Don&Kelly

I've heard it said "What gear ratio you have can also effect your dyno readings. The bigger the rear the lower the dyno reading will be. For example the same car with 3.42s will have a higher dyno reading than the same car with 3.73s."

So I called my tuner pro guy & he said yep, but did not know why.

OK, so this makes the second time I have been told this. The question is why and how much of a difference?

Thanks



:cool: :cheers: :smash: :seeya

Mike Mercury

I asked this same question about a year ago and never got a consensus answer.

If the Dyno took RPM into account (such as a timing pickup) then it could compensate. But, I don't think inertia dynos have such feedback capability.

.


[Modified by Mike Mercury, 9:49 PM 11/18/2002]

F8LPONY

Try and do some searches on LS1Tech.com in the internal engine section and Drivetrain section. I know there was a rather large post concering this about a year ago.

Suffice to say that it does affect your dyno #'s negatively with lower gears.

Bill Dearborn

Not sure why it would. If you are using a Dynojet both drum speed and engine speed are measured and used to take out any effects of gear torque multiplication.
Bill

rwj383

In short, your dyno #'s are most accurate when final drive is exactly 1:1. This is why you have to be in 4th gear on the manuals and in D for autos. the further you stray from 1:1, and depending in which direction, the less accurate your dyno readings. This is a very succinct explanation. As an experiment, do a pull in 2nd, 3rd or 5th gear (MN6) or L2 (A4) and see what happens! I see it a lot; first timer on the dyno jumps up and down because he thinks his stock motor is putting out 480rwhp when instead he forgot to shift.

Terry Humiston

It has to do with the inherent frictional loss of gear sets. The larger the ratio multipier, ethier up or down, the greater the frictional loss. We always dyno in 4th with a 6 speed because that gear is straight through or 1 to 1. Any other gear would produce lower numbers. Gear set frictional losses are also proportional to the torque being transmitted. The differential gear set is the major part of driveline loss.

The right angle turn in the shaft power also contributes to increase loss. Front drivers with transverse mounted engines have less driveline loss because of this.



[Modified by Terry Humiston, 12:28 AM 11/19/2002]

NEPTUNEBILL

I had my 390hd installed at Roger Vinci's and he told me my numbers would be lower. I assumed it was because it takes a little more power to push the lower gears, like the previous post, friction loss. Roger and Joe told me they went to a 410 on an f body and lost apprx 4 to 5% on the dyno even though the car accellerated like they added considerable power.

Mike Mercury

Bill; that is what I was trying to find out a year ago. I had asked if during dyno runs if there was a connection to the engine to determine RPM. The few that answered said they didn't remember/think so.

How does the DynoJet aquire the engine speed data?

hogurt

I don't believe that the gears have anything to do with dyno numbers on a dynojet.

As long as you are performing the dyno in the appropriate gear. For instance in a manual, 4th gear is the 1:1 ratio. If you have a 2.73 gear on an A4 then you need to run in third to have a 1:1 ratio. If you run in second, your dyno numbers will be higher because your rear wheels are being assisted by the gear configuration. If you run in OD, your dyno numbers will be lower because your gear configuration is hurting your performance.

Unfortunatly, I do not know which gear is the 1:1 ratio on a 3.15 or 3.42.

Terry Humiston

Any rear wheel dyno will measure engine RPM and rear wheel RPM and can compute overall gear ratio. The dyno will not know if the multipication is coming from the transmission or the rear axle unless there is an operator input to the program for that data. Reguardless, any gear multipication up or down will generate loss. The more the multipication, the greater the loss.
A dyno test run in first gear (Z06) multipies 3.42*2.97=10.16. 400 lb/ft of engine torque would put close to 4000lb/ft to the rear wheels. The dyno would measure the ratio between engine and wheel speeds and divide the measured torque by the computed ratio. The frictional losses of turn the gears would show up and the final computed and adjusted torque would be lower than a like run in fourth.


[Modified by Terry Humiston, 12:05 PM 11/19/2002]

Mitch Alsup

Answer: HP and TQ measured on a rolling wheel dyno go down, but acceleration and performance go up!

Why?

The higher gear ratio makes more of the rotational inertia of the driveline visible to the dyno. This higher inertia saps TQ from the engine BECAUSE it is ACCELERATING faster. This is one of those little conundrums with physics. It really is faster, but the dyno really reads a slight loss in HP and TQ!

