Transmission Ratios

[rmcmullin]

I guess I really don't understand this last post. Maybe someone could help me out.

I do have a Dyno sheet on the engine, and the HP and torque curves leak at 7000 RPM. The engine ran on the dyno at 8000, but HP and torque are diminishing. From playing the numbers it seems to make sense to peg the shift point at about 7500 which will keep the engine higher on the power curve at the gear change.

I'm beginning to think that first gear isn't too critical since it will be difficult to prevent wheel spin. The shif to second will likely come early thereby giving a long pull in second gear up to 7500. 3,4, and five result in progressively smaller RPM drops and, therefore, keep the RPM well up in the power band, 6000 to 7000.

[Interested Observer]

Sparky is a big fan of tractive effort (TE) and prefers to deal with that and torque, probably because it is a more intuitive quantity to him.  Since the torque and power are directly related, either method can be made to work.  Most deal with power, since that is what makes things go and is generally what is quoted off the dyno.
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The torque and power peaks cannot both occur at 7000 rpm unless the torque just STOPS there.  Otherwise, your general approach makes sense, it is just a matter of how accurate you want to be as regards optimizing gearing and shift points.

[SPARKY]

"I'm beginning to think that first gear isn't too critical since it will be difficult to prevent wheel spin. "
I do not run a TQ converter because I have a fairly light car  3500#

I run a PG/GV   which gives me 4 trans ratios  1.76   1.37  1.00   .78

 I run this at B'ville with a 2.14 RA with a 16" Mickey which is about 24.75 diameter

MY FDR in 1st is 3.77   1/od 2.93  D 2.14  D/od 1.67   

  I do this because I have two other tire sets  23.5" on 15" and  26" on 18" wheels 

I assign the diam. of the 16" a value of 1 and put all this on a "Dream Chart" spread sheet by multiplying the TQ by the FDR by the tire factor.  I plot TE and Speed for each gear by using the dyno TQ numbers

With your Liberty you can do with gear ratios what I am doing with tires. 

The bottom line is you need the biggest number of TE at the Speed you are needing to run  in my case my target speed is 315  so I need the biggest number at about 320  I changed my car to be cleaner and I changed my eng to move the TQ curve up a little higher in the RPM band.

RPM    1st/TQ   1st/speed   2nd/TQ  2nd/Sp  3/TQ  3/sp   4/TQ   4/sp    5/TQ    5/sp

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putting this on a spread sheet will give you a greater understanding of what your car is doing on a run especially if you have a peaky eng with some big jumps or drop off in power


I/O  said it best  this is what helps me "see" the direct connection from the info on my dyno sheets to the race track speeds I am shooting for.

others see better with graphs---others with HP    OUR "Mileage Varies"

[WOODY@DDLLC]

Here's my spin on it - pun intended! Torque & traction equal acceleration and HP equals top speed. This is true in drag racing, land speed and everything in between. Traction varies between the venues but torque for a given power-plant would stay the same. The torque applied at the contact patch produces the force of thrust that gets us down the track. Torque is the ability to do that work and HP is the amount of that work that gets done in the time or space allowed for doing it! If we plot the torque curve vs the gear ratios [torque multiplication] we can determine the maximum acceleration possible in each gear if the traction is 100% available. For our purposes three examples are given. All three have the same transmission gears and torque but the overall gear ratio and vehicle weight are changed to see the effects. First chart just runs out of gear at 175 mph but we want to go 200 mph and our wind tunnel tests and dyno results say we can do it so we change the overall gear ratio. Notice the curves are the same they just get a little flatter [less acceleration] and the end points go faster. We shift at the same rpm but at different mph. When we add the weight then the end points stay at the same mph but we get less acceleration. Maybe have to move from the short course to the long course? If we plot force (thrust) instead of acceleration the curves and shift points look the same. If we have wider transmission ratios then there are no intersection points and we just shift at red line. If we assume that 0.6 g is good B'ville traction and 0.4g is not so good then you can see that each combination will have different launch and run results. For some the first three gears are totally useless. [Think push start!] You can see where the throttle has to be managed in the other cases! These charts also assume the course conditions are the same all the way down - good luck with that assumption! These trends agree pretty well with the seat of my pants experiments - how about yours? I am always amazed at how well empirical rules of thumbs work (most times/some times?)! While this exercise is useful for planning or refining a particular setup sometimes you just have to run what you brung to have fun! Theory and reality do agree that once the limits are reached it's still add gear & weight then mo' torque & HP!

