LT4 LT4 Tuning (long)

[rcdk]

When I bought my LT4, it was a huge step up in performance for me.

After I bought it I started frequenting corvette forums and listening to people talk about their LT1 and LT4 vettes. Comparing their experiences to mine, I keep coming to the conclusion that I must have a serious tuning issue.

I am just not getting the kind of tire-roasting torque other people talk about from LT1s and LT4s. Are they full of it?

I have changed the fuel filter, plugs, and wires. With each one I there was a noticeable change in the performance level, but nothing like other people brag about. In fact, I still rarely end up triggering ASR, even in the rain, and never for strait line acceleration.

Actually, I have noticed that there is a lot of difference from one day to the next. But even on the best days I am somewhat underwhelmed.

Other than a lack of power, the only other symptom is a slightly less than perfect idle, especially with the a/c on. It sounds like the engine is struggling to provide enough power for the a/c. I get no codes, and about 17 mpg mixed highway/city driving.

So what to do next? Opti? O2 sensors? Buy a real OBDII diagnostic scanner (not just a code reader) and figure out what the problem is?

 

[Mad-Mic]

hmmm....

i'll give it a shot....there are tricks to the trade to find a few extra ponies in the motor with minor tuning that some if not most don't know about. 1 is even installing plugs (besides the bosch plats +4's) make sure the gap of the plug is indexing towards the middle of the cylinder. Very simply take a marker and mark a line down the ceramic resistor part and line up that mark with the middle of the cylinder. Believe it or not when you index the plugs in that fashion it does improve horspower! This will also help lean the car out, which you want (from the factory they run very rich) it's not much leaner but it helps 2. Gap the plugs a bit more say if factory calls for .045 gap them .047 this gives you a longer hotter spark even though your spark plug that is recommended is good but this burns the fuel air mixture more efficient. this is a good trick for guys that may be having probs passing emissions by a smidge 2. try using a k&n filter, i don't see that you got it listed, with a open air box. and maybe a good set of mufflers. with these simple tricks and mods your car is still basically stock but probably would see close to 30+ hp in difference and i would say 20 lbs more torque

i'm sure someone will chime in and give more advice but if your car is stock look at the numbers i think it's 330 hp and 340 lbs of torque where do you want to go? the ones i listed are basic easy stuff i'm sure you know it's about money....lol from what i hear that thing will run! With what i recommened you'd be at around 360 hp and 370 lbs of torque Click Here for 96 LT-4 motor specs for more info on your car

Here for General 96 info

Hope this helps!!!

 

[P-ColaVette]

LT4 won't engage....

Hello rcdk !
I just wanted to say that based on what your telling us in your post, there most definitely is a problem with your LT4. I drove several LT4 powered vette's before buying mine, and they all would trigger the asr when full throttle was applied from a standing start. I would strongly urge you to have the car's computer looked at, as well as the catalytic converters, as they may be stopped up. You didn't mention the mileage of your car, that may be an indicator of where to look. I have a '96 LT4 CE vette, and with very limited mod's, ( see my posting in "going to the track on sunday") my car has run a best of 13.1@105mph in the quarter mile. My car has 87,000 miles on it as well !



Steve.

 

[rcdk]

I have about 41k on the car.

It has a K&N, but no open lid.

Fuel pressure to about 42 with key on, engine off. Drops to 35 when the engine is idling, 45 when regulator is unplugged.

Vacuum is increases as engine speed increases, and jumps when you let off the throttle. I have read that jump is an indication that your exhaust system is not plugged. Not sure if that holds for an LT4 since it doesn't have an EGR valve.

Oh well, I need to get a real scanner anyway. Wish I had known there was such a thing before I spent $150 on a code reader.

Any suggestions for a tuning scanner setup? Looks like $300+ for most commercial versions, with some hobbyist produced systems running $100+.

 

[BubbleHead]

Like cola said, check your cats!!!! If they are clogged, they will kill your power.

Your LT-4 is able to destroy the tires off the line.

 

[rcdk]

Just hooked up a pressure guage to the air injection lines on both sides.

I get _NO_ exhaust backpressure. If I leave the lines open, I do get a little exhaust flow, but it isn't much and it isn't loud at all like an exhaust leak would be.

Thing sounds like it has picked up a miss now.

 

[Trance_LT4]

Is there a computer program or plug-in you can use to mess with the timing and fuel map, ect ? Because I can't really notice it when I'm driving, but the dyno graph shows my car has a slight stumble above 5000 rpms, like it has a fuel or spark problem. It still puts out over 300 rear wheel horsepower, but I'm losing a good 5-15 hp from what it should be making max-wise. Any suggestions? I disconnected my MSD6a and it actually ran a better dyno.

 

[rcdk]

Yes, you can use these things to get sensor data.

Check out www.carputing.com, but you better be sitting down when you price it.

