I understand Spencers point, but the MPH on the Dyno read-out negates his point.


-Rogue

 

I voted a modest 350-400 for my upcoming 3.1L. I don't plan to run outrageous boost or anything like that.

 

I don't see a MPH...or is that the idea?
wouldnt that only be like 15lbs on that size engine?

 

Horizontal axis...

 

the graph could still be wheel T or HP (uncorrected for gearing) vs. MPH (in 5th gear?)
overall ratio is still ~2.8 (or so), therefore if gearing is not factored in the T ~ 820/2.8 ~ 285 lb-ft

 

LOL, you guys are way over-thinking this.
Its a dynojet - big heavy roller. Isn't going to matter much which gear you are in...
The 'uncorrected / corrected' is simply for atmospheric conditions.


-Rogue

 

A dyno should be run in as close to a 1:1 ratio as you can get. Thus the reason for making your dyno run in 4th or 5th gear.

 

Yes, but it really doesn't make that huge of a difference.

 

If you are 1st or 2nd gear compared to 4th or 5th, the drive train has a significant mechanical advantage. That is why you have gearing.

 

Yes I know this.
The dynometer knows this as well. Go dyno in 2nd gear, then do it again in fourth (back to back without changing anything). I bet peak HP will be within 5% of each other.

 

i would imagine youd have to tell the computer what the gears were so it could give a good number.

 

no

 

generally yes...
depends on the dyno though...

some measure axle torque, which must be converted to crank by dividing by gear ratio...

some measure the time it takes to accelerate a mass (the drum)...
then using F = ma and a = dv/dt or the change in speed over time, F will be in lbs and coverted to T based on the machines geometry...

this again gives you axle T, NOT crank...
the driving force in any case is thrust a force in lbs...
assume a car makes 200 lb-ft at the crank and tires OD = 2', r = 1'
and it's overall ratio ~ tranny x diff ~ 5
Torque at axle = 1000 lb-ft
thrust or motive force 1000 lb-ft/1 ft ~ 1000 lbs

it's a trick some dyno operators use, put the wrong ratios in and manipulate the numbers...

older dynos actually turn a generator and make power (watts), which 1 HP ~ 746W

some turn a pump and make pressure which is measured and converted to force/torque...

but they all measure the force of the engine multiplied by the gearing (unless gearing is 1:1 overall), so the gearing must be factored back out
all except an engine stand type dyno

 

The sheet was a printout from a dynojet, not a load-type dyno.
Meh - do the math if you want, but I've done back-to-back pulls in different gears on a dynojet, and know from experience that it really doesn't effect peak hp.

 

Rogue is correct. The 3rd and 4th gear runs will come out within a few hp of each other (for example, on my car approx. 300rwhp, 3rd or 4th came to within 10hp of each other). I use to have a mustang 500SE dyno at the shop.

As far as making hp numbers, all you need to do on a mustang to make high (or low) numbers is change the weight of the dyno in the parameter settings. Takes 3 seconds. Boom 600hp out of a stock 2.5L NA Instant bragging rights.