3g hemi gas mileage

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5.7 hemi

5.7 hemi

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Some of ya might have been over to FBBO and seen my early b body build with a 5.7 hemi and 545rfe trans. Well I've been throughly dissapointed with mpg.....13.5. This was checked with a full tank, drove 70 miles a day for 3 days, all freeway and filled up again to check.

My combo is an 03' 5.7 hemi truck engine, 6.1 hemi injectors, cnc made adapters to clear the timing cover so I could use a car intake, TTI headders with their 2.5inch x-pipe exhaust, 742 sure grip center with 3.91's, and 275-60-15 rear tires (28 inches tall). It shifts through all gears and I can feel the tc lock up. The car weighs about 3600 lbs with me in it and a full tank. This is flippin' sad to say the least.

I called Chris at hotwireauto and he said that I need to move the o2 sensor to the passenger side from the drivers side and that my gears are to low, need to be 3.55's. Sorry, but I'm calling B.S. on both of those. I checked the o2 sensor and it was sooty and I cant figure out why. I have double checked the speedo against my wifes Iphone and it's on the money.

I will say the car moves with authority and will boil the tires if you look at the accelerator pedal wrong. So WTF!!!
 
Have you talked or heard from others with similar combo?? What are they getting?? Doesn't sound too far off to me.. But what do i know??
 
My 2007 Charger R/T gets 13+ around town with a normal lead foot. It gets 16-17 when I take it easy and let the MDS take affect.
On the highway it gets up to 25 mpg with the MDS and cruise control engaged for most of the time.
 
If a Ram truck that weighs in 5000+ lbs can get 20 on the hwy, no reason why my car that weighs alot less should not get better than 13.5 period.
 
The tps gone bad that or something isn't reading right with the ECM.
 
5.7 hemi said:
My combo is an 03' 5.7 hemi truck engine, 6.1 hemi injectors, cnc made adapters to clear the timing cover so I could use a car intake, TTI headders with their 2.5inch x-pipe exhaust, 742 sure grip center with 3.91's, and 275-60-15 rear tires (28 inches tall). It shifts through all gears and I can feel the tc lock up. The car weighs about 3600 lbs with me in it and a full tank. This is flippin' sad to say the least.

I called Chris at hotwireauto and he said that I need to move the o2 sensor to the passenger side from the drivers side and that my gears are to low, need to be 3.55's. Sorry, but I'm calling B.S. on both of those. I checked the o2 sensor and it was sooty and I cant figure out why.
Click to expand...

Are you dead sure the TC is locking up?
Is the tire diameter the same?
How is the exhaust configured on each vehicle?

5.7 hemi said:
If a Ram truck that weighs in 5000+ lbs can get 20 on the hwy, no reason why my car that weighs alot less should not get better than 13.5 period.
Click to expand...

So it would seem. But the few, but noticeable differences are there and this could be the issue.
 
Sounds to me that your not tuned? Those 6.1 injectors will dump fuel if you are running a 5.7 tune.
 
And your only running 1 O2 sensor so I'm assuming you full time open loop? What PCM are you using?
 
5.7 hemi said:
If a Ram truck that weighs in 5000+ lbs can get 20 on the hwy, no reason why my car that weighs alot less should not get better than 13.5 period.
Click to expand...


I have an '03 5.7 Hemi in a Ram truck and I get nowhere close to 20, more like 10.5 to 11 on a good day.
 
I drove an 03 5.7 truck with 2 dirt bikes in the back on a 1500mile round trip and it never got 10 mile to the gallon the whole trip at 70 mph.
 
What computer are you running? I have a few different fuel mileage numbers from my experience so far, but I'm not even fully dialed in on my tune yet. When I had my Hemi carbed to start with it was getting between 13-16 depending on what jets and rods I was trying out that day, but I couldn't get the top end out of it I wanted. I just switched over to fuel injection and during initial tuning I saw 6, lol. But most of that was spent running in the driveway or wide open throttle trying to get different parts of the map tuned in (running a Megasquirt). The last tank I put in I was up to 12-13, but I've been having trouble with my accel enrichment kicking in because of a noisy TPS signal. Just got that sorted out, but don't really have any runtime on the car with the fixed signal. GDemon has pretty much the same combo as me (5.7, non O/D 833, 3.55 rear gears) and he got 20 on the highway over a full tank, so that's what I'm shooting for. Talked to a guy at a show with a 74 Cuda that did a 5.7 swap and he was in the same ballpark of high teens low 20s. I think my tires are around 26" inches tall right now and I cruise 70 around 3k rpm or a little over. I haven't weighed my car after the FI swap, but after the Hemi swap I was coming in around 3250 with me not in the car so I don't think it has changed too much.
 
