Just do it!

[Bill Verburg]

Originally Posted by Tom W

Bill: You've probably noted it somewhere (but I can't seem to find it). How are you calculating the thrust? You've piqued my curiosity and I want to make the equivalent calculations for my car and potential gearing.

It really is fascinating to look at this stuff in different ways
you need;
1) overall gear ratio, individual gears and cwp
2) rear wheel torque #s @ rpm from a dyno
3) revs/mi of the rear tires to get the tire constant

tire constant = 5280/(2*Pi*revs/mi)

thrust = ROUND((overall gear ratio*torque@the wheel)/(tire constant),0)

to graph you need to determine the other axis, I chose speed

speed = (rpm*60)/(revs/mi*overall gear ratio)

[Tom W]

Thanks. I have the torque at the flywheel, but that should be good enough (numbers will change but shape will be the same).

[Bill Verburg]

Originally Posted by Tom W

Thanks. I have the torque at the flywheel, but that should be good enough (numbers will change but shape will be the same).

if using flywheel torque multiply the numerator of the thrust equation by ~ .85 to account for driveline losses

please post your results it would be educational.

in the last charts above I had to multiply the hp overlay by 100 to see it reasonably w/ the thrust scale on the y axis.

[Bill Verburg]

Curiouser and curiouser
When I overlay hp on thrust vs rpm in all gears it's obviuos that it does no good to take the engine past 6.5k and in the upper gears it appears that it's advantageous to shift even earlier.



*note hp does not equal torque @ 5250 here because the hp had to be scaled by a factoor of 100 to fit on the chart and be reasonably visable

[kurquhart]

Originally Posted by Bill Verburg

Curiouser and curiouser
When I overlay hp on thrust vs rpm in all gears it's obviuos that it does no good to take the engine past 6.5k and in the upper gears it appears that it's advantageous to shift even earlier.

My engine and trans are nothing like this setup (more drool), but I found the exact same thing during my last track weekend: shifting at 6k produced lower lap times than my historic 7k. Not only did it pull better, but the shifts are smoother and produces less chassis upset.

[North Coast Cab]

Bill,
Can you overlay thet stock G50/20 box so we can more clearly see how different your box or a custom short box is? Also help those with shallow pockets to maximize what they have.

[Bill Verburg]

Originally Posted by North Coast Cab

Bill,
Can you overlay thet stock G50/20 box so we can more clearly see how different your box or a custom short box is? Also help those with shallow pockets to maximize what they have.

That's what this one is all about, the #S to the right of the gear description are the speeds were peak torque occurs


1st & 2nd in the /20 gives the best acceleration but at a lower speed
3 -6 in the /30 give the best acceleration but again the cost is at a lower speed.
6 in the /30 is just a little taller than 5 in the /20


You can make them so complicted that you can't get any info from them, so I try to keep the info to 2 or 3 topics

[xwcgt]

I know this is an old thread, but im surprised this particular post wasn't corrected.

stark contrast to a page earlier where Bill said, shift at 7000rpm to maximize thrust in the next gear. the curves here show that Bill might be fixated on a powerband like look of the torque/thrust curves, which is not the best place to focus on . even here, the redline or 7000rpm shift is an advantage over shifting at 6500rpm as was noted on the graph.

as was said earlier, anytime you have more thrust in the current gear, stay in it before you shift. clearly here , a short shift at 6500rpm will have near 500lb-ft of torque of loss consequences at the rear wheels. (about 2000lb-ft vs 1500lb-ft post shift levels from 1st to 2nd on an earlier gear shift at 6500rpm vs 7000rpm)

unless im missing something here , it looks like this graph is contradictory to the previous graph on:

Just do it!

It would be interesting to know why.

Originally Posted by Bill Verburg

Curiouser and curiouser
When I overlay hp on thrust vs rpm in all gears it's obviuos that it does no good to take the engine past 6.5k and in the upper gears it appears that it's advantageous to shift even earlier.



