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Rob Liebbe
05-08-2006, 08:30 PM
Does anyone know if a change in rear axle ratio will give a corresponding chassis dyno torque reading change. Seems to me like it would have a direct affect on torque and maybe a small affect on horsepower given no other changes.

Rob - the curious racer - Liebbe

CMC17
05-08-2006, 08:55 PM
I will see if I can dig up a debate or two regarding torque and gear ratios.

Todd Covini
05-08-2006, 09:36 PM
The dyno is supposed to compensate for it and give negligible differences.

-=- T

donovan
05-08-2006, 10:28 PM
my understanding is that if everything is set up correct on the dyno you will only see the powerband (curve) move in RPM's left or right... your peaks along the rpm range should not change but they will move.

example, if you peaked at 300 with 3.55's at 5000 and you install 3.73's your peak might be at 5200 but it should still be 300, if you install 3.27's you would still peak at 300 but at 4800...

that is my understanding... and the rpm's are just guesses.
donovan

Rob Liebbe
05-09-2006, 08:47 AM
David,

Are you talking peak horsepower numbers or torque? My thoughts are that you would be right for horsepower, but it seems to me that the torque curve ought to change. You're basically giving the engine a better mechanical advantage with a steeper gear and that should directly affect the amount of torque produced at the rear wheels and therefore the amount of torque measured.

I'm looking forward to learning something.

AI#97
05-09-2006, 09:26 AM
The truth be told the rule of thumb, and actual mathematics is as follows.

Using a 3.55 rear end as a base line, HP/TQ numbers go DOWN as the ratio goes up....3.73's will show lower numbers on the graph at peak due to a higher parasitic loss of spinning the drivetrain at higher rpms.

3.27's will show higher numbers as you aren't having to spin the drive line as fast.

there was a discussion on this over on the corral a long time ago and even though the wheels are spinning slower, the drum is spinning slower, and the increase in mechanical advantage is greater, HP/TQ numbers are always marginally less..... However, on the street, WITH TRACTION, the car with numerically higher gears usually pulls harder....


Suggestion, add 100#s to the car and don't worry about HP/TQ! :P

mitchntx
05-09-2006, 09:36 AM
So changing a gear ratio would REQUIRE a recert, correct?

donovan
05-09-2006, 12:03 PM
David,

Are you talking peak horsepower numbers or torque? My thoughts are that you would be right for horsepower, but it seems to me that the torque curve ought to change. You're basically giving the engine a better mechanical advantage with a steeper gear and that should directly affect the amount of torque produced at the rear wheels and therefore the amount of torque measured.

I'm looking forward to learning something.

I have two charts from a gear swap... I just happened to have changed gears and did my dyno cert back in 2004 to 2005 season with no other changes...

the peaks were the same on HP&TQ, the curves look the same as well... just the RPM ranges changes... as in the curve moved lateral, nothing up or down.

I have most of my dyno run files, I will see if I can overlay them and post.

I never questioned it, since HP is calculated from the TQ... it just made since, I might sit down later this week and think about it some more.

donovan

oz98cobra
05-09-2006, 05:31 PM
Matt, it's a good thing you mentioned the Corral in that post - at least we know it's something you read and didn't deduce that yourself ;)

Actually, it's the other way around - the loss Matt is aluding to is due to frictional losses of the tire on the rollers, which will be higher with 3:27s for the same transmission gear than it will for 3:73s since the wheels are turning faster - therefore the 3:27s will show a slightly lower numbers than the 3:73s. Any change in drivetrain inertial or frictional losses with a swap from 3:27s to 3:73s are going to be miniscule compared to the frictional losses between tire and roller since tire rolling resistance increased inversely proportional to speed. Large variations due to driveline inertia only really comes into play in very low or high gears - like 1st or 6th - which is why chassis dyno runs are invariably done in 3rd or 4th gear. Note that most rear drive cars have a 1:1 4th which should have less trasnmision loss, but this would be negated by the increased wheel speeds on the rollers anyways.

In theory, it shouldn't matter a damn what rear gears you have in the car if it is properly compensated for in the dyno software - but that is rarely the case as David D's experience shows. A chassis dyno calculates torque by the rate of change of speed of the rollers (acceleration) over time - and the HP at the rollers is calcuated from the torque using the formula "HP = Torque x RPM(of the roller) / 5252" - but ... the plot of those numbers is referenced back to engine RPM, not time (or roller RPM).

