LS3/LS9 Fuel Economy MPG

Discussion in 'High Tech Retrofits' started by SMCZ28, Jun 14, 2013.

  1. SMCZ28

    SMCZ28 Veteran Member

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    Alright I'm trying to make a decision between an LS3 BLOCk or an LS9 BLOCK. I know the LS9 is based off of the LS3, the LS9 is stronger due to the supercharger and it also has oil squirter to cool down the pistons, also it has a dowel for the main caps and uses 12 mm bolt size for the heads. I have no clue which to go with but I'm also thinking about possibly supercharging if I go with an LS9 but there is a LS9 supercharger kit for the LS3 and L92.

    One thing I'm trying to figure out is if a N/A LS3 that produces the same power as an FI LS9 gets better or worse fuel economy, reason being the vehicle will be my DD but I also want to try out pro-touring starting out with auto crossing and than moving to track when my skills increase a lot from doing auto crossing.

    I've tried looking, I see a stock LS3 corvette engine can get 26-30 MPG HWY, the LS9 gets about 22 MPG HWY but it does have a lot more power than the stock LS3 so if anyone with a modified LS3 knows their fuel consumption it would be of great help. :D or if anyone has suggestions I'm all ears since I'm confused about which block to go with, the LS9 has nice features but is 1,500 USD more.

    Set up I'm thinking of is either LS3/LS9 with a T56 Magnum with either an IRS or maybe a 3 link suspension.

    Thanks,
     
  2. Air_Adam

    Air_Adam Veteran Member

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    If you're worried about the strength of the block, then skip the production stuff and get an LSX block. Its substantially stronger than either the LS3 or LS9 blocks. The LSX block is engineered to handle 2500hp or something ridiculous like that. Fuel economy will be partially how you build the motor (heads, cam, induction, etc), partially how you set up the car (gear ratios, weight, wheels/tires, etc) and a BIG part in how you actually drive it.
     
  3. SMCZ28

    SMCZ28 Veteran Member

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    Sorry I forgot to mention I would like to go with an aluminum block over the iron LSX block but I think they make an aluminum LSX block but I don't see how that would be better. lol Honestly I don't know how I would drive, I really haven't driven anything with power except the 454 in the Suburban I had but even than I kept it at around 2k because I did worry about it since it has 180,000+ miles on it so didn't want to push it and bugger it up. Wheels I should have mentioned as well, I'm not certain but they will either be 17s or 18s and see if I can find a design as light as possible. Transmission gearing I don't think I will mess with, rear end gearing I was thinking however close I can get to 3.73. Since the 3 link I'm looking at comes with a ford 9 inch I'll get what gearing I can to be close to 3.73. Weight, I will try to get as light as possible with out sacrificing luxury like A/C, I was thinking lighter seats, rear seat delete, battery in the truck without a doubt, maybe a fiberglass or CF hood and trunk. I know the dry sump system will add some weight since it holds 10 quarts for the LS9. I might add some speaker stuff to the trunk. I was initially thinking 500- 600 TQ but I don't know how well that would do around the corners in auto crossing or the track.
     
  4. Protour-Camaro

    Protour-Camaro Veteran Member

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    If your plan is to autocross, you don't want the iron LSx block weighing the front of your car down. You want an aluminum block. Additionally, there is such thing as "too much" HP for flying around corners. I have seen lower horsepower cars get around the track quicker because the drivers can handle the cars better. I feel 550 to 600 rear ponies is plenty.

    There more to MPG than the engine's horsepower or induction...How do you drive the car? Overdrive trans? overdrive ratios, rear gear ratios, tire diameter, weight of the car etc....All effect MPG! I would tend to believe Force Induction would fare the worst MPGs with all things being equal.
     
  5. Thunderkyss

    Thunderkyss Veteran Member

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    The Corvette is going to get that kind of gas mileage because it is so light, it has a high rear gear & a 6 speed transmission the low gears give you the grunt you need to get moving & the high gears put you in overdrive to get your gas mileage.

    But power costs. You're not going to make HP with air, 500hp is going to take so much gas whether it's NA or FI.
     
  6. Protour-Camaro

    Protour-Camaro Veteran Member

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    :D

    Fuel economy and high horsepower don't belong in the same sentence.
     
  7. SMCZ28

    SMCZ28 Veteran Member

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    Well I didn't think there was going to be a BIG difference between the two but if the HP/MPG was worth it I would go with N/A or FI like if I got a little bit more power over N/A with FI and only losing a tiny bit of mpg I might go FI. I never really planed on going FI but if it helped and got a little better MPG I might consider it and I'm getting fond of the sound when it starts to go but really in a muscle car you're only going to hear it when you first take off and than the exhaust will take over from there. lol I knew HP should be in about the 400-600 range but what about TQ? I look at torque more than HP.
     
  8. BonzoHansen

    BonzoHansen Administrator Lifetime Gold Member

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    not sure why MPG is a big factor here, this doesn't seem like a budget build and you talk about racing. And after you initially spelled out BLOCK twice you went on to compare complete engines, so I'm kind of confused. Are we comparing complete engines or blocks?

    Quick look at MPG since the topic is at hand. Let's assume 22 and 28, 15k a year at $4 a gallon.
    Code:
    MPG		 22		28
    Miles per year	 15,000 	15,000 
    gallons used	 681.8		535.7
    $ per gallon	 $4.00 	 	$4.00 
    Fuel cost	 $2,727.27 	$2,142.86 
    Difference	 $584.42 
    This is probably best case scenario - it assumes all HWY miles, which of course is not the case. Drop gas to $3 and you savings is only around $438. Drop to 5000 miles and you are talking under $200. IMO MPG is a non-issue unless you are talking about 8 vs 28 mpg. It's hard to think saving $500 max a year impacts a 20k-35k drive train decision
     
  9. SMCZ28

    SMCZ28 Veteran Member

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    I was comparing bare blocks since everyone compares the complete engines when I was trying to do research of which bare block would be a better option to go with since there is at least a 1,500 difference between the two and I plan on buying a bare block to work with not a full crate engine and I explained it to see if someone would know if that 1,500 would be worth spending on the features the LS9 has over the LS3, I just wanted to factor in MPG to see what I would be getting into since when I first thought about building an engine I didn't think of doing FI since I was going with an LS3 and after reading more about LS engines and finding out the LS9 is the same as the LS3 I started wondering if the features were worth it and if so I'd go for it but if I go with an LS9 block I might consider FI.
     
    Last edited: Jun 15, 2013
  10. 71RS/SS396

    71RS/SS396 Veteran Member

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    I've built a similar car to what you are trying to build. If you plan on taking the car to a road course and running it hard I would stay away from any forced induction, trying to manage the heat they generate is nearly impossible, you will have good power for about 3 laps and then the IAT's will rise and the power will fall off, you simply can't package a large enough intercooler into the front of a 2nd gen. If you want to use an LS3 use the dry sump version, controlling the oil will be one of the most important things to consider if you make the car handle real well. You need to spend money on an oil cooler, diff cooler, and power steering cooler if you seriously track the car. My car with the LS7 makes 515 rwhp and it's plenty of power for the road course and too much for auto-x. There are very few cars that ever pull me in the straights. I can manage about 20 mpg on the hwy but my gearing is optimized for racing not mpg, I'm running a close ratio 6 spd with .74 6th, 3.89 gears, and 26" tall tire so my rpm's are fairly high @ 70 mph.
     

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