hey
I gutted my cat a few days ago...car works much better now. Thing was pretty cloged up. But should i replace it with a high flow cat or just put a pipe through the gutted one? I have to have one on there for visual inspection. Would the car work better without a cat or if I used a high flow?
86 Iroc-Z 305 tpi 700r4 manifolds (for now) 3" to cat and 2 1/2" cat back (for now too)

 

The car will obvisouly perform better without one, cause it decreases backpressure, but a hi-flo one will hardly rob you of HP.

 

Agreed.

 

no smog = no cat

put a pipe on there instead
you will be happy for doing so

 

(smog + no cat = Failed Emmissions) Get legal, you will not be any faster without the Cat. and any Horsepower gains are only your imagination.

 

To follow on the above regarding horsepower gains are imagined - I think you are seeing gains by eliminating an older, possibly partially clogged cat.

When I bought my 88 L98 IROC, it had a gutted cat. I knew that it wouldn't pass emissions, so I researched and decided on the Random Technologies cat. After putting it on the car, the butt dyno told me that the car seemed slightly faster?! Interesting, but made no sense to me at that time.

I researched this (to see if there weren't some explanation for this unexpected result) and found that the computer is tuned for a certain amount of backpressure, basically the backpressure created by the stock cat. Greatly reducing the backpressure could have a detrimental result on the computer's settings' performance results. It doesn't know that it doesn't have stock backpressure and keeps running engine as if it has some backpressure. Supposedly, Random Technologies came up with a better apparent backpressure to work with the stock computer settings. This seems like a plausible explanation to me. If it true? - don't know. But it is a possible explanation for the butt dyno improvements...

I would be interested in hearing Vader's opinion on this...

 

ttt, I want to know what guys have to say about the previous reply

 

Post a similar topic or a topic with a link on the Tech Board. They will know the answer.

 

No flame intended, KeithO, but that explanation is possibly the biggest load I've heard in all my years. The ECM does not know backpressure, greatly reduced or not. There is NO such thing as good backpressure. There is also NO way you could possibly get back pressure "too low" with stock manifolds. Before cats, with cast iron manifolds, and free flowing mufflers, there is NO WAY that cars had "too little" back pressure. If you allow the engine to breathe better, intake, exhaust or both, the computer , with its sensors, will taylor the fuel, timing, etc to optimum for the conditions. If you do any real tuning changes, its always a good idea to "clear" the ECM by disconnecting the batt for a few minutes, anyway. This will give it a chance to "re-learn " any changed/ new values that apply to the upgrades you performed.

 

1991 RS 305 -

Just a minor point, if you think the ECM is going to learn and adapt to everything that you do to your car, you are dead wrong. The learning values are "clamped" and are only permitted to move within a narrow band. This is so that car doesn't adapt to a malfunctioning sensor. I do know a thing or two about the design of ECM controllers...

 

True, but as far as back pressure goes, I've never heard of or read any example of where less back pressure ever hurt an engines performance. Even with stock, cast iron manifolds,and no exhaust system from there back, you'd still be left with some backpressure, possibly about as much as if you had headers running through a full system. However, the headers would still offer a scavenging effect that the cast iron manifolds do not.
As for the ECM, I'm not saying that hard values are eraseable, but when making many minor/moderate mods, ( exhaust,porting plenum, ram air, fuel pressure) clearing the computer will give it a chance to relearn certain values that could/will assure max benefit from the mods.

 

I think we basically agree. Backpressure is bad. The subtle point is that the ECM is tuned with the assumption of stock backpressure. I consider it possible that removing the ECM-assumed backpressure will possibly cause the ECM to contol the engine in a non-optimal way. How "unoptimal"? I don't know. That was my understanding of the above explanation.

 

FWIW, I "gutted" my cat. This is the wide mouth cat with the 4 bolt flange on both ends. The next morning going to work, on the freeway , I immediately noticed crisper throttle response at light throttle, and at WOT, it felt stronger from 4000 to 5000. Not to mention a much raspier, throatier exhaust note thrown in as well. That was with the stock G.M. muffler. With the Dynomax Welded, it sounds even more so. BTW, when I did this, I pulled the fuse from my ECM for a bit, to clear it, "just in case".
For the finer points of the ECM, the EEC, Chip Burning boards would know more about the backpressure thing.

