PTRocks

I believe it's the opposite. In a lower gear a greater portion of the work output goes to spinning up the rotating inertia of the motor, clutch, and trans input shaft.

908ssp

Does your car accelerate faster in 4th gear or 5th? 4th of course so that is interpreted as high HP.

BLKCLOUD

Let me ask you experts this:

I dyno my car with 3.27 rear gears, with a 1:1 tranny gear. It makes 300 RWHP.

I decide to swap my rear gears to 4.56s. I do another dyno, again in the 1:1 tranny gear, no other changes, same dyno. Will the chassis dyno show more, less, or the same as the previous 300 RWHP? Why?

And for the super-duper experts, will it produce more consistent results on the dyno with 3.27s or 4.56s, and why?

I ask that only the experts (or those that consider themselves such) reply. I am far from an expert, but to quote a line from the movie "Stripes", I am 'willing to learn'.

Thanks.

sqidd

It has nothing to do with “Being the Man”. Your approval means nothing to me.

It’s about misconceptions and the spread of them as "fact" on forums.

BLKCLOUD

Ok. So please enlighten me and kindly answer my questions above.

Thanks.

sqidd

Yes, the 4.56 will show higher numbers than the 3.27. It is a rare occasion that a situation as extreme as this will come up because no one is running a 4.56 with a stock height tire. They are running them with tall tires which evens the “gearing” back out to a certain extent.

With a Dynojet dyno you will get the most accurate and repeatable results with the lower (numerically) gearing because the acceleration rates are slower and easier for the programming to calculate/interpret. This same reasoning is why runs should be done in 1:1. It is also a lot harder to get a clean run with higher gearing because the car will want to spin the tires on the rollers, hop around, etc.

Inertia dyno’s are far inferior to “Eddy Current” because they calculate power based off of the acceleration of the drum where the Eddy Current dyno measures actual “resistance”. You can fool a Dynojet dyno into giving higher HP numbers simply by putting really light wheels, driveshaft, flywheel, etc on. Because the wheels, driveshaft, flywheel, etc are lighter they will accelerate faster and the programming will interpret that as more HP.

Dynojet dyno’s are great tools, but only as good as the operator. You can get wildly different results simply by changing the testing procedure. 90% of the dyno information/claims I see in magazines, forums, etc lack solid testing procedures and therefore are “suspect”.

As an operator I can easily show “Mr. Customer” a 10% hp/tq swing without making one change to the car. And I can show another 10% hp/tq swing by making runs on a cold car compared to one that is heat soaked. I’m not saying that there are droves of unscrupulous dyno operators out there. Just that there are a lot that do not account for all the variables in the testing procedure and therefore end up with misleading results.

BLKCLOUD

I disagree on both counts. All other things equal, the 4.56s will show slightly lower numbers than 3.27s. I have seen this on my own car and many others. Also, I ran stock height tires on my 99 Cobra with 4.56s.

Additionally, it would take a VERY large tire to "even out" 3.27s vs 4.56s.

I would argue that the 'repeatability' differences that you cite would be well within a margin of +/- 1%, which could easily be the difference of 1 or 2 degrees on the thermometer, a few percent of RH on the hygrometer, or a few hundreths of a MB on the barometer - all readings that the dyno needs in order to calculate SAE results.

Spinning the tires is an issue with very high HP cars. If that is the case, then yes, it would certainly help to have higher (lower numerically) gearing.

You can fool it other ways, too. But again, I don't care what the actual number is, except how it relates to the previous pass so that I can see what good (or bad) I did with whatever mod I made.

Couldn't agree more with that statement. I have been an operational test director for a variety of USN programs. I have a fairly reasonable understanding of 'solid testing procedures'.

Again agreed.

But I shall stand by my statements - to wit: more gear will show less RWHP (which I really don't care about anyway), and repeatability with a 1.33 transmission gear will be functionally identical to repeatability with a 1.00 transmission gear (which I absolutely do care about).

xkornkidx01

Take it to the track guys! HAHA

xkornkidx01

I'd still like to see DYNO sheets... regardless if they sound "in line" or not, anyone can make up numbers.

xkornkidx01

Either way, if and when the dyno sheets prove that... its a great power increase! Much like we've seen with the '05-'09 S197

t-ray

In *theory*, the gearing should be irrelevant to a dynojet 248. Overall gear ratio is calculated by dividing engine RPM by the drum's RPM. Therefore it shouldn't matter in which gear the car is dyno'd, nor should it matter what final drive is used. Ditto for tire height.

However, in the real world, it does matter. The previous statement was predicated upon a reliable rpm pickup, as well as a reliable tire>roller interface. That's why pulls in a taller gear show less bounce between runs - as has already been stated, it takes longer to accelerate the drum in a taller gear. Longer time = smaller rate of change in rpm pickup = more accurate measurement.

I'm no operator, but I've observed hundreds of dynojet pulls for modestly powered cars(~265whp). The 1:1 gear almost always produces marginally higher numbers. For my own car, a 4th gear pull (1.271) averages 264whp, whereas a 5th gear pull (1.00) averages 266whp. TIFWIW

BLKCLOUD

Concur with your thoughts. My only comment would be that the difference in accuracy stated in your second paragraph would be marginal and functionally inconsequential.

t-ray

Agreed - both marginal and inconsequential. However, when people race dynographs on the internet, even something as small as 2whp is in contention. And that is unfortunate.

sqidd

I don't need to. He could add 250hp to his dyno sheet and my car will still pound the guts out of his.

And yes, I can drive it.

BLKCLOUD

That wouldn't be saying much, as I don't currently own a car. And my Ram 1500 could dust my bike (which wouldn't be saying much either).

I'm curious though...what does your GT run? The blown one, of course.

falhulk

Funny how guys say they know everything but refuse to explain what they do then talk trash to freaking Bob Cosby! Do a google of that name and see what ya get. If you know as much as you say help the rest of us. Dont talk down to everyone...........we are all trying to figure this car out

xkornkidx01

+100000000000000000000

sqidd

About 200

BLKCLOUD

That's kind of what I thought. You are indeed the man.

Have a great day!

RedCandy5.0

Bob everyone is the man on the internet!