I was wondering why the RWHP and the RWTQ always cross each other at 5250 RPM's. I noticed this on alot of dyno sheets and was wondering WHY ???

 

It has to do with the formula for calculating horsepower. Horsepower equals torque times speed divided by 5252. This is true only if the torque is in pound-feet and the speed is in revs per minute.

 

http://vettenet.org/torquehp.html

 

The equation is HP=(T*RPM)/5252
or it can be rewritten as HP=T*(RPM/5252)
so when RPM=5252, the equation becomes HP=T*(5252/5252)
Since (5252/5252)=1, the equation becomes HP=T, but this is only true for RPM=5252. The two numbers have to be equal at 5252 RPM as dictated by the equation or it is a bogus dyno graph.

The 5252 constant comes from the conversion of the units of T*RPM to HP. If we have 400 ft-lb of torque at 5000 RPM, running through the equation gives us 380.8 HP. Without the equation, we have the power (not the unit horsepower) of the engine as P=T*RPM
or P=400 ft-lb*5000 RPM
so P=2000000 ft-lb-rev/min, since there are 60 seconds in a minute
P=2000000 ft-lb-rev/min(1 min/60 sec), the min terms cancel giving
P=33333.333 ft-lb-rev/sec, now to convert from rotational motion to linear motion, there are 2*pi (or 2*3.1416=6.2832) units in one revolution
so P=33333.33 ft-lb-rev/sec(6.2832/rev), the rev terms cancel giving
P=209439.5 ft-lb/sec, finally, 1 HP=550 ft-lb/sec or 1=sec-HP/550 ft-lb
so P=209439.5 ft-lb/sec(sec-HP/550 ft-lb) the sec, ft, and lb terms cancel
This leaves P=380.8 HP which checks
Written out, the conversion is (1 min/60 sec)(6.2832/rev)(sec-HP/550 ft-lb) or (1*6.2832)/(60*550)(min-sec-HP)/(sec-rev) or 1/5252.1131 (min-sec-HP)/(sec-rev). As you can see, it's not exactly 5252.

 

Who are you,Rainman!

 

Along those lines I read that there is a average 10 hp/tq loss from changing from 3.42 gears to 4.10. I dont understand how the dyno
graphs a loss when actual torque is actually more ?

 

First of all, you really didn't get an increase in torque, just an increase in acceleration rate. Secondly you're losing top end which you may not use on the street but the dyno likes. Especially a dyno that measures inertial loads. Against popular belief your car may be faster at lower rpm which is fine on the street but your most likely running outside of the stock cam's power range because of turning higher rpm with the gears and the dyno needs that higher rpm acceleration to graph an accurate result when on an inertia machine which is 95% of dynos in use. So in short your car is most likey faster on the street without an increase in engine output but on the dyno it's going to show less because now you've changed the final gear ratio and gave away some top end. Hope that helps.

Ed

 

Kind of yes and no. My car peaks at 6300 rpm and the dyno test is to 6500rpm. No loss of top end here in 4th gear 1:! 6300 rpm puts me at about 128 mph. Its a little hard to distinguish the difference between acceleration and torque when I gained 0 HP and it feels like I gained 50 HP. ??

 

I haven't seen an LS2 yet with the stock cam that made power to 6300 rpm. You had to lose some top end acceleration going to 4.10's, it's impossible not to. I'm sure you're losing numbers on the dyno because most dynos measure inertial data and going to high with the rpm hinders the acceleration of the drum if the car isn't making power at that rpm. Unless the dyno has an absorber and a strain gauge it's measuring rotational speed of the drum, going past peak hp does nothing but give away a few numbers. I know it feels like the car gained a ton of hp but what you feel is the gears accelerating the car at a faster rate. Thats why Nascar puts higher gears in the car at short tracks, so that they can accelerate out of the turns, they don't need the top end speed. At superspeedways they put lower gears in the cars because they don't need the acceleration they need the top end. Same motor same power, maybe a little different cam for the difference in rpm range but it's still 800 hp either way. The LS2 cam stops making power at around 5600 maybe a little more or a little less but you can't change that with gears.