 

yeah I know this is going to come off as a smart a$$ but what do you expect it's ME


maybe when you had the high flow cat on there going from lets say a 2 1/2 inch pipe into a big open chamber (a.k.a. a gutted cat) that the velocity inside the exhuast slowed down. remember velocity will go up with a smaller exhuast pipe but go down with a larger exhuast pipe... also restrictions make the velocity go down.

well with this gutted cat is goes from that small high velocity system into a large area.... the gas will slow down... then hit kinda hits the end of the cat, and since the cat is not shaped very well in getting rid of the exhuast due to the very sharp turn any gas would have to take to get off the outside edge of the cat
this would again slow down velocity which would reduce scavaging, and could also block make the next exhuast pulse back up into the hollow cat...... reducing total flow

now granted it might be better to have this vs the stock cat, though that would depend on how the stock cat works. some are just like a plug..... and if yours really was plugged then yes this would be better

but a high flow cat is better at keeping that velocity up, and makes the change in direction a lot easier so it gets rid of that problem.... still not as good as a straight pipe though


1991 RS 305 the reason you might have noticed a imporvement could have been b/c either the stock cats SUCK on your car or your car had plugged cats....

 

The exhaust, as it goes through the restrictive "monolith block" inside the converter, is making its way through the "housing" of the converter the same way whether or not the "monolith block" is present. The only difference / benefit is the exhaust has NO RESTRICTION inside the converter housing, which clearly is the entire point . True, the exhaust is directed back down to the outlet size opening of the cat (3 in oval in my case) , but IT DOES THIS REGUARDLESS.

I see no possible way ANY hi-perf cat could outflow a gutted cat .The only reason to keep the empty converter housing in place is to pass a visual inspection. Oblivious tampering with emission equipment (test pipe in place of cat, etc) is a $$$$$ Federal offense. A empty piece of pipe "might" flow better, but clearly does not look OEM ,and would raise much suspicion at a smog test station.

At least gutted is a "free" improvement that looks stock, would pass a visual inspection, and without removal and examination, cannot be proven to not be stock.

BTW, I've heard of guys passing the sniffer test at idle with a well tuned engine, running around 210*-220* with no cats. .

 

whoever says running a cat (high flow or not) is better than just a straight pipe should get their head examined. (no offence)

 

Thats a very well thought out, articulate response, but I think you may have missed the point. No cat housing of any kind WOULD be best, for sure. My point was that a "gutted" cat is a nice, cheap compromise. Much better flow than any aftermarket cat, I assume, and will satisfy "at least" the visual part of an emission inspection.

 

why arent drag cars and many other forms of race cars just begging the rules guys to make it mandatory to run a 50 state legal catalytic convertor? hmmmm, i think i know why they arent begging them to do that.
and lets just say that by running a cat you can GAIN 5 hp. i'd rather not run one just cuz of the sound difference.

why do you run headers, 3" pipe, high flow muffler/s? why, cuz they flow, so you gonna tell me you want to put a cat in your system rather then a pipe. WHEN the pipe CLEARLY flows more then a cat (any cat). sounds weird to me

 

and this is the important thing to remember

 

I don't think serious drag guys are running stock computers so this it not a relevant point.

This is my last post to this thread. There is no technical dialog here, nor a willingness to think about what others are saying.

 

i agree totally...whether or not that crap is inside the cat, the gas still has to go from small to large area..well said 1991RS

 

may be

just figured that with a high flow cat it might be able to direct the flow of air better so as not to slow down the velocity as much as a gutted cat


also with a high flow cat most of them make the change from the small to large area and large to small area at a shallow angle so as to not induce much turbulance inside the cat

 

I've had a gutted cat, and a hi-flow cat on and off my car like 5 times. Can't tell any power difference between them, sound definitely changes a bit with the gutted cat though.

 

http://goingfaster.com/spo/290hp305.html

READ

 

Reguardless,

Gutted cats = less backpressure/free power/better sound.

High flow cats = less backpressure/costly power/weak sound.

The choice is yours.:lala:

 

im not running a stock computer either, and im not a hardcore drag racer. in fact the garbage man crunched my computer (and i couldnt be happier)

 

dont need to...

 

so... by your logic... eveyone should run a 4" mufflex catback?! you guys are wack..... there is such a thing as low exhaust velocity.... and that hurts low end tourque. runing a stock 305 w/ straight pipes not only sounds like ***, but i would be willing to bet it would hurt u in the quarter. look at this
------> http://www.dynomax.com/techsupport.stm

 

who are you referring to? i know guys with dual 3" exhaust on sub 300 HP cars, not saying it is the best choice but they still have plenty of low end torque and didnt feel any difference when they put it on.