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Discussion Starter #1
Is there anyone out there with an aftermarket TQ converter with minimal other mods to their SS? I'm wondering something. I've noticed when I fill up and reset the mileage gauge on the DIC, when I come to a stop with the trans in gear, the mileage drops like a stone. If I put it in neutral it drops, but at a quarter or less of the speed.

A lot of my drive time is spent sitting in traffic, so I'm wondering if a higher stall converter (3200 or 3600) would actually improve my mileage under the same driving conditions.

Obviously the real reason I'd be changing the converter is because of the performance gains, but I don't want the trucks daily driving to be significantly affected.

Thoughts?
 

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Traffic with a stall torque converter will lower your gas mileage.
It will lower it because essentially the transmission is slipping more, so you need more RPM to move the vehicle in slow traffic.
 

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2008 Imperial Blue
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I put one in two weeks ago and notice a significant drop in mileage, but I do a fair amount of city driving, and this city has really bad traffic (and some hills, not like SF, but not flat).
 
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Discussion Starter #5
Traffic with a stall torque converter will lower your gas mileage.
It will lower it because essentially the transmission is slipping more, so you need more RPM to move the vehicle in slow traffic.
Right, but that's exactly why I think my mileage might improve. When I'm sitting still the stock trans is loading up the engine. I'm proving that by seeing different behavior on the DIC with the trans in neutral. With a higher stall the engine would run with less or no load while sitting still in gear. Of course the engine would rev to a higher RPM to move the vehicle, but my thinking is that wouldn't necessarily reduce the mileage since the engine has less load on it.

That doesn't sound exactly right, but you get the idea.

What kind of mileages are people getting with a 3200 and 3600 stall?
 

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06' TBSS 2WD
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I run a Yank SSTB 3600, and do nothing but city driving only, so tons of stop and go traffic, and I average 13.5 around town when I keep my foot out of it. I also run non-ethanol 93. When I run the ethanol blended 93, my mpg's drop to around 12.
 

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Right, but that's exactly why I think my mileage might improve. When I'm sitting still the stock trans is loading up the engine. I'm proving that by seeing different behavior on the DIC with the trans in neutral. With a higher stall the engine would run with less or no load while sitting still in gear. Of course the engine would rev to a higher RPM to move the vehicle, but my thinking is that wouldn't necessarily reduce the mileage since the engine has less load on it.

That doesn't sound exactly right, but you get the idea.

What kind of mileages are people getting with a 3200 and 3600 stall?
Why you're see a difference between being in gear or in neutral gas mileage is your idle airflow values set in the tune of the truck. There's a park/netural table to set for idle airflow, and an in gear airflow table which increases idle airflow for being in gear. More airflow=more gas being used, since at part throttle or idle the truck's air to fuel ratio is 14.7.


With a cam and a 3k stall 8-9 mpg average.
 

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07 TrailblazerSS Imp
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I watched the Aeroforce guage when I sat at light vs driving and when it was set on HP there was a difference of 2-3 HP; so if you shift to neutral it would be at 7 HP when put in gear sitting still, it jumped to 9 or 10 HP.... thats known as parasitic drag. The Alternator does the same thing in a lesser amount-the more electronics you have on the more work the alternator has to do to keep up.
 

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Discussion Starter #9
I run a Yank SSTB 3600, and do nothing but city driving only, so tons of stop and go traffic, and I average 13.5 around town when I keep my foot out of it. I also run non-ethanol 93. When I run the ethanol blended 93, my mpg's drop to around 12.
Interesting. I average about 14.5 mpg with ethanol blended 89 octane. I suppose 13.5 mpg with the stall converter isn't that bad, but of course if there was a way to have both better performance, and better mileage that's really what I'm after. Thanks for the input.
 

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Discussion Starter #11
Why you're see a difference between being in gear or in neutral gas mileage is your idle airflow values set in the tune of the truck. There's a park/netural table to set for idle airflow, and an in gear airflow table which increases idle airflow for being in gear. More airflow=more gas being used, since at part throttle or idle the truck's air to fuel ratio is 14.7.

