Gear Effect On Dyno Results

Got your popcorn? No...it's NOT an Oil Thread. LOL
According to the chart below Kenne Belle did a dyno test on a stock 2014 GT500 with 331's. On 91 octane it pulled 583rwhp. They swapped to 373's. Results were 565...down 18rwhp. Of course the car likely felt noticeably faster on the highway thru 373's. But it's still interesting how rear gear ratios may effect what guys might say when their "belly is leaning on the bar".
See first 2 line entries:
http://kennebell.net/wp-content/uploads/2019/04/Shelby_2013_TechTips.pdf

Always wondered how a different rear end ratio affects dyno power numbers.
Obviously the engine is putting out the same HP so it must be in how the dyno calculates HP.

in for the education. good topic

Gears, tire height, tire pressure and sidewall stiffness I have seen all make difference.
I just watched Alex Flores from Lund make like 18whp difference or something to that nature by upping tire pressure from 20 to 35 on his tires.

It's not the dyno. It has everything to do with the angular acceleration of the wheels, brake discs, axles, and anything else that is being rotated by the ring gear. More torque is being applied to those components with a lower gear set. More torque equals faster angular acceleration and faster angular acceleration requires more power. So, the engine is using more power to "spin-up" those components faster and that leaves less for the dyno to measure. I'm sure friction plays a small part in this, but it is minimal compared to the losses due to rotational inertia.

Here are some links if you want more in-depth information.

Dynamics | Boundless Physics

The Effects of Rotational Inertia on Automotive Acceleration

I just watched Alex Flores from Lund make like 18whp difference or something to that nature by upping tire pressure from 20 to 35 on his tires.

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Yup...highest psi while still maintaining traction can give the customer just a bit more to smile about. I once did some eval pulls before a track day with my little 427SC Cobra kit. It came off my trailer with tires preset at 16 psi for the track. Numbers were a bit low and you could see the tire squatting on the rollers during the pull. The shop pumped them up to 26psi for the pull. After that event no matter what I drove around on ...all my dyno pulls were at 26 psi for consistency.
SteveWK....the "Boundless Physics" was very interesting !

Is this like the test KB did where they declared authoritatively that a BAP will get you to 1000whp??

ROFL

Dyno results are most accurate when the final drive is 1:1. The further you go from 1:1 the less accurate your dyno results will be. It has to do with the frictional loss of gear sets.

The car really is faster. Higher gear ratio will make more rotational inertia visible as the dyno reads it.

Motor HP does not change when you change gear ratios. The only thing that changes is the torque. When you change gear ratios both RPM and torque readings will change but horsepower will not.

The dyno doesn't care about the gear ratio. It's not getting info from the drum its getting it from the engine. An inductive wire is attached to a spark plug wire to determine RPM. The Drum only measures TQ.

That's it, I'm ripping out my 3.73 and installing 2.73, that's like a 100 whp gain right!? LOL

New That's it, I'm ripping out my 3.73 and installing 2.73, that's like a 100 whp gain right!? LOL

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Ha-Hah! Or do your pulls in 5th and fool that silly ass dyno lol!

I’ve always Dyno with 4.10 and wondered as most our cars have 3.31s.
Actually the last 1050rwhp runs were with a 9”, 4.00 gears, 29.5” MT Rs deflated to 17psi so not to spin. All should make it lower.
Oh well still ran hard at the track.
Interesting find.
-J

Here are 2 videos of my car getting dyno'd...the only difference mechanically is a change from 3.55 to 3.73 and a change of tire from 335/30 to 345/30.

These are on the same dyno, but done 1 year apart so the weather conditions may have been different....either way they are pretty close.

2018 3.55s / 335 tires

2019 3.73s/ 345 tires

DutyCalls said:

Here are 2 videos of my car getting dyno'd...the only difference mechanically is a change from 3.55 to 3.73 and a change of tire from 335/30 to 345/30.

These are on the same dyno, but done 1 year apart so the weather conditions may have been different....either way they are pretty close.

2018 3.55s / 335 tires

2019 3.73s/ 345 tires

Click to expand...

Maybe I missed someone saying the number but otherwise it's impossible to read the dyno graph in the video. Would you mind just posting what the numbers were?

merkyworks said:

Maybe I missed someone saying the number but otherwise it's impossible to read the dyno graph in the video. Would you mind just posting what the numbers were?

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2018 739 rwhp 670 rwtq
2019 747 rwhp 651 rwtq