Its pretty well established that a ls1 corvette can reach or exceed 30mph. I would like to make the most efficient LS engine that I can. My goal is high, 40 mpg and 400 hp without NOS or a turbo. A DD racecar! I don't plan on hitting both simultaneously due to VVT and AFM. Is it possible....maybe but I going to try.

The car I will be using is a 1987 porsche 944 which I plan on getting a custom geared 968 transmission with LSD in the future which will give me a tall over-drive gear along with a decent gearing ratio all the way through. The car is currently 2900lbs with nothing touched. There is no reason I couldn't drop the car weight by 200 lbs within the next year. The weight difference in the engines in not much at all. Its an iron 4 cylinder to an aluminum 8. I believe the frontal area is smaller than a corvettes so I should be starting ahead of 30mpg. Other motifications to happen: A/C delete, race seats, electric fans, fiberglass aero body panels, lightened flywheel, light clutch, etc.

My questions for the ls1tech forum are about the engine build itself. I'm thinking a ls2 with a lightweight internals. I'm thinking of honing it out to 4.030 with very tight tolerances for a good seal. Then I'd destroke the engine down to 5.7L-ish. Less friction than a ls1 5.7L and I believe an over-square cylinder will give me a more complete burn. I plan on road racing the car so the small the cubes, the better since some classes are engine size dependant. If I were to run an AFM system and VTT, I should be able to detune the car on the highway to attain a better mpg. I could run L92 heads for all the air I'd ever need from a fast intake and a ram-air intake. Would I needed the heads ported and polished? Would there be a large advantage to an underdrive pulley?

As far as the cam, valvetrain, etc. I'm all ears. I won't pretend to know a lot about timing but that's why I'm posting in the advanced engineering. I cannot seem to find a whole lot of info on mpg builds and I figured you guys are the pro's.

Please keep the flaming down to a minimum. Its not entirely impossible but maybe I don't quite reach my goal. We'll see. This project is a major stretch that I'm hoping to finish in the next year or two. Any comment, especially if you have experience with a similiar build, would be greatly appreciated.

 

It may be doable but there are two problems i see in this theory.

The first is your exception to tolerances. tight tolerances and high RPM's do not mix. tight tolerances and aluminum do not mix. Small motors need to spin faster to generate more horespower which produces more heat which does not mix with tight tolerances. Aluminum has a stretch or heat distance much greater than iron. Tight tolerances can also cause rings to clamp against cylinder walls and cause piston ring seals to crack faces off of pistons.

You will be playing the game of efficiency more than the game of lightweight as far as the motor is concerned. MPG as a whole deals with how much weight the motor has to move around. a 2600 pound car versus a 3800 pound car for example. I would look at how much you can remove weight wise to also better help you achieve your goal.

This is my personal understanding of VVT but i could very well be wrong. Ignore it if there is a better suggestion or take from it what you like. In order to to take advantage of VVT you would first need to map two different cam phases on opposite extremes and phase between them. How you want to do this is very subjective - there are camshaft profiles at each extreme that vary differently on individual platforms as it is - to have both extremes on a single platform while also throwing in the desire for economy before HP then it becomes a bit complicated and very personalized to your build. Intake valve sizes, piston face design, changing dynamic compression ratios, static compression ratio, and a load of other variables like intake efficiency and port size, rear axle ratios, fluids throughout the vehicle, etc.

Economy is a hoshposh of many many variables all working together as a whole - even tire profiles and wind resistance. Just my .02

 

Damn, I was hoping this was in a Camaro.

I want to build a reasonable lighter weight Camaro with higher MPG; it has to run 11s though.

 

Your comment on VVT and combustion chamber shape is exactly why I posted in the advanced tech section. I'm not sure even where to start with establishing any of the many valvetrain and combustion chamber variables. Every variable seems to be dependent on all the others. It seems like there should be a certain extreme that lets me hit that 400hp mark and then the other extreme will be a detuned version of that setup.

As far as my tight tolerance point, I was thinking more along the lines of getting a good hone that's more circular allowing for a better fitting ring. You do have a very good point though, I hadn't thought of the fact that a high spinning motor would not allow for as tight of a tolerance.

I do have a large majority of the other car details worked out as far as non-engine economy details. Wind resistance is a huge factor that I will definitely be working on since its probably the largest factor of economy.

 

most of your goal is going to come from a incredible TUNE. with a higher afr like a 18:1. on the highway and a little richer around town. but the prob with a higher afr is heat so to keep the engine cool would be the best thing along with vaporizing the fuel as best as possible as it comes out of the injector and as it makes its way into the combustion chamber. so good heads with a good cam will be ideal but your gunna need low end power to so you can run low rpms. i had a 2000 ss that made 35mpg on the highway. wasnt to hard but you had to work for it haha

 

off topic, but are you the same guy who has gotten flamed several times for asking this same question?

Good luck with the question at hand tho!

As far as mpg goes, weight weight weight.. gears gears gears, tires tires tires.

 

18:1 is incorrect and too lean. 16:1 is considered the absolute maximum and is dangerously lean. 14.7 is the stoichiomic 1 to 1 ratio of gasoline to oxygen and therefore 14.7:1 AFR is technically the most accurate to offer the best burn in a perfect bubble. With friction and inefficiency involved this number is realistically around 13:1 for power. Most modern cars rolling off of the lots are sitting around 14:1 to 14.7-1 AFR however, and the timing as well as plug temps are tuned for the added heat and response. If you're tuning for economy then it is not something you can really tune for ahead of time but something that must work together as a whole. Efficiency of one part does not guarantee efficiency of another.

