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Combat 11-11-2012 02:05 AM

Backpressure Issues?
 
After installing my exhaust a couple months ago, I noticed my Jeep was running a little slower off the line. My buddy told me this was probably because they deleted the resonators, which resulted in a loss of back pressure. What I want to know, is once I install my spartan plus cam and get a custom tune, will the issue of having a loss of back pressure fix itself? Will I see my power return? or should I put the resonators back before the cam swap?

Thanks in advanced

Scottina06 11-11-2012 02:37 AM

Re: Backpressure Issues?
 
Dont buy into the back pressure BS. Back pressure is a myth. An engine is nothing more than a big air pump and all laws of fluid mechanics apply. The only thing that matters is the exhaust gas velocity. When you put a pipe that is too large.....you lose gas velocity.....which results in a loss of performance. Most people like to think of this idea as a loss in back pressure.....but thats crap. If back pressure was needed to make horsepower......then race cars would not have straight through headers and exhausts.
Removing the reso's should not have hurt performance at all. If anything.....your performance should be exactly the same because the cats are the biggest blockage of the flow anyway.

LandEagle 11-11-2012 03:06 AM

en.m.wikipedia.org/wiki/Back_pressure

Good example for bacpresure issue is a cat. Engine have to work and spend/loose HP to get gases trought

In some cases, gas velocity is presented/explained as back pressure and backpresure as gas velocity.

They are close together in same pipe, but different kinds.

Loosing gas velocity causes also unburned fuel/air mixture passing correct
through the cylinders into the exhaust.

So if you pipe is too big, could cause your airpupmp wont work like desired.

For higher velocity the headers and exhaust pipes are often bend with a thermo tape. One of the tape manufacturers name is thermatec as far i could remember.

dmsfun 11-11-2012 10:10 AM

Re: Backpressure Issues?
 
Aftermarket CAI and exhaust lose low end. and overall throttle response. If there are any gains, they are just a few HP higher up in the RPM range, but street drivability suffers, making it a lot less fun to drive. Back pressure is not a myth, and to prove this, if you go back to stock intake and exhaust, you will regain all the lost low end. Contrary to popular belief, An air pump does not function better by opening up the intake and exhaust, it is worse until operating at a high speed. Race cars are not driven on the street, they are only driven flat out, and have peak HP figures high up in the RPM range, with very poor low end torque. If you put your stock intake back on, much of the low end will return. Many have done this, including myself.I dimped the MAgnaflow and went back to stock and it is so much better on the street, plus none of that noise. Adding a cam and porting heads will make it evern worse at low end, unless it is a grind with longer duration with not much more lift.

Bikemobile 11-11-2012 12:47 PM

Re: Backpressure Issues?
 
Your performance cam will most likely have more valve lift and a bit more lobe overlap. This will cause a bit more loss of low end but hopefully a boost in mid range and high end.

LandEagle 11-12-2012 02:53 AM

Quote:

Originally Posted by dmsfun (Post 709726)
Aftermarket CAI and exhaust lose low end. and overall throttle response. If there are any gains, they are just a few HP higher up in the RPM range, but street drivability suffers, making it a lot less fun to drive. Back pressure is not a myth, and to prove this, if you go back to stock intake and exhaust, you will regain all the lost low end. Contrary to popular belief, An air pump does not function better by opening up the intake and exhaust, it is worse until operating at a high speed. Race cars are not driven on the street, they are only driven flat out, and have peak HP figures high up in the RPM range, with very poor low end torque. If you put your stock intake back on, much of the low end will return. Many have done this, including myself.I dimped the MAgnaflow and went back to stock and it is so much better on the street, plus none of that noise. Adding a cam and porting heads will make it evern worse at low end, unless it is a grind with longer duration with not much more lift.

...installing CAI on factory TB could sometimes cause this. I modified 2 jeeps for myself. Aftermarket TB transform your wehicle and make the base solid for other mods on airpump. On my current 3.7wk alot of lowend torque gAin trought Ryans TB and some very noticeable mpg loss in my wife city trips.

LandEagle 11-12-2012 03:01 AM

...i spend aprox 13% less gas now for city, and every stop and go makes fun, cause TB is sensible and pulls from the bottom.

Combat 11-12-2012 03:01 PM

Re: Backpressure Issues?
 
I have a ported throttle body to near 85 mm already, so I don't have a stock one, nor do I think my throttle body is causing the issue. Maybe I should lessen the diameter of my pipe size. I currently run 3.0 inches front front to back. That might be the issue? I could be wrong

LandEagle 11-12-2012 04:23 PM

Quote:

Originally Posted by Combat (Post 710164)
I have a ported throttle body to near 85 mm already, so I don't have a stock one, nor do I think my throttle body is causing the issue. Maybe I should lessen the diameter of my pipe size. I currently run 3.0 inches front front to back. That might be the issue? I could be wrong

Ok, end of theories :). Just checked you profile Combat. The other one i modded, was 4.7ho

LandEagle 11-12-2012 04:27 PM

02 4.7ho. Also 3inch from Y to tip. Dont thing 5.7 is not matured for 3inch. ;)
What you really need, is a dataloger. And play around on the track with several.setups. Thats the only way to get out of feelings. Some like dashdaq.com

Combat 11-12-2012 11:22 PM

Re: Backpressure Issues?
 
What about a custom tune, does anybody think it will fix the issue?

LandEagle 11-13-2012 01:04 AM

Quote:

Originally Posted by Combat (Post 710400)
What about a custom tune, does anybody think it will fix the issue?

Vendor here, flying ryan, he makes custom for this jeep. Just ask him.


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