Technical How do you determine shift points?

G-Meter

 

As the "not traditional" cat is out of the bag already...
Just to see if the principle worked, I hooked my Picoscope oscilloscope to the AC generator on a 50cc twostroke just to get a good, clear signal with a frequency proportional to the RPM (lots of other sources can be used such as ignition, it just happened to be the easiest and safest one to go to).
Used the maths function to make the scope calculate a trace for RPM (green). Just using that and the
rulers you could see how much time it takes to accelerate from one rpm to another. As shorter time means more power, it can be used as a basic dyno to compare power output between engine adjustments for example. (My laptop that the scope runs off has no battery, so I couldn't do any road tests, so sorry, no good power comparisons with different jets in the carb or something like that.)

Anyway, I went deeper into the maths functions, and added another magenta trace for the derivative of the frequency, i.e. the rate of change of the rpm. Above zero means increasing rpm, and the highest peak means the rpm where it accelerates the fastest, i.e. peak power rpm.

If I were to use a road speed signal instead of a engine rpm signal as input (well, I'd probably want both to see what rpm to shift at) this trace could be used to show when acceleration in gear one drops off below acceleration in gear two, and so on.

Using the green RPM trace and rulers to check acceleration time between different speeds is just a refined version of doing the same with a stop watch and the speedo/tacho/a distance of road. A traditional method to tune any car with basic equipment, I think?

 

A very convincing theory I've read is that you want to shift somewhat after the power peak, at that point where the power after the shift equals the power before the shift. It was shown how repeatedly having rpm increase from a bit before the power peak, through the power peak, to a corresponding bit after the power peak gives the most power for the greatest time.

Argument's sake: suppose our power peak is 300bhp at 6000rpm. Suppose the ratio interval between 1st and 2nd is 28%. Looking at the power curve, we see that we're making 280bhp at 5000rpm, and also 280bhp at 6400rpm. The second point is 28% more rpm than the first, so the second point is our shift point. Finding that is probably a bit of trial and error, but with a plot of the curve in front of us it shouldn't be that hard.

So, we shift at 6400rpm in 1st, where we're making 280bhp. That lands us at 5000rpm in 2nd, where we're making 280bhp. That way, we're never making less than 280bhp at any time after passing 5000rpm in 1st gear.

Shifting at the power peak, or at any other point less than 6400rpm, means spending some time making less than 280bhp.

 

Do some math. You want to use as much power as you can, you aren't really concerned with peaks...you're more concerned with area under the curve. And since power is the product of torque and RPM, you're likely to find that shifting after the power peak makes you go faster.

Anecdotally, my Chevy keeps going faster the later I shift. It hasn't blown up when I accidentally took it to 7500, but it has kissed the valves a few times. The cam supposedly runs out of power around 6500.

 

One pops up occasionally
https://www.ebay.com/itm/324221444086?hash=item4b7d1a9ff6:g:SswAAOSwt2te7~wi

 

I just get in the "Zone" and become 'One' with the car.
I'm not sure if I tell the car or if the car tells me when to shift.

Hold a cardboard core from a roll of paper towels to your mouth and say, "The Zone Is strong in you, Luke!"

 

Every engine is going to react differently even if its the same/similar. The weather and elevation are also going to change results. You might try getting one of those inexpensive GPS units that tells you speed and et. Then perform a series of tests for each shift point. Generally best acceleration is going to be shifting somewhere after the torque peak........but where?
I'd wind the engine to say 5,000 rpms and shift into second and see how long it takes you to get to a predetermined point on the track/road that will be near your next shift point. By that, I mean if you pick a certain arbitrary distance that is approximately where you expect to shift again.........then the GPS will tell you exactly how long it took you to reach that point on the track.
Try driving to this same exact point, but raising your shifting point each time and see what rpm yields the quickest et.
Then do it again and set another arbitrary distance for shifting to third. Its possible that the rpm will be different for each gear given the different spread between gears.

 

I ‘d say it’s the apex of gut feel and seat of the pants reaction , with a healthy dose of street smarts thrown in…that and listen to what your engine is saying to you.

 

Way back before we had all the fancy electronic equipment and I was drag racing, I simply felt the engine not pulling as hard and shifted. By looking at my time slips I tried shifting a little earlier and found out that doing so I got better ET's. Not having any shift light, I simply put my pointer lower on my Sun tach, and shifted ealier.
For the street I shift way too early for maximum performance, and just do so by ear.

 

I’m not discounting what you’d say. I’m sure you have way more info that i do, but wouldn’t a chassis dyno yield better results for a vehicle? Or are you looking at the engine, the selecting proper gears in a transmission and rear end to maximize it?

 

I grab the next lower gear after it falls on its ass and the truck behind me gets real big in the rear view mirror
that is kind of how it is driving a banger...

 

No , you should build your combo from data of the engine & mathematics for gearing, if you have the $$

Most that go to a dyno or chassis to
tune for what you already have,

Then there is those that use dynos to build most efficient combos
(research and development)

There are chassis dyno that a typical business will have, ones with rollers tires on top & some that bolt to axle ,
Those dynos only give data from a certain rpm /speed start No gear change / shifting, = torque and horsepower @ out put ( tire , axle )
a typical chassis dyno can not give data from dead stop ,gear change ,down force , Weight transfer etc..

Then there is Chassis Dynos used in research and development equipped
with all bells & whistles
Manufactures & race teams with very deep pocketst,

There is a HAMB member here with a Very interesting build thread & on going
been in mags in last few years , his son I believe works for a manufacture or has access to a wind tunnel. If I remember correctly.

 

Of course, another way to tell when to shift is by the size of your wallet......

