Why do we dyno in 4th?

D

Don Hardcastle

Busa Ridin' Sailor (ret)
Donating Member
Registered
Ok,

In a car we run the car in the gear that is closest to 1:1 so that you can get as close to a true wheel horse power figure as you can get.





Everytime that I have seen a HP dyno run done, it was in 4th. The gear ratio for 4th is 1.285:1. The gear ratio for 6th is 1.043:1.



Sixth gear is closer to a 1:1 than fourth. Having said that, why is the Dyno run for HP done in 4th gear?





Don
 
Because they want to!

Don't argue about it!


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It's the gear that represents the closest to a 1 to 1 ratio engine to transmission. (If you know what I mean)

All cars and bikes are dyno'd this way because of this ratio thing. Thats what I have read. Also heard the same when I supercharged my Bullitt Mustang and had it dyno'd in Buda Texas south of Austin.
 
If you dyno'd your Busa in 2nd.... imagine what numbers you'd end up with.... people would think your dyno sheet was that of a turbo Busa....
 
(Ruben @ Apr. 19 2007,14:45) It's the gear that represents the closest to a 1 to 1 ratio engine to transmission. (If you know what I mean)

All cars and bikes are dyno'd this way because of this ratio thing. Thats what I have read. Also heard the same when I supercharged my Bullitt Mustang and had it dyno'd in Buda Texas south of Austin.
Click to expand...
That still doesn't explain it. If the ratio in sixth is 1.043:1, isn't that closer than 1.285:1?
 
a quote from www.modularpowerhouse.com

I believe its because its close to a 1:1 ratio. I never heard about bikes though, wished I would have known that. I had my bike dynoed one time and they did it in sixth gear

So I guess bikes are done in 6th... I havent had mine dyno'd.... just the mustang
 
(USN05LE @ Apr. 19 2007,08:54) Ok,

   In a car we run the car in the gear that is closest to 1:1 so that you can get as close to a  true wheel horse power figure as you can get.





Everytime that I have seen a HP dyno run done, it was in 4th.  The gear ratio for 4th is 1.285:1.  The gear ratio for 6th is 1.043:1.  



Sixth gear is closer to a 1:1 than fourth.  Having said that, why is the Dyno run for HP done in 4th gear?





Don
Click to expand...
Is that just the tranny or the over all final drive ratio? I think when you calculate in chain and sprockets 4th is closer to 1:1 and 6th is a slight overdrive. Don't know for sure but that would be my guess.
 
Had mine dyno's last week with the new wet nitrous kit and it wouldn't hook up in 4th at all, it just spun the tire on the dyno. The shop was about to close so we didn't have time to take the back apart and strap it down so he ran it in 5th just to check the A/F and it still spun the tires at over 220hp.

The best answer I have for your question is that everyone does it in 4th so it has become a "standard" like adjusting for elevation, you could run it n any gear you want, the lower gears would read higher if you want a cool dyno sheet for looks.
 
Powers739: What were your numbers?

I was hoping JohnnyCheese would pipe in on this thread...You there JC?
 
I was told from a highly respected tuner that bikes should be run in 6th.
I asked why and went home thinking about it for a while. I ask JC and he tryed to explain it but it didnt make sense to me . I ran a dyno for years.
I worked for a cummins engine dealer and we always ran in the closest direct gear. Any higher gear the tractors would fall on its face ( lug ) and you couldn't get a hp reading. so i wondered why wouldn't you run a bike in 4th gear right? Right. Then I kinda figured , hey, The bikes could sustain rpm with out lugging in top gear and get it's highest hp reading which is what you want to know. right? Therefor I feel this tuner knows what he is talking about. Also I did alot of research and found that a dyno doesn't really care what gear you are in as long you are not lugging the engine as to make it impossible to get a max power reading. Just my 2 cents .
 
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