Exhaust restriction/velocity and torque

[Pranav]

I have to re do part of my exhaust to improve ground clearance.

Currently I have my intake restricted all the way down to a single 2" hole to make 297/260. Missing the 308/258 I had with lt1.

My current exhaust consists of 2.25" pipes that Y into a single 3" pipe through a dynomax straight muffler. Pretty open.

I was thinking maybe running some 1.75" pipe straight off the manifolds, for a short distance before going back to 2.25" to connect with the rest of my system

My thinking is this could possibly increase low/mid range exhaust velocity/scavenging, and gain me some low/mid range torque while acting like a restrictor at higher RPMs? All done in conjunction with taking out some intake restrictor of course.

Worth a shot?

[Al Fernandez]

Craig is the only one I know to have done any real exhaust restriction experimentation, but his was all restrictor right at the end. Maybe start with his lessons learned before investing money and time?

[39PitCrew]

I know Sean uses a tailpipe restrictor and I think Paul is using one now too.
I'm hoping to get David to install one soon. I dislike the throttle body plates,
messes with the airflow into the intake. We constantly have oil sitting in the
space right behind the restrictor plate because of screwy turbulence.

[Pranav]

Yeah my restrictor is in my airbox since you can't use throttle plates on ls1s, the throttle blade opens into the intake.

I do believe too much intake restriction causes more oil than usual to get sucked in through the valve cover to TB breather at WOT. Another benefit of being able to take some plate out of the intake.

[Supercharged111]

I'm told it's because it creates a vacuum where that breather enters the back of the TB. Were one to relocate that upstream of the intake restrictor, it ought to reduce that oil consumption cause by the suction on that hose. We shall see, I picked up a 5.3 and am slowly gathering the bits to do the swap. I don't think I want anything other than a stock y pipe for ground clearance. I know on my truck that ditching the 1 7/8" pipes off the manifolds for some dual 2.5" pipes made more torque from an idle up, so I'm hesitant to think that choking the flow will really boost torque. What I really wonder about is the difference in torque produced by an x pipe vs a y pipe.

[Pranav]

I'm told it's because it creates a vacuum where that breather enters the back of the TB. Were one to relocate that upstream of the intake restrictor, it ought to reduce that oil consumption cause by the suction on that hose.

Oh yeah I forgot the LT1 has that crappy TB breather port. I never moved my breather hose to the intake elbow on the LT1 because I was always scared of making that part of the intake oily and then suck contaminated crap into the opti since it feeds from the same area.

LS runs off the intake hose, way better. I'm down to a few ounces of oil in the catch cans per weekend outside of COTA and Cresson.

[Rsmith350]

I'm re-doing my exhaust due to a couple landscaping adventures. I will be doing a 2" all the way back and use baffles for restriction Pranav. Maybe we can collaborate.

[Pranav]

Last year I ran on a 54mm restrictor on a brand new motor, 297/260.

After a full season, this winter I reduced my 2.25" pipes to 1.875" for about 18", and put in thicker gear lube, ran 304/261 on a 57mm.

I'll take it.

[drecords]

Last year I ran on a 54mm restrictor on a brand new motor, 297/260.

After a full season, this winter I reduced my 2.25" pipes to 1.875" for about 18", and put in thicker gear lube, ran 304/261 on a 57mm.

I'll take it.

7 ft-lbs of torque is impressive

[Supercharged111]

Last year I ran on a 54mm restrictor on a brand new motor, 297/260.

After a full season, this winter I reduced my 2.25" pipes to 1.875" for about 18", and put in thicker gear lube, ran 304/261 on a 57mm.

I'll take it.

Did it nose over harder up top though? You running stock y pipe?

[Pranav]

Yes on stock y pipe, my "restrictor" pipes are simply what connect the manifolds to the y pipe in place of the cats.

Downstream of the y pipe is a 3" pipe and dynomax straight muffler.

I need to pull up.my old dyno to compare the curves.

[Pranav]

Comparing my charts it did not nose down any further up top; same peak HP at the same RPM. I just gained more lower range torque and the it looks like the peak torque number is a few hundred RPM earlier.

Same HP at the same peak RPM, just more torque and earlier/lower in the range.

Remember, it wasn't JUST the pipes, I took out some intake restrictor and also did run thicker gear lube this time for more protection.