Calculating Engine HP from Rear Wheel Figures

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70DusterBob

70DusterBob

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I recently made a modification that got me 27 hp in third at 2800RPM, which someone told me is x 2.45 for first gear, which is 66 hp. I got a 50 ft lb in torque at 2800RPM also increase in third, so in first that would be 122 ft lbs. What would the engine hp and torque ft lb increases be with those numbers? Is there a way to be able to calculate it? That was a 20% increase in torque and hp for my set up, w/a 360.

I am running 3:55 gears in the rear, but have very tall 15 rubber on 15 in rims. When the speedometer was working, it was reading close to if not perfectly accurate. I assume, since it was born with a 225/904 w/14" rims, that what ever the gearing would be for that set up would be the same as what it in it now. I'm guessing they came with different gearing in the rear, how could I find out what the original gearing was if I needed to, the VIN?

I can get the tire size off the tires, but don't have it handy. 727 trans.

Thanks
 
I think, you're giving too much credit to dyno results. It's only a tool, not scientific fact. If you figure drivetrain loss at 18% for 4sp, and 20% for automatic, you'll be fairly close to crankshaft output. Multiplying by gearing and trans ratios still doesn't take everything into account, so I've always just use those 18/20 numbers.
If you show more power, that's great. If you feel that power, and it's proven out at the track (where fact can be calculated) then you're golden.
 
I think your right about the dyno being a testing tool, not a scientific fact. I do need to get to the track and try it out. Problem is the track is a good 80 miles one way. I will save up my pennies and see if I can get out there.

The Duster puts me in my seat "much" harder than before. If you ask me it is more than the 20% increase, but I gotta get it on the track like you said... But it does test before and after fairly accurately, under the same circumstances (on the dyno) there was a 20% increase, on the track it may be more or less, but I can't prove it is more or less until I get it out there...

However, how do you calculate an increase in HP and/or Torque from a difference in ET's? I gotta get the car weighed for one thing, right?

Thanks
 
The best feeling is the stronger pull. that's the "butt dyno".
Yes - car weight and mph can be used (online claculators, or the formula'a online too). Race weight is important so the car weighed with you in it and the same amount fo fuel if possible.
 
70DusterBob said:
I recently made a modification that got me 27 hp in third at 2800RPM, which someone told me is x 2.45 for first gear, which is 66 hp. I got a 50 ft lb in torque at 2800RPM also increase in third, so in first that would be 122 ft lbs.
Click to expand...

Your engine will always have the same 27hp increase. The effective torque to the rear tires can be different via gearing, but since the gearing is steeper, it will travel through the power band quicker, making you arrive at a higher gear sooner. Eventually you'll have to shift, nullifying the gear ratio multiplier.

As for flywheel to rear wheel horsepower, 20% seems close from what I've seen. Maybe 25-30% if you have a big converter (4,000rpm+ stall).
 
Thanks ya'll. I do need to git 'er out on the track... Then I can see what the mods "really" did/do.

Thanks for the link to the calculator...

I will try to post the results. But it may be a while.
 
All the figgerin in the world aint as good as your butt feels when you stab the gas. I wouldnt worry about it as long as it feels good.
 
RustyRatRod said:
All the figgerin in the world aint as good as your butt feels when you stab the gas. I wouldnt worry about it as long as it feels good.
Click to expand...

Hey Rusty,

This is a homemade modification, not something I bought. I need to know what it did so maybe I can improve it or at least find out what I did and how it affected the HP/Torque.

It "feels" like more than a 122 ft lb increase in torque, and wayyyy more than a 66 HP increase, but I won't know until I get it on the track and do an overall, 0-100 mph calculation of ET.

Your right though, it does put me in my seat alright. A LOT more than it did.

Thanks...
 
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