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Dyno test done in 3rd or 4th gear, or does it matter?

jeff

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#1
I'm going to be visiting a dynamometer later this fall and wanted you guys' perspective if you have experience....is there any difference on our cars between testing it in 3rd gear vs. 4th? I've noticed some talk about how 4th gear will get slightly higher results and I think I understand why, but the place I'm going to says they do 3rd gear runs.

If anyone who can speak to this or has experience, please chime in, maybe [MENTION=930]dyn085[/MENTION] or some of you other dyno pros.

Thanks!
 


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Ravenna
#2
I've always been a firm believer in running whatever gear is as close to a 1:1 ratio as possible. In our cars that's 4th gear. If you dyno in 3rd you actually have a mechanical advantage that will cause the dyno to read higher. I see people all the time running in 3rd and I never understand why. 4th will show the most accurate numbers.
 


Truth in Ruin

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#3
I've always been a firm believer in running whatever gear is as close to a 1:1 ratio as possible. In our cars that's 4th gear. If you dyno in 3rd you actually have a mechanical advantage that will cause the dyno to read higher. I see people all the time running in 3rd and I never understand why. 4th will show the most accurate numbers.
^^^Truth.

I suspect that people use 3rd for two reasons:
1) less stress on the drivetrain.
2) the possibility of bigger numbers.
 


dyn085

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#5
Fourth is the closest to 1:1 and will yield higher numbers, but I don't think the added stress to the drivetrain is worth the 3-5% unless you're strictly shooting for dyno brags. The amount of time to spin up to full rpm and overall speeds are significantly higher in fourth, which ultimately is a significant heat difference. I think that you'll find that most tuners agree, but whichever you choose you just want to ensure you choose it again in the future if you're ever measuring delta due to a modification.
 


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Ravenna
#6
Fourth is the closest to 1:1 and will yield higher numbers, but I don't think the added stress to the drivetrain is worth the 3-5% unless you're strictly shooting for dyno brags. The amount of time to spin up to full rpm and overall speeds are significantly higher in fourth, which ultimately is a significant heat difference. I think that you'll find that most tuners agree, but whichever you choose you just want to ensure you choose it again in the future if you're ever measuring delta due to a modification.
I guess I'm confused here. How would 4th read higher numbers? Running in 3rd would give you a mechanical torque advantage. Since dynos read torque and calculate horsepower based off that, your numbers would be higher in 3rd. I would think more than 3-5% especially. Unless I'm completely wrong here, but every dyno sheet I've seen where someone runs in 3rd is always higher than a consecutive pull in 4th.
 


shouldbeasy

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#7
^^^Truth.

I suspect that people use 3rd for two reasons:
1) less stress on the drivetrain.
2) the possibility of bigger numbers.
I'm gonna go with the top reason being top speed - even with our short gearing I think I'm in the 180kmph plus range when I do a 4th gear pull. Closed course of course... At that speed you're covering some serious ground and not everyone has that open road to do it on.
 


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#8
I'm gonna go with the top reason being top speed - even with our short gearing I think I'm in the 180kmph plus range when I do a 4th gear pull. Closed course of course... At that speed you're covering some serious ground and not everyone has that open road to do it on.
I think, in my case anyway, we're talking about being on an actual dyno. Speed is irrelevant at that point. That's why I'm so confused as to why anyone would dyno in a gear that isn't as close to 1:1 as possible. That's the most accurate numbers.
 


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#9
Jeff is going on an actual dyno, not doing a v-dyno run.
As most have said on here, you should be using whatever gear is as close to 1:1 as possible and people say that's 4th.
I've never known a dyno operator ever do a pull in anything other than fourth but I do know a few people who run 1:1 fifth gears in their rally cars so maybe they do?
 


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#10
4th gear will give more realistic load and boost conditions. Every dyno is different though, our 4th gear braked run pulls can last up to 22 seconds, this isn't ideal for showing big numbers but does show how well the car copes under pressure.
 


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#11
I have never dynoed a car, but doesn't the operator have to enter in a bunch of parameters specific to the car in order to get an accurate reading? Presumably, the gear or gear ratio would be one of them, no? Unless the software just defaults to a 1:1 ratio, in which case using 4th would seem to be most accurate.
 




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