Dyno tips?

[happy and angry]

our cars are geared differently depending on what gear you're in...our final drive ratio is 3.941 in 1st-4th gears, and 3.350 in 5th-6th gears. use 4th gear because it is the closest to 1:1 of the gears that have the better final drive gearing.

How does final drive ratio affect a dyno run? I would expect a dyno run in 5th to post similar numbers to a run in 4th, only with numbers a bit lower because of the gearing ratio difference. The final drive ratio just means the 1.171:1 or 1.085:1 ratios apply to a different final drive gearing, but will have the same final ratio of tire revolutions to transmission revolutions, which means the degree of inaccuracy in 4th on an inertial dyno would remain the same.

 

[Sacrilicious]

How does final drive ratio affect a dyno run? I would expect a dyno run in 5th to post similar numbers to a run in 4th, only with numbers a bit lower because of the gearing ratio difference. The final drive ratio just means the 1.171:1 or 1.085:1 ratios apply to a different final drive gearing, but will have the same final ratio of tire revolutions to transmission revolutions, which means the degree of inaccuracy in 4th on an inertial dyno would remain the same.

i was assuming that we would be able to put down noticeably more power in 4th gear because:

in 4th gear:

gear ratio = 1.171
final drive ratio = 3.941

this implies 1.171 x 3.941 = 4.615 engine revs per tire rotation.

in 5th gear:

gear ratio = 1.085
final drive ratio = 3.350

this implies 1.085 x 3.350 = 3.635 engine revs per tire rotation.


the difference between 4.615 and 3.635 engine revs per tire rev seems like it would make a reasonable difference. i don't know for sure simply because i have never tried dynoing in 5th gear, but i think it's a safe bet to say that it should be noticeable.

in the end, i think the most important thing is that 4th gear pulls are easier on everything involved simply because it puts a lighter load on the engine and the dyno. i forget what absolute engine load maxes out at in 5th gear, but it's noticeably higher than in 4th gear. also, it's a lot easier to run up to ~105mph than it is to run up to whatever speed 5th gear maxes out at.

 

[Darksun280]

um no way in hell I'll start a dyno pull in 5th at 3300 rpms the way my car sits now. no no no no no......

 

[AutoXRacer]

5th gear is way too tall to load up the car like that...use 4th...some people even use 3rd...but you'll get abnormally high readings.

 

[aaronc7]

I want to see a 1st gear dyno, lol

 

[happy and angry]

5th gear is way too tall to load up the car like that...use 4th...some people even use 3rd...but you'll get abnormally high readings.

It doesn't really matter what gear you use (although first or second here would be dumb...) so long as you keep using the same type of dyno, use the same gear, and use it as a tool to track progress with your car rather than an absolute definitive number. I was just looking at it from a purely hypothetical "strictly speaking wouldn't 5th be more accurate on inertial dynos?"

 

[Sacrilicious]

It doesn't really matter what gear you use (although first or second here would be dumb...) so long as you keep using the same type of dyno, use the same gear, and use it as a tool to track progress with your car rather than an absolute definitive number. I was just looking at it from a purely hypothetical "strictly speaking wouldn't 5th be more accurate on inertial dynos?"

in this case, then i would say yes...just as long as no one attempts this on our car! (eekdance)