Backpressure

Rogers · Post by **Rogers** » Sun Jul 13, 2008 6:00 pm

I have a 2008 mustang GT and I installed a X-pipe without mufflers. The mid-pipes are stock and I have Pipes muffler deletes on the rear. Is this setup helping me or hurting me? Do I need backpressure for my engine to run properly?

MadBill · Post by **MadBill** » Sun Jul 13, 2008 8:20 pm

No engine ever needs backpressure. Sometimes however retuning is required to take advantage of a reduction.
True story: My first summer job in a gas station, I noticed the mechanic crimping the tailpipe nearly shut on his '48 Chrysler. He explained that he figured if the gas couldn't get out, then it couldn't get in, and his mileage should improve. Even as a green 16 year old, I was pretty sure that wasn't right and told him so, but what, as he pointed out, did I know? Next morning, as he was beating it back into a semblance of round, I asked him how it had worked out. "%@# thing won't go over 25 MPH..."

panic · Post by **panic** » Sun Jul 13, 2008 9:14 pm

Re: "he figured if the gas couldn't get out, then it couldn't get in, and his mileage should improve"

I heard the reverse: putting on dual exhaust would ruin mileage because "it stands to reason that if more comes out, you have to put more in".

Pretty bad?

Source: local BOCES auto instructor

Rogers · Post by **Rogers** » Sun Jul 13, 2008 9:14 pm

I was just wandering because My mustang is less than 2 months old and I'm experiencing a slight nudge at exactly 3,000 rpm. It doesn't do it at medium throttle or wide open, just while slightly on the gas while climbing to 3,000 rpm and at exactly 3,000 rpm every time. Could this be because of exhaust setup. No mods other than exhaust. Setup is over a month old but notice this about 4 days ago. Could be normal or just paranoid don't know? I don't think I ever felt it before. I bought the pipes mid-muffler system and decided not to use the straight through design because I like the rumble of the no mufflers. Does these mufflers have backpressure? Should I reinstall the mid-mufflers or baffle similiar to stock. I'm a torque man and I'd like as much bottem in torque for what I got. A good setup is recommended. Thanks! And if anyone else know about this 3,000 rpm thing please comment, like I said I could be just paranoid with this backpressure confusion.

headman · Post by **headman** » Sun Jul 13, 2008 10:37 pm

Backpressure never increased power.

Yet if you install too large or too short of a pipe the exhaust gases can slow down and actually lead to an increase in so called backpressure exiting the cylinder head. Opposite the effect that was anticipated.

eg.
So the hot rodder loses power with a big pipe and comes to the conclusion he need more backpressure.

Actually he may have destroyed the "tune" of his exhaust system for a given rpm, where the exhaust was scavenged pulled from the cylinder...
Pulled so well the exhaust flow aids in creating a depression for the intake charge to fill, increasing VE. The exact opposite of backpressure.

Rogers · Post by **Rogers** » Sun Jul 13, 2008 11:40 pm

I'm using 2 1/2 piping all the way back to the rear bumper with 4 inch tips. changed out h-pipe with x-pipe and using axle back muffler deletes. mid pipe from x-pipe over rear wheels to muffler deletes is stock. it was easier to install and less problems especially when it came to centering the tips than piecing the whole pipes mid muffler system together. kind of noticed that it felt like it had more torque in the lower rpm range with the mid muffler setup than no mufflers. The first day I took off the mufflers off I got in to it with a charger SRT. downshifted to third and it was bye bye on the highway. I noticed the higher rpm range had a very noticeable increase and aggressive but in my opinion mustangs are designