If I wanted to make my 3 cats removable for track days, how much HP could I expect to get from 1990 L98, and will it cause issues for the ECM?

 

can't tell you how much hp, but from my readings, the factory removed cats from the factory race cars as a safety issue. They over heated under racing conditions.

 

A few years later when OBD-II appeared, yes, it would matter because the ECM would be seeing a noticable difference in "before & after" cat exhaust thru the o2 sensors mounted ahead and downstream of the cats...specifically to see whats changed. Thats why smog states require the older OBD-I to do the dyno test and the OBD-II can self diagnose and print the report to the shops computer directly...it tells on itself if there is something not right. Worst case would be having to install a heated o2 sensor so the sensor would be hot enough to send its signal. With no cats, the possibility of cold exhaust exist but I do not think your yr had the o2 and ECM to see that. Thats definately OBD-II stuff...Around '93-94 I think it was.

3 cats? if you have pre-cats, one on each tube just south of the collector under the manifold, I'd find a way to LOOSE 'em or hollow them out and make the main cat removable with a "test pipe". You can buy a new Y pipe thats a bit larger ID and its got no pre-cats...of course it comes with a paper that clearly states this is for OFF ROAD USE ONLY, and you have to promise Mr. Gore that you won't pollute the air that his gulf stream jet has to fly thru.....

HP..? some. The true benefit is how the engines power band is now distributed. The BIGGIE is throttle response.

Right now it hesitates a bit when you floor it. You may not think so, but it drags because there is a ton of resistance to a sudden gulp of air.That gulp has nowhere to go when the exit is full of pressure...so everything slows down.
Take those cats out (especially pre-cats) and you'll $hit yer britches then fall down in it !
The increased throttle response is dramatic and you won;t want to go back to "legal"..ever again.
It'll rev like a chain saw and that alone makes it seem like there is an extra 50 hp...while its more like 20...(honestly do not know for sure) the difference is very exciting to feel it respond like that. Tap the pedal, it jumps.

Just a word of caution....IF someone were to attempt to beat out a pre-cat or any other, that material is toxic and creates a bunch of fine dust as it breaks apart. If there is access to good welding equip, it might be worth it to cut it open then weld it back up, BUT that leaves evidence that someone did exactly that and you do not want to be caught with hollowed out cats.. For that reason and others I bought the "OFF-ROAD" Y-pipe...and keep forgetting to swap it out come monday...

 

I'd gut the pre-cats and use a magnaflo main cat.

 

The rest of the exhaust on L98 cars sucks so bad you're wasting your time screwing with the cats. 5hp will not make a bit of difference at a track day.

What's between the ears, brakes, suspension (alignment), and tires will have a far greater impact, with the emphasis being on what's between the ears.

 

Have you considered http://www.corvettecentralexhaust.co...mance_dual.php. ??

 

x2!

 

Are you running the stock Y-pipe(s)? front and rear

Have your exh manifolds been ported?

 

x3

BUT- if you want to pick up power from the exhaust system the FIRST thing to do is to dump the crappy stock headers. I did LT-1 manifolds on mine a few months ago and it made a significant improvement. Minor fab work required, but you should still pass smog.

Look here: http://marcmedina.com/LT1manifolds.html
and several threads like this: http://forums.corvetteforum.com/c4-t...discovery.html

http://forums.corvetteforum.com/c4-t...-part-3-a.html

 

Back in the day, when I first started doing small mods to my car, I decided to hollow the main cat. I had already had my car to the track many times in completely stock form. With no other changes except a hollowed center cat (still had pre-cats at that point) the car consistently ran about .15 seconds quicker in the quarter. I can only deduce that the factory L98 main cat is pretty damn restrictive. Ive heard LT1 cats are much less restrictive.

I'm not sure what kind of track days you're refering to, but if you're talking quarter mile, you will see quicker ETs by eliminating the center cat.

Also, as others have said, you'll see no codes, as there are no oxygen sensors after the cats on L98s.

 

the mufflers are long gone on my car and the exhaust flows out both sides. The Cats however are 130K miles and 22 years old.

 

Stock manifold and front y pipe with California required pre-cats, stock to the main cat, after market from their back.

 

Primarily I autocross, but also like to do occasional HPDE.

 

Are you allowed to convert to high flow units -- placed in factory locations? What about 2.5" front Y-pipe?

Points issues? Sniffer Issue?

With the age of your converters and geographic location, 2.5" front Y with high-flows seem like the best option.

You won't have any code issues no matter what you do. With an aftermarket rear Y-pipe and old cats, I'd think you'd be looking at a 15-20rwhp bump with removal. Porting those manifolds and/or converting to LT1(s) is another option. (You can even have them extrude-honed if desired.)

 

So do you live in California? If so, you could run into problems with removing the pre-cats and either removing or hollowing out the main cat. I had read somewhere that CA is using portable sniffers on freeway on-ramps where they can detect exhaust gasses that do not meet the state standards. Tickets are issued through the use of cameras that take pics of license plates.

On thing to note is that federal law prohibits the removal of a working cat even if you just want to exchange it for a high-flow version. Should a shop get caught removing cats from a car, they could face a 5-figure fine

As far as removing just the pre-cats and using a "test pipe", you may not see much of an increase in power. I run true duals, an x-pipe, and race mufflers on my 87 autocross car and there was a small gain but not much more than 5-8HP. Headers are on the install list for springtime and I would expect that this could bring as much as a 15-20HP gain. But I don't have to go thru any emissions testing.

 

Hmmmm....That's all compared to the stock exhaust? Seems like you'd get (and we've seen) closer to 5hp per element removed from the L98 exhaust "chain". I thought the conversion from a stock exhaust -- to headers with true duals would easily exceed 15-20hp. Of course, one of the issues is what the area "under the curve" looks like before/after the swap.

Wouldn't be surprised if the intake becomes the HP limitation one exhaust get's better. Actually, you'd have to expect that.

 

I was concerned the Cats are going bad. I would rather be legal, but I if there was a gain I could see swapping them out for racing. I haven't done headers because no one makes a Cali legal set.