mark kibort

I have been having quite a bit of discussion with Icarp over the past few weeks off line, and the interaction has been quite good. Lots of tech racing talk .
There is always a much easier time to get points and ideas across over the phone, vs here on the list. So, in bouncing some of the "Laws of Physics" that I promote, it became obvious that my statements might have been too white or black. So with that, i want to apologize to those that were put off by my conclusions for the shift point discussion.

Now, to keep this really clean and simple. Let's talk about straight line acceleration. (Not in a curve, approaching a curve, or with traction limited)
What i had meant to convey and what i should say when talking about shift points is that shifting past max HP RPM levels, usually provides the most amount of acceleration overall . (Both at the shift point and after the shift).
However, as most of these discussions need to have as a baseline rule , "It depends". Its a term often used in the "what gear ratio is best " discussion as well.

I thought i would start a thread that speaks to the resultant forces to the wheels at all the different power levels and how they relate to what the engine torque levels are.

Generally, it's pretty easy. If Max HP is found at max RPM levels (Redline) shift at max RPM). However, if max RPM (Redline) is beyond max HP, then it depends if you can get more average HP (area under the curve) by short shifting. I posted a curve that actually showed this situation. Those curves are usually found with heavily restricted. engines, or turbo-diesels.
Below is a graph is a common BMW E36 3.2 liter track car. before and after mods. I've superimposed the stock BMW gear ratios for all forward gears to show where the resultant RPM , post shift, will be.
Ive also posted a curve of a 928 where it would PAY to short shift. 6200rpm or so. the good news for that car, is it is forced by performance to be nice to the engine!

If you look at max RPM HP valve, peak HP value and then post shift HP value, you can clearly see that by taking this particular engine to reline, you maximize HP which in effect maximizes rear wheel torque at any speed. again, this is not speaking to anything other than straight line, full traction acceleration.

If you want to discuss that here, along with how measurements of the dyno runs correlate to what is seen on the track and other observations and thoughts. by all means, add to the discussion.

ProCoach

I like my ShiftRPM app. On iOS. Built by my friend and colleague, David Ferguson. Info here: http://veracitydata.com/shift-rpm-app/

Need to enter the engine dyno sheet, intermediate ratios, final drive and rear wheel diameter or circumference.

Doesn't hurt my head and helps me when I have a LOT of different choices on my purpose-built race car.

wgn

The key that some folks forget is that acceleration doesn't care what hp or torque the motor is making, only what force the tires can apply to the track surface. So the torque multiplying effect (leverage) of the gearing needs to be considered.

Looks like the veracity racing app does this nicely, though I'm not an Apple guy so I can't test it.

Jake951

For maximum acceleration, it boils down to one simple rule: choose the gear that maximizes hp to the driven wheels. If you do the math, that is exactly equivalent to maximizing torque at the driven wheels when you take torque multiplication of the gearing into account.

mark kibort

Yes, this is the bottom line right there. what's nice about looking at just HP is that it already does the tedious math for you. (gear ratios and tire diameters) Just find the max hp or maximize hp over any entire gear, and you automatically get the greatest force at the rear tires. as you say, automatically takes into account the gear ratio torque multiplication at any vehicle speed.
well said too.. right to the point!

I think the thing i see most often, is the confustion of rear wheel forces and torque. (as you point out, that's what we really care about) vs engine torque values. the nice think about the newtonian identities, is that it shows that acceleraiton is proportioal to power (at any vehichle speed) so, its a very easy indicator to either show where your greatest acceleration will be or the forces that can be generated, becuase:
1. power = force x speed.
and
2. acceleration = power/(mass x speed
Both of those simple equations say that if you maximize power at any speed. you will get the greatest acceleration rate possible.
Power vales makes it very simple to find the shift points, as all you do is make sure that the post shift HP level is equal before you shift, based on gear spacing. If you do that, the rear wheel forces will always be greater at the wheels

