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LS1 intake manifold and throttle body options?

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(I'm not planning on starting this project now, just gathering information for fall projects.)


99 Sierra 1500 GMT800 5.3L V8 4l60E(auto) 3.42 G80 RWD, 79k miles, fuel injected, brand new fuel pump - all stock


Yes, some people say the 5.3L is too small, get a bigger engine. I want to keep my truck for 10 years and I do not want to kill fuel mileage (for normal driving) by putting a massive engine in it. Besides, with a larger engine I'd probably want/need a better transmission, new driveshaft, etc. -- lots of money just to waste more money at the pump. I'm happy with the 5.3L; I just want to add some more power for passing, towing (later), and bad highway merges/ramps. Plus the projects are fun.


This is a Vortec LS1 328 ci small block engine, right?


1. How about a large intake manifold and larger diameter throttle body? My stock throttle body has a small diameter of ~75mm, which limits how much air can enter the engine, and the stock intake manifold has the same limitations. I found an article mentioning a FAST LSXRT intake manifold, 102mm diameter, but they mentioned trouble getting a throttle body that size for an LS1. What are my options here?


2. Engine internals: camshaft, pistons, springs, etc. what can I swap in to get more power on the low-end without killing durability. I don't care if high end horsepower drops off sharply. The tach rarely says > 3,000 rpm even with a tune. I haven't tried the 91+ octane tune (need to get the fuel) yet but most likely I would see no benefit from anything > 4,000 rpm except occasional towing. Some people say drop down, 112 something. Currently my 1st gear acceleration is underwhelming except when mashing the pedal; gears 2/3/4 have plenty of power with even light pedal pressure.


3. Forced induction, maybe 1-2 years from now. Only way I could do this would be to just find parts from junkyards (or rebuild) and piece together a kit. This would be a huge project for me, probably take me a year just to find everything. But I would like to do other upgrades first to increase capacity and get the engine set up properly, that a forced induction system could later take full advantage of. Not looking for too much boost, probably below +10 psi.


I will be doing an electric fan conversion, but I have another thread about that.

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I thought the 5.3 was or are 325ci, at least mine is, the way your talking your going to spend some serious money if swapping internals and forced ind even though that's further down the line, best bet right now would be to slap on some headers with exhaust and tune into that which will put some pep into it, as far as forced induction I think you can put up to 3psi safely on the motor without swapping internals and tranny parts but tthe kit is expensive. And im sure there are plenty of posts on here about all this justt need some research.

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Thanks, yeah I'm just learning and have lots of reading to do...thought maybe someone would chime in with a good idea. :ninja:


My exhaust system's fairly new, probably a factory replacement since the muffler is HUGE - not looking to change the sound, quiet is fine for me. A better-flowing exhaust sounds like it would complement later engine work. I'll look into it. I've read that increasing flow decreases backpressure which reduces torque. Is it true? Also, the truck is 12 years old -- any improvement to be had from replacement cats? How do I tell if they were replaced recently, lack of rust?


The main reason for not looking at exhaust upgrades is that I wasn't sure what kind of forced induction setup I wanted later on -- no sense in buying a shiny new exhaust setup if it has to be cut up in a year or two. Electric chargers are junk (except the expensive real ones, but they require big battery packs and function more like "electric nitrous"). I like the idea of a turbo, but most don't add HP until like 3000+ rpms, plus they require intercooling. A supercharger would be a better fit.


The FAST intake manifolds I mentioned are expensive and 102mm sounds a bit large. Some other models are 90-92mm, which should still be a noticeable improvement and the throttle bodies should be a little less expensive. I've been told that this is good prep for a high-lift cam -- gotta read up on the number and how they affect my engine. That kind of cam may be complemented by a new valve train. However, a supercharger often installs in place of the intake manifold, no sense in paying for one if the supercharger would just replace it.


More air means more fuel is needed, so I've been told injectors should also be upgraded. Not sure about the stock fuel pump.


Someone said Camaro/Corvette parts are direct bolt-on and make good upgrades. Any ideas?

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Hmm...you got me reading about transmissions now. :D


The huge rpm drop between 1st and 2nd gears is apparently characteristic of these 4 speed transmissions. The newer 6-speed (4 + 2 OD) has so many advantages over the 4-speeds (3 + 1 OD). Better fuel mileage, more consistent power, smoother shifts. 1st gear is like 4.00 and it's a computer-controlled double-clutch unit.


For what I want (mpg, smoothness, and power), upgrading to the 6-speed would be much better than the engine upgrades. Too bad the 6-speed won't work in my Gen. III engine (specifically, it's PCM, sensors, and harnesses). Two ways reported to work are to swap in a compatible engine + transmission or to upgrade electronics on my old engine so that it can communicate with the newer transmission. (The old PCM is retained to work the water pump and some gauges.) Either way would require a lot of time and money -- I would have to cancel other projects and save up for a while.


I'll read a bit more about engine swaps. Is it possible to arrive at a significantly better acceleration:mpg ratio by swapping in a larger engine (like the 6.0) and changing the rear from 3.42 to 2.7x gears, or would the difference be negligible? Someone reported an OD cruising 30-35 mpg w/ 400 hp stock engine (~6.0?), 6-speed transmission, and 2.7x gears, but the swap was like $10,000 at the time. Maybe costs have come down...

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