With a cam and a 3k stall 8-9 mpg average.
Hmmm. So based on that, it shouldn't be any different between a stock and higher stall converter vehicle because the tables would be the same. Am I understanding that correctly?

Does my explanation of the mileage differences between stock and stall make sense? I'm saying that under normal driving the stall vehicle gets to it's powerband quicker and doesn't have to work as hard to move the vehicle. Less work should equal less fuel, right? That's the way I'm thinking of it.

Thanks.
 

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Me>Matt
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I get 12.4-12.6 city on 91 awd with a ptb3600, before on stock stall I got 13.4-13.8. I haven't noticed much of a difference and I'm 90% city driving. Hwy I get about 17mpg now
 

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Discussion Starter #13
I put one in two weeks ago and notice a significant drop in mileage, but I do a fair amount of city driving, and this city has really bad traffic (and some hills, not like SF, but not flat).
Did you do any other mods to the trans when you did the converter swap?
 

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When I did my converter swap, I also did the Corvette 1-2 servo, and the PCM4LESS tranny cooler, as well as upgraded to the GM Dexron VI tranny fluid.
 

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TrailblazerSS
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Interesting. I average about 14.5 mpg with ethanol blended 89 octane. I suppose 13.5 mpg with the stall converter isn't that bad, but of course if there was a way to have both better performance, and better mileage that's really what I'm after. Thanks for the input.
You run 89? interesting to see what your numbers are with 91/93 like there supposed to have.

with my cam and 3200 i get about 8.5
my cam isnt 'that' agressive
 

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Discussion Starter #16
You run 89? interesting to see what your numbers are with 91/93 like there supposed to have.

with my cam and 3200 i get about 8.5
my cam isnt 'that' agressive
Yes, I have a PCMforLess tune for mid-grade fuel. I don't like running the ethanol blended gas, but those damn quick-trips are everywhere here and regular gas is hard to find.

The 14.5mpg average is in the city, on the highway I usually get 16-17. 8.5mpg is just crazy. I sure hope that's not your daily driver, or you don't have far to go. What do you get on the highway?
 

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Hmmm. So based on that, it shouldn't be any different between a stock and higher stall converter vehicle because the tables would be the same. Am I understanding that correctly?

Does my explanation of the mileage differences between stock and stall make sense? I'm saying that under normal driving the stall vehicle gets to it's powerband quicker and doesn't have to work as hard to move the vehicle. Less work should equal less fuel, right? That's the way I'm thinking of it.

Thanks.
You will see a loss in fuel economy going from a stock converter to a higher stall converter over 3000 rpm, assuming everything else is the same (tune, driving habits, etc.).

Even though the delivered torque to the rear wheels will be higher with the aftermarket converter, you will lose fuel economy due to the slippage in the transmission.

Put it this way...you realize that higher stall converters slip more at low speed, and that they put more heat into the fluid (hence the need for a trans fluid cooler).

Heat is energy, and that energy, when put into the fluid, is dissipated out to the environment via the cooler, thereby not putting into the driveshaft to turn the wheels and ultimately accelerate the truck.

As such, you will use more fuel.

Now, an argument can be made that since the rotational mass of the converter is less, plus it's smaller diameter lends itself to a lower moment of inertia, so theoritically, you may see an actual increase in highway only economy, again, everything else being unchanged.

This last point could be determined best with accuracte instrumentation, and you may see an actual difference over a long drive, but there are so many variables that I wouldn't count on it (fuel variations, wind, tire pressure, driving style, etc).
 

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My truck on the highway gets 16-17. Its not my daily driver either. i havent put 3000 miles on it this year.

are you calculating fuel mileage or going by DIC
 

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Discussion Starter #20
My truck on the highway gets 16-17. Its not my daily driver either. i havent put 3000 miles on it this year.

are you calculating fuel mileage or going by DIC
I'm going by the DIC. I should calculate it just to see how far off it is.
 
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