Greetings C4 Members,

I have been thinking of doing some mods to my 96 LT4 Vert that would provide a bump in performance and sound, without modifying the internals of the engine. The only current mod is a Borla cat back system.
I would like to do the following mods:

1) ECM Tune - Any recommendations in the lower New York area?

2) Long Tube Headers - I was thinking of going with Exotic Muscle 1 3/4 primaries with high flow cats with extension pipes.

3) X Pipe - Replace center channel Borla section with X Pipe... Any vendors that make this for a 96 vert LT4?

4) Cold Air Intake - Any ideas?

5) Gearing - I was thinking of changing the gearing to 4.10s, but not sure if too steep.

Appreciate any guidance.

V

 

I don't think you're going to get a lot of extra power from a PCM tune on that engine. It's a mass-airflow design and will account for extra air pretty well, within the range of mods you're considering. The only mod I see that will actually increase power (flow) very much is the headers, and even that won't be more than maybe 20hp gain.

The car starts out with a cold air intake, doesn't it? I'm not sure what exact mod you're thinking about for this, but it already draws in air from in front of the radiator. I guess those claw intakes or cutting open the air filter housing would reduce restriction, but I don't think the stock intake is a major restriction at stock-ish power levels. It will make more noise, though, I guess.

The overall gearing on a 96 car with the ZF6 transmission is pretty tall, especially in the first couple gears (it's a fairly close-ration tranny) and in 6th (0.49:1!). A change to lower rear gearing would be a noticeable change favoring off-the-line and low-speed acceleration, without killing your highway economy and comfort too much. This question really boils down to the way you use your car.

As for the PCM tune, you may find it hard to find someone who can even plug in and communicate with your computer. It's a one-year, bastard-child OBD2 setup from GM and most modern OBD2 apps won't talk to it. The only tuning app I know that works these days is Jet DST. I think you'd be better off buying that for yourself (~$400) and using it to learn how to tune your own car. Your stock tune will work fine, and you can experiment with small changes to volumetric efficiency setting and/or MAF mapping from there. I believe you can also account for gearing changes within its constants as well, although I've never messed with that.

 

Go for a 4:10 gear and the long tubes connected to a factory cat-back.

No need to spend $$ on a cold air intake, the stock unit will be just fine.

 

 

4.10s all the way. I don’t find them too steep in an LT1 and with a higher revving LT4 they’ll be even better suited.

You may may want to start with the gears, they’ll give you the biggest seat of the pants gain for your dollar. You may even decide on passing on the other stuff.

You'll need some kind of tune to recalibrate the pcm for the gears or the speedo will be off.

 

Thanks for the insight! If I were to go with LT Headers and high flow cats, would the computer throw an engine code due to the increase in exhaust flow? Or would the computer automatically adjust for the change?

 

I believe it would adjust for the change in flow well enough. The problem you'll encounter with this mod is that you'll probably lose function for the two rear O2 sensors. They will be too far from the exhaust ports and not heat up quickly enough, and/or you'll need to extend the wire harnesses, and reportedly that doesn't work very well.

Those O2 sensors' only function is to assess that the cats are working properly on an OBD2 car. They don't affect the actually running of the car at all. But they'll set CELs and you won't be able to pass inspection. But with the Jet DST tuning software, you can turn that diagnostic test to "do not report," and it will no longer check them. So you'll probably need to do that if you need to pass a plug-in emissions inspection and/or you want to turn the CEL off the correct way.

 

I agree with Mr Miller. You are not going to get much of anything with a PCM tune unless you decide to drop in something like a hotter cam. Your LT4 is already pretty highly tuned. You will not need a re-tune with the headers as the computer system will account for it.

Coupling the long tube headers and bullet cats to your stock cat back exhaust is going to sound REALLY GOOD. The headers should not give you any codes.

From all that I have read over the years the stock dual exhaust on the LT1/LT4 is really pretty non-restrictive up to the cfm of air necessary for about 400RWHP.

