same dyno different diff ratios

[Celica RA45]

been tuning my new motor with E85 and 14.5 to 1 comp and it made 204kws at 9000 rpm diff ratio is 4.3 and 205 x50 x15 rims about 584mm dia
went back last week end same motor but 4.55 ratio and also changed the tyres to 225 x50 x15 or 603mm power is now 176 kws
but also at 100 kays with 4.3 and 205 tyres revs were 4250 rpm in 4th
with 4.55 and 225 tyres revs are 4370 ,so a little more
so most dyno dynamics change as soon as you change little things

[Sam-Q]

shouldn't make a difference unless you used a different ramp rate. Are you sure the atmospheric compensation isn't set wrong?

[Matt]

I wouldn't panic about the numbers... They are just that and depending on the temps etc. one day you'll get say 200, the very next it'll output 150.

Never been a fan of dyno outputs, given the variance. You'll find the "performance workshops" readings are higher after they work on your car.

[maxhag]

I hear so much about dyno differances and numbers that they put out..

I am no dyno expert, but my 4AGTE has dyno'ed pretty much the same, on 3 differant dynos at 3 differant workshops..... 152kw, 148kw, 150kw

[Tim.duncan]

ever had this on a main line glen?

[Celica RA45]

std motor made 156 on a dyno dynamics and made 154.7 on a mainline ,same tyres and same diff ratio but with 48mm quads plus 2stage injection and race pipes

[Celica RA45]

maxhag on the 3 dynos you used 4kilowatts difference is ok ,but i cant except 204 down to 176 with just different diffratio and different size tyres
also same ramp rate and same temps on the day ,we even check the barametric ,and humity as well .we think it must be as the car goes up on the dyno it has different load with different diff ratio and also bigger tyres

[assassin10000]

Wow, that's a huge difference. Could it have been strapped down differently? Iirc that can make a big difference.

Andrew

[Celica RA45]

talked to my mate at motec ,as i have gone up to wider and bigger wheels ,this now creates more friction on the rollers and a lower reading same with diff ratio as well ,they all add to different readings

[SHEPPO3930]

wider tyres will create more drag, causing a lower reading... but not a cause a drop as big as yours. There's a couple things to take into account, firstly the fact the dyno sessions weren't on the same day (so that can cause different readings straight off the bat!), then there's other factors.

IMO, it's the larger diameter tyres that is causing a majority of the loss. Warning, the following has some maths involved...

Lets simplify the wheel & tyre down to a round solid wheel. It has a thing called the Polar Moment of Inertia (PM). Simply put it's the wheel's resistance to being rotated. The formula for PM = 1/2 x mass x radius^2. Increasing the mass will increase the PM, but not as drastic as increasing the radius. You have done both (larger tyres will be both heavier and larger in diameter). Thus, increasing the wheel's resistance to turn.

Now when you accelerate a wheel, that takes HP, which is worked out from torque (this is how inertia style dyno's work). The torque (T) required to accelerate the wheel is:

T = (PM x "RPM per second") / 9.551. "RPM per second" is the rate of acceleration.

From that we can work out horsepower (HP) as, HP = (T x RPM) / 5252 <-- This is why inertia dyno graphs almost always have HP and Tq graphs crossing at 5252rpm.

So....... what i'm saying is, the larger the wheel, the larger the Polar Moment of Inertia, the more is resists rotation (slower to speed up), and thus requires more energy to accelerate up to speed.. regardless of if it's on a dyno, or driving down the road. By putting larger diameter wheels & tyres on it, it will cost you HP. No if's, but's or maybe's. It's physics.

The diff ratio swap should have given you a *little* more power at the wheels, for the same reason running a car on the dyno in 3rd gear always makes more power than in 4th. The motor has more mechanical leverage (shorter gearing) to act against the rollers - i know this from my own experiences with my own cars & bikes on various dynos. Yes dyno's can run correction and all that to accommodate this, but they always read higher when a shorter gear is used.

At the end of the day, it doesn't really matter what the dyno says if you have had 2 different runs on 2 different days. Dyno repeatability is always a big problem, regardless of what modes and/or corrections are run. The only way to tell is to have a dyno session where you change ONE thing at a time, and kept the conditions as close to identical as possible.

In your situation, you have multiple factors causing a lower reading: 1) wider tyres = more drag, 2) larger wheels require more power to accelerate, and 3) the conditions on the day may have been less suitable than on the day you made more power.