 

Dont shift unless you are in front

 

When you're sure it's your wheelie bars touching the ground but it turns out to be your crankshaft ... you waited too long to shift.
It's a very fine line ... sometimes you go over it and sometimes you don't

 

I shift by ear , I can tell when power is about to fall and know it's time to bang another gear . After rowing the gears for 6o years in the same hot rod it's comes naturally !

 

My favorite racecars didn't have a tach or a transmission !

 

I do not give a shit what anybody says. You have to feel the machine, what it likes and doesn't, seat of the pants, listening, and hoping you have your tune on and your game on! These old dragsters are ALL about the feel which comes with experience and knowing your car. Period! Ok, I'm done, carry on gentlemen.

 

I have always shifted when it stops pullin. Engine sound and vibration are part of the process. When your momentum falls off, throw the lever. When you get used to a manual transmission, you don’t really need to think about it.

 

I always went with short shifting lower gears.

 

I'd say it is what your engine supports just as Glbird stated. I don't remember ever watching the tach to know when to shift while racing but the feel of the pull of the engine told me when to shift. I had to be doing something right as my times were consistent on the same track.

 

I don't do drag racing at a drag strip. I haven't had a functioning tach in a manual trans car for 40 years.
I did a lot of street racing when I was younger, and I had a garage full of broken parts (that was when the tach went away). I discovered back then that high RPM launches and high RPM shift points broke more parts. Low RPM launches gave me better traction, and lower shift points won more street races. That was when you found out just how good your clutch was.

As stated earlier, you have to know your car and all its stuff. Adjusting shift points to what the car tells you was essential to win street races, back in the day.

These days, the shift point is selected mostly to hear the tires chirp after the shift, if I'm in that mood. The games some "kids" play.... LOL!

 

Before this happens.

 

I've always done just the opposite. Short shifting lower gears where the engine can pull lower gears easier loses a lot of power. Long shifting in taller gears puts much more strain on the engine as it takes longer to wond out the taller gears.
I can redline my engine in 1st or 2nd extremely fast, but redlining it in 3rd and 4th takes much longerr, so tougher on the engine.

 

With a 2.5 low gear 500 rpm will get you 2.5 tines in get farther in high gear. Most cars I've run were quicker by shift 500 rpm quicker in low gear

 

Here a short list of gear ratios from just one manufacture,
So how serious are you to get the Max from your power plant & set up!!
Most seem to just want to be bench HotRodders or Racers with what they have.

 

We ran F/MP car with a Dana 60 out back. 287 C.I., 3,444 lbs. Tried a set of 7.17 rear gears one weekend. Shift points came pretty quick. Mostly by feel.

 

It's a ZEN thing, maaannn...

 

Hello,

@Moriarity changed his automatic back to a 4 speed. Now, he has a "really desirable" 61 Impala. Shifting is like anything else in time, the more you shift, it just becomes second nature and is a part of driving.

My brother did not want a tach for his new 1958 Impala as he learned to shift with the “sounds” of the motor, as taught to him by several well known speed shops in our neighborhood. (Reath Automotive, Joe Mailliard, Jack Ewell/Mickey Thompson) The time it takes to eyeball the tach is wasted time in all things involved in starting line procedures.

So, the sounds are a part of vision forward and hand on the shifter. A great start with power and no tire slippage is a fast start. Then listening to the peak performance of the motor to shift to the next gear. The eyes are on the starting light or the drop of the arm at the elbow. The elbow moves first, not the hands up.

The column shift did not hit or get close to the tach, so it shifted smoothly every time. When we were able to get the C&O Stick Hydro installed later, the stick just did its positioning for each gear or just in "R" for race… ha, just kidding… “D” was more like it. It shifted like no other and now, it was stomp on the gas and hang on in a straight line for the win.

The C&O Stick Hydro gave us the advantage over other stick shifts, ( 3 speed/4 speed) as they had to content with coordinating everything from the floor, pedals, and shifting levers. The Stick Hydro just jammed forward and did its own shifting when necessary. We could do manual shifting, but the power to gear ratio was too short and the advantage was to use the “D” for all gears in the quarter mile adventures.

Jnaki

Through many times of trial and error, practicing the three speed column shifter in our neighborhood streets allowed me to get better with each slam of the shifter lever. But, the key was listening to the sounds of the motor when it arrived at the peak. At first, I thought listening to those experienced racers were talking “jibberish.” The were older and had more experience in things drag racing during these early days.

As my brother got better in his starting line routine and fast starts, he started winning more races and finally brought home those coveted trophies. When it was my turn to drive, everything I had practiced came to the final point at the drop of the flag or green light. Smoothness and knowing your motor as to when it was “time to shift.” For all of those early lessons to learn from those masters, it has paid off in our long line up of stick shift cars in our family.

In our married life, there was only one time my wife was with me when a lowered car full of teenagers pulled up next to our car. My wife said those guys are saying something, pointing across the street and the green light. I laughed and said it is teenage bravado without knowing what the outcome will be.

At the 2nd light, I accelerated at the disappearing of the green and was into the intersection, before the lowered teenage car even left the crosswalk. I shifted into 2nd and then 3rd as we accelerated away. At the next light, they kept at it again, wanting another challenge. So, one more time, the same idea, get a good start, power up and shift when it was at the peak performance. No charts, no tach, no theoretical ideas and power points, just pure power to accelerate away.

“Wow” was what my wife said when those teenagers turned at the next, empty, late night intersection and drove away. Embarrassed by an old couple with learned skills as a drag racer from those long ago early days. YRMV

Note: during the 20+ years of stick shift driving, my wife learned to shift when it was time... or when the motor was winding up to a peak in sound. So, for all of her stick shift cars, it was sound to do the correct shifting and not worry about power loss. She just looked at the tach as a big dial on the dash, just an added accessory not for her.