jb35007

today my theory of power rated on a dyno got shot down in one of the tv hotrod shows...

plz someone that uses a vehicle dyno plz answer this question

my thought was when doing a dyno pull that the correct reading would be best read with the vehicle (stick shift) being in what ever gear was 1:1 ratio that being 4th gear in my 2001 6spd corvette...

now my theory has always been that if you use a lower gear say 3rd gear then your dyno numbers would read a little bit higher as the gear reduction would multiply the power a little

and if you put the gear into overdrive then the dyno would read that you had less power to the rear wheels.

so my question is is there a difference in the dyno readings depending on gearing???

95rtturbo

Not 100% sure, but I think you have it backwards.. If you dyno the car in 2nd gear as opposed to 4th gear, you will get lower numbers since the load isn't as high. 1:1 is the standard gear to dyno the car with.

jb35007

yes my 4th gear is 1:1 in the 6spd vette

schpenxel

Pretty sure you have to put in a final drive ratio into the dyno so that gear ratio is taken into account

The key is to be able to see changes you should use the same gear between pulls/rounds of mods/etc.

jb35007

no one on here uses a dyno in there day to day job?

RonSSNova

I do. Nearly daily.
Mostly late model Dodges. :-)
The dyno doesn't know what gear you use.

Because there are more parasitic losses in the trans in the lower gears, you get less power. Also can see more tire spin on the really high powered cars.

On the auto trans dodges, we do use third. Simply because the 4th gear clutch isn't as strong as the 3rd clutch and we don't want to hurt the trans.

If you are after the truest RWHP, you do want to use whatever gear is 1:1.
But when tuning and looking for changes, 3rd works fine.

Ron

tblu92

The most important reason to run on the dyno where the trans is in a 1:1 ratio is not necessarily the HP and TQ readings but the AFR readings---
When and engine in under driven like in low gears the load on the engine is very low--
Typical AFR readings in 1st and 2nd gear can read much richer--So they are not accurate when the engine is under a max load like in a 1:1 or overdriven
Lets say you have your car tuned to have a 12.8 AFR at WOT
In all the under driven gears the readings may read as rich as 12.4-12.5 So it you adjusted your AFR in your tunes based on these readings it would be horribly lean in 3rd gear or in a 1:1 ratio------Being a tad rich in 1st or 2nd gear won;t usually affect ET's on a drag race but being too rich or too lean in the 1:1 ratio will --with the added side affect of possible engine damage if too lean
PS--The new C7 Corvettes have NO gear ratios that are exactly 1:1-----
Dyno HP and TQ readings are simply window dressing and for bench racers to brag about how much power their cars are making---( not to mention all dyno operators have the ability to alter in factors to make the dyno read whatever they want) Trust me if you got your car dyno tuned--the outcome will ALWAYS read a major increase--as they want to verify the $450 you just spent to prove it was worth while
A real racer uses the dyno as a tool only to simply adjust his WOT fuel ratio and BEST timing while the car is under MAX load--- The HP an TQ numbers are to subjective

enoniam

Max load is max load. Doesn't matter what gear you are in. If you are WOT then the engine is giving all its got and the AFR is the AFR. Only way its different is if you are holding back on the throttle due to tire slipping on the roller, and then it isn't really max load.

schpenxel

Transient fueling can come into play differently depending on how quickly it's accelerating, i.e. different gears

RonSSNova

I've never fudged the weather conditions to give a customer a big number. Ever.
So watch your statement Tom.

ron

enoniam

Transient fueling, being transient, is only going to come into play for a very short time after going WOT, assuming you are doing a dyno run for max torque and hp measurement.

