Thread: More fuel economy woes.. 2004 2005 GTP

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  1. #1 More fuel economy woes.. 2004 2005 GTP 
    SE Level Member HuffNPuff's Avatar
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    Any (relevant) thoughts welcomed! I currently have an 05 GTP with the following mods: 3.5", AL 104's, CAI, ZZP stainless downpipe/hi-flo cat, 180 Tstat, 1.8 Yella Terra Full Rollers, ZZP PCM 1.0 (with their default, custom tune... this was all ordered from them at the same time, so their tune is supposed to have taken these mods into consideration). All of that being said, I had the same poor fuel economy--for all intents and purposes, identical--on my bone stock 2004 GTP and my current 2005 GTP before mods.

    I ALWAYS run 93 octane. My mods have made virtually no difference in mileage (but a damn sight difference in power when the foot goes all the way down!).

    I have tried all variations of driving styles (had the 04 from @2007-2011, and the 05 from @2008-present). Short of keeping my foot on the floor at all times, there is only a slight difference in average economy between being very consciously light on the throttle and what I would consider "normal" tendancies--ie: not grannying from stop lights, but not gunning it. Usually just beside or slightly ahead of other cars leaving lights.

    I NEVER average more than 20MPG, with a fair mix of city/hwy, 40%/60% to 30%/70% respectively on "normal" tanks (no long trips). I have city driving 5-10 miles at a time, and a highway commute of around 40 miles, round trip. On long trips--where I would stay on the highway throughout an entire tank, stopping only to refill--the BEST long trip I have ever averaged is 25, with long trips usually being in the 22-23 range. Average driving with a 1/2 tank or so trip yields a best of ~21mpg.

    All of these numbers are from actually calculating every fill up. I track every fill up with a Fuel Log app on my phone. The DIC is consistent, but generally off on the average by about 1.5mpg. At 60-ish MPH highway speed, I will see the instant economy reading in the 30MPG range, but that has yet to effectively translate into a decent average... I'll even see a mid-30 instant if I'm behind a few cars or a large truck, but again... my average is not affected.

    Tires: Current setup is 18x7.5 wheel with Yokohama Avid Envigors, 245/45-18. That size is the same overall height as the stock 17" Wheel/tire combo, so there is no effective gearing change. The fuel economy numbers didn't change--on either car--when moving from the stock combo to the 18-inchers. Nor has the type/brand of tire affected the mileage. 2 different sets on the 04--Goodyear something or others and BFG Touring. On my 3rd set (2nd 18 inch set) on the 05--BFG Touring, super-cheapo 18s (Sunew?), Yoko 18s. I have fiddled with pressures to no avail, currently having run from 33psi to 41 psi, with the only difference being a slightly bumpier ride (surprisingly, not awful at 41 psi. I have been VERY pleased with these Yokohamas!).

    Again, same mileage circumstances on both cars from the time I got them (~40,000 miles on the 04 when I got it, 22,000 on the 05 when I got it, currently at 98k). Mods, tires, wheels have made no mileage difference. Driving style has negligible effect. MAF has been cleaned. No codes/lights.


    My last fill-up was just under 18 mpg... :-\
    Last edited by HuffNPuff; 12-12-2012 at 10:33 AM. Reason: Correcting rocker ratio from 1.7 to 1.8
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  2. #2 Re: More fuel economy woes.. 2004 2005 GTP 
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    My GXP regularly averages 20ish with a good amount of highway. I have gone as high as 19 in town and 30 highway...that was some pretty flat road though...
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  3. #3  
    GT Level Member RivMan's Avatar
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    Sounds about right to me... These aren't advanced fuel sipping cars, they're inefficient, heavy, powerful cars designed when fuel was ~$1.00 a gallon. My 99 Regal got around 19 mixed, with a max (road trip) of 28.

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  4. #4 Re: More fuel economy woes.. 2004 2005 GTP 
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    A 3.5 on a GEN V with only a downpipe and rocker. Sounds like alot of trouble too me.

    Those plugs are too cold and that pulley is too small for your "other" mods in my opinion.


    Never been a fan of ZZP tunes, its very possible that is a cause to the poor mpgs, along with the plugs.

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  5. #5 Re: More fuel economy woes.. 2004 2005 GTP 
    SE Level Member HuffNPuff's Avatar
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    Thanks for the reply. The 3.5 is a little bit of a stretch. When I mentioned wanting to go 3.5 instead of 3.6, they said I would need at least either a cam or rockers. Rockers being the easier option, that's what I went with. In hindsight, I think I would have been fine with the 3.6. Even with the 1.7s, a full-bore run pushes 15-15.5 lbs of boost... which, as I understand, is about the most these small Gen 5 M90s can even come close to putting out with any semblance of boost-efficiency remaining (heat created vs. boost). And, I don't think my tranny likes that a whole lot. haha. As I understand it, the Gen V's can produce more boost for the same pulley size versus the Gen III's due to some design improvements bringing about greater boost efficiency.