I want to check out Peter's software IOS find.. that looks really interesting.
but heck, im still trying to get the race lap software to work properly. i have several, both seem to crash half the time at the track when im on the pregrid

thats really interesting. ill check it out and give it a try. I think there is also a G-meter type acceration app that allows for testing 60-100mph runs that basically acts like a true rolliing dyno. if you can get the speed sensor accurate enough, you can basically replace the dynojet ! (as long as you use the same road section, winds are the same,, etc ) i did that for many years to detect major problems (ones that were worth 15hp or more). (stop watch, video of the speedo)

mark kibort

Here is a rare example of when short shifting (in a straight liine, not traction limited) helps with maximizing acceleration.
I posted this earlier with the M3 dyno curves, but this graph is from a higly anemic engine that pays big dividends to short shift . this is a classic example of when the engine just makes "noise" in the higher RPM ranges. the nice thing about this curve is that you never really ever need to shift past 6000rpm, unless the gear ratios are wider than 71% post shift RPM drop. in other words, if you have a closer ratio gear box, short shifting would happen at an even lower RPM.

a stark contrast to the BMW e36 3.2 liter engine that never pays to be short shifted in straight line acceleration. (shift points are marked for all its gears)

Bill Verburg

Not quite, what it does is maximize the rear wheel torque in the next gear(ideally but not always as sometimes the transmission engineers have other items to prioritize as they did w/ the g50/20 used on US 993 and several other street oriented applications too)
graphed here is the rear wheel torque in all 6 gears and the hp and where a shift to the the next higher gear takes you, engine is 993 X51, trans is stock US g50/20, aero has not been factored in here.

mark kibort

Bill, what do you mean by "not quite". can you explain? as i understand it, if you ever have more HP at any speed, you will have more rear wheel forces. (rear wheel torque after the gear ratios as your graph is showing for the respective gears). Any time at any vehicle speed when you have more HP available, you will always have more torque available. It has to, and as it was stated, it will mathematically proportional. (power =force x speed)

so, what your graph shows , on this particular engine and transmission,is that a redline shift (or max RPM on the graph) gives the most HP before a shift, and the post shift HP is greater. It also accurately shows that the rear wheel forces in any gear is either greater near max RPM than the following RPM post shift (except for the last shift from 5-6th where it gets a little unclear unless we have the exact gear ratio because there could be some overlap.)

what i show on your graph is that for each max RPM shift , the post shift RPM is less thrust.. so, it pays to keep it in gear. as long as the red line i drew doesnt go horizontal, the thurst force is higher at the higher Hp level pre shift than post shift by not shifting until max RPM.

every one of the brown arrows you draw, loses about 100 to 500lb-ft of torque and thrust 100 to 1,000lbf of force by shifting just 500rpm early



[QUOTE=Bill Verburg;13446093]Not quite, what it does is maximize the rear wheel torque in the next gear(ideally but not always as sometimes the transmission engineers have other items to prioritize as they did w/ the g50/20 used on US 993 and several other street oriented applications too)
graphed here is the rear wheel torque in all 6 gears and the hp and where a shift to the the next higher gear takes you, engine is 993 X51, trans is stock US g50/20, aero has not been factored in here.

Bill Verburg

[QUOTE=mark kibort;13446229]Bill, what do you mean by "not quite". can you explain? as i understand it, if you ever have more HP at any speed, you will have more rear wheel forces. (rear wheel torque after the gear ratios as your graph is showing for the respective gears). Any time at any vehicle speed when you have more HP available, you will always have more torque available. It has to, and as it was stated, it will mathematically proportional. (power =force x speed)

so, what your graph shows , on this particular engine and transmission,is that a redline shift (or max RPM on the graph) gives the most HP before a shift, and the post shift HP is greater. It also accurately shows that the rear wheel forces in any gear is either greater near max RPM than the following RPM post shift (except for the last shift from 5-6th where it gets a little unclear unless we have the exact gear ratio because there could be some overlap.)

what i show on your graph is that for each max RPM shift , the post shift RPM is less thrust.. so, it pays to keep it in gear. as long as the red line i drew doesnt go horizontal, the thurst force is higher at the higher Hp level pre shift than post shift by not shifting until max RPM.

every one of the brown arrows you draw, loses about 100 to 500lb-ft of torque and thrust 100 to 1,000lbf of force by shifting just 500rpm early



How did I know that your response would be to not read.
and further change the data by using an arbitrary shift point above the redline shift point
and ignore the comment about the design criteria of this transmission, it is not designed to maximize track performance but rather to maximize fuel economy

mark kibort

Im sorry, sometimes the interaction on the list can be hard to follow (maybe its just me) but be patient.
i didnt read or comment that the "design" of the transmission was to maximize fuel economy. if that is the reason to short shift, im all on board with that too!
But, i was commenting on how the "math" doesnt add up based on your comment. why did you say that and what did you mean.

