Guest 73TPIZ Posted March 11, 2005 Share Posted March 11, 2005 Guys, My '93 GMC Sierra extended cad is just how i want it in the looks department (customized by myself years ago), but it has 198K miles on it the 350 w/ auto trans is beginning to show it. I will be towing a trailer and cars from time to time and was looking at a rebuild that would tow 4000-6000 lbs and get decent gas mileage. I know a diesel would be the best but i want to keep this truck. Currently i get 16mpg empty and ~14mpg with a moderate load. I was thinking maybe a stroker 383 (torque) with a TPI system on it (more torque) and a towing cam. Peak torque at ~2500rpm. Mate that to a later T56 trans with the .50 sixth gear. I've heard peak torque is where an engine is most efficient. If i could cruise say 1400rpm @ 80mph in 6th empty and 1800-2000rpm in 4th or 5th with a load maybe that would get decent gas mileage. Would this yield maybe 25+mpg empty and ~20mpg loaded? What do y'all think? Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted March 11, 2005 Share Posted March 11, 2005 i dont think 20mpg-the problem with a truck is aerodynamics.i think the 6th in the t56 will be to high unless you have a 4.10 or better rear gear.late model corvettes get 25+ mpg on cruise conditions but it slides through the air.chevy makes a 383 box motor designed for truck replacement engine.the t56 might work with a 4.56 rear gear.you would have to drive it with a vacuem gauge.use the tpi manifold with a some kind of tunable efi and a wide band o2.my 96 ford half ton van with 351 gets between 14 and 15 mpg towing my car on a very light 1 axle trailer.i am happy with that.you dont have the crappy oxygenated gas that commifornia has. Quote Link to comment Share on other sites More sharing options...
Guest 73TPIZ Posted March 11, 2005 Share Posted March 11, 2005 Yea Randy, kinda like pushing a barn down the highway. Has anyone heard what i mentioned earlier about a motor being the most efficient at peak torque? The 2005 corvette's peak torque occurs at 4000rpm, which is awfully high. If the saying's true, the 'vette would get even better than the 28mpg if the torque peak was lower. My '91 CRX HF (high fuel edition) is rated at 48-53mpg. Its peak torque is at 2000rpm and that's exactly what it turns at 60mph in 5th. That would seem to not be a coincidence and Honda planned it that way. The DX and SI cruise at 3000 rpm and peak torque is at 4000+rpm. Thanks for any thoughts you guys have. Quote Link to comment Share on other sites More sharing options...
Tim240z Posted March 11, 2005 Share Posted March 11, 2005 What about some of the newer LS based truck engines, like the 6.0L? Quote Link to comment Share on other sites More sharing options...
Pyro Posted March 11, 2005 Share Posted March 11, 2005 Slow down your towing speed would help. A 383 with a set of vortex heads would make tons of low rpm power. You need small intake ports and a long stroke to make low rpm torque. A short rod, 400 sbc would be even better for low rpm torque. Then a short duration high lift hy-roller cam would be worth 20 more ftlb over a regular hy-cam. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 11, 2005 Share Posted March 11, 2005 My new 5.3 gets ~18 unloaded. I was thinking of trying a different cam and bigger exhaust for better mileage. On my Toyota 4 banger I got about 25, then I installed a high lift low duration cam and a header and 2.25" exhaust and I got over 32mpg. I don't know exactly what effect this type of mod would have on the 5.3, but I expect I could get it into the low 20's at least. Quote Link to comment Share on other sites More sharing options...
johnc Posted March 11, 2005 Share Posted March 11, 2005 Slow down your towing speed would help. Exactly. I can get a 27% improvement in fuel economy in my F350 by dropping towing speed from 75 to 65mph. Its hard to do, but I just put the cruise on and try not to notice all the trucks passing me. Quote Link to comment Share on other sites More sharing options...
Phantom Posted March 11, 2005 Share Posted March 11, 2005 I have a 2002 Suburban with the 5.3 Vortec V8 with the 4-spd overdrive automatic. It averages 18-19 MPG unloaded at 65-70. Pulling my car on a U-haul trailor (5,000 lbs) at 65-70 MPH it gets 12-13. The 6.0 vortec will handle the hills better but will get significantly worse fuel economy. Quote Link to comment Share on other sites More sharing options...