    The Autolite 104's are what comes with that kit (even with the larger 3.6 pulley). You would run hotter, even with the boost I'm getting (again, 15-15.5lbs per Autometer monster live boost gage, recessed in place of the round, top vent to the left of the steering wheel)? Or, do you--like me--feel I shouldn't really be getting that much boost? lol

    What issues have you had with the ZZP PCM/tune? When I got these bolt-ons (I come from a world of Holley/Pro-Form carbs on SBC/BBC...), I had no intention of becoming a "super tuner," nor desire to constantly fiddle with the tune. I do not "race" this car, nor go to the track at all. I was just very disappointed with the performance of these cars, stock, and wanted to add my best bang-for-the buck add-ons, and stop there. I read many good things about ZZP prior to making the purchase and felt their Stage II kit, with the addition of the 3.5 and the rockers, was that bang-for-the-buck. Performance-wise, the car is an entirely different creature with the mods. I was very pleasantly surprised.

    Is there a different setup/mod you would recommend... other than or in addition to dropping to a 3.6? I did get the modular pulley system, so it is not much trouble to change and I have been considering that anyway.

    The current tune is per TurboTim, who seems to have a good reputation.

    The ZZP PCM notwithstanding, I had the fuel econ issues on both the 04 and 05 when completely stock.
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  6. #6 Re: More fuel economy woes.. 2004 2005 GTP 
    SE Level Member HuffNPuff's Avatar
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    True... they are not exactly designed to be misers on fuel economy, but I get nowhere near the rated economy. Also, I had a carb'd, 84 Z28 with a 5 speed that would get 31MPG on long trips. To be sure there's been improvement over the subsequent 20 years between production! Especially with a fuel-injected V6 having only 75% of the displacement of the 5.0 (H.O. no less! haha. 195 HP! lol). But, what SHOULD be capable--power and economy-wise--from this 3.8 is not the point. The problem is that it doesn't come near what it is reported to do, factory or otherwise. :-\ Most of the time, I don't even average the estimated city number with my combined 30/70 city/hwy.
    Last edited by HuffNPuff; 12-10-2012 at 07:21 PM.
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  7. #7 Re: More fuel economy woes.. 2004 2005 GTP 
    GrandPrix Junkie spazzz's Avatar
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    Attachment 10321this is about what I've seen in my 2004 compg..Usually slightly more in the city and about 22-25 highway
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  8. #8 Re: More fuel economy woes.. 2004 2005 GTP 
    GXP Level Member txslow6's Avatar
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    The positive side points to the obvious-You gained power, but didn't sacrifice mpg to do it.

    The issue lies mostly with in the tune. ZZP's, like other shelf tunes, are purposely tuned to have your car run rich. If you want mog's to increase you will need a custom tune to accomplish the biggest increase.
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  9. #9 Re: More fuel economy woes.. 2004 2005 GTP 
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    Holy boost, how much KR does that thing see, I bet its hurting big time.

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  10. #10 Re: More fuel economy woes.. 2004 2005 GTP 
    Gun nut and III percenter TexasTanker19kilo20's Avatar
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    Try getting your tires Nitro filled, but after all the reading about the PCM. Interested to see how this works out. Heard (not sure) the Nitro fill helps with keeping constant tire pressure which results in fuel savings.
    Goldie 98 Topaz Gold GTP (retired)/ Shadow 06 GTO 6.0 Phantom Black Metallic- traded in. Now cruising a 2005 GMC Sierra 2500 HD 4x4 with the always fun 6.0L
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  11. #11 Re: More fuel economy woes.. 2004 2005 GTP 
    GTP Level Member wa04compg's Avatar
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    Do you regularly scan for KR? I had to run headers to have no KR
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  12. #12 Re: More fuel economy woes.. 2004 2005 GTP 
    SE Level Member HuffNPuff's Avatar
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    I do not regularly scan. I did, however, just order BT OBDII so that I can start. Should be here Friday (and, wouldn't ya know it, we are headed out of town for the weekend and taking the "bus" with the kids, so no updated scans this weekend... )
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  13. #13 Re: More fuel economy woes.. 2004 2005 GTP 
    Max Cook
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    I wouldn't go WOT again on it until you do some scans. I cant believe you wouldnt have massive KR on that build. I have practically everything but heads on a genV build and I get KR blips occasionally when its this cold and before they Dyno tuned it I hit 6 KR. GenVs are ridiculously efficient. I was getting 8-9 PSI of boost on a 3.25 pulley and 10-12.5 on a 3.1 pulley. Pretty big jump. You should stick with a 3.6 at the least. There is no reason to ever run 15.5 pounds of boost anyhow. 10-12 and then bump up the timing.

