarily increase torque by 19.9% at every given speed, in fact in some instances you may LOSE torque. This is why you can't just go 'bigger' and get a 'better' result.

you see similar results when you have to dyno cars with a 2.56 rear. you have to dyno in 2nd instead of 3rd which skews the graph. This is regardless of eddy current or inertial dyno

Real world example:

Two M6 Corvettes are going to race from a rolling start, and decide to go from 45mph. One car has stock 3.42's and one has a set of 4.10's installed.

The car with the 3.42's can start in 2nd, but the guy with the 4.10's has to start in 3rd because 2nd gear is already close to redlining. 2nd gear has a gear ratio of 2.07:1 where 3rd gear has a ratio of 1.43:1. So now, the 19.9% torque advantage of 4.10's is actually now a 20.7% torque DISadvantage at this mph. See the problem?

So how much more 'power' do you get?

Instantaneously, you would have to run the numbers at each specific point in time to figure that out. You would have to combine the instantaneous numbers of a specific set of points in time to figure out average power.


One other point I'd like to make. When you run a heavy wheel and tire combo mileage suffers. Why is that. if it takes no power to spin those wheels once they are running, then why does freeway mileage suffer so badly.

As an example. A swap from factory GM 17" wheels to a set of factory GM 20" wheels is a loss of ~1.5mpg running steady state down the freeway in a Suburban or a Tahoe. I've personally tested this.

lionelhutz

There is still an inertia factor as the drum speed is constantly accelerating during the test though. The eddy current braking might slow down the acceleration lessening the effect but it's still there.


This is a very interesting.

How steady is the RWHP loss over the whole test range? The curves I have seen show the new RWHP just below the old RWHP pretty much across the whole test range.
You must mean the torque @ a certain road speed changes and not the torque vs engine rpm?


Are you being vague and trying to imply the mass is the cause? Larger wheel and tire combinations are also usually wider and have a softer tread compound. So, don't neglect the extra air drag and rolling resistance this causes.

Peter

deimis

I thought this was settled the first page. Although I must admit I did learn quite a bit by reading most of this

J-Rod

Lionel, have you ever seen an Eddy Current Dyno? It doesn't have a huge drum. It has a pretty small drum with a large enough diameter for the tire to make good contact.


Also, I'm not being vauge. I've already stated my point numerous times. The entire discussion is around weight. Heavier driveline mass eats up HP. Its not just on an inertial dyno, its on load based dynos as well. Also in just driving around on the street, it takes more HP to drive a heavier mass farther away (MOI). Its not just in takeoff, its in steady state driving.

As for argument about tread compound, or section width. In the examples I have, the change in width and compound were negligible at best.

In fact many on the CF have made changes with no changes in either area and seen large losses in RWHP by only changing one varaible which is changing the wheels.

lionelhutz

And your point is what?

That seems really odd considering that the scientific community has widely accepted that to keep a mass moving a constant speed you have to overcome the losses. I'm sure you know these losses. Most of them are commonly call friction. Examples would be things such as air friction, wheel to asphalt friction, bearing friction etc. In a car, there are also other losses such as pumping the engine or auto trans oil. The mass does not have these losses but the mass can cause more of these losses. MOI (which I assume equals moment of inertia) has absolutely nothing to do with steady state. NOTHING. Yet, you seem to believe the mass itself is the loss...

MOI affects the power required during acceleration but it retarded to say something like this - "the heavier wheels caused me to lose 15RWHP". The power required to accelerate an inertia is dependant on the acceleration rate - it is not a fixed loss. You can try to tell me the inertia causes a fixed loss but then you would be wrong. No gray area - 100% wrong.

Peter

lionelhutz

FYI, compared the RWHP measurements from 2 different dyno's when the tests were a year apart is completely pointless.

Peter

J-Rod

Peter, let me try to clarify for you what I am saying.


As we all know, HP is a measurement of acceleration. The ammount of time it takes do X ammount of work.

