Dynoed my engine yesterday

[Bruce Dorsi]

RPM TQ HP

5200 445.6 441.5
5400 431.2 443.5

And you all know I can not resist asking silly questions, so not to disappoint you here it is.  

What would have happened at 5252 and why?

=================================
It DID happen, Peter.....Didn't it??  

[1FATGMC]

Out of curiosity (not criticism),  to what do you attribute the relatively-large drop off in hp and torque between 5700 rpm and 6000 rpm?  .....Could that be attributed to a too-small carb, the cam profile, or ?

Also, how did you arrive at the dynamic compression number?  ....Is it calculated, or from a computer simulation?  

I'll be interested in his answer also, but Comp gives a 2000-6000 rpm operating range for that cam.  My guess is that with a 350 vs. his 383 it might have made hp a little higher as the 383 needs more air/fuel and that cam and heads are probably restricting the 383 a little above 5700.  Now in a 350 it probably would have made power a little higer, but still no where near the power he is making with the 383.  He has a very impressive motor.

You can download a DCR (Dynamic Compression Ratio) program HERE.

I lucked out on the static and dynamic ratios on my motor and can run 86/87 oct. gas with no problems.  I picked a cam and the heads without knowing at the time anything about DCR.  Now I would never build a motor without checking what the final DCR is going to be.  Since I put a lot of miles on a motor I'll give up some HP for cheaper gas and gas mileage.  My current DCR is about 7.5 :1 and I have no problems even pulling the trailer.  Like Rumrumm said his is on the high side for even high octane, but those AFR heads are really good and he will probably be fine.

c ya, Sum

[Bruce Dorsi]

Thanks, OZRod and Sumner, for the DCR link.  ....I learned a lot there!  


BTW, Sum, thanks for the GREAT write-up of Hooley's exploits with the Stude.  .....All involved should be rightfully proud!
:b-d:

[enjenjo]

RPM TQ HP

5200 445.6 441.5
5400 431.2 443.5

And you all know I can not resist asking silly questions, so not to disappoint you here it is.  

What would have happened at 5252 and why?

So anyone have the answer?

[1FATGMC]

RPM TQ HP

5200 445.6 441.5
5400 431.2 443.5

And you all know I can not resist asking silly questions, so not to disappoint you here it is.  

What would have happened at 5252 and why?

So anyone have the answer?

The torque and HP would have been the same number since:

Horsepower = Torque X RPM divided by 5252

so at 5252 rpm when you divide that by 5252 you get 1 thus Horsepower = Torque.

c ya, Sum

207 mph with Hooley

[Ohio Blue Tip]

HP and torque would be the same at 5252 RPM.
5252 is the constant used on most dynos to figure HP and Torque.  It's just easer than trying to figure radians.  The 5252 constant is used in most Engineering environments.  

[PeterR]

The torque and HP would have been the same number since:

Horsepower = Torque X RPM divided by 5252

so at 5252 rpm when you divide that by 5252 you get 1 thus Horsepower = Torque.

c ya, Sum

Correct!   Now the reason why.

Power = (the rate of doing work) = (work done)/(time taken)

But work =force x distance

So, power = (force x distance)/time

and in the imperial system the unit of power is generally the horsepower,
where 1 HP = 550 ftlb per second.

HP = (force x distance)/[(time in seconds) x 550]  

Now this real easy to calculate for straight line work, just multiply the distance the force moves through, divide by the time taken, divide by 550 and there is the answer.  Calculations involving rotation are not so obvious, so I use an illustration with straight line and rotary power at the same time.

Imagine for a moment a crane lifting a heavy object at an even speed.    

F = the cable force in pound
D = the distance the object is lifted in feet
t = the time take to lift it in seconds

HP = (force x distance)/[(time in seconds) x 550]
= (F x D)/(t x 550)

Now look at the winch drum which has a radius R
Distance cable is pulled = (circumference of drum) x (number of times it turns)      

Circumference of drum = 2 x Pi x (drum radius)  = 2 x Pi x R
Number of turns in t seconds  = RPM x t/60

So D = (2 x Pi x R) x (RPM x t/60)

Now replace D in the power equation

Power = F x (2 x Pi x R) x (RPM x t/60)/(t x 550)
The t cancels out and the 60 can be pushed to bottom line,

Power = 2 x pi x F x R x RPM/(60 x 550)

Now, F x R is force by radius which is torque,

Power = 2 x Pi x torque x RPM/(60 x 550) = (torque x RPM)/5252

As Sum indicated above, when RPM =5252,
HP = (torque x 5252)/5252 = torque
Similarly at 2626 RPM the power is half the torque.

Now why bother with all this?

First it demonstrates that any rotary power source whether it be a mouse on a treadmill or a Cat D9, the power is always (torque x RPM)/5252.

This is worth saying in words, because it then highlights something else.

The power at any point of the curve is equal to the product of the torque at that point by the RPM at that point.  

THE POWER CURVE IS DETERMINED TOTALLY BY THE TORQUE CURVE.

[rumrumm]

Sorry, I have been out of town and have not gotten back to you. I really can't explain the difference. I only got a read out on one other pull as their computer went down. The other pull was 463 ft. lbs. of torque at 4300 and 439 hp at 5700--ten hp less than the best pull. But that was an earlier pull and they told me the more an engine is run on the dyno and broken in, the better the figures tend to be. The other pull did have the same drop off between those numbers--from 439 hp to 429. I think the cam simply creates peak power at 5700 and then falls off. This is the first time I have had an engine dynoed so I can't give you a good answer. Perhaps tweeking the carburetor more would make some difference. With a shorter stroke 350, peak power would be at a higher rpm and the drop off may not occur until after 6000. My desktop dyno was very close--only 7 hp off (457 hp at 5800). Hopefully, when the car is done, I will put it on a chassis dyno and retune the carburetor. It should be interesting to see what the figures are.