    Also your gas milage is fine. Thats pretty much what people get when things are doing things right lol. I dont get anywhere close to that anymore.
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  14. #14 Re: More fuel economy woes.. 2004 2005 GTP 
    SE Level Member HuffNPuff's Avatar
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    @Max Cook: Advice duly noted! lol. I've been in this configuration for right at 2 years, though. Lucky??? Engine built on a good day at the factory??? Definitely not one done late on a Friday afternoon! haha. I don't do a whole lot of WOT--it's not usually necessary... Most of my hard acceleration bumps the 12lb range. But, there is the occasional high-gear pull. I was very surprised at the boost I was getting after the mods. 15lbs is a good deal more than what I was looking for. I was expecting to get in the 12lb range, maybe 13. Not 15. I trust my boost gauge but will re-check with a different gauge, too. Before mods, the gauge would show a max right around 7.5-8lbs, which is about right for stock from what I have read.
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  15. #15 Re: More fuel economy woes.. 2004 2005 GTP 
    Max Cook
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    Probably a little bit of luck but if youre not going WOT very often at all especially near 3rd gear shift area thats probably whats saved it.
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  16. #16 Re: More fuel economy woes.. 2004 2005 GTP 
    GTP Level Member AjL227's Avatar
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    ZZP most likely set PE to come on at ~30-40% TPS to prevent your **** from blowing up, because they know you are risking your engine with that poor mod selection.
    Goodbye fuel economy.
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  17. #17 Re: More fuel economy woes.. 2004 2005 GTP 
    I live here. SlowNA06's Avatar
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    Burning any coolant?
    Irridium spark plugs last 100k mi and work just as well as copper. Copper is a waste on N/A and only lasts 15k mi. Don't use Platinum.
    Use 195* tstat unless you can thoroughly explain why not; 99.9% don't need a lower temp.
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  18. #18 Re: More fuel economy woes.. 2004 2005 GTP 
    GXP Level Member GTPpower's Avatar
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    Wow. It would be a good idea to put a bigger pulley back on. That's a lot of boost for being non-intercooled.


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  19. #19 Re: More fuel economy woes.. 2004 2005 GTP 
    SE Level Member HuffNPuff's Avatar
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    Quote Originally Posted by AjL227 View Post
    ZZP most likely set PE to come on at ~30-40% TPS to prevent your **** from blowing up, because they know you are risking your engine with that poor mod selection.
    Goodbye fuel economy.
    So, PE mode puts it closed loop... working from presets and disregarding 02? Not sure where they have any of that set. I would think that, if it was going to be so dangerous as to require PE at that low TPS, they would have advised as such...??

    Bear in mind, before this purchase was made (3.5 instead of 3.6), I asked ZZP IF there was a relatively simple, additional mod I could do to support the 3.5. I was told, by them, that the 1.8 rockers would enable me to run the 3.5. Maybe not ideally, but relatively safely was my impression. This was not done as a "Send me the 3.5 because I want MORE!" I wanted a little more, but not without sufficient support. Had I been told that the 3.5 would NEED headers, heads, cam, more significant mods, etc., I would have stuck with the 3.6. I was told, "You would need a set of high ratio rockers(or 95-96 octane) to be able to safely run a 3.5” pulley without any KR." So, I ordered the rockers believing that to be enough. I'm glad my **** has not blown up! But am now more interested in optimizing what I have.

    Also, never had the fuel economy to start with!
    Last edited by HuffNPuff; 12-12-2012 at 10:35 PM.
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  20. #20 Re: More fuel economy woes.. 2004 2005 GTP 
    SE Level Member HuffNPuff's Avatar
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    Quote Originally Posted by SlowNA06 View Post
    Burning any coolant?
    Yes, actually. However, head gaskets check out ok on both Compression and Leak-down tests. Cooling system (LIM gaskets, included) hold pressure wayyyy above minimum required. I thought they were leaking for sure and when they held pressure at 16 lbs (pressurizing system with test kit), I pumped it up to 30 and let it sit. It lost about only 1/2 lb after being left for an hour at 30 psi. No oil intrusion into cooling system, nor vice-versa.

    If you have other thoughts on that, I'd love to hear them!
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