I have not spec'ed a VVT cam before and really have no clue how - only how they work and what extremes you need to consider independently to make it work. As far as actual grind numbers? No clue - i would be giving comp-cams a call and talking to them directly as they understand the phaser as well as its limitations.

Also, this might be worth reading:

http://www.gmhightechperformance.com...l/viewall.html

 

^ i got that afr number from a 12' grandsport on a wide band with a factory set cruiselean right nexted to a gm tuner that said it was completely normal for that car on the highway. i think if gm put that on a factory car its fine.

 

bwkmaro: Could you give me the exact specs on your car that got 35 and the grand sport that was running 18:1? I'm guess they weren't running completely stock especially at 35mpg unless you are a god at eco-driving.

01blue: I am not the guy that got flamed before but that guy did get pretty hosed. That's why I was a little tentative in making another post of mpg but I tried to learn as much as I could before I posted. Though I will admit, I have posted some things in the past that I should have gotten flamed for like asking about a DD 550hp 5L engine for $3,000.

 

I got my numbers from books - one of them from Jeff Hartman and the other chevycraft.

I will say if anyone attempts to tune a new motor to 18:1 on purpose they are asking for trouble. Try it and see for yourself if you want to tempt fate with nothing but boltons. Running lean is no laughing matter when you start throwing timing into it to make power. Detonation, combustion chamber temperatures skyrocket, and hot piston faces no longer cooled from the IVO event of atomized fuel droplets, compression rings get hot and begin to rob horsepower putting extra strain on the pistons. Next thing you know piston chunks are in the bottom of your oil pan. When fuel injectors go on big cube builds, nasty things happen all because of lean mixtures.

Pull the timing and run the proper spark plug and anything is possible - but that does not mean it is more efficient. The OP should be careful temping this state of mind as it is very possible to run into problems on pump gas.

 

I have my 408 I'm my 04gto setup for lean cruise and and while in lean cruise it sits right around 17-17.5:1 AFR, but I also am running a fully forged bottom end and coated pistons.

A big thing safer as cooling goes during lean cruse is good pistons end they have lots of great coatings out there to help control heat.

 

I will be running a set of forged pistons for sure. Its probably not entirely needed but it gives me a piece of mind.

Has anyone messed around with Active Fuel Management or DOD? I forget which one is the term GM uses. What would stop a person from having it controlled by an on/off switch. If I were to just keep four cylinders running on the highway and then turn the others on later for racing, what could go wrong? I haven't seen any of these systems on a manual which is what my car will be. Is this a problem?

 

the camaro wasnt my car i work on the weekends for a gm tuner in his shop learning how to take his customer base . and it was just a small cam car (idr the specs but it still used beehive springs it sounded stock) with bolts-ons, a fast 92 and 3.27's. it had a stock block. but i spent like 3 days tuning the thing 95% of that was idle, cruise, light acceleration and deceleration. the guy paid a bunch of money to get a "better than factory tune" as he put it. that car ran great and the cruise lean i actually found that made the most power without heat probs was 17.3:1. so im not saying 18:1 is better or a sweet spot or even ideal but i saw it with my own eyes. i could have written that book....you read for all you know

 

I think getting 40mpg out of an LS car would include at a minimum these things:
- ALL IN THE TUNE!
- An expertly designed camshaft
- A fairly lightweight vehicle
- Rather skinny tires for the rolling resistance factor (225's or less)
- A maximum of 1400rpms on the freeway.

I like this idea, and I think usually manufacturers have found the best mpg area to be in the 55mph range. Good luck!

 

-2.73 rear end ratio
-.50 6th gear
-low rolling resistance tires
-lower your car for slightly better aero
-as mentioned, a well designed custom cam and complementing tune

I don't see why you wouldn't be able to hit 40mpg with taller gearing, narrower tires, better aero, and less weight than a vette. You may want to check out some hypermileaging (read: prius enthusiasts) forums for some tricks we can't think of.

 

I have been on the ecomooder.com website quite a bit. I have to give them a ton of credit for keeping a ton of data. They plot the changes in mpg for every change. I wish there was data like that for HP on a ls1 though, no one in their right mind would pull apart an engine to only change one part at a time.

I do understand all of the aero and transmission stuff. I really need help understanding how one would modify VVT and DOD along with what the optimal engine setup would be. Hence why I posted on ls1tech.

 

Honestly, I think you're better off with a custom non-vvt grind then an off the shelf VVT grind. DOD is going to severely limit cam selection also. Plus, can you even get DOD to work with aluminum block LS motors?

Edit: you could take a aluminum 5.3 block, put a 4.8 crank in it, high compression, some trick flow 205s, a FAST, custom headers and exhaust (small primaries and small exhaust diameter and ceramic coated), custom grind cam, and tune it for gas mileage. That would be a pretty economical imo, it won't be lightning quick... but economical.

 

This ^^^

 

cheat and use EGR.
by cooling the combustion somewhat, your options of running timing advance will be more open.

 

I'm not exactly familiar with EGR on lsx blocks. Do explain.