After reading various articles in numerous publications on dynos and horsepower, I feel I should put forth a few observations:

Chassis dynos are great tuning aids but they only give a approximation of power output as some of the important variables are not accurately controlled. Certain magazines seem to think that results obtained from chassis dynos are the gospel. They are not. In one recent independent test, hp figures varied by 11% simply by doing the runs in different gears and in another test, results varied by almost 4 % by doing the runs with a different wheel/tire combination. Tire alignment has been shown to affect results up to 3% as well. Note that Engine hp DID NOT change here yet the dyno recorded an increase in hp at the wheels. A change in wheels/tires also does not affect true, wheel hp either, only the rate of change on the rollers and the vehicle acceleration on the road. One can only conclude that inaccurate moments of inertia and correction factors are being used....

G35_TX - Good info. Thanks for sharing.

 

Nice cut and paste from some previous discussion that is mostly irrelevant to what is being discussed: why the 1:1 gear is chosen. If you'd like to talk about any specific of the points in that text, I'm happy to discuss it. Rather than spew a load of info, you should take the time to focus your argument. I'll sum up your argument for you.

All you had to say was that there is no main gear set for the 1:1 ratio, and therefore no main set losses, done! I admittedly was not familiar with this and just assumed there was a 1:1 main set just like my last car (which was FWD).

So clearly running in the 1:1 gear minimizes main set losses. Do those losses outweigh other losses (inertial, rolling resistance, other geartrain) that decrease or increase in the next lower or higher gears? That's not clear, but in this case I agree that dyoing in the straight-through gear would be best from a standards point of view!

I argue to get to the bottom of something. I'll gladly admit when I'm wrong, just state your argument!

And getting back to the very original argument, I never claimed that ANY dyno was gospel in any sense, only that a Dynojet is completely unrealistic.

 

That depends how you define "true wheel hp". Is it steady state power otuput at the wheels at constant speed with no acceleration? Okay, sure. But you're never going to be able to accurately back-calculate to an accurate crank power number, so why try.

Personally, when I think about "true wheel hp", I think about the power that is actually accelerating my vehicle on the street, or the physical net force realized at the tire contact patch pushing agains the road. In this case, all of the driveline losses as well as inertias of all rotating components are parasitics and directly impact this relatively measurable force at the ground. The point is obviously to construct a repeatable experiment (same dyno, same run on the street, same gear, etc) so you can get true relative gains.

 

You have no basis for a arguement. You are wrong and get over it.

I gave you detail info on how the Dyno works and how the drivetrain works. But you obviously want to ignore it.

Obviously you didn't read what I posted. So I am not wasting my time anymore with you. In my book you have no clue what your talking about.

 

I have no doubt that you have more knowledge on many subjects than I, but I can assure you that physics / mechanics is not one of them. But you are right, I have absolutely no clue what I am talking about because I assumed there was a 5th gear main set. Please. I haven't studied the layout of our transmission in detail, but I have studied losses in gear sets. I could write a book on dynos and losses. Your entire cut and paste is completely congruent with everything I've said (you should know, you read it) except for your argument which I had to make for you after reading through it: no main set for 1:1! Admitted! I clearly stated the assumption of a 1:1 gear set in my conclusions. Next time you should practice making succinct arguments. It is okay to admit that another person has made valid points.

If you truly understand the text here that you've pasted (not that I think it is gospel, the hard-coded percentages are definitely questionable), you would clearly understand my argument as to why the 1:1 gear will not necessarily give you the best power numbers on transient measurement systems (on the street, inertial dyno, etc). Steady-state zero-acceleration measurements would obviously be different (where inertial losses drop out) and 1:1 would then likely always be the ideal (if rolling resistances weren't too large). If you ever have any technical questions about anything in that text or need anything explained, feel free to hit me up. No hard feelings.

 

OOPS!!! I must have slipped with my cursor!!! So Sorry Please.

 

^^^ Cute, but I am missing your point.

 

RubbinOneOffInRI is just a little upset because he doesn't like my suggested power numbers and would prefer to wrap his fist around something rather than wrap his mind around something.

 

I have a question. This is a tangent, and i'm not taking sides in the debate.

Ok, here goes - everyon egrees that for tuning purposes, it doesn't matter what your dyno # is, its the change in that number from mod to mod that will matter.

