Why is Throttle Not Linear?
Thread Starter
Burning Brakes
Joined: Apr 2002
Posts: 1,105
Likes: 114
From: Lawrenceburg, KY
Why is Throttle Not Linear?
Specifically why is the Accelerator Pedal Position % vs Throttle Position % not 1:1?
Looking at the stock C5 table for APP vs TP, even at 75% pedal the throttle is only commanded 46% open. It's only the last 25% of pedal travel that the throttle goes wide open (and then it's pretty much all at once). Can someone enlighten me on the reasoning behind this?
It would seem to me that the throttle should follow the pedal near 1:1.
Specifically, I'm looking at the table on a 2004 manual C5 coupe using HPTuners (though files from other year C5s seem to be near identical).
Looking at the stock C5 table for APP vs TP, even at 75% pedal the throttle is only commanded 46% open. It's only the last 25% of pedal travel that the throttle goes wide open (and then it's pretty much all at once). Can someone enlighten me on the reasoning behind this?
It would seem to me that the throttle should follow the pedal near 1:1.
Specifically, I'm looking at the table on a 2004 manual C5 coupe using HPTuners (though files from other year C5s seem to be near identical).
Thread Starter
Burning Brakes
Joined: Apr 2002
Posts: 1,105
Likes: 114
From: Lawrenceburg, KY
Yes... yes it is (on a whim I decided to try true 1:1 and yeah, it's not drivable like that).
However, I did set the 50%+ APP = 50%+ TPS and then scale the lower half to transition smoothly. By keeping the lower range (<25% APP) near stock, it's very drivable (to me) and yet much more responsive.
However, I did set the 50%+ APP = 50%+ TPS and then scale the lower half to transition smoothly. By keeping the lower range (<25% APP) near stock, it's very drivable (to me) and yet much more responsive.
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Melting Slicks
Joined: Jul 2015
Posts: 2,296
Likes: 232
From: Round Rock Texas
Yes... yes it is (on a whim I decided to try true 1:1 and yeah, it's not drivable like that).
However, I did set the 50%+ APP = 50%+ TPS and then scale the lower half to transition smoothly. By keeping the lower range (<25% APP) near stock, it's very drivable (to me) and yet much more responsive.
However, I did set the 50%+ APP = 50%+ TPS and then scale the lower half to transition smoothly. By keeping the lower range (<25% APP) near stock, it's very drivable (to me) and yet much more responsive.
You might get silenced for this secret telling
Last edited by sirdano; Sep 24, 2017 at 09:09 PM.
Race Director
Joined: Apr 2007
Posts: 11,152
Likes: 890
From: South Western Ontario
It's not been mentioned, but moving the angle of the throttle shaft linearly does not cause a linear increase in the throttle body cross sectional area. Even the older TBI, LT1 and cable LS1's had a cam that caused the throttle shaft to move slower at first and then faster as the cable pulled more. Still, you'd be right that the curve doesn't make the area open linearly.
Drifting
Joined: Nov 2014
Posts: 1,978
Likes: 110
From: Switzerland
are you talking about "max. rotation versus pedal" in hp tuners? (i only have the demo version.)
what difference does it make if you depress your pedal slower or faster? i found a tune where you get 100% throttle from 60% pedal up. no idea how that makes sence.
what difference does it make if you depress your pedal slower or faster? i found a tune where you get 100% throttle from 60% pedal up. no idea how that makes sence.
Melting Slicks
Joined: Feb 2001
Posts: 2,804
Likes: 358
From: Northville, MI
In the drive by wire TB, you have a built in delay, if your tune gives you wide open throttle at 60% pedal, it will still scale the the acceleration rate back to the standard set in the nonn programmable portion in the PCM. There are ways around this system but changing the relationship of throttle to TB response is not an easy tuning solution. What works at one setting will cause twitching, bucking or undrivable conditions in another situation.
Thread Starter
Burning Brakes
Joined: Apr 2002
Posts: 1,105
Likes: 114
From: Lawrenceburg, KY
See attached screen shots. The first two are stock 2004 C5 throttle maps. The second two are what I have mine set at. Notice that from 0% to 9% they are very similar. This was necessary to have any low speed resolution. At 13% mine starts to start a gradual ramp up. This was needed to keep from having a very abrupt transition from "stock" to "modded" values.
Heel & Toe
Joined: Sep 2016
Posts: 24
Likes: 0
Geometry
See attached screen shots. The first two are stock 2004 C5 throttle maps. The second two are what I have mine set at. Notice that from 0% to 9% they are very similar. This was necessary to have any low speed resolution. At 13% mine starts to start a gradual ramp up. This was needed to keep from having a very abrupt transition from "stock" to "modded" values.
Thread Starter
Burning Brakes
Joined: Apr 2002
Posts: 1,105
Likes: 114
From: Lawrenceburg, KY
Look at the bore as a circle, the disk(throttle) closed , it is a circle with a large area. As the disk opens it becomes a elipse, with each degree open it becomes a smaller area. As it gets closer to wide open it has a very small control of the conductivity of the of the tube it is controlling. If I continue explaining, I will probably misspeak, hope this helps
... At 0% (fully close) the throttle plate is flat against incoming air and event a 1% change in throttle plate angle will yield significant change in air flow capacity. By contrast, at 99% angle, the throttle plate is almost insignificant so the difference between 99% and 100% is near zero change. The result is that small changes at "low throttle" yield significant changes in air flow capacity, which in turn affect driveability.
Melting Slicks
Joined: Feb 2001
Posts: 2,804
Likes: 358
From: Northville, MI
The 97-03 PCM has 512kb of information of which 96kb can be change/altered. The remaining 416kb is part of the GM adaptive strategy for running the engine management system. (2004 has 1056kb but still only 96kb programmable). The throttle lag between the DBW and TB opening is more than the two dimensional relationship of APP and TB opening. The work around is more involved and, of the 3 or 4 different tuning systems on the market for the LS engine, each have different methods of making the change. Be carefull when making those changes on your system and always save and notate each step of the changes.
Le Mans Master
Joined: Apr 2012
Posts: 7,814
Likes: 2,266
From: Brighton MI
My understanding is that it has more to do with meeting EPA requirements, both mpg and emissions. It has the added benefit, at least from a mfg warranty POV, of reducing drivetrain shock loads.
When I had mine dyno tuned at Livernois Motorsports, I asked them to make it linear...they told me I didn't really want that, but they would remap it for better response...and they did. Much improved over stock.
When I had mine dyno tuned at Livernois Motorsports, I asked them to make it linear...they told me I didn't really want that, but they would remap it for better response...and they did. Much improved over stock.