OK, heres the story. back in early june i installed a set of pacesetter longtubes and had a custom 2.5" catted y-pipe made up for it. after that i have a 3" cutout and an SLP 2OTL. With this setup the car seemed to run great (ran a best of 13.6 at 104). this gave me a really good improvement throughout the entire power curve, most noticeable on top. (over stock manifolds and cat).

Recently though one of the brand new cats i had (catco 2.5" cats) went bad and started rattling. i was never happy with the volume of my car with the cats so i just had them cut. the car sounds GREAT now and is really loud with the cutout open, but i really think it feels slower. I know the seat of the pants test doesnt mean anything, but im not gonna be able to make it back out to the track this season to find out.

long story short im just wondering if a stock LT1 performs better with a little backpressure. and if so, is this only characteristic of cars with stock heads/cam/tuning?

 

All internal combustion engines need back pressure to run at their best. To a point back pressure increases low end torque. w Most likely, with your cats gone you've traded some low end for some top end. SOTP horsepower guestimates are influanced mainly by tourque.

 

i wouldnt say it "needs" it but usually the lack of shifts the power band up. exhaust backpressure acts similarly to a restriction in the intake.

 

No, you dont need "backpressure." Hooking the engine up to any exhaust is going to create some backpressure anyways.

If anything, you gained a little bit of power by loosing the cats and your powercurve shifted upwards. By shifting the powercurve upwards, you probably feel like you lost a little power down low, but its all perception.

It's similar to how a LT1 "feels" faster than a LS1 even though it's not.

 

I really don't think you lost any power with the loss of the cats, I was driving around my car for awhile with no cats and just long tubes and an off-road y-pipe. When the cats were finally installed, I didn't notice a difference.

 

None of our engines "need" backpressure to run their best and fastest. You want a completely restriction free exhaust, the best being simply open long tube headers (I run an open y-pipe personally). In OBD2 cars removing the cats will set a code however because the EGR will no longer function - it's backpressure operated and would be the ONLY reason our car's "need" backpressure.

Opening the exhaust will tune the RPM band upwards with your peaks higher - also leans out the car a little (a nice plus if you aren't tuned). Capping it up will lower the powercurve to a lower RPM giving the illusion of "more torque down low." You don't.. you actually have less, you just make it sooner and drop it off faster.

 

Upping the power band isn't always fastest and "the best thing" to do. Remember torque is what wins races. 10 ft/lbs of torque at the low end is going to do alot more for your timeslip than 10 hp at the topend. Looking at your power under the curve is just as if not more important than peak numbers.

 

My 2 cents on this statemant.
Low end torque is most effective during the initial launch and only in 1st gear.

High end torque will be used for acceleration in 2nd, 3rd and 4th gears.

So for an actual 1/4 mile run with maximum performance and if i had a choice of where to place my torque I would prefer to use in the powerbands of 2nd through 3rd gears rather than 1st.

 

i agree, LT1's already have good low end torque

 

Most people I've talked to really mean part throttle acceleration when they say SOTP whether they know it or not. Ferrari, Lamborghini, Yamaha, Honda, Suzuki and plenty of other OEM's put valves in the exhaust of their high output engines to alter the exhaust harmonics or "back pressure". Throttle response, fuel economy and power are all improved at part throttle when it's done properly.

You very likely lost power at small throttle openings, say 1/3 throttle or below. If you did it would make for a SOTP feel of less power or responsiveness as you roll into the gas. I've noticed less roll on response many times when going with a less restrictive exhaust, but peak power at WOT generally increases.

If you rarely run at WOT this type of power tradeoff might not be a good deal. I find crisp throttle response gives me more enjoyment more of the time considering the way I drive. Different strokes for different folks.

Later, Michael

 

Neat reading on variable exhaust and its benefits:
http://autospeed.drive.com.au/cms/A_0883/article.html

 

actually i noticed the difference at WOT. it seemed to take a little longer to pull through the gears.

 

i tryed my 95 z28 with open ypip it fell on it face ..way way way faster with it hooked up.thats why people that run open headers put a 1 and a half foot extention on there headers ..and that makes a world of differance..and if you don't believe this try it ..and then you tell me ... Yea thats what i thought faster haha

 

They add extensions so fresh air from outside doesn't hit the 02 sensors and mess up the a/f, it has nothing to do with backpressure.

 

The only back pressure you need is in the headers, you want the best flowing exhaust possible.