as far as the red -lines i drew, they were not arbitrary , they were specifically pointing at the max HP that was measured by the dyno and then multiplied via the gear ratios. I wasnt making a specific shift point, i was showing that the lines that were drawn showed how much more rear wheel forces are available up to max HP and beyond vs shifting before the hp falls off to max RPM

you said what i have pictured below. you commented on the prior poster's equating of HP to rear wheel forces and now you say that you were talking about "fuel economy" and how the design was not for track performance.. sorry, i didnt read that in your response

EDIT: I updated the graph to show more exact RPM drops between gear shifts .
Bill ii'm curious.... what is the blue highlight on the HP curve that shows it starting at before HP peak to HP peak. it seems to be called "power band" but i want to know what that means to you . to me, it looks like the author of the graph mistakes thrust curve band, with power band... two entirely different things.

mark kibort

Here is an old thread that addresses the concepts very well.. Bill and i had some good interaction back then. He had those graphs way back then too!


https://rennlist.com/forums/racing-a...ml#post3599116

Veloce Raptor

I am not getting into this rathole again. However, there is a very interesting article in the current PCA Club Racing News, copyrighted by Christopher Brown ("Making Sense Of Squiggly Lines"). Again, much of this is way too academic versus what works in the real world, but he does address various types of engines, and how sometimes optimal shift points are at RPM peak and sometimes they are well before...

mark kibort

Dave, its not a rat hole.. its a really good discussion if you come at it from a perspective of possibly learning something new.

Icarp and i were actually on the phone for a long time talking about all sorts of conditions. when we narrowed it down, we were exactly on the same page, with no further questions and actually started to talk about dyno runs vs actual performance (inertial dynos vs static dynos that dont take into account HP losses) that conversation didnt effect the outcome of the main conversation regarding shift points.

Ive actually posted a dyno run of a car that pays to shift before redline, in a BIG way. (redline is 6600rpm and best shift point is near 6000rpm). this is really a small % of most engines, but they do exist.

The real point is the thrust curves you can multiply out very easily and get the answer. This will give the acceleration characteristic of shift points at redline vs max HP (if not the same point) and whether it makes sense to short shift. This has nothing to do with short shift conditions, such as through or approaching certain sections of track or traction limitations. We are talking ONLY straight line acceleration characteristics.

Bill has the thrust curves as posted above. I don't think anyone could provide a condition where his thrust curves would prove to provide gains if short shifting.
The reason is, the post shift RPM hp value at max HP is less than the redline post shift level. If that is the case, it makes sense to shift at redline ... if not, then shift at max HP or best said, at the point where you maximize average HP.

If Im not describing the conditions clearly or if there is a error in what i say above, please be the first to let me know. i don't want to be propagating any mis-information in case I'm missing something here.

mark kibort

Dave just read the PCA magazine article...

https://www.pca.org/sites/default/fi...16.3%20WEB.pdf

very easy read, only a page. it uses an example of a spec racer ford engine.. (very similar to the engine Ive owned that pays to be short shifted). this is in no way really representing most cars you see at the track. Read the article Dave.

I posted viper, M3, vet, 911, etc etc curves and all of them would be clearly benefiting your acceleration by not short shifting and shifting at max RPM levels.
if you read the article, its exactly what we have been talking about. Maximizing HP over the RPM range of the gears you have to use over any operating speed range.
It's the easiest way to analyze the proper shift points. Even Carrol smith agrees pretty clearly in his book, and also says how simple it is. you dont need to use thrust curves... just gear spacing, and HP graphs and you are good to go with eyeballing the greatest level of HP produced and used.
this is NOT too academic and completely relates to "real life" track performance. I dont like to use testimonials, unless i know the facts behind them as well as be able to prove them.. in this case, a well known expert agrees.


Check out the curves i showed you before in your thread and see if you disagree. below is the link to the curves of the:

viper
cayman
spec boxster
911
e36 BMW
Try and find a spot on the HP vs torque curve where short shifting would ever produce more rear wheel forces at any speed, as long as you were straight and not traction limited.

https://rennlist.com/forums/racing-a...l#post13404098

MSR Racer

What do you call it when Kibort 'kiborts' his own thread???