Id do the 383 over a 408 any day as well. Much cheaper to build and for a DD truck is a perfect combo of torque and power. 4.8/5.3 blocks are a dime a dozen where as a good 6.0 block will set you back 4-500 bucks from the start. I bought the 5.3 block that will be the 383 for my Sonoma GT project for 60 bucks.

 

well since your replaceing everything.
find a junk yard 4.8, take your time and do it right.

 

Better gas mileage AND more torque simply by using smaller pistons??????? PLEASE explain how this is possible???? lol

 

Hmmm. It made sense to me when I was constructing that post, but allow me to explain myself.

Of the popular big-cube swaps, we have the 370ci, the 383ci and the 408ci. Between SOLELY the 370ci and 383ci, the 383ci will be the better daily driver setup. The 370ci will make more peak hp than the 383ci mainly because of the 370's bigger bore/valves/head potential but peak hp is not what a daily driver motor is after, average torque is the goal.

Between the 383ci and 408ci, no doubt, the 408ci will demolish the 383ci, and is actually my favorite engine combo (the 408ci is). But, when solely discussing daily driveability, the 383ci will do the job PLUS SOME, and return better fuel mileage than both the 370ci and the 408ci. Quick reasons; more average torque than the 370ci therefore less tps %, more efficient burn thanks to the tighter bore, yet doesn't have the gas guzzling thirst of the 400+ cubic inch motors.

 

The 408 has a metric assload of torque due to the big stroke -- in fact my high hp build netted more torque than horsepower despite making all my choices for high hp.

Essentially, he's saying the torque on the 383 is higher that a 370 because the stroke is .388" longer, but it has better potential gas mileage than a 408 because it has less cubes.

I kinda thought this is what you were trying to say. I think some got confused by the "better gas mileage than the 370 and 408" followed by "better torque than the 370". I initially read them as 2 separate statements, which I think is what you mean.

I buy this part. I'm not sure I buy that the 383 will get better mileage than the 370 because of more torque. After all it has more cubes to feed to generate that torque.

 

yea, that makes a bit more sence to me....

 

Indeed.

Not solely because it has more average torque, but specifically the smaller 3.898-3.905" bore will typically have slightly better thermo efficiencies which promote a more complete and efficient burn. That is why GM originally designed the first LS1 with a small bore, as GM was worried a large bore like the old school 350 would not pass emissions which were predicted to tighten up after year 2000. The larger the bore, the more difficult it becomes to pass emissions requirements.

Think of it this way: Pour two lines of equal amounts of gas out on the ground. One 3 feet long, one 4 feet long. Light them at the same time and you'll find that the flame hits the end of the 3 foot path quicker than the 4 foot path. Gasoline does not "explode" in the cylinder, it actually has a measurable burn time. Since an engine has a limited amount of time to burn the fuel, it becomes more efficient if the flame does not have to travel as far a distance.

This is why the 383 is my favorite DD engine when heavy SUV's are concerned, like my wife's. (No my I'm not saying my wife is heavy ) The 383 has the small, efficient LS1 bore with the torque and displacement you'd come to expect from a big cube motor. Whether or not the 383 will be more efficient than the 370, I will find out shortly. I believe it can and will. I see the extra displacement of the 383 over the 370 being negated by it's smaller, more efficient burn potential while the extra average torque will bring tps % down during errands and daily driving, hopefully requiring less fuel.

 

To get a 383 starting with a 5.3/4.8 motor, don't you have to bore the block out a bunch? A full tenth of an inch. Bore it a bunch and add more stroke and I can see a receipe for an expensive failure. I'm assuming you're investing in a custom crank, better rods, and forged pistons. You're talking about starting with a cheap block, but pouring a bunch of expensive parts into it. If I'm going to build a motor to make power, it doesn't make any sense to me to scrimp on the foundation. Spending an extra few bucks on the front end might be the best money you'll spend.

I know some folks that won't bore a 5.3 to LS1 specs without sonic testing to determine cylinder wall thickness. Do you guys building the 383's go this far? Or do you just assume the 5.3/4.8 block will be fine making big power with a big overbore and a bigger crank?

 

ALL 4.8L/5.3L blocks have the potential to be machined further than LS1 bore specs to 3.905", but sometimes you will find through sonic testing that certain individual bores cannot be bored past 3.898" because of the casting tolerances at the factory. If 6 of the 8 bores can go to 3.905" but 2 cannot, they must all then be the same. From my experiences with boring 5.3L blocks over, there seems to be a common trend with cylinders #2 and #6 being thinner than the others. Why? I don't know; but that doesn't matter to me. A 3.905" bore is not going to make any noticeable power increase over a standard 3.898" bore that is worth putting the cylinder wall strength at jeopardy by going that extra James Bond .007".

There is nothing wrong with a 5.3L block bored .118" over as far as strength goes. Yes, the 5.3L block was stronger left alone, but have you seen a bare 6.0L block? There's even less material! I'd bet top dollar the iron block is still stronger than any aluminum LS block that comes from the factory, except for the LSX and C5R block.

 

ok guys, in my research I have found a 4.1" stroke crank. Will this fit in my block and still be reliable? I did the math and came up with a 3.905" bore and the 4.1" stroke will net 393 cubes. Is this possible in a 4.8/5.3 block given all the cylinders allow the bore? Will it be worth the cost? The cranks i have found are the same price, not real sure on which length rods I need yet. Still researching as I wanna gather as much info BEFORE I start this project. Thanks for all your input.