Im at a turning point where i need to you know what or get off the pot.

Truck is a 99 Sierra Z71 gen1 vin r 5.7l/4l60e/3.73 rearend and engine is getting tired at 250K miles.

originally rated for around 255hp the engine is pathetic by todays standards and barely gets decent gas mileage (when new).

Truck is used to pull a 5400lb 32ft camper or a boat and my key goals are better gas mileage than the original engine and keep the RPM's around the same power band of the original engine.

My thoughts on engine setup. (keep in mind that i am not wanting a dragstip truck at all)

5.3l (lm7) bored and rebuilt to 5.7l with a suitable cam for my needs
5.3l (lm7) rebuilt completely with a suitable cam for my needs
6.0l out of the question because of gas mileage

My worries of the 5.3l is that i am going to have to rev the engine alot higher than what my original 5.7l would have been running at. (gas mileage and engine life suffer)
The 5.3l bored to a 5.7l will yield atleast 350hp and would keep it in a respectable rpm range compared to my "old" engine but how bad would the gas mileage be compared to the 5.3l.
Thinking either way going with a ls6 intake just to clean up the engine compartment. Either way i will get rid of the god for saken vortec distributor cap and rotor! Obviously i will have to go with a LSx wiring harness and computer.
I guess my main goal is gas mileage without power loss sufferage when truck is being used to tow approximately 10 times a year or so. Truck is used on the highway/interstate a substantial amount throughout the year. (traveling without towing)
rearend rebuilt 2yrs ago, tranny has 125k on it since its last (first) rebuild (tranny will be ran through again when engine is pulled) all valve body updates done, beast sunshell, kevlar 2-4 band, etc. already installed)

Your thoughts/opinions or foresight that i haven't considered above???

 

I wouldn't rule out the 6.0L.. having more displacement might get you better mileage than the smaller engine.. having more Torque to move the heavy loads of the boat/trailer would be a +.

With the smaller 5.3L.. you might need to step on it more to get up the hill and moving with heavy loads. As you noted, having to rev it higher to produce the same output (TQ) and to be moving them heavy loads. Where the 6.0L higher output well do the job better.

With the 6.0L. .say a LQ9.. your already at 385+ HP. LQ4 would be 345 HP.. so.. put in a mild cam and one that is designed for your towing needs, have it tuned.. your way pass the HP and TQ easily...!

I would stay with the truck intake. You well get better low/mid range power and torque. At the low and mid range RPM, the truck intakes will flow better than and car intake.

my 2 cents.. go with the 6.0L.. !

BC

 

If i drove the truck a substantial amount of time towing then yeah i maybe could see your point but since 95%+ of the time the truck will be highway/city driving(without a load) i just dont see the cost associated with the 6.0l (engine cost, gas mileage) being an economical choice. The camper usually is not towed more than 250 miles away 2 times a year(in southern IL which isnt exactly hilly), IL isnt called the prairie state for nothing.

(i have 2 complete lm7's sittting in the garage right now at a purchase price of $250 each)

I appreciate your thoughts, but i just dont think i can be convinced that the 6.0l would be a wise economical choice. It would be like telling someone to use their diesel chevy half ton as a goto work vehicle and it will save them money in fuel cost vs. a gasoline v8.

 

You tow a boat, want more power and be nicer on gas? You may want to look at the newest 5.3's and 6.2's in silverado/sierra trucks. The VVT and DOD technology are what get those trucks the advertised 21 mpg.

Get the complete pull-out, transmission and all, and wire it up in your truck. Throw in some free flowing exhaust and a cold air intake, and that should get you pretty close to meeting all of your goals with a gas V-8.

Those engines your already sitting on, you should have no issues re-selling them for the same price if you go this route.

My 2 cents.....

 

I dont see any reason a 5.3 cant pull what a 5.7 does, you said the 5.7 is mostly stock right? The 5.3 will put about 300-325 HP stock, well built it could put 400+HP out. If you cam it properly 400HP is very obtainable.

a Z71 is really not going to be the MPG king, but I bet you know that huh? The 5.3, I think, could get a respectable MPG in your truck but alot of variables come into play also. If you wanted MPG you would be driving a hybrid SUV and pulling a 1200lb pop up trailer. The 5.3 WILL do what your 5.7 is doing and has done.

 

Interedting, you see that is why I asked for all your thoughts. I hadnt considered the vvt ad dod engine. I'll have to do some looking into that. The 2 i have in the garage will have homes....one in my vette and one in a 2000-2005 tahoe/yukon when i buy it!

 

Oh the std truck 5.3l's are rated for around 285+ HP. But when comparing any engine to another (not alike displacement) you are comparing apples to oranges. Sure the 5.3L will PEAK more HP out than my old 5.7L 350 Gen1 engine but that peak will happen at alot higher RPM than my Gen 1. That 400hp build would yield pathetic gas mileage which is what I dont want.