Rob, HP is calculated directly from the TQ so if the torque is affected, so will be the HP. If you are looking at it from a CMC/AI rules perspective, the bottom line is that rear gear change could affect the peak numbers a little - and in theory, you may "gain" power as you go up in gears, or "lose" a little if you go down in ratio?

oz98cobra
05-09-2006, 05:34 PM
... and we better not have too many more tech posts like this here, or folks will start to think they are on corner carvers ;)

AI#97
05-09-2006, 06:10 PM
Matt, it's a good thing you mentioned the Corral in that post - at least we know it's something you read and didn't deduce that yourself ;)


Well, for the record, I never claimed the corral thread was totally accurate given it was probably 4 years ago when I read it....however, 5 years of dyno tuning nearly stock cars and seeing first hand the effects of gear swaps on cars at the dyno tends to give me a pretty good REAL LIFE account.... :wink: don't know how many times I have seen a guy put $2000 in mods in a car with a 3.27 or 2.73 rear end and get X rwhp/rwtq, then watch him throw 4.10's in it and get pissed that he lost 5-8 rwhp/rwtq......then go to the drag strip and run slower cause he couldn't put the power down OR had to shift one more time.....Been there, done that myself. Now apply some great traction in there and you will find that steeper gears kick the snot out lower gears! I was worried about my 3.73's for AI but they seem to have been the best choice over the 3.55's I thought I might need.

It's kind of like that Verner guy is saying...screw the HP, drive the car and tune the combo/driver. Besides, you only have to dyno if someone gets pissy right?! :wink:

Rob Liebbe
05-09-2006, 11:34 PM
Interesting read guys, thanks. I'll just continue to beat the snot out of the car and have a good time. If I ever have to go into the axle, I may try a different ratio.

oz98cobra
05-09-2006, 11:56 PM
So maybe what you were observing there Matt may have been 4:10s + street tires + metal dyno roller = tire slip = lower numbers :?:

CMC17
05-10-2006, 07:44 AM
I will have to post the dyno sheet and video when I lit up the tires on the dyno. T-trim w/21lbs of boost + 150 shot of NOS = scared dyno operators. Had to get three large guys to sit in the hatch. Yep, site to see.

BTW.. the Mustang dyno is more accurate than the dyno-jet.

jeffburch
05-10-2006, 08:39 AM
I will have to post the dyno sheet and video when I lit up the tires on the dyno. T-trim w/21lbs of boost + 150 shot of NOS = scared dyno operators. Had to get three large guys to sit in the hatch. Yep, site to see.

BTW.. the Mustang dyno is more accurate than the dyno-jet.

I happen to have video of that pull.....
http://www.lingenfelter.com/C620054271038hp320.mpg


jb

AI#97
05-10-2006, 10:14 AM
So maybe what you were observing there Matt may have been 4:10s + street tires + metal dyno roller = tire slip = lower numbers :?:

Entirely possible given the greater mechanical advantage but I would say half the cars were on drag radials or atleast an MT ET street which are probably marginally better than a street tire when cold. Tire slippage will also show an eratic graph with mini peaks and valley's where the tires are grabbing/slipping....sort of like a pull with rapidly fluctuating a/f ratios.

One thing I will comment on is that MOST dyno operators don't go through the details to ensure proper setup for a dyno run. Most cars that would roll in on ET drags would still have 10lbs of air in the tires and they would not bring them up to 30lbs which causes a ton of rolling resistance.

consistancy in dynos for the most part is just NOT there. I think that is why NASA does a pretty good job in detailing the checklist for an "approved" dyno pull to try and make it consistant.

CMC17
05-10-2006, 11:36 AM
I tried 30psi in my ET Streets... once. After filling the dyno area with tire smoke, it was deemed not a good idea. Of course this was with a ~700rwhp car.

Load type dyno's seem to be more accurate after reading about the difference in how they operate.

michaelmosty
05-10-2006, 12:00 PM
I tried 30psi in my ET Streets... once. After filling the dyno area with tire smoke, it was deemed not a good idea. Of course this was with a ~700rwhp car.

Load type dyno's seem to be more accurate after reading about the difference in how they operate.
Yea, I had that same problem in the CMC car. I unfortunately destroyed a pair of Toyo's. :?