89IrocZ350TPI

Would a car with 2.77's dyno the same as a car with 3.45's or would the lower gear affect the dyno? If so by how much.

thx

Atilla the Fun

they don't. try it yourself. dyno the car with 2.77s, then re-dyno with 3.45s. Your results printouts will show within about 2%.

89IrocZ350TPI

Are you sure. I was told lower gears have a way of tricking the dyno. Hmm

Atilla the Fun

So long as you tell the dyno guy what gears you have, and if you corrected the speedometer, and if not what gear it was correct with, you will get an accurate power number.

Apeiron

You don't even have to tell the guy what gears you have. It'll infer the total drive ratio itself.

85MikeTPI

iTrader: (2)

Higher (numerical) gears will increase the TQ multiplication of the engine, and thus will increase the dyno reading on a rotating drum dyno (dynojet). This is also one reason you always dyno in your 1:1 transmission gear. The rotating drum dyno only measures the TQ turning the drum, the Horsepower is a calculation done using TQ and RPM.

The affect is basically the same % as the ratio difference, but slightly less because of the increased losses as gear ratios increase.

So your 2.77 car will see about a (20 - ~5) 15% increase with the 3.45 gears.

Apeiron

No it won't, because the dyno knows both the engine speed and the speed of the drum and it calculates and corrects for gearing.

85MikeTPI

iTrader: (2)

What does it correct?

It's simple physics.

Sorry, but a gear change will show on the dyno.. Been there, done that.

Apeiron

Yes, it is simple physics.

Put an engine with 300 ft-lbs of torque at the flywheel into a car with on a rear-wheel dyno with 3.73 gears. Assume 15% drivetrain loss. Notice that the dyno doesn't read 950 ft-lbs at the rear wheels.

Atilla the Fun

My local dyno shop doesn't use a DynoJet. The guy types the axle ratio into the computer. I got the same results with 3.08:1 and 3.73:1 ratios. Of course, most guys change to synthetic lube when they change ratios. I changed the 3.08s to synthetic before the trip to the dyno.

89IrocZ350TPI

Ok how much of a change will it show. man this thread has confused me even more lolzz sorry but I really didnt understand the last sentence of your previous post. Did u mean a lower gear car will dyno higher pretty much?

89IrocZ350TPI

I just read on a mustang forums that the gear ratio is entered into the computer. So it shouldnt effect the hp/tq reading.

85MikeTPI

iTrader: (2)

My Optimal Shift point analysis from a Dynojet:



Same car, same dyno, same motor, same tune. The only thing that changes is the drive ratio from the gears. Dynojet's have a calibrated drum, and can't correct for all the nuances of gear changes.

Shagwell

This is correct. We've had my buddies stang on a dyno numerous times, anytime he has made a performance change to the car he has re-dyno'ed before changing gears, we then optimize the gearing after some passes have been made. The gear changes have yet to show a variance of more then 1.2%, which could have easily been an atmospheric difference or a slightly differently adjusted tie-down.

85MikeTPI - you are 100% certain that all atmospheric variables were the same(temperature, pressure, altititude) and that the car was tied down exactly the same way with exactly the same tire pressure? - and the same SAE correction factors were used? If so, then either your dyno opertator is doing something wrong or he needs to have some refurb work done on his equipment.

A dyno reads wheel speed and engine rpm vs a calculated resistance(load). Gearing plays no part.

85MikeTPI

iTrader: (2)

These pulls were made within minutes of each other, no variables..

I'm actually surprised that this was such a mindset. I've been doing dyno tests for years and never knew there was such faith in dynos.

I've done similar tests in the past using different motors, different dynos, different operators, etc., all the same result.

Shagwell

The pulls were made within minutes of each other with different gears? If the proper correction factors are being used to calculate rpm vs wheel speed, then the numbers should have minimal variance due to a gearing change. - It's not a mindset, as I stated my buddies stang has been on multiple dynos, both mustang and dynojet and has never had a variaince of more than 1.2%(comparitively, we all know the results between a dynojet and mustang vary) from a gear change, and that was even testing on various different days.

I can't see needing to change the shift point due to gears. The head-flow, intake, and cam size will always dictate an engine's peak power rpm and the optimum shift point will always remain within a few hundred rpm of that, no matter what gears are in it.

My only faith in a dyno is that it is a great tuning tool. We don't race dynos for a reason.

antman89iroc

Ok I'll throw in my .02. I have not made numerous back to back dyno runs changing gears so this is just my opinion.

Obviously, gears have nothing to do with the horsepower an engine makes. The question is whether a dyno would read a different horsepower with only a gear change (assuming all other variables are the same).

Assuming the dyno is properly calibrated and used- Absolutely not!!!

The dyno doesn't "care" what gear is in the rear. Or what gear the tranny is in for that matter.

A dyno simply calculates the horsepower based on the HP=torque x RPM/5252formula. End of story. If the gear is lower, yes more torque is delivered to the wheels, but the RPM of the wheel is less. Think about above constant in terms of wheel RPM and engine RPM.

In the case of the dyno that simply spins a big drum, after a gear change the drum would accelerate faster, but to a lower speed, therefore regestering the EXACT same horsepower at the wheel. The program simply associates the horsepower measured (at the wheel) to the engine RPM monitored when the wheel generated THAT amount of power.

If a dyno "sees" more power somehow it must be attributed to something else (besides conditional changes) such as programming in the software or possibly operation manipulation, intentional or otherwise.

There is no additional horsepower so the dyno cannot say there is.