AVB

iTrader: (9)

Every-so-often I'll run w/o cats and have always put them back on due to the loss in torque. These are the cats that are part of the FLP system and the loss of torque is really noticeable when their off.
I've heard that high-flowing cats aren't supposed to make a difference, but if that's the case should the difference be that great? Or, would I get it back plus some if I get it retuned.

XtinctHawk02

You would need some "high-flow" cats to gain some of your torque back. All the cat's do is build up that back pressure and release it to the rear wheels.
Without cats you have nothing to build up the back pressure, therefore you lose you torque.
So the question is?
Do you want more rear wheel hp? if so get rid of the cat's..
Or Do you want more torque to the rear wheels?-If so leave your high flow cats on.

AVB

iTrader: (9)

I do understand the tq to hp relationships will change some.. but I had always hoped the increase in hp would make it "seem" minimal..

So, even after getting re-tuned for a less restrictive exhaust/possibly a leaner afr, one couldn't expect to get all/most of the torque back?

Thanks for the reply, btw.

ohhiitznik

iTrader: (-1)

Hahahhaha here we go again. Backpressure is a myth, it doesnt "build" torque. Backpressure just moves your torque band lower in the rpms, so you "feel" it more. Resonant scavenging is what you want. You want exhaust velocity. The way the exhaust waves move through the pipes determines your torque band. Open exhuast will always give you the most torque, hp. if you put your cats on, you can feel your car drop off at higher RPMs due to reduced exhaust velocity. Straight pipes cut to a specific diameter is what you need. You can put bullets but you will gain the most. 2.5 for stock cube motor, and 3 for big h/c or big cubes. trust me, its only SOTP feel. Put it on a dyno n youll see you have more torque and more hp, its just higher up in the band where you need it while your racing

AVB

iTrader: (9)

https://ls1tech.com/forums/showthrea...highlight=cats

That post (reply #6) is what really led me to ask this question.. Your response btw, is alot easier to understand. Thanks and I'll try it and dyno..

jimmyblue

iTrader: (11)

Backpressure on the power stroke would aid
torque - under very specific conditions. The
exhaust pressure would have to be higher
than the cylinder pressure, the exhaust valve
open. But this is not how the power stroke is
set up. The exhaust valve may open before
BDC slightly, more on a "big" cam, but by BDC
your crank angle has really cut the cosine
torque contribution to about nothing.

Backpressure on the exhaust stroke -steals-
torque, a negative shaft torque is taken to push
the gas out against any backpressure.

Reduced backpressure equates to more cylinder
emptying, then more cylinder filling. This will
increase dynamic cylinder air, pull back spark
timing unless corrected. It will pull in a bigger
charge with less dilution, possibly making a
faster burn, maybe not wanting more spark
anyway. The true VE at higher loads will go
up but the PCM knows nothing, this will likely
change transient throttle mixture by the SD
tune. Anyway, you have multiple mixture and
timing effects that come from a change in
exhaust efficiency and the PCM needs to be
educated.

But the pressure as a simple actor, is not
giving torque.

AVB

iTrader: (9)

Man.. my eyes are still glazed.. With all due respect, can you please simplify?

moving forward, what about dyno's of before and after.. I've always liked pictures.

SouthFL.02.SS

What you're experiencing is peak torque moving higher in the rev range without cats.
Your tq peak is now at higher revs. You actually may have gained peak tq numbers without cats.

FI FREAK

lol...man run no cats..is that wasy to understand

LS1LT1

iTrader: (16)

It's all about perception, how the car feels (in this case, less torque) might not be how it's actually performing (probably faster).

jimmyblue

iTrader: (11)

Maybe the problem is the torque to noise ratio, the
same sound level that used to mean hard acceleration
now corresponds to less.

Quantitative comparisons are better.

AVB

iTrader: (9)

I had thought this once during one of those time I was driving w/o the cats.. definitely less pull at a given decibel but taking sound out of the equation it still felt like a noticeable loss. Here's a my last dyno.. What do you guys think removing the cats will do based on this.. also, I'm still playing w/alky to get it from runnin so rich.