Back pressure

[omega_rugal]

i know any restriction in the exhaust cuts preformance on the high end, more air coming in needs to go out, but when it comes to low and mid range do you think a leak in the exhaust and thus, less or no backpressure will affect performance?

 

a friend of mine just got a 2001 WJ, a 4.7 v8, and asked me to make some tweaks since he´s new to all jeep stuff. i found a leak on the passenger side manifold, bad idle and a small low end hp and torque loss (above 1600 RPMs it works as it should) and i was wondering if they are related. note: good spark on all cylinders, air/fuel mixture is correct, no vaccum leaks, all good except for the exhaust leak

 

i read somewhere that some backpressure helps on the low end, any ideas?

[gogmorgo]

"Backpressure" is an imaginary concept used to explain some fluid dynamics phenomena, but doesn't really do a good job of it.

The idea is that when you're shoving air down a pipe, there's a specific diameter of pipe where the full volume of air is moving, which helps pull the exhaust out of the engine as the hot gasses cool and contract. Larger pipe diameters than necessary won't necessarily have the full volume of air moving, which leads to turbulence and reduced velocities, so less natural suction helping to evacuate the exhaust gasses. Smaller pipe diameters than optimal mean the engine has to compress the gasses to force them through the pipe, which robs horsepower.

At lower rpm you notice benefits to smaller pipe diameters because you're moving less air, but it's not because you're developing more pressure in the exhaust or anything like that. 

 

I doubt a small exhaust leak would affect performance much just because it's an exhaust leak. I would say if it is an issue, it would more likely be due to the exhaust leak acting as a venturi and pulling in a small amount of fresh air that skews the O2 readings.

[yxmj]

I have a 4.7 in a 2002 2WD Dakota ( NV 3550 / Corporate 9.25 with 3:55) 

The PO had chopped the exhaust off after the cats (on this truck it has 2 cats, one dropping directly  off each manifold, so that would be a major leak )

And then he tossed on as pair of elbows turned sideways......basically wheel well headers.

 

IMHO research the motor a bit.....they do not make power like a 4.0 down low....mine likes to get above 25...2800 RPM then it starts to pull hard.......

 

The only thing you mention that I do not have is a bad idle.......aside from the leak maybe something else is effecting it....I would check that out first

 

The other question for my own curiosity......if he has a tach ...where does the red-line start....................my tach has no red line??????

[JMO413]

My 05 WK redline is 5600 with the 4.7.

It's also gutless under 3000 RPM give or take a couple hundred RPM.

[Eagle]

22 hours ago, omega_rugal said:

 

a friend of mine just got a 2001 WJ, a 4.7 v8, and asked me to make some tweaks since he´s new to all jeep stuff. i found a leak on the passenger side manifold, bad idle and a small low end hp and torque loss (above 1600 RPMs it works as it should) and i was wondering if they are related. note: good spark on all cylinders, air/fuel mixture is correct, no vaccum leaks, all good except for the exhaust leak

 

 

What is your baseline for diagnosing a torque and HP loss at low RPM? Loss compared to what?

 

I briefly owned a 1999 WJ with the 4.7L engine. It spent more time in the shop than it did with me. During one lengthy shop period, the dealership gave me a 5.2L ZJ as a loaner vehicle. Based on the seat-of-pants dyno, the older 5.2L had GOBS more low-end torque than the brand-new 4.7L. The 4.7L didn't start to wake up until it was over 3,000 or 3,500 RPM. Since I never flog my rides like that, the only times I ever saw those RPMs were in passing situations on 2-lane roads when the tranny kicked down as I was accelerating to pass. On those occasions it would take off like the proverbial scalded cat. Under normal driving, it was like driving a brick.

 

I have one of those cheap, Harbor Freight Tools folding utility trailers. I had that behind the WJ with some bags of topsoil from Lowe's for my then-girlfriend's garden. The load was WELL under the rated towing capacity for the vehicle, but it felt like the parking brake was on the whole way. To be honest, my 4.0L XJs and MJs pulled better when towing than the 4.7L.

 

Bottom line: IMHO, the 4.7L engine is a passenger car engine that should not be used in Jeeps or pickup trucks. It's not a torque engine, it's a high-revving RPM-oriented engine. I hope I never own another one.

 

[omega_rugal]

18 hours ago, Eagle said:

What is your baseline for diagnosing a torque and HP loss at low RPM? Loss compared to what?

 

compared to any other vehicle, unless you step on it it refuses to move, like you say, as if the parking brake was on...

 

so you mean is normal? i didn´t know they sucked that bad...

[JMO413]

The number really aren't very impressive. Mine does run well at high RPM. The flatline in torque from 2000-2400 is interesting.

[Eagle]

2 hours ago, omega_rugal said:

 

compared to any other vehicle, unless you step on it it refuses to move, like you say, as if the parking brake was on...

 

so you mean is normal? i didn´t know they sucked that bad...

 

It is what it is. But you wrote " i found a leak on the passenger side manifold, bad idle and a small low end hp and torque loss ..." A "loss" means that on Thursday that particular vehicle has less torque and/or horsepower than that particular vehicle had on Tuesday. If that particular vehicle doesn't perform the way some other vehicle performs, tjhat's not a loss -- that's a difference between two different vehicles.

 

Look at the chart that JMO413 posted. The horsepower peak is at 5200 RPM. That's higher than most engines I've ever driven -- pretty much anything I've ever owned was redlined at 5000 RPM. And the torque peak is at 3600 RPM. It's no surprise that it feels flat at low RPM. I don't think it "lost" anything. I don't think it ever had what you were expecting to find.

[Minuit]

35 minutes ago, JMO413 said:

The number really aren't very impressive. Mine does run well at high RPM. The flatline in torque from 2000-2400 is interesting.

And that's the HO, which has like 30 horsepower more than the standard 4.7L.

[omega_rugal]

16 minutes ago, Eagle said:

I don't think it ever had what you were expecting to find.

 

that´s a relief, it´s not mine , and the owner is happy revving it up