Technical How do you determine shift points?

All the talk about Moriarity converting his '61 Impala to a 4-speed got me thinking.... Is there a 'best way' to determine shift points for best performance? More specifically, for manual transmissions. Now I understand that individual engines will have different power curves. A big block that peaks at say 6K RPM in HP but has a big mid-range torque hit will be quite different from a lower torque high-strung small block but a HP peak at 8K. Transmission gearing has an effect also.

Falling back on the old adage that 'HP sells motors but torque wins races', it seems to point to shifting into the torque peak would result in the best acceleration, but is that true in the real world? As an example, an engine that makes peak HP at 5500 RPM and peak torque at 4K would favor a roughly 25% drop in gearing when you upshift. Shift at 5500 and that drops engine speed to about 4100, very near the torque peak. This would appear to be borne out when looking at gear spreads on 'typical' 4-speeds. A wide ratio will be about a 30% reduction between 1 and 2, 28% between 2 and 3, and 26% between 3 and 4. Close ratio is similar but has larger percentages of change between gears; 28, 24, and 20 percent respectively. This seems to suggest that each gear change may want a different shift RPM depending on the transmission if trying to hit the torque peak.

Thoughts, opinions, experience?

 

That's good thinking for going down the track but not practical or needed for a normal street cruiser.

 

When the valves float, shift.

 

^^^^^what he said^^^
Gm designed a rev limiter in their hydraulic lifters

 

Super hi volume/high pressure pump ... smallest pan available ... shift when the oil light comes on

It works like this, you pump all the oil up into the valve covers, the pan goes dry, the oil light comes on. It's not exactly a science but it does work.

Remember kids, I am a trained professional, do NOT try this at home

 

My head hurts. In theory shifting into the torque curve should be beneficial, but it would depend on the engine and torque curve. Nailhead Buick engines for example- wide torque curve, and most "experts" favor a wide ratio 3 speed over a 4 speed, especially a close ratio one. High revving SBC would differ greatly, even in the same car would want a close ratio 4 speed.
I guess the best test would be a measured mile, a tachometer, and a stopwatch to get where the best shift points are. The drag race boys have figured all that out for their particular car.

 

I’m looking forward to this one, I’ve often wondered the same thing.

 

When it sound like it's workin', I shift.

 

Had a Mopar with a 383, dual quads, 4 speed and 4:56 gears.

Had to shift like... right now.
WHAAAAAA!
WHAAAAAA!
WHAAAAAA!
WHAAAAAA!

 

I like to keep my mill in the torque curve. This is not always easy with wide ration transmission. You really do not want to drop out of the torque curve.

For general driving I think probably where it starts to climb up on the cam. With a close ratio transmission you do not have to get really deep into the torque curve, on a wider ratio transmission you need to dive a little deeper into the pool so that you don't drop out of the torque curve when you let the clutch out.

For performance driving I like to use the entire torque curve up before shifting. Unless I am having a traction problem then I may short shift it to keep it moving forward and use the torque curve up on the big end.

 

Is GRRRRRR!!!!! a bad thing?

 

Optimum shift points aren't a problem with my car. By the time I manage to complete the gear change, it's a wonder the car doesn't start to lug.

 

We had a small block with all the good stuff ( aluminum rods, rev kit etc. you could turn it to 9800 rpm’s but the car was just as fast shifting at 7800. The heads did not support the air flow. Long story short, it depends on what you got!

 

My best pass ever was what I call "climb shifting", launched at 6500, 2nd at 6500, 3rd at 7000, 4th at 7200

This engine made peak hp at 6600
Peak tq at 5100

So in theory, as gear multiplication gets lesser you compensate with more rpms..

 

Yes it would depend on what you have and what you want. Only practice with your machine will get you there.

 

I shifted when I felt the motor stop pulling. After awhile I quit looking at the tach, all seat of the pants.

Same way manual shifting auto transmissions too.

.