BrandonsZ Posted March 11, 2005 Share Posted March 11, 2005 The sick thing about SMOG now is that they tune the car for lowest possible smog. This does not coincide with highest efficiency and definitely not highest power. Mid 20's is possible with a Chevy body an acquaintance of mine did just that and it was otherwise stock. It had a 350 and 4 speed auto trans. He disconnected all the smog stuff, changed the chip and tuned it for efficiency. It was a Silverado 2WD. It was registered out of state of course, the bastard! Quote Link to comment Share on other sites More sharing options...
mtcookson Posted March 11, 2005 Share Posted March 11, 2005 Yea Randy' date=' kinda like pushing a barn down the highway. Has anyone heard what i mentioned earlier about a motor being the most efficient at peak torque? The 2005 corvette's peak torque occurs at 4000rpm, which is awfully high. If the saying's true, the 'vette would get even better than the 28mpg if the torque peak was lower. My '91 CRX HF (high fuel edition) is rated at 48-53mpg. Its peak torque is at 2000rpm and that's exactly what it turns at 60mph in 5th. That would seem to not be a coincidence and Honda planned it that way. The DX and SI cruise at 3000 rpm and peak torque is at 4000+rpm. Thanks for any thoughts you guys have.[/quote'] i've read that as well and believe it. my maxima's peak torque (n/a) is around 2800 rpm which is where it cruises at in the proper gear at highway speeds. even with the turbo setup now, i've had up to 28.5 mpg keeping it at the peak torque (more or less peak torque, this engine has a lower cr so the peak torque is probably different off boost than the 9:1 n/a engine... but i'm not sure. boosted the peak torque is a bit higher but i still get good gas mileage keeping it at right around 2800 rpm.) the 28.5 is pretty nice... but i know it can be much better with better tuning and a bit of repair things that need to be done (fuel filler recall, leaky intake, broken exhaust studs). i bet i can get well above 30 mpg on this thing Quote Link to comment Share on other sites More sharing options...
Guest 73TPIZ Posted March 12, 2005 Share Posted March 12, 2005 jmortensen, the mpg your truck gets seems in line with my buddy's 5.3 (18mpg) and my dad's 4.8 (~20mpg). Just seems a shame my 198k mile 5.7L throttle body FI truck only gets 10% less than the new multi-port FI 5.3s. I tow at ~65-70mph and get maybe 1-2mpg less than when empty at 75mph. That's just weird and seems like there should be some room for improvement. If I could tune a torque motor for low rpms, I just wonder if it would do better on mpg. Thanks guys for the replies. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 12, 2005 Share Posted March 12, 2005 jmortensen' date=' the mpg your truck gets seems in line with my buddy's 5.3 (18mpg) and my dad's 4.8 (~20mpg). Just seems a shame my 198k mile 5.7L throttle body FI truck only gets 10% less than the new multi-port FI 5.3s. [/quote'] I agree, but the 5.3 is getting quite a bit more power for the 18 mpg. Mine is rated at 285hp/305tq, so there is some payoff for not having any better mileage. I do really think these 5.3 engines are choked from the factory, but I'm not really a V8 or a Chevy guy so I don't know that for sure. More of a gut feeling. Of course my Z has triples and a big cam, so I not be the most objective person on the issue... Quote Link to comment Share on other sites More sharing options...
Jolane Posted March 12, 2005 Share Posted March 12, 2005 My dad's 99 4x4 GMC Sierra Extended Cab with 5.3L gets 20-22 regularly, but the speed is a relatively constant 45-60 mph all the way (speed limits). This is with the 4L60 auto. Not too bad in my opinion, although I am sure if he stepped on it everywhere, it would go down. Does any think that the altitude would make a difference? We are 4500+ feet ASL, maybe the computer is compensating for the altitude (based on O2), so it just will use less gas, and have less power, at our altitude... Just a thought. Joshua Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted March 12, 2005 Share Posted March 12, 2005 I drive a 92 ford explorer 5 spd at 35mph and 850 rpm... thats for cruising on those 35 mph streets. Mash 1st to 35 then fifth not a good idea but its quirky and i like it, plus it doesnt require pretty much any gas to run at that speed but uh, acceleration, shift to 2nd... is this bad for gas milage just wondering? thats the main reason i do it. (i never push the gas to where u start to hear that "groan or moan" so it always sounds normal just like its idleing nothing like itss struggling to spool up) Quote Link to comment Share on other sites More sharing options...
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