 

4.10's are a good choice, long tubes with an x pipe and Borla mufflers not so much. I had the same exhaust and the drone inside the car drove me friggin nutz, even if many compliments from others on the outside. If you still have the stock exhaust those mufflers work very well in that setup.
If you do the 4.10's you well need to reflash the PCM for the speedo so you may check on the cost of that vs a full tune.

 

One thing to consider since you have a convertible is the under-car X Brace (assuming you still have it). It's my understanding that it's nearly impossible to do long tubes with cats and retain that brace. And you REALLY don't want to lose the brace, the vert body is like a wet noodle without it.

I agree with the others, do the gears first, that'll give the best bang for the buck. I have an LT4 vert as well, but I went the cam route, installing the LT4 hotcam. It's a blast to drive but could really use shorter gearing, with the extended rev limit it'll do 70 at the top of 2nd gear. I think stock was still over 60 in 2nd. The 4.10 gears would be my choice if I were to regear. Stock is 3.45 so going to a 3.73 just wouldn't be enough of a change to be worth the time and expense.

 

Thanks for your response. Can the computer be recalibrated for any gear set? i.e. 3.73s, 3.90s or 4.10s? I might go with the 4.10s... Thanks again!

 

I have ARH headers on my LT4 and the computer has never thrown a code. I am using high flow cats and have the after cat O2 sensors working fine.
I also have a cold air system. The stock tune will be fine with those mods.
I purchased a JET tuner off ebay a few years ago at a decent price and started playing with a few things such as cooling fans, CAGS elimination, a slight timing advance.
It has provisions to adjust the computer for gear changes as well (I have the stock gears though so I have not adjusted that).

 

On another note... Just played your YouTube video going down the track.
Sounded awesome! What shifter does your Vette have? Exhaust?

 

I imagine yes with the right cable/software. Mine is a 94, OBDI, it was easy to reflash with Tunercat and the proper cable. Your 96 is OBDII, not sure what cable/software you would need.

 

There used to be two software/cable packages for 96 cars: LT1-Edit and Jet DST. I think LT1-Edit is defunct, so Jet DST is the only option left. DST is literally Tunercat for OBD2. They purchased the software from Tunercat. So it's got the same user interface and all.

 

Thanks! It’s got ARH headers with cats going to 3” magnaflow mifflers. Drones like a **** bit sounds great. I’ll be going back to the factory cat back next year. Shifter is a hurst unit, not sure if they make it anymore

 

Is there any way to fix this problem? Some ideas that come to my mind are modifying to use self-heating O2 sensors, trying to bring the catalytic converter closer to the header pipes to act as the collector, or giving up on the long tube headers and trying to find mid-length headers or something in-between to permit functional use of the O2 sensors.

 

You can try bringing the cats right up to the collector, but you lose some potential tuning that way because that will become the effective termination point of the total collector volume (see Vizard's long article on exhaust tuning). Some people say they've gotten it to work.

But again, there's absolutely no reason to even run the rear O2 sensors at all if you just get the Jet DST tuning software. You can just turn off the self-diagnostic for the rear O2s and pass any plug-in emissions test you need to pass. They don't affect the actual running of the car at all - only the fronts are needed for proper close-loop operation, and those work fine if they are housed in the long-tube header collectors.

 

Well, even though I can run without the rear O2 sensors, I want to use them. I don't want any rigging of my car when I have it restored and modified to like a new GM production car.

I figured I was going to have problems with the long tube headers when I stumbled upon information about using catalytic converters with them. Maybe that is why they won't foreseeably be legal in any of the 50 U.S. states. I will give that article a good look over.

 

I had 3.91s in the Ghost and it was perfect for that time. I drove an LT4 with 4.10s and didn't like it. I stopped doing the 'spirited driving" and installed 3.54s to get better MPG on freeways.