 

I had an OT Ford 429 4 speed with a factory rev limiter. I unhooked it at the drags as I was going to race my BIL in his '66 GTO and I wanted to win. I don't know what the rev limiter was set at but I apparently exceeded it and had to drive home on 7 cylinders. Had to pull the manifold to retrieve the bent pushrod and lifter laying in the galley.

 

Ideally you want the rpm drop to be at or just below peak torque. But not all transmissions have ratios to complement the exact drop.

The other thing is to shift when peak hp drops off. Either due to heads or other items (cam, valve float, exhaust, carb cfm) limiting airflow. You don't want to keep rpm climbing once it starts dropping off, it will be slower than optimum acceleration.

Just to add, for a pure drag race car, the best stall speed is the same rpm as peak torque.

 

I shift by this method, due to track conditions, tune, how the car is working etc. You have to know your car very well, and how she reacts to conditions.

 

RRRRRRR - 2 - RRR - 3 - RRR - 4 - RRRRRRRRRRRRRRRRRRR

 

I find graphs like these very informative. They are easy to create in Excel. I created this one for an OT truck equipped with a T19 4 spd and a 2 spd overdrive range splitter. I was mainly curious to see were the gear splits fell. Other details that go into the math are the 4:11 rear axle and 31" tall drive tires. The engine is an old mechanical diesel that makes peak torque at 1800 rpms, and peak Hp at about 3200. I like to shift it around 2800 which causes it to drop back to 1600 on the next gear. Occasionally it will get wound out to 3200 if all the boost is required. Splitting 2 and 3 is advantageous if there is a heavy trailer behind it, 3 and 4, not so much.

 

If you're fast enough it becomes "GRick"

 

Learning how a "She" car acts to conditions is like impossible.

 

But my race car only had one speed, you just put it in gear, press the loud pedal and go fast.

We actually geared the car to almost bog as you got back into the throttle about the middle of the corner. That was probably close to, but a little below peak torque. Once the track got slick it was all about momentum and not spinning the tires.

 

How Extreme do you want to go and spend the $$ & wanting close to correct gearing .

Pull engine , Dyno ,
Then several Math equations ,
dyno #s , weight of vehicle, , tire size & other ext.
After several math equations ,
You will have a Ideal of what gear should be use per gear change in gear box @ that elevation
If you want it efficient ( power & torque)
This is why Liberty , Lenco B&J other bands
are the Ultimate

 

..... and if it goes bang, you need to back it off a little bit.

 

Torque is how big a bucket you can carry, RPM is how many buckets you can pour into the pool every minute, horsepower is torque multiplied by RPM = the total amount of water you get into the pool.
Torque in itself is just a static force, if there's no movement there's no work done at all, and if you add movement (i.e. RPM) you have jsut multiplied torque and RPM so it's horsepower making the work happen.

Don't believe me? Go grab a torque wrench, attach it to your cars wheel and leave the parking brake on. How much torque can you apply without the car moving at all? Just torque, no movement, no work done.

Sure, there's the large volume, low rpm engines (i.e. tractors) with high torque figures for a given power output and there's the small volume, high rpm engines (i.e. motorcycles) that may produce the same peak horsepower, those engines have VERY different characteristics and it's true that the low rpm, high torque motors have more "grunt" - that's because low rpm motors have power spread over a wider rpm range, while high rpm motors get a high power over a narrower range. Has nothing to do with the torque "working", just a matter of different engine character. 100hp at low rpm equals 100hp at high rpm - but use the rest of the engines rev range and there will be differences as the power is different on other rpms.

So, it's horsepower you need, if you have the dyno sheet for the engine and the gear ratio for the gearbox you can calcylate at what rpm the power on one gear goes down below the power in the next gear at the rpm you get to at that gear.

 

That would have been a good question to ask Bill Jenkins.
You wouldn't have heard me asking it though!

 

Same with dirt modifieds. We do have transmissions, but once ready to go, high gear. Quick change rears are perfect for getting there.

 

engines are like musical instruments....shift when the octave peaks out,,,take a little practice....the five senses are not trusted like they use to be....