What aspects of a tune influence torque?

[John Smith]

Hi,

I recently had my car retuned for the summer as well as the addition of a catted x-pipe (SLP) The results of the tune were surprising to me, in that I actually picked HP but lost a lot of torque. I know x-pipes tend to sacrafice torque for HP, but the loss was bigger than I thought it would be. Before I go on I will list the variables: The biggest influence on my dyno numbers is probably that the initial tune was done on a 50 degree day, where as the most recent tune was done on a 90 degree day. I am sure this is responsible for most of the descrepancy.(But I want to hear what you guys think)

On the cold day tune (stock midpipe, 50º) my numbers were: 425HP/405TQ
On the hot day tune (SLP X-Pipe, 90º) my numbers were: 437HP/385TQ
A/F was mid to high 11's across the rpm range.

These numbers are peak numbers, which wouldnt be that bad except for the fact I lost a lot under the curve. I lost 20 lbs/ft of torque across the board under the curve, until 5000 rpm where the numbers from the original setup and the new setup intersected. In some places I lost as much as 40 lbs/ft down low. I also lost about 20 hp across the board under the curve until 5000 rpm where I actually started to make more power. My question is how much of this loss is related to the tune vs. the weather? What aspects of a tune most influence torque?

On a seperate note; the car sounds freekin awesome with the SLP pipe and Borla Stingers.

 

[chuck943]

Well ok here's my theory on it.
First torque is a twisting force applied to an object. Now say for instance you are loosing a bolt with a torque wrench if the bolt is frozen it will take more torque to loosen it but once you get it started the amount of torque required to turn it decreases. Now same thing would apply to an engine. An engine is nothing but a big air pump, it pulls air in and pushes air out. Now if you lower the back pressure and increase the flow then you should lower the torque required to turn the engine. Kinda of like water flowing thru a pipe,as you turn the valve and increase the flow the pressure drops.

 

[40th GT]

I would have to say it's the air:fuel mixture that influences torque. When a tuner adjusts the PCM's parameters, they're usually playing around with the fuel tables.

And I'm guessing that you lost torque in the heat b/c you're running slightly leaner than you're supposed to when it's hotter.

 

[Stan Weiss]

The weather should have had very little to do with the numbers as they should be corrected. If the numbers are not corrected and the Barometric Pressure and Vapor Pressure were the same you would make about 5% less HP on the 90 Deg. day. At what RPM were each of the HP and torque max. at? As for the lost torque I would look first at the x-pipe.

 

[John Smith]

Sorry I didnt respond to any of your posts (I subscribed to the thread, but never recieved a notifications about responses) Thanks to all who replied. I will check my Dyno sheets when I get home and post what rpm peak power and torque happened at.

Thanks

 

[ADRENLN]

no matter what the reason was, if u r lost up to 40rwtq anywhere in the lower rpm something is wrong!!!!!! that is huge. i would take the damn thing off if that is the case.

that is way more lose then gained.

dont make sence to me.

 

[BobHyatt]

There's more to it than that...

Torque can be computed if you know horsepower @ rpm. And vice-versa. Torque is simply a measure of how hard your motor tries to turn at a specific RPM. In fact, there is no way to measure horsepower on a dyno. It measures torque and converts that to HP. They are related very closely. HP is the measure of lifting 550 pounds one foot in one second. Torque is a different measure of the same force. One foot-pound is equivalent to hanging a 1 pound weight on the end of a 1 foot long bar. That "twisting" force is torque.

As far as a "tune" goes, here's what can influence torque.

1. obviously if you run it lean or rich, you lose power/torque. A "tune" should not do that.

2. spark advance. When the motor runs at high RPM, you need more spark advance to get the cylinder to fire early enough that the flame front has propagated sufficiently by the time the piston reached TDC so that max force is developed there. Retard the spark and the full cylinder pressure develops too far after TDC and you lose power. Advance it too much and it fires too early and part of the energy is wasted trying to turn the motor backward, as well as making that famous "pinging" sound. It is likely that this is the part of the tune that lost you torque.

3. Other usual things like camshaft overlap and duration will affect torque, for the same reason. As the motor winds higher, things need to happen earlier in the stroke. But sometimes this causes low RPM problems (as very high spark advance would). That's why street cams are different from strip cams. On the street you want to be able to pull at 1500 RPM. On the strip you want max power, no matter what RPM range it develops at, you will just adjust your shift points and rear end ratio to take advantage of it wherever it is.

There's nothing else the "tune" can change. air/fuel ratio at various throttle settings/ rpm levels, and spark advance. camshaft profile is fixed when it is ground unless you drive one of the rice-burners with variable (electronically adjustable) valve timing and intake runner length.

Losing low-end torque in return for high-end horsepower is as common as rain. Shoot, many race motors won't even idle at 800 RPM, much less produce enough torque to pull your hat off your head...