The logic is: if dynojet #1 gives you 230 and dynojet #2 gives you 240, and you add a mod then dynojet 1 will go to 240 and dynojet 2 will go to 250. I can go along with this - you get 10 dynojet hp.

But my question is this:
if dynojet 1 gives you 230 and mustang dyno 1 gives you 210, then you do the mod, and dynojet 1 goes to 240, will mustang dyno 1 give you 220?

I don't know the answer, but logic suggests that it won't. It takes more "real" power to get 1 mustang dyno HP, as compared to 1 dynojet hp. Thus, shouldn't a change of 10 dynojet HP yield a change of less than 10 hp on a mustang dyno???

My point is, that people think that it doesn't matter what dyno you use to tune. That you can just look at the increase. But i think on a mustang the increase of any given mod would be smaller than the same mod would yield on a dynojet. Thus, this creates incentive for tuners to use dyno's that yield the highest numbers.

Now i'm not saying which one is "real" and which one is "better" i'm just saying that even if you only look at the margin, the delta, the change in HP, different dynos will still yield different results. By implication, just because a mod says it will increase power by 20hp on a dynapack, the mod will yield less hp on a dynojet and even less on a mustang dyno. How much less, i don't know. But it will be less.

Someone tell me if i'm totally off base or if my logic works?

 

Your logic is correct. The issue is that you can't assume that the transfer function from crank power to measured wheel hp from dyno to dyno varies just proportionately (that is, that measured power at the wheels is always just proportional to crank power). We know for a fact that it is not from how the various parasitic losses vary by load / speed / etc. But to first order, one rough way to relate the numbers would be to scale your gains relative to the crank hp rating. That is, if your baseline dyno is 240 whp and with mods is 250 whp, for a crank rating of 298 hp, your crank gain is roughly (250 - 240) * 298/240 = 12.4 hp. Given that it appears that Dynojet numbers have a percentage fudge factor added to true measured output, this scaling would work rather well.

A more scientific way, in my opinion, would be to measure air massflow rates instead. Do a baseline run on the same stretch of road (or on the dyno) in a set gear while measuring the MAF sensor and assume it correlates with the given crank hp numbers. You can then measure changes in air massflow for various mods as potential power increases (assuming reasonable tuning). If I measure a 5% increase in air massflow from an aftermarket exhaust, I know it's doing something. If I then slap the car on the dyno and actually lose power, I know that it is a tuning issue, because the potential for power is there in the added airflow. I took MAF sensor data on the dyno for this very reason.

Here's my bone-stock run on the Mustang, with corresponding MAF data. Notice anything about the relation between the HP and MAF curves?

 

Mistermojorizin

I don't know what MechEE wrote because I don't give a F, but you're right. Some shady manufactures do what ever it is to get big #'s. Knock off company's are really bad for this. Companies like "To......" Nah never mind, will do anything for a sale. Some do it in perfect conditions which we never have in the real world, but yeah, their product did do it.

The point you made about the difference of the "changed #" between the two dyno's is true but to what extent? You dyno'd your car and came up with 240HP and if you had difference of say 20HP between the two, you're talking 12% basically. Now you to add a Mod that's supposed to yield 12 to 14 hp, you now could get a 1.44 to 1.68% difference. So if your buddy got 14HP and you got say 12.5HP, I don't think you'd be too upset would you? Then you would say..... "Yeah but we didn't dyno it on the same dyno".

And to you MechEE............... You have made go back 25 yr's and I'm a teenager again. I could argue all day and throw insults back and forth with you. I come here because I bought a new car and want to find out anything I can about it. I have been modding cars for a long time but this is my second *** car and I bought my first in 02 so I have a lot to learn. I don't come here to argue, I come here for enjoyment. You can certainly continue to slag me if you wish. That's OK, I don't mind. If someone says something that is incorrect, yeah we should correct them in a NICE way. Most intelligent people will take note and try to find out the real truth on their own. But you have to keep in mind the purpose of the thread. My was to try to make a controlled environment and post to the community the numbers I got. That was it. I could say F it, and show people crap!!! I just didn't like what I saw on the board. Too many people saying "Yeah I can feel it" How much is "it"? Was it worth it to pay X dollars for ?hp difference. That was all. So if our paths cross again I will say my point then finish with the thread. You can say what ever you wish. I'M DONE!!!!!!

BTW: I have a great sex life and don't need to "Ruboneoff".