There are some hobbyist projects out there for around $150.

http://www.abcwc.net/accounts/quanta/order.html

 

[P-ColaVette]

No back pressure...

Hello rcdk
I think the best thing to do now, given that there is no exhaust restriction, would be to take it in to GM, to let them run a diagnostic on it as a good baseline. As for a scanner/programmer, I would suggest e-bay. I actually bought a Hyper Tech Power Programmer Three from a fellow CAC member for about 1/3rd of the price of a new one. Someone may have had a power program of some sort loaded into your cars computer, and did not fully or succesfully reload all of the factory parameters back into your car prior to selling it. By taking it to a Dealer, they would reload the factory programming in order to be able to run a complete diagnostic. Please keep us all advised as you search the problem out, as it may be something other LT4 owners will encounter in the future !



Steve.

 

[Trance_LT4]

Hypertechs don't let you mess with anything important though, just rev limiter, speed limiter, fan on/off temp, and shift points (on crapomatic transmissions). Nothing important like timing advance, air fuel ratios, ect ect ect

 

[vetsvette]

rcdk,
I agree with P-Cola. Let your dealer get a look with his scanner/computer. Get it running up to snuff in stock configuration before modifying anything. It does sound like you are way down on power.

 

[rcdk]

Going to the dealer tomorrow at 7. I will post the results.

I loaned my compression tester out so that is one thing I haven't checked that has me a little worried. Two of the AC plugs were missing the pucks, but I am not burning any oil.

 

[Redbob]

Connecting a pressure gauge to your exhaust air injection ports won't tell you a thing at idle - there has to be a significant amount of exhaust flow. Best would be to check it with both the engine and the exhaust system fully warmed-up at say 4000 RPM, at WOT.
6000 would be better; wide open throttle is key.
You'd like to see less than 15 in. Hg backpressure at 6000 RPM, less than maybe 8 or 10 inches of mercury at 4000. If you see much, much higher than that, expect to find catalyst melt-down, possibly even mufflers plugged with bits of ceramic catalyst substrate.
But you really shouldn't have a catalyst problem unless there was an engine problem causing it such as a stuck injector dumping in lots of excess fuel, or an ignition problem also resulting in unburned fuel.
I think I'd start looking for an ignition problem.
Good luck;
- R

 

[rcdk]

Got a call from the dealer.

Water pump and opti are bad.

They also wanted to do the intake (oil) leak.

They wanted $1600 for the work, so I will be doing it myself.

Any recommendations?

 

[69MyWay]

rcdk,

This is a convertible, right? Chances are you don't have a performance rear axle in it. Is it possible it is a standard LT1 with red paint on the intake and LT4 nameplates?

The convertibles don't have the explosive acceleration as they usually don't have a performance grade gear ratio.


The opti spark is a common problem. The intake leak is also common.

This work is not too hard to do yourself.

 

[rcdk]

I suppose anything is possible, but if it is a fake someone did a good job.

It is a 6-speed, and has the codes for a performance axle.

I am considering port-matching the intake while I have it off, but it sounds like that doesn't really do much for it until pretty far up in the RPM range.

 

[vetsvette]

If the 8th digit in you VIN is a 5 it is an LT4. If it is a P it is an LT1. When you do your intake, replace the gaskets with GMPP#12367777. These more closely match the port sizes than the stock gaskets.

 

[Wheelie]

I had the same lack of performance on my LT1. Turned out to be carbon on the pistons. This caused detonation and the computer set the timing way back. GM top ngine cleaner did the trick.

Port matching the intake will net you very little gain. But if you can do it yourself then by al means do it.

Steve

 

[Mad-Mic]

i think it does help

I am considering port-matching the intake while I have it off, but it sounds like that doesn't really do much for it until pretty far up in the RPM range.

i have to disagree you'll probably see a small amount of change BUT with any other mods you do you will see a Greater change from this mod. and since you already have it off the car and for a few extra bucks it's worth it in the long run....Rule #1 to get more horsey's. Got to get it IN and OUT as fast as you can! if you add a good hi flow cat and exhaust that all ties in together like a puzzle. one piece out and it just don't look right or in this case feel like it should have more horseys than it does. If you ever decide to upgrade to a bigger throttle body ( not sure what LT-4 throttle body size is)and fuel reg you'll be glad you did it now but it would be nice to match it up on the throttle body stock or bigger one with the port and polish job only other thing with out rebuilding the whole motor is port polish maybe punch out the intake valve alittle and valve train and upper is done......for that 1600 you should be able to get that done if your willing to spend the money....then that thing will run faster than you thought it ever would

Do yourself a favor if you do the heads make sure you use NEW push rods! (do them there LT-4's have pushrods? I'm just a redneck :L)

 

[P-ColaVette]

Crapti-Spark !

Hello rcdk !
I am glad that you at least have a good cause for lack of power, and that it should be rather economical to fix, by doing it yourself ! I know that when you get your car back together, you will be simply amazed at what your car is capable of !



Steve