Realistically I believe other than usually higher transmission losses in the other than the 1:1 gear a big reason for not running in lower gears is so you don't blow through the run in just a second or two. There's going to be a finite amount of time it takes the exhaust gases to get from the heads to the end of the tailpipe and that amount of time compared to how quickly the rpms are changing is going to affect how accurately the dyno is placing your measured AFR against rpm.

tblu92

Sorry but I dis agree---I have done 100's of street runs both with auto and manual trans using a WIDEBAND to verify actual AFR readings in all the gears used approaching 1/4 mile trap speeds ( up to approx 120 MPH) The wideband AFR's prove that in 1st and 2nd gear the AFR's are always RICHER than when the trans is in a 1:1 ratio--Meaning that even though the ECM is commanding the exact same fuel ratio in all gears--it becomes leaner when the trans is 1:1---Only explanation is that the engine is under more load and using more air
Data logging proves the same point
On a typical C5 the cylinder air will max out in 1st and 2nd gear at about only .60 grams--But when you hit 3rd the cylinder air will jump to .68 to .72 grams---again proving more load and more airflow---

RonSSNova

Yeah?
Made 4 passes tonight.
1st and second were leaner than 3rd.
Always are.

A4 car.
11.30's at 121 tonight

Wanna see a log?

Cyl air maxes in 3rd. .9g/cyl. Lower in 1st and 2nd......

schpenxel

I always see lower gears leaner than higher ones, so your results match mine

RonSSNova

Thanks. But what do we know?
I've become tired of reporting my real world results.
Orig posters leave the thread and a certain someone has every answer.
I'm done here.

Ron

schpenxel

I'm with you

enoniam

The engineer in me always respects empirical data but when the data seems to contradict my understanding of physics I quickly become suspect of the quality of the data measurement.

In this case if your premise holds true there is a strong inverse correlation between the acceleration of the crankshaft and all parts mechanically attached to it and how well the engine sucks in air, as well as how well it measures the air going in, assuming all if its cyl air calcs from that data are consistent regardless of gear and hence crank rpm acceleration. The physics-based engineer in me has trouble swallowing that premise.

We have two other forum members posting differing AFR results relative to what's happening in different gears, one of them a professional tuner and dyno operator. Clearly their empirical data is not matching yours on what's happening between gears relative to AFR changes.

What is consistent is the increasing cyl air numbers. The engineer in me also has to understand what's happening there - I'd suggest that at the start of a run the air in the intake and underhood about to be sucked in the intake is warmer than the air it sees after running up through the gears - hence it becomes denser as the temp goes down and therefore more grams/cyl stroke going into the engine. That said going from .6 grams to around .7 is a large increase (over 10%) causing me to wonder if you've got something going on in your tunes causing the cyl air calcs to be not consistent across intake air temp changes???

As a final note, if limited to 3rd gear doing street tuning (up to 120 mph) you wouldn't be accurately tuning my car at peak hp. My car hits 120 at under 5100 rpm, with the optional 3.15 gears and automatic trans. Car pulls hard up to 7000 rpm so if limiting to 3rd gear and 120 mph you can't be tuning at max hp and it would be worse with the standard automatic rear ratio.

Also to note, my car, while not stock, flies through 1st gear in 2 seconds, second in less than 4. Given limited data acquisition rates through say HPTuners scanner software that equates to huge rpm differences between data points and from what I've seen data that isn't collected at the same point in time for each scanner data point. I don't believe you use HPTuners but I would suspect that your software if collecting engine parameter data through the OBDII port would be similarly limited.

romandian

a few things.

the operator can adjust the acceleration of the dyno (at least on a bosch) so the gearing choise probably boils down to losses in the transmission.

you have to accelerate all rotational mass like wheels, rotors, flywheel, engine or driveshafts. that eats up power.

a turbo car will build more boost with higher load.

Joshboody

Not sure if OP's question was answered, but let me try simplifying it
- Gearing does not matter for HP
- HP is rate of work... mph + acceleration.
low gear: <mph + >acceleration = xHP
high gear: >mph + <acceleration = xHP
- engine torque is calced from mph and rpm

Any HP diff between gearing is due to heat lost through gears and engine load.

enoniam

Nice simplification Josh, but a bit of clarification regarding the torque calculation:

Torque (in foot-pounds) = Power (in horsepower) times 5252 / rpm

That is why foot-pounds and horsepower are always the same at 5252 rpm.