Ed

 

Im still making power at 6300 rpm 408 but looking at the dynograph
that peaks Torque at 412 @ 4800 rpm I can see the torque dropping off dramatically after 5200 rpm. That might explain where the 4.10
gears just make it happen that much faster. It runs now about like
a c6 zo6 stocker with the GEARS & the FAST

 

So that means that at 5250 your making close to your peak hp because hp and tq cross right there so 412 tq at 4800 where is it at around 5200 if it's at 408 than that is where you're making you peak hp also you're just holding it there for 1100 rpm with no gain. I'd try another dyno, it seems a little skewed by my standards. If your hp and tq don't cross at 5250 I'd question it. And if it does than your not making peak power at 6300, you're just beating your car up to run it up that high. The cam is the cam, the FAST, the gears, the thermostat, whatever isn't going to change the motors rpm range. If you want to do that get some new spings and rocker arms and you might be able to run it up higher. It might be time to try a new dyno.

Ed

 

NO NO what im saying is at 5200 rpm the torque is already falling from its peak (412) at 4800. The HP is still rising and is at 390 at 5200 rpm and rising to 408 at 6200 rmp and starts falling off at 6300 rpm. The tq & HP are at 390 at 5200 rpm. The dynograph is good and have their crossover points at 390 each. Im not trying to run it higher , its just that it gets there quick w 4.10. Another interesting thing is the rev limiter is set to 6500 and chuck COW said it should be set to 7000 because TM starts to kick in 500 rpm before the limit so there may be a
little more for me . Leon

 

So from 5250 to 6200 you gained 18 hp. I'm not saying your car isn't making that power, but I don't care if Chuck says it or not you shouldn't be running the motor that high with the stock valve train components. If you do you're asking for a failure. What's the hp at say 5800 where stock LS2 cams run out of gas. There is no reason in the world to run it up to 7000 at all. Believe me there is no legitimate horsepower left in your car at that high of an RPM on a accurate dyno. I wish it were not that way but the stock parts are the stock parts and just don't make power up that high.

Ed

 

You're correct that torque at the wheels is more when comparing let's say 4th gear with 3.42s vs 4.10s but torque/hp at the flywheel hasn't changed. Contrary to post #7 above, "gears" have no affect on low end or top end HP of the engine. In fact, there is so much wrong with post #7, I would ignore it. With more torque at the wheel in 4th with the 4.10s on an inertia type dyno, acceleration is quicker meaning the length of time to complete the dyno run is shorter when using the same RPM range...let's say 2500-6500 RPM for your car. This is a post of a calculation I did last year to determine how much HP was used to overcome inertia to accelerate the rear wheels/tires on a Z06:
http://forums.corvetteforum.com/show...&postcount=421
You can see the length of the dyno run was used to calculate HP, so it's intuitive that a shorter dyno run would increase the HP required to overcome inertia of all rotating components from the harmonic dampener to the tires and everything in between. Most of the "indicated" HP loss with 4.10s on an inertia type dyno goes to overcome inertia in the rotating engine/drivetrain components.

There's another part of the HP loss that's a real loss and that has to do with the gears themselves. When going from 3.42s to 4.10s on the same diameter ring gear, the ring gear has to gain teeth. The only way to do that is to increase the pitch/angle of the gear teeth. The rear end gears are called hypoid gears as opposed to spur, helical, worm, etc. Spur gears are the most efficient with a good quality involute spur gear having pure rolling motion at the pitch line giving less than 1% HP loss. Worm gears are the worst with pure sliding action defining the motion between the two gear teeth and very high HP loss associated with low efficiency. Hypoid gears have a combination of rolling and sliding and are in between spur gears and worm gears in efficiency. As the pitch increases, sliding action increases resulting in less efficiency. It's not a lot but it represents a real loss on the road vs the "measured" loss due to inertia on the dyno.

If you were to run your car with 4.10s against one just like it except for 3.42s in a roll race from 45 MPH to let's say 130 MPH, the two cars would be dead even at the end...or very close to it. The 4.10 car will be in 2nd gear while the 3.42 car is still in 1st meaning the 3.42 car has the best overall gearing and will jump out to a lead. But when the 3.42 car shifts to 2nd, the 4.10 car now has the best overall gearing and begins to pull back ahead until it has to make the 2-3 shift and the advantage goes back to the 3.42 car. The two cars see-saw back and forth to 130 MPH where they should be about even because the average HP put to the road is about equal. The advantage to the 4.10s is better acceleration from a dead stop because it multiplies torque at the lower RPM and allows the engine to get up on the HP curve sooner. 1st gear is the only gear where you're very low in RPM and substantially off the HP curve...all of the other gears drop to 4400 RPM or more after the shift making the average HP put to the ground the same. The 4.10s increase the average HP put to the ground in a 1/4 mile run by getting up on the HP curve quicker in 1st gear and running out close to redline in 4th at the end to capture almost all of the area under the HP curve in that gear. While 4.10s help in the 1/4 mile to run with stock Z06s, don't do a roll race against one from 45+ MPH or you'll get walked badly.

 

I didn't say that gears have an effect on top end power of a motor???? I understand that the motor makes the same power regardless of the final gearing, but with the 4.10s you're going to run out of RPM at a lower speed than you will with 3.42s.

About the dyno, an inertia machines formula is based off of rate of acceleration, but it's not the only determing factor. Believe me the dyno takes gearing into account whether it's making plain to see or not. It knows if the car is turning the drum at 70mph @ 2200 rpm with the 3.42 or 55mph @2200 rpm with the 4.10s. So yes you're going to accelerate faster from whatever mph with 4.10's but you're not going hit the same top speed either. If you are losing top end speed because of the gears, you're losing power on the dyno. You're not getting curve necessary to give you an accurate hp reading. I don't really care to get into an argument with you, my dyno has an absorber and a strain gauge and the gearing is out in the open and there is no fudge factor, so I don't have to worry about things like that. You've got what you got 4.10 gearing or 3.42 gearing I'll get the same number and that's the way I want it. You can't put a fast accelerating 2 stroke dirtbike on the dyno and run it and come up with 400 hp. On the street though that bike might be faster from 0-30. Just because it can turn the drum fast for a short time doesn't equal a lot of hp.

It doesn't matter though I'm more interested in how his stock LS2 cam makes power up to 6300 rpm.

Ed

 

Great thread! Thanks to both glass slipper and bad427 for your input. Both your posts are helping to increase my understanding of the subject...your time and expertise are appeciated.

 

The stock ls2 cam in certain combinations can make power increasing
to 6500 rpm. I dont know where you got the idea that a stock cam
will stop making power at 5700 rpm. Most that do make the 400+ are boltons with ported FAST, TB , LGLT's and of course the tuner involved.
on a bone stocker I can believe it. BTW the dirtbike on the dyno is not a very good analogy.

 

Ok, if you think it's going to make power up there, I'm going to concede here before this turns violent. I just know what I know about LS2's, their cam and their spring combination and where the stock combination makes true power. If you think a ported fast is going to do it though more power to you.

Ed

 

Chevy claims:

400Hp @ 6000rpms
400Ft @ 4400rpms

 

So what does the lower speed with 4.10s have to do with anything??? The engine still runs through the same rev range

An inertia chassis dyno uses the weight of the dyno drum and the rate of acceleration of the dyno drum to determine HP and that's it. The dyno doesn't know/care what the gear ratio is...all it needs to know is the RPMs of the engine so it can calculate torque and plot the HP/torque curves. Other chassis dynos need gearing and tire info to calculate HP after measuring torque via a strain gauge. And again, the lower speed doesn't mean anything.

Your first sentence here contradicts your very first sentence of this post. Your second sentence not only doesn't make technical sense, it doesn't make grammatical sense.

You'll get a higher number with 4.10s because the absorber holds engine acceleration constant no matter what your gearing is meaning the lower wheel RPM correlates to less kinetic energy added meaning less HP goes to overcome inertia of the rotating parts after the differential. With the engine making the same HP, the chassis dyno reading increases. It doesn't matter which way you go as far as the type of chassis dyno, there is going to be a difference in rotating inertia either before or after the differential/gears and it will have an affect on RWHP readings. It may be the extra friction from the higher pitch of the gears is offsetting the gains in inertia and you end up with the same number but I doubt it. Within the realm of +/- dyno error, it may be the "same".

A fast accelerating 2 stroke dirt bike accelerates fast because the bike and rider weigh all of let's say 350 LBs and the bike has let's say 50 HP for a 250 cc bike. If you think you're going to put that same bike on a car chassis dyno and turn a 5400 LB two foot diameter drum fast, you clearly don't understand how an inertia dyno works. That inertia dyno doesn't care where the HP is coming from and a 50 HP bike will take a long time to accelerate that big dyno drum...there's a reason bike chassis dynos have 450-650 LB 18" drums. The bike is fast on the street/track because it's light...bike inertia chassis dynos have lighter drums because there's not as much HP available from the bike to accelerate the drum. In other words, the size of the drum determines the load placed on the engine...the heavier/larger diameter drum has a higher inertia for the engine to overcome requiring much more HP to spin it up properly so the inertia loads of the engine rotating parts don't get completely neglected. An overly long dyno run makes for an essentially steady state HP reading...not a good thing when an engine's job is to accelerate the car and itself. Here's an excerpt from my post a year ago I linked to above:

Work is done ON the wheel/tire by the engine to increase their KE with the amount of work done (KE added) given by the KE (after) minus the KE (before). KE=1/2Iw^2 (where w is the angular velocity) from the link I gave above and presented here again. http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html
The rate at which work is being done on the wheel/tire is equal to the rate at which the KE is increasing. HP is the rate at which work is being done so we're left with finding the KE added and dividing by the time it took to add the KE to the wheel/tire to find the average HP required to accelerate it from one RPM to another. (This is from my old Physics book which I tried to scan in but my scanner stopped working.)
We'll use more realistic numbers for the C6Z06:
Tires- 36 LBs @ 24" average diameter
Wheels- 20 LBs @ 15" average diameter
I for the tire is I=(16.33 KG)(.3048m)^2=1.517 kg-m^2
I for the wheel is I=(9.07 KG)(.1905m)^2=.3292 kg-m^2
Tire+wheel I=1.517+.3292=1.846 kg-m^2
We'll use the video from the following link:
http://www.livernoismotorsports.com/....phtml?t=video
The 15th one down is a stock LMB C6Z06 with the engine running from ~1700 RPM to ~6900 RPM during the run which took 18 seconds total.
1700 RPM @ the crank=(1700/3.42/60)(2)(pi)=52.05 rad/sec at the wheel.
KE (before)=1/2(1.846)(52.05)^2=2501 Joules
6900 RPM @ the crank=(6900/3.42/60)(2)(pi)=211.3 rad/sec at the wheel.
KE (after)=1/2(1.846)(211.3)^2=41201 Joules
KE (after)-KE (before)=41201-2501=38700 Joules
1 Joule=.738 ft-lb
38700(.738)=28560.6 ft-lb which is the total work done to increase the RPM from 1700 to 6900.
To get the rate at which work was done, divide by the 18 sec dyno run giving: 28560.6 ft-lb/18 sec= 1586.7 ft-lb/sec
1 HP=550 ft-lb/sec, so we have 1586.7/550=2.885 HP for one tire/wheel or 5.77 HP for both tires/wheels.

If you replace the wheel/tire in the example above with a drum from an inertia dyno, it's basically the same thing. The engine is doing work on the drum by adding kinetic energy (work/kinetic energy in FT-LB, not to be confused with torque which is LB-FT) to it. The time required to add the kinetic energy is the rate the engine is doing work (FT-LB/sec) which is the definition of HP. The lower "top speed" with 4.10s has absolutely nothing to do with the rate at which kinetic energy is added to the dyno drum. You've got a good dog in the race but I think he's just barking up the wrong tree.

As far as the LS2 making HP above 6000 RPM with the stock cam, you do realize you can shift the peak HP RPM several hundred RPM with bolt ons such as intake and exhaust that "tunes" the engine either up or down in the RPM range. Kind of like the difference between a dual-plane and single-plane intake manifold on a carbureted engine. Or a long tube/small primary diameter header vs a short tube/large primary diameter header. The cam will indeed set the RPM range but the intake/exhaust tuning determines the exact RPM within that range.

Again, this is just a discussion.