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Discussion Starter #1
I made my first dyno pull today in 3rd gear 1.36 ratio (toploader 4 speed). Obviously 4th gear is 1:1. What affect does this have on the horsepower results if any?

I used third because i have a 3.0 rearend.
 

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Results depend on the type of dyno and software. Some are reactive dynos, and others are inertial, and every company has it's own way of calculating. Effectively, it doesn't make much difference overall as lower gearing accelerates an [example] inertial dyno faster, but to a lower speed, giving a calculation result similar to a higher gear accelerating slower but to a higher speed. However, I won't go into that as it doesn't matter for what a chassis dyno is for. Chassis dynos are NOT for measuring horsepower, and should never be used for that - even though that's the most popular use.

Chassis dynos are for tuning with more chassis variables than an engine dyno. It would be much better if the output was in letters or Greek, as it would be just as useful as HP numbers for the intended purpose. For example, if you did a pull and got "J" power at one RPM, then made an adjustment and got "K" power at the same RPM, that's tuning. More power relative to other pulls, under the same conditions on the same dyno is the point. Raise your tire pressure or align your U-joints and make 20 hp more. It's all relative.

So, my most useful suggestion is to ignore the gear, but always use the same gear so you get relative results. I prefer higher gears only because they give a longer and more readable pull on an inertial dyno, or less jitter on a load cell. Otherwise I don't care. Just always do it the same, ignore the HP, and tune to increase the average numbers across the useful RPM range. Hope that helps.

David
 

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It wasn't asked but a stick car is normally dyno'ed in a lesser gear (3rd in this case) to help prevent damage to the clutch (and possibly other driveline parts) which can occur if the motor was directly driven. This is especially true with sticky tires on the rear.
 

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Discussion Starter #4
I appreciate the replies. I will stick with 3rd upon making changes. Because of the 3.0 rearend - they explained that 150 mph speeds on the dyno is excessive and dangerous if something came apart.

now i need to figure out why my HP number was so low. Thatll be another thread.

Thanks for help
 

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It wasn't asked but a stick car is normally dyno'ed in a lesser gear (3rd in this case) to help prevent damage to the clutch (and possibly other driveline parts) which can occur if the motor was directly driven. This is especially true with sticky tires on the rear.
Does your dyno guy have you start rolling in the pull gear? I never do that, and lightly run through the gears just above engine-lug RPM to the pull gear, stabilize RPM, and punch it. The TQ and HP to the trans is identical in any gear, so there should be no difference in load to the clutch - unless you start rolling in a higher gear. Is that it?

David
 

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Discussion Starter #6
If i understand you correctly... yes... he runs 1st thru 3rd.. once in third cruises up to 3000 rpm then punches it
 

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Does your dyno guy have you start rolling in the pull gear? I never do that, and lightly run through the gears just above engine-lug RPM to the pull gear, stabilize RPM, and punch it. The TQ and HP to the trans is identical in any gear, so there should be no difference in load to the clutch - unless you start rolling in a higher gear. Is that it?

David
My bad I called the dyno guy and the dyno used was a Mustang with a load cell which can use any gear for pulls. There was a concern about slipping the sinterned iron clutch disc with a low "max" base pressure and so the car was up shifted and the pulls were done in 3rd gear.

I would suppose that a Dynojet would give the most accurate readings in direct drive 1:1 mode.
 
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