[SPARKY]

Yepppers Mr. Woody !!!!!!!!!

I have head B'ville called a 4 mile drag race with a 1 mile trap  there is not much difference between my last mile speed and my exit speed and if ther were an exit speed at the 4 I bet there would not be much difference there either for my car.

[jacksoni]

Though not exactly on topic, you suggest that your engine makes PEAK HP and TQ at the SAME rpm. I am not an engineer and only a very amateur engine builder/assembler but this is very unusual and my sense from reading is not likely and/or not desirable-even possible?. Do others have this experience? Dynoroom? As Woody has said, TQ accelerates you but HP is where your speed is overcoming aero and mechanical drag. Sparky mentioned 4 mile drag race with a long trap. In that setting, your gears and shift points want to rock over the HP curve so the HP before and after the shift are more or less the same. If you shift at peak HP RPM, you lose-looking for max HP under the curve. Drag racers for sure shift in this fashion and though there are arguments about it, they always shift this way, more or less.

[rmcmullin]

OK, I guess I "misspoke."  Peak power is 711 at 7000 RPM  Peak torque is 540 and occurs at 6500 RPM but is 533 ft. lbs. at 7000 so the curve is pretty flat.

I didn't anticipate my question would generate such a quality and quantity of comment.  You guys are a lot smarter about this sort of thing than I am so I'll have to spend some time digesting the info you have provided.

[jacksoni]

And like some ( not all) of the responders) I like your gear split, especially the 4-5 jump as you want to keep the engine as close as you can to peak HP and as Midget said, enter your last timing trap pretty much all in. My engine is small and peaky. 4th is 1.23 and drops me farther off my curve than I like. Depending on engine set up, power peak for me is 9000 or 9500 and I shift at 9800-10K  (digital tach has some trouble keeping up with the engine sometimes so not always sure) but it drops close to 8000 on the last shift. I'd much prefer 8800 or so which your 1.10 gear would do. In another car we used a Jerico out of a Nascar Pocono car where they run in 4th part time and 3rd part time. 3rd gear was 1.09. Worked well but 1st was about 1.50 so we had trouble launching that particular set up. Adjust your final with the QC and tire size.

[SPARKY]

RMAC  with an eng like yours--you may well benefit by the torque multiplier tables you can see and compare your TE over a wider range than one might think  in my old car I was able to run almost exactly the same speed at 7800 in direct on 28" tires  as I was on 24.5" tires in od at 6850 ..  the chart said I had 15'# more the smaller tire in od at the same speed and lo and behold I went back with the smaller tires and ran a little faster and got the hat.

[rmcmullin]

Woody:
Are you computing the acceleration shown on your graph by using A=F/M where F is the product of engine torque multiplied by the final ratio in each of the gears, and M=weight if the car?

This has been a really interesting discussion. It's my first entry in the forum. Are they all this enlightening?

[Interested Observer]

Calculating the “acceleration” possible from merely the tractive effort and car mass would seem to be a pointless and potentially misleading exercise.  Such a value would only be reasonably representative at push-off, before significant drag forces enter the picture.  And even then, it would be erroneous since the low end acceleration would likely be limited by the friction coefficient.  So then, what is the point?  Just stop at tractive effort or do it including all the pertinent forces.

Torque is the ability to do that work and HP is the amount of that work that gets done in the time or space allowed for doing it!

While I am reluctant to criticize Woody’s notion, and his concept stated just above may “work” for him, it is in conflict with the actual definitions of torque, power, and work.  Torque, per se, isn’t the ability to do anything.  Torque acting at a rotational velocity is (horse)power, and that is the ability to do work.  Further, horsepower is not an amount of work.  An amount of work is just that--work or, energy.  Horsepower acting over a period of time will expend an amount of energy and do an amount of work. 

When people adopt different and erroneous notions as to what these quantities really represent, it is no surprise that they become confused and distressed when discussing them with others.

[SPARKY]

" Are they all this enlightening?"

  Well no--- some can be a lot more confusing    But that is the good part---it is trying to lift the veil of confusion that lead to a deeper understanding  of how ones best way to get there..

ultimately it just all boils down to---Total Drag vs TE and or thrust---