*note hp does not equal torque @ 5250 here because the hp had to be scaled by a factoor of 100 to fit on the chart and be reasonably visable

[xwcgt]

Originally Posted by Bill Verburg

Curiouser and curiouser
When I overlay hp on thrust vs rpm in all gears it's obviuos that it does no good to take the engine past 6.5k and in the upper gears it appears that it's advantageous to shift even earlier.



e

Bill,
Can you explain this. it seems to be in contrast to a graph you posted earlier, saying to shift at redline (7000rpm).
Just want to see if this is a inadvertent error. I'm Explaining the HP vs shift point to a couple of my students today and was using the very handy, thrust curve graph.
It seems obvious that the graphs of all gear produced thrust, have more thrust in the prior gear right up until redline.
Thx

[Bill Verburg]

Originally Posted by xwcgt

Bill,
Can you explain this. it seems to be in contrast to a graph you posted earlier, saying to shift at redline (7000rpm).
Just want to see if this is a inadvertent error. I'm Explaining the HP vs shift point to a couple of my students today and was using the very handy, thrust curve graph.
It seems obvious that the graphs of all gear produced thrust, have more thrust in the prior gear right up until redline.
Thx

That particular engine has a rev limit of 6700
here you can see where a shift at 6700 takes you, this trans is not optimized for the engine or for track use in general. it is a fuel economy trans, as such short shifting in 3,4,5 will take you to a better part of the torque curve, 1 & 2 shifts rpm drops are good and take you to a decent place on the torque curve

[xwcgt]

Originally Posted by Bill Verburg

That particular engine has a rev limit of 6700
here you can see where a shift at 6700 takes you, this trans is not optimized for the engine or for track use in general. it is a fuel economy trans, as such short shifting in 3,4,5 will take you to a better part of the torque curve, 1 & 2 shifts rpm drops are good and take you to a decent place on the torque curve
[img]img]

Ok that explains part of it (the part that the redline is really not 7000rpm, but 6700rpm.. the graph i copied however said to shift at 6500rpm, which didnt make much sense to me.
Next, what do you mean by short shifting. is the 6700rpm the rev limit and also the "short shift" or do you mean to shift at an even lower RPM?
If so, that would not optimize acceleration thrust, as you can see from your graph, every single gear shift has maximum thrust before a shift up to the next gear. (even 4-5 and 5-6th, however, not as critical as the prior shifts)

mostly i want to understand the "blue power band " area highlighted. this is the max thrust force in each gear, but based on the gear spacing vs speed, certainly not the "powerband" for the car... that would be in the higher RPM ranges, all the way to whatever your redline is.
This is because at each shift at 6700rpm the thrust force is lower in the next gear, but you have to shift anyway, because its at its RPM limit.

[Bill Verburg]

Originally Posted by xwcgt

Ok that explains part of it (the part that the redline is really not 7000rpm, but 6700rpm.. the graph i copied however said to shift at 6500rpm, which didnt make much sense to me.
Next, what do you mean by short shifting. is the 6700rpm the rev limit and also the "short shift" or do you mean to shift at an even lower RPM?
If so, that would not optimize acceleration thrust, as you can see from your graph, every single gear shift has maximum thrust before a shift up to the next gear. (even 4-5 and 5-6th, however, not as critical as the prior shifts)

mostly i want to understand the "blue power band " area highlighted. this is the max thrust force in each gear, but based on the gear spacing vs speed, certainly not the "powerband" for the car... that would be in the higher RPM ranges, all the way to whatever your redline is.
This is because at each shift at 6700rpm the thrust force is lower in the next gear, but you have to shift anyway, because its at its RPM limit.

the light blue is the max available thrust region for this engine w/ this transmission, as such you get the best acceleration in that region.

the red shift lines are determined at the right end by the red line in the graph, the left ends are determined by the drops inherent in the gear stack. As long as the red shift lines have a positive slope you are gaining the ability to go faster, if the slope is zero then there is no gain in ability to accelerate, as the slope approaches zero the less is gained.

A full appreciation is requires the incorporation of aero data

here is a 993 RS/CS in max downforce configuration which is also max drag, the thrust is net including aero, the trans here is a g50/30 used in 993Cup and RSR, the full benefit of the closer gearing isn't derived because the street toque curve falls off too quickly at higher rpm, this is seen in the shift lines that go to zero slope on the 4-5 shift, the closer to zero slope the more benefit from short shifting


to get the full benefit of this gear stack a motor which sacrifices some lower rpm torque for some more higher rpm torque would be desirable.