Torque to accelerate wheel is based on WR^2. Since the radius is squared, the location of the weight is very important. A heavier wheel tire combo with all the mass at the hub could perform better than a lighter wheel that had all its mass concentrated at the tread. Weight times radius squared.

Lower ROTATIONAL INERTIA (flywheel effect) is the thing that increases RWHP, not just lower weight. A heavier wheel tire combo would produce more RWHP if it's Rotational Inertia were lower than a lighter setup. For example a small diameter aftermarket wheel may WEIGH more that a larger diameter OEM wheel. But when you calculate the rotational inertia, the smaller wheel may have a lower inertia than the larger wheel, so the heavier wheel will produce more HP. It depends on how the mass (weight) is distributed. The formulas that calculate Rotating Inertia include the OD to the 4th power...If the OD is larger, the flywheel effect could also be larger.

So optimally you want to use the lightest, smallest OD (stock size) wheel/tire you can get if HP is your primary concern.

But, I will say that when you add add additional mass or something else that creates loss in the driveline you go to measure the HP at the rear wheels by either an inertial dyno or a chassis dyno you will see a correpsonding loss in HP.

This can be from heavier tires, a transmission with more rotating mass, or a host of other factors.

At the end of the day, loss is still loss. You can get in here and argue that its because of wind resistance, or frictional losses from the tires, or whatever argument you want to make to not accept the fact that additonal mass takes additional HP.

I've also done some back to back testing where I take component X out of the driveline and do a back to back dyno and see that I lost X hp from that one change. How else can you correlate that change except to say that it took X ammount of HP?

I've done plenty of my own empirical testing on the dyno and the track. You make a switch from a 10 bolt, to a 12 bolt, to a 9 inch, to a Dana 60.... Same gear ratio in the rear and you will measure completely differently based on the gear offset, and the weight of the components in the rear itself. But, the loss in HP, and also a corresponding loss in ET and MPH is there. In fact we fight a constant battle in many of the competetive classes to look for components with the least ammount of mass that will survive behind the HP the engine makes. Often times its a balancing act.

The same thing happens in the engine itself. People seem to think that a "forged motor" one using aftermarket rods and pistons will make more power than the stock engine. The fact is it will not. Most aftermarket components are forged. Thus they are heavier than the stock rods and pistons. Cartianly they are more durable, but they eat up power to spin them. I went with some of the lighest generally avaliable components for a 347 building for friends motor. We went with superlight rods and pistons. But, the mass was still more than the GM PM rods, and the factory HT pistons. The motor made less measured HP.

You go from a light factory wheel to a cheap import wheel. Here is a quote from Piano Prodigy on this one with some empirical testing he did with back to back tests on the same dyno.


Another test with a low HP car, again disputing the argument about needing heavy wheels to act like a flywheel and store energy.

This comparison was made in '98 with an Integra Type R. That car was making 178 FWHP, and ran some 5.6s 0-60 with the stock wheels and tires.

The stock 15" wheels vs. 17". The 17" wheels and tires were about 5-6lbs heavier. Using a G-Tech Pro meter to test the 0-60 times. The owner averaged 5 runs on each setup and the 17s added almost 1/2 sec. to his times.

Here is what Herb Adams has to say in the matter in his book an Chassis Engineering.

Something else people forget is big heavy wheels increase braking distance as well too.

lionelhutz

That statement is flawed. The acceleration rate is a measurement of acceleration.

You could more correctly say power can be a measurement of the work done when a mass is accelerated.

Oh wait, that can't be right.

If HP is a measurement of the work done when a mass is accelerated then RWHP would be the measure of the work done accelerating the car's mass, which would include the mass of the wheels.

This is better. To be more correct, power is the amount of work done per unit of time.

When the car is moving at a steady speed the mass of the wheels has no direct effect on the power required. I believe I stated that in my last post after you insisted that the mass itself caused a steady driving loss.