Alrighty lets try to get some of the questions answered.

03' hemi 545rfe trans, stock computers (althougth I raised the shift points and firmness)
28 inch tall tires, 6.1 injectors, TTI headders with x-pipe exhaust, car intake manifold, 3.91 rear gears.

I switched the o2 to the passenger side per Chris at hotwireauto.

After much thought on the injector size, I might be to big with the 6.1 injectors so I'll switch them back to the factory 5.7 injectors as johnparts has eluded to.

The 03 hemis use only 1 o2 when using the aftermarket harness from hotwireauto, 04 and up use 2, again per Chris at hotwireauto, besides, I did try both and the engine ran like ****.

Gonna back flush the block tomorrow, so I'll switch the injectors at the same time.
 
Just curious...what RPM are you turning at 65-70 mph on the highway? I had planned on 3.91's in my Duster with the 5.7/545RFE....but may rethink that back to 3.73's or 3.55's.
 
At 60 mph its turning 2000 and at 80 pmh 2500 rpm.
 
I was going to use 3.91 gears as well. Drove a 2010 ram with 5.7, 545RFE, and 3.91 gears, at 70mph, It turned 2000 RPMs, thought that was a great number.

I was hoping to see at least 20mpg on the highway from my 05' 5.7 and 09' 545RFE.
 
My 2011 Challenger R/T is heavier, has 3.91 gears (super track pac) and the 6 speed manual. I believe it turns similar RPM's and doesn't have the cylinder shut off being a manual trans. I do get mid to lower 20's on the freeway cruising 80 or so MPH
 
talk to bob Reams here in AZ... it gets 27mpg...

imagine injection
602-377-4093
 

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5.7 hemi said:
Alrighty lets try to get some of the questions answered.

03' hemi 545rfe trans, stock computers (althougth I raised the shift points and firmness)
28 inch tall tires, 6.1 injectors, TTI headders with x-pipe exhaust, car intake manifold, 3.91 rear gears.

I switched the o2 to the passenger side per Chris at hotwireauto.

After much thought on the injector size, I might be to big with the 6.1 injectors so I'll switch them back to the factory 5.7 injectors as johnparts has eluded to.

The 03 hemis use only 1 o2 when using the aftermarket harness from hotwireauto, 04 and up use 2, again per Chris at hotwireauto, besides, I did try both and the engine ran like ****.

Gonna back flush the block tomorrow, so I'll switch the injectors at the same time.
Click to expand...

Right now you fuel map is opening injectors longer then they need to be opened because the 6.1 injector flows 32.3% higher flow then the 5.7 injector this is problem #1. Problem #2 is I don't think the system your running is allowing closed loop fuel trim adjustment with a single o2 sensor. So you need a wide band o2 to see how rich you are running and adjust the tune accordingly if you have the ability to modify the tune. That's why I really want to run a factory PCM and harness in my build and that's why I don't have a running engine right now. Chrysler spent years on the factory calibration to get 20+mpg. So if you can't tune it make sure it's completely stock or you will never get the mpg you expect.
 
Iam running factory computers. Factory Rams use 1 o2 before the cat and 1 o2 after, so since no cat you use only 1, thats the logic as it was explained to me when I got the harness. The only tuning I have done was a 91 octane, raised the rpm limit, and shift firmness. I took out the 91 octane after figuring out the mpg, but have left the rest, so its basically all stock computers, no aftermarket such as fast, mega squirt, accel etc.
 
Got the 6.1 injectors swapped over to the 5.7 injectors. The throttle response seems a tad better. Filled up the tank, will check mpg in a few days.
 
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