hypothetic dyno hp question

[1320quick]

Ok,so a group of us gearheads were drinking beer the other night and a question came up about what the effects of a gear ratio change would have on rear wheel horsepower numbers seen on a dyno. For example, If you had a car with a sbc, 4-speed and a 3.55 rearend ratio and you ran it on the wheel dyno. Lets say it made a max of 250hp at the rear wheels. Then you take the same exact car, change the rear-end gear to a 4.56 and run it again on the dyno with NO other changes. Would it make the same exact 250hp as it did before? Some said it would make the same hp and others said it would make more. So tell me guys, who is right?

[sc racing]

It would reach peak HP sooner but the HP would'nt change.

[sunsation540]

would peak at the same time but the mech advantage of the lower gear would show an increase it would be small but it would be an increase ... same if you change the converter it is also a torque multiplyer small changes prol less than 5% but it's a mech advantage.

[96Mustang460cid]

I remember an article in Car Craft or Hot Rod Magazine that done exactly this. I think they found a ~5 hp difference, but I don't remember which direction the difference went.

I would think the 3.55 gear would make more hp due to less frictional losses. I don't know if the losses would be measurable, but the 4.56 would have more tooth contact due to it's higher number of teeth { ===> more sliding in the ring and pinion}.

Have a good day!
Michael

[doctor408]

why do all dynos hp and tq graph cross each other at 5225 rpm's??????

[sunsation540]

that is the math at work rember hp is derived by a multiplyer of torque and for the math to work 5250 is the magic number where they cross,

[cashier]

if i remember right they input the rear gear ratio and how tall the tires are into the computer before the run. so there's no differents between them.