Yes, a change in drivetrain inertia will change the chassis dyno measured RWHP since the dyno measures RWHP while sweeping the drivetrain through a speed range. A change in the drivetrain inertia will also change the performance of the car on the street. But, you won't have that measured RWHP available when you drive because you won't be accelerating at the same speed as the dyno test.

Did you notice that Herb Adams book uses "eq. hp" or "worth X hp". He didn't claim the mass changed the HP.

Peter

J-Rod

Lionel,

HP is a measurement of acceleration.

A combination of two laws of physics, force equals mass times acceleration and work equals force times distance, gives us this equation: W=m X a X d. "W" is the work, in pounds-feet, the rear wheels are doing, "m" is mass equivalent (in the case of a dyno jet its the mass equivilant of the drums), "a" is acceleration (increasing drive wheel speed) and "d" is distance (the dynojet's drum circumference).

Once we have the work, we can find horsepower. One horsepower is 550 pounds-feet of work done in one second (again, that whole thing about time I was referring to) so, we divide the work number by the length of time measured, then divide the number we get from that by 550. To simplify: we get horsepower by multiplying the mass, acceleration and the distance, then dividing that product by time multiplied by 550. This can be expressed by: hp = (m X a X d) ÷ (t X 550).

We talked about MOI earlier. Everything that has mass has a moment of inertia, the energy required to spin an object about its axis. The greater the moment of inertia, the greater the energy required. Two factors affect an objects moment of inertia: mass and the distance of the mass from the axis of rotation. This means that a one-pound solid rod with a one-inch diameter will have a smaller moment of inertia than a one-pound disc with a 12-inch diameter.

So, whether its lighter wheels, or lighter driveline components like a lightweight aluminum driveshaft in a rear drive car. They all work together to decrease the amount of power that needs to be produced to initiate what is called the MOI, or moment of inertia.

Let me ask a couple of simple questions. Have you tested anything you've posted in here. You keep telling everyone that it has no effect, yet when presented with evidence that it does in fact make a difference, you have explained that its a flaw in the way the dyno measures HP.

You've explained that its because of the use a inertial dyno. Yet, when presented with evidence that it is not just a inertial dyno, but that it is also on a load based dyno (which was part of your earlier argument).

A key principle of the Dynojet is that the on the dynojet the drums’ inertia acts as a sort of passive power absorption device. "Mass equivalent" is a term engineers and physicists use to quantify the difference in inertia of a mass in linear or, more properly, "translational" motion and one in rotating motion. The mass equivalent of a rotating drum is quite different than its mass for translational motion so the weight simulated by the drums when rotating is different than their actual weight.

Torque on the dynojet is figured by multiplying the horsepower by a constant, 5252, then dividing that product by the speed at which the thrust force was measured. This is why when your spark plug lead falls off on the dyno that you don't get a Torque reading on your dyno graph.


In an engine dyno (like a Superflow) are a "brake-type" dyno measures the power that is actively absorbed by a water, oil, or eddy-current brake or by a generator. These are also called hydraulic dynos. The Hydraulic dyno measures torque, then you can extrapolate horsepower from this data.

As I've pointed out previously. I've actually tested what I'm posting in here. Have you? Have you actually tested the performance of a vehicle, added or removed mass to the driveline, and seen a loss/gain in HP and performance?

As I said before. I have. So, you can argue over the sematics of what I'm saying. You may not liek the way I use a perticular term. But, you can't argue the simple fact that I have the data to back up my claims and to date all I've seen thus far in response is a bunch of circular arguments.

I would also like to point out that Tony Mamo has done some testing in his own vehicle with 2-3 different engine combinations. Those engines were run on a load based based Dyno at Westech. That engine was then placed into his vehicle and re-run on Westech's chassis dyno. In each case, the driveline (transmission, tires, clutch, etc...) remained a constant. In each case, a fixed ammount of loss ( I believe 75HP) was recorded going from the engine dyno to the chassis dyno.

