Question about dynocharts & shift gears

[FenixBirdWing]

I've been reading up on dynochart and dynotuning and there's still one question that's been bugging me and I haven't figured out - all of the dynocharts I've seen so far display a number of trial runs or 'pulls'. Each pull has a HP plot and a Torque plot over the same RPM range.

Why does each pull only show one set of HP & TQ plots over the RPM range even though there are multiple shift gears the transmission can be in? Why wouldn't a HP & TQ plot have to be generated & displayed for each gear the car transmission can be in? I'm thinking that the the TQ value at 4000 RPM in 1st gear would be different than the TQ value at 4000 RPM in 2nd gear and so there would be value in measuring and plotting the HP & TQ curves in each gear. For example, I could use those plots to determine the best shift points from gear to gear when driving a manual tranny.

Was wondering if anyone might know the answer to this or be able to correct me if I'm making a faulty assumption somewhere along the way? Is it maybe the case that a dynochart for testing in one gear can be used to represent the HP & TQ curves for any other gear (maybe by throwing in some type of scaling factor) and so one set of HP & TQ curves for a single gear is all the information you really need? When dynotuning is performed and dynocharts are generated, what gear is the car typically put in for testing?

 

[kooldawg6]

Cars are normally in 3rd gear for a dyno pull. They usually start the pull at around 35 MPH and go until redline. 3rd gear is a good gear due to it can pull good from 35 all the way up to and over 100 depending on mods. I'm not real familiar with all the different auto trannies, but the newer ones need a specific tune written to lock the converter during the pull or otherwise it locks and unlocks and creates "spikes" on the graph and makes it difficult to get the most accurate reading in the upper RPM range. The best shifting technique is to run it up near redline before shifting. Eventhough you'll pass your peak power levels (usually), you want to carry it higher so that when you shift, the RPMs don't drop way below your power curve.

The power levels at a certain RPM stay the same no matter what gear you're in. The reason it feels different is due to the gearing of the trans and rear end on how it gets transfered to the tires. Think of with a set of rear gears how the car accelerates. Now change those gears and it changes the rate of acceleration eventhough it didn't affect the engine power levels...only how it gets transfered.

Here is my dyno graph from last summer. You can see how the first pull had "spikes" near the top from the converter locking/unlocking. The next few pulls they were eliminated as he wrote in the tune to keep the converter locked the entire time.

 

[jonnieguns]

I've heard two different reasons.

1st, runs are done in 4th because of the 1:1 gear ratio. I don't know enough about dynos to verify this for myself.

2nd explanation is that each pull must take a particular amount of time, from start to finish. This is because the dynamometer is taking a finite number of readings per second. If the run is done in 1st gear, it will be over far too quickly, and fewer total readings will be recorded, resulting in a more inaccurate result. This is why a higher gear is selected.

If anyone can shed some light on the truth behind either of these, I'd appreciate it.

 

[NastyStang113]

4th gear is used since it'll be a 1:1 gear/drive ratio. The way the torque multiplication works makes this the 'correct' gear to do the pull in so it gives you a 'correct' reading of horsepower or torque.

Irregardless if your dyno chart says you make peak power at say 6,200 RPMs that doesn't mean you want to shift at 6,200 RPMs. If you shift at say 6,500 RPMs you'll have more RPMs in the next gear and have 'more' power at the time. Most S197s with stock cams should be shifted at 6,400-6,500 RPMs for the best times.

Cars are normally in 3rd gear for a dyno pull.

No, the transmission is in 4th gear on a 5th speed transmission during a pull. It's not about the mph that the car is at either. The run is started when the car is at about 1,500-2,000 RPMs if it's a manual. If it's an automatic it's a lot different.

 

[kooldawg6]

Makes sense. Thanks for clearing up the gear used