Cat. converters and torque

Cat. converters and torque

I am debating whether there are gains to be had by removing or upgrading my cat. converters. My options are basically
-leave them
-gut them (for off-road racing use only)
-replace them with high flow cats
-eliminate them with a straight pipe (again, off-road racing use only) :)

I would really like advice because I am approaching this dilemma scientifically (is that a word?) and my
strategy is the following:

- absolutely no loss of torque is acceptable
- I am on a very tight budget

I understand just enough about "exhaust aerodynamics" (that's definitely not a word) to be confused and undecided.

*I have experienced great loss of low rpm torque from reducing back pressure in small engines
*I understand that back pressure is somehow needed unless we are talking full race high rpm launching geared & slicked race vehicles
*I heard that velocity is good for creating or encouraging a good intake charge
*I know there are rythmhs to intake and exhaust pulses, but don't know much about how to optimize them
*Chevy trucks is my experience often sound smoother and meaner with the stock cats

On that note a restriction that chokes power on my 110k mile factory cats and a cheap mod that could drop a few tenths of my 1/8 mile ET is irresistible. What do you recommend. Stock truck with 3.42 gears and a 4.8 Vortec, would like 4.10s in the future. Dreaming of heads and cam but that will not happen soon unless I win the lottery. Plan on a cold air intake, under-drive pulleys, electric fan, and new oxygen sensors plus wide-band to perfect my tune. I have advanced timing and overall "aggression" and it has responded well.

Current exhaust setup: (please don't laugh)

- cheap Summit shorty headers with stock old gaskets, wrapped with DEI thermal tape and spray.
-stock cats, seem ok 110k miles
- stock midsection, couple tiny rust leaks in this :(
- rusty but not badly leaking catback with OLD Flowmaster but sounds good

Thanks guys! :)

Boost, my opinion is that if you have the 'right' exhaust/headers high-flow cats will make almost as much power as no cats. Maybe a total of a 1% loss of total combo power..I.E., 4-5 HP.

While perhaps it is not of much of a concern for your set-up, I prefer the sound of a nice-set-up through cats. No cats does not necessarily sound that great.

Thats my thought. I am not sure exactly how much HP your set-up might be cat-less..but good high flow cats can be part of a nice set-up.

Good luck..

Quote:

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Originally Posted by Boost

Cat. converters and torque

I am debating whether there are gains to be had by removing or upgrading my cat. converters. My options are basically
-leave them
-gut them (for off-road racing use only)
-replace them with high flow cats
-eliminate them with a straight pipe (again, off-road racing use only) :)

I would really like advice because I am approaching this dilemma scientifically (is that a word?) and my
strategy is the following:

- absolutely no loss of torque is acceptable
- I am on a very tight budget

I understand just enough about "exhaust aerodynamics" (that's definitely not a word) to be confused and undecided.

*I have experienced great loss of low rpm torque from reducing back pressure in small engines
*I understand that back pressure is somehow needed unless we are talking full race high rpm launching geared & slicked race vehicles
*I heard that velocity is good for creating or encouraging a good intake charge
*I know there are rythmhs to intake and exhaust pulses, but don't know much about how to optimize them
*Chevy trucks is my experience often sound smoother and meaner with the stock cats

On that note a restriction that chokes power on my 110k mile factory cats and a cheap mod that could drop a few tenths of my 1/8 mile ET is irresistible. What do you recommend. Stock truck with 3.42 gears and a 4.8 Vortec, would like 4.10s in the future. Dreaming of heads and cam but that will not happen soon unless I win the lottery. Plan on a cold air intake, under-drive pulleys, electric fan, and new oxygen sensors plus wide-band to perfect my tune. I have advanced timing and overall "aggression" and it has responded well.

Current exhaust setup: (please don't laugh)

- cheap Summit shorty headers with stock old gaskets, wrapped with DEI thermal tape and spray.
-stock cats, seem ok 110k miles
- stock midsection, couple tiny rust leaks in this :(
- rusty but not badly leaking catback with OLD Flowmaster but sounds good

Thanks guys! :)

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LTs would do better than no Cats , but LTs and no CATs , cam later on good start , shorty headers are not much better than cast iron manifolds . And loose the Slowmasters (yes SLOW Master) , just not a good Muffler for Chevy's . My $.02

I agree with WeathermanShawn’s comments. The highflow cats of today are approaching “straight pipe” flow – minimal loss of power output on a street driven vehicle. Plus, it’s the responsible course of action.

Regards,
Taz

Hello Boost,

Sorry for being scattered, trying to do too many things at once. I chimed in on the cats portion of your question earlier, but missed the rest. The LR4 4.8 L is a great little engine. I have one in my Sierra with over 420,000 Km (265,000 miles) on it – doesn’t use any oil, always starts in -40 C weather, and still has good power. The cats are also original - no flow issues as of yet.

Your plan to go from 3.42 gears to 4.10 gears should help. Full size trucks are heavy, and the 4.8 has shorter stroke crankshaft than the rest of the Gen III family - disadvantageous to torque output. The crankshaft stroke is similar to Gen I 327 engines of legend - making it a quick revving engine.

On a tight budget an ’02 LS6 cam is a decent upgrade. The cam and springs from Scoggins-Dickey are around $350. The idle will be bone stock (no lope) but the torque curve is smooth and broad. You’ll also need slightly longer pushrods (7.450 inch) for correct valve train geometry with the smaller base circle LS6 cam.

I prefer Lunati VooDoo series cams for the Gen III cathedral ports heads - but the overall cost is lot more.

For a greater performance boost - put the 4.8 L in an S10 !


Regards,
Taz

Thanks everyone, great detailed comments. I went with shorties because they are supposed to target low rpm torque specifically opposed to the long tube headers, which are also much more expensive. I got mine new for under $200. Yes I am falling in love with the 4.8, not much torque but sounds like a beast and feels like a 5.3 or LS1 on the higher rpms. The Z06 cam is great but while this may sound silly, if I go through the trouble of installing a cam, I WANT lope :)
The 4.10s are definitely going in sooner or later.

Back on the topic of the cats, I am pretty sure I will go with high flow cats, but only when they don't flow any more. Catless sounds raspy and there is no need to pollute for minimum gains. Some of the cars I've had in the past that were turbocharged and smaller displacement, there was a need to pollute, or lose 100 hp.
My main concern was really less how well cats flow, more like what is the relationship between the way this engine produces torque and the amount of bacpressure it "requires"?? Ultimately, I would remove all backpressure for 1 horsepower, if I didn't lose any low rpm torque. That's just me. Thanks again