I did some extensive testing on a C5 driveline several years ago where we removed each item from the driveline to see what effect it would have on vehicle performance. This included a swap to a C/F driveshaft, swap to lighter brake rotors, etc... In my testing, mass removal equated to both more measure HP, and also greater performance of the vehicle.

So, say whatever it is you want to say Lionel. I have the data to back up what I'm saying. Can you?

J-Rod

Again,

Combo 1 first (Stock 17"). The best pull was 421 rwhp and 395 rwtq.

Combo 2 (18" Z07). The best pull was 409 rwhp and 386 rwtq.

So, the 33 lbs extra weight on the rear hubs robbed 12 rwhp and 9 rwtq.

Dudeurgettnavette

Mark C5

A couple things; Comparing best pull to best pull not really a legitimate method. Much better would be an average of pulls.

33 pounds is a huge difference and not likely to be seen changing wheels and tires on a street car.

Margin of error between pulls. Being conservative, say 3 HP, gives you a difference as low as 6 HP. I doubt any but the most experienced driver could "feel" that.

So to answer the OP's question, in a purist sense yes. In any sense that matters to the non professional crowd (most of us) not really.

J-Rod

Mark, you might want to go back and read. You seemed to have glossed over a few facts. There were 6 pulls done. 3 on each set of tires. They were in fact street tires going froma lightweight factory wheel to a heavy aftermarket reproduction wheel which is EXACTLY indicative of what the average end user will see.

Mark C5

I certainly did read, apparently you, not so much. Your quote says the numbers are "BEST PULL". I said an average of the pulls for each set would be a more legitimate method.

They went from a factory FRONT wheel and tire size to a non factory REAR wheel and tire size. I haven't seen anyone running front sizes on the rear on the street, so no, not indicitive of what the average user will see. Even so, the average user will NEVER feel a 6 HP difference.

And lastly, the post says at least 2 pulls were done on each set with no mention of how many total, so where do you get 3 on each set and 6 total?

J-Rod

That quote was only part of the total post. The details were explained about pulls, etc...


You alos made up this arbitrary "average" number. I've done quite a few dynos and in my own car the dynos only vary by 1 HP at the most in 3 pulls.

And, there are quite a few folks who can feel 12RWHP.

nickolbag

When I went from stock Magnesium rim/non runflats to 20" Ruff Racing rims I felt a huge difference the first time I stomped on it on the highway. Don't know what the weight difference was between them but it was easily felt.

AU N EGL

with that one J-Rod.

Again my rear street wheels

18x12 CCW Classsic Street with 18x335/30 Mich PS
= 51 lbs

Track ( road race wheels )
18x13 CCW Classic Track with Good year G19 18x25.5x11.5
= 39 lbs
or with Hoosiers 18x335/30 at 41 lbs

12 lbs difference per wheel or 24 lbs total

Street wheels
Street wheels 393 rwhp and 395 rwtq

GY Race wheels 405 rwhp and 407 rwtq

Hoo Race wheels 403 rwhp and 405 rwtq

Right at 1 hp diff per 1 lb difference or 1 hp : 1/2 lb per total rear wheels wt

Dropping 12 lbs ~ 6 HP increase.

This is what we found each of 4 winter tests sessions.

yes I could even feell the difference between the GY and Hoosiers. But that was a big rubber compound and grip difference,

Mark C5

The post still says best pull numbers, not a legitimate method.

No, I didn't make up a number. I used a very conservative number for margin of error. Ask anyone who operates a dyno for a living. 1 HP run to run with the car remaining on the dyno is very difficult to achieve over three runs. Take the car off the dyno (like when you change the tires) and no way.

Mark C5

A good post. Again this is going from a RACE wheel and tire to a STREET wheel and tire and only a 24 lbs difference. A street car going to different street wheels and tires would see even less. I would think about 5-10 lbs per wheel and tire combo.