Terry Humiston

Mitch,

If this were the case we would see higher numbers if we dyno in fifth (with it's overall ratio of 2.87*) as opposed to fourth (with it's overall ratio of 3.42). Real world examples shows this clearly not the case. We always see the best or highest numbers using fourth gear. A run in any other gear will always produce lower computed HP and TQ numbers. Sorry.
*numbers quoted for a Z06

C5Noir

Here's the reason, I'm quoting Patrick G on LS1Tech.com:
"Higher ratio gears will give you lower chassis dyno numbers for a strange, but logical reason. In essence, the DynoJet calculates hp based on the time it takes to spin up the 2800lb roller assembly. It's basically work divided by time and rpm. Think about this: If you car is at idle in neutral and you stab the throttle, it will take time to accelerate to redline...let's say 1.1 seconds. Now let's say it takes 8.2 seconds for your car to accelerate the DynoJet from low speed to top speed with 3.23 gears and 7.3 seconds with 3.73 gears. Dyno printout says 355 rwhp with 3.23 gears and 346 rwhp with 3.73 gears...why?

Think aabout this: In the 8.2 seconds it takes to spin the rollers with 3.23 gears, it would still take the motor about 1.1 seconds to overcome its own inertia (idle to redline). There's about 13.4% of the work used just to accelerate the motor itself. With 3.73 gears, the time to reach redline decreases to 7.3 seconds. Divide the 1.1 seconds into the 7.3 seconds and you will see that overcoming the internal engine inertia costs 15.1% of the work with 3.73 gears. There is less hp available during this time period to spin the rollers so the DynoJet will read a slightly lower hp figure. Make sense, or did I lose you?"

TooManyIDs

Moundir

Say a car dyno's 400rwhp with 3.42's what will it dyno with 4.10s? ;)

Terry Humiston

C5Noir,

Its’ easy to get lost in this crazy logic. What an inertial dyno measures at the rollers is rear wheel torque based upon the time required to spin up to a certain RPM. The dyno also MUST measure engine RPM. It then computes over all gear ratio and uses this number to calculate the final HP and TQ numbers. When you change to a lower (numerically higher) axle ratio you will increase rear wheel torque. The dyno will compare engine RPM to the rear wheel RPM and see the gear change. The dyno program will then use this higher ratio number to compute the final numbers. While the switch to 4.10 gears (for example) will show higher torque as measured at the rear wheels it will also result in a larger correction factor. This change in the correction factor will offset the increase and then some. Part of the problem is understanding the terms. The maximum rear wheel torque as measure by the dyno is not real world RWTQ, but a computed number useful for comparing one engine to another. A 4000 RPM clutch drop in a Z06 in first gear will generate over 4000 pound feet of initial twist at the rear wheels. :crazy:

Mike Mercury

I keep hearing this; but when asking how the dyno aquires engine RPM... no one seems to know for certain without guessing.

Of the people I had asked a year ago; they didn't remember any cable or sensor being connected to the car to determine the engine RPM.

So (again)... how does a inertia dyno sense the engine RPM?

Terry Humiston

From the dynojet web site http://www.dynojet.com/a248.shtml

MODEL 248 DYNAMOMETER INCLUDES:
Measures up to 1,200 and 200mph
Dynamometer Chassis (drums, shaft, bearings, etc.)
Software and Hardware to interface (PC, printer, etc. not included)
Auto Tie Down Package (straps, ratchets and hooks)
Ignition Wire Inductive Tachometer Pickup Leads
Anchor Bolts with Level Adjusters
Four Rubber Chocks
Air Actuated Brake System
Remote Software and Brake Control From Driver's Seat
Automatic Conditions Measurement (absolute pressure, air temp, etc.)
Training Included.

Terry Humiston

One minor clarification. Strickly speaking an inertial dyno could measure raw rear wheel horsepower without any engine RPM data. The data would not be very useful as you could only plot it against time and not against engine RPM. Increasing the spin of the drum mass from one RPM to another within a set period of time would produce a measurable amount of work and from that you could calculate HP. The chart plot would not look like the typical one you see as the engine RPM would be unknown. And, I suppose,:eek: if the program doing the calculations had operater supplied fields for tire size & gear ratios it would be possible to compute the engine RPM. The ingnition pickup is what I have always seen used as it is simple and mostly error proof. But it would seem you could do without, contrary to what I said earlier.

Mike Mercury

ahhhh, an ignition pickup :)

Funny; that the others I had asked said they didn't remember the dyno operators getting under the hood to connect up anything!!!!!!

:cheers: