mark kibort

We have been over all the equations, formulas, theories, emperical data, and misconceptions.

The purpose of the dicussion was to be able to understand Hp and torque so that decisions regarding performance changes could be made using the proper information. (whether we are talking about gear ratio changes, cams, changing torque or HP curve shapes, etc)

As was stated, HP is equivilant to Torque, multiplied through the gears at the wheels, at any vehicle speed. (ie Acceleration = Power / (mass x velocity)

You want to know what curve on a dyno graph is most useful, try this quick test.

THE TEST:
Tell us, using the HP /Torque curve below, where the shifts points would be, from 3rd to 4th gear, knowing that the gear spacing from 3rd to 4th is 74-75% of an rpm drop (ie 4.24:1 3rd to a 3.2:1 4th gear) to provide the greatest acceleration for your car. (edit, thanks Glen)

see what curve gives the answer the quickest. Did you use the torque curve, or did you use the HP curve? show your work!

TOP two correct responses win the highlight video CD of the last 4 years racing seasons video clips.
The answer will sound something like. "For max acceleration your shift point will be at X,XXX rpm based on this engines HP/torque curve and gear box spacing because........" , "I calculated this by ....................."
"the HP curve - the torque curve (select one) was the easiest to use to figure this out because.................

Mk

acceleration=power/(mass x velocity). as in the words of glen L. "ts the LAW! "

GlenL

No fair, Mark!

It's Sunday and I don't have Excel at home!

The answer is: shift at 5333 RPM

The easy way is: Look at Torque (But of course!)

The reason is that it was easier to mathematically model the torque curve than the power curve. (Where'd you get that crappy torque profile, anyways?)

Here's the work:

Model the torque curve as a straight line running from the torque peak to the max RPM shown. That'd be a line from (4200RPM, 230ft*lbs) to (6500RPM, 125ft*lbs) Those who can do the algebra will confirm that this line is Torque = RPM * (-0.0456) + 421.4

This step involves two assumptions:
1) The torque curve over the region is relatively straight.
2) The answer will be found in the linear region

Then find the shift point by knowing that what's important is the torque on the rear axle. As the ratio of the shift is given as 0.74, the 3rd gear torque will be effectively higher by 1.0/0.74 or a factor of 1.35. Also, the initial fourth gear RPM will be 0.74 time the 3rd gear RPM. So, we need to find the RPM where the RPM wich is 74% lower also has 35% higher torque. The actual ratios and the final drive are not needed for this analysis.

That is:

T3 = (RPM * ( -0.0456 ) + 421.4) / 0.74
T4 = (RPM * 0.74) * (-0.0456) + 421.4

We want T3 = T4. This happens when the RPM is 5333 RPM. That should be the shift point. The fourth gear is started at 3922 RPM.

Depending on the nature of the data and analysis techniques available this could be done in power as well. With that linear torque curve, it was easier. And don't forget, Power versus RPM really doesn't mean anything, it's power versus time that has value and the dyno graph doesn't show that.

(I'll PM you my address.)

(Bonus: 5333 is a good number, but who sees my error?)

John Welch

For max acceleration your shift point will be at 5,893 rpm based on this engines HP/torque curve and gear box spacing because........" , "I calculated this by the HP curve which was the easiest to use to figure this out because, torque doesn't matter'

Brett928S2

Hi

Well I "REALLY" hope that isnt a 928 dyno chart lolllllll...If so....Id sell it for parts...

All the best Brett

928 S2 AUTO V8 4.7 LTR 1986 IRIS METALLIC BLUE WITH X-PIPES ,RMB ,PROMAX CHIPS, KICKDOWN SWITCH.K&N.
PCGB UK 928 FORUM MODERATOR

mark kibort

Close! Lets let a few more trickle in. How about the "power guys" out there! (and glen, i know you are a little of both!) Hey, its sunday, racings over, football isnt for a week, so, what the heck.
a real life example. oh, i chose this set of curves due to the range of the curves. It will make the final point, clear as day!

so, yes, we are strictly looking at the HP/torque curves, and not even looking at the duration of torque or HP applied over the speed range, which could shift things up or down slightly in real life applications. (i.e. hp/seconds /kw/hours, or torque/seconds, etc)

any more takers??
MK

m21sniper

Hockey is on.

Go watch hockey before you start another torque-HP war.

Better yet, go drive your shark.

LOL..

MilesOrbell

5050rpm, so it drops you back into the rising, near peak, torque.

SharkSkin

I'm gonna say 5750, because that will drop you to ~4300, maximizing the "area under the curve". To fit this into your verbal formula:

For max acceleration your shift point will be at 5750 rpm based on this engines HP/torque curve and gear box spacing because the ratio drops by .74 when making this gear change" , "I calculated this by taking the hp peak at 5100 and adding (5100x.13) to it, which is half the gear spacing. I checked my work by subtracting 13% from 5100, which gets me about 4400. The hp curve is at it's highest between ~4350 and ~5750 so on average, this maintains the highest HP through the gear."

"the HP curve was the easiest to use to figure this out because DUH... that's the curve that represents ability to accellerate mass over time.

IcemanG17

MK
Hmm.....since most of this discussion is far over my head...I will guess a shift point of 5500rpm....no sense reving higher since the power drops off really bad....what kind of engine is this graph from? No torque past 4500rpm and no power past 5500....my guess....is a L98 vette motor?

GlenL

Someone point out my error! Sheesh, no engineers on the board?

(Also, the real answer is 5420)

mark kibort

Without giving away the answer before a "torquey" chimes in, Dave A has the lead with John right there too!


Glen, Here is a hint. Hp is equivilant to torque through the gears at any vehicle speed.
Does 5420rpm down to .74x 5420rpm (4010rpm) provide the torque x 3.2:1 equal the greatest torque to the wheels to accelerate with vs the engine torque produced above 5420rpm through a reduction of 4.24:1? ( you see how torque can be a little more work to deal with?)
Also, HP vs RPM tells you just as much of the story as torque vs rpm, however, HP incorporates more of the information. afterall, fs=P
So, if you really want the accurate answer for speed ranges, based on your point, you will need to know the time spent in the power curve. (using a unit of work, such as kilowatt seconds, hp seconds, whatever) maybe use the last graphs we got from the drag comparisons, showing these datap points. (ie the HP or ft-lb seconds used from up to 118mph in 3rd, going to 118mph in 4th . As i said, this will only shift the shift point up or down slightly.

this is more of a simple analysis, showing that one of the curves is far more easier to work with and gain comparative information from. (torque or HP?)

MK

PS I see the error. you assumed a straight line for the torque curve and used min and max values, to find torque per rpm. that doesnt work, as its very important that you use the torque curve as a curve! A quick confirmation of the error would be to get the rpm at the middle use the formula, and then look at the torque on the curve and see it if matches your formula.

By the way, the curves are from the 4.5Liter 928. Its a heck of a lot more than i had with my 84 starting out (178rwhp bone stock, going to 200 with headers and race exhaust!) I was still happy as a clam. (ignorance is blisSS!)

928Quest

Trick question!!!!

Marks fourth gear is higher numerically than third!!! When you shift from 3rd to fourth the engine EXPLODES!!!

Ignoring the fact the your original question is goofy ie ....(ie 4.24:1 4th to a 3.2:1 3rd gear)... you have fourth at a higher ratio than third.

Mark wants us to fall into two camps, the equally spaced around the HP curve or the equally spaced around the Torque curve.

Here's my guess.

The answer is closer to 6250-6350 or thereabouts. That is the RPM at which if you continue in third gear you will actually have less torque than if you had shifted to fourth and therefore you would be accelerating at rate less then you would be if you had just shifted to fourth.

I used the torque curve, just to **** Mark off.

Acutally, I think the real answer is to dump that car for a GT

mark kibort

ha ha. no, not a trick question, and i fixed that typo. yes, 4.24 3rd, 4th at 3.18.
.75 or .74, it really doesnt matter.

yes, you will have to fall into two camps, but its the HP curve (which is nice to work with an almost perfect arc) or take the torque curve, and find the cross over torques from 3rd to 4th gear.

If you used the torque curve. (and that doesnt surprise me, so im not "pissed off"), how did you arrive at 6300rpm?

I think we all get the fact that we want the most amount of torque to the wheels through the gears at any speed. so, where does that happen in the torque curve. the HP is easy to see visually.

6300rpm gives 572ft-lbs through the gears. shifting to 4th, you are now at 4725rpm, at 690ft-lbs . you can work out all the values, but shifting this car that high would give someone a very poor acceleration rate. want to check why visually. look at the hp curve in the range of 4725 to near 6300rpm. Notice anything distinctive?

Mk

928Quest

Dammn looked at the wrong curve, when I was looking at torque I accidently read the HP graph on the right side. Damn HP graph!! I would need to adjust my rpm down

928Quest

Just recalculated and yes it falls symetrically around the HP peak, about 5750 in third to 4300 in fourth.

So for this example I fall into the look at the HP graph catagory.

I still stand behind the comment, dump that car and get a GT.

Isn't there a famous quote out there that goes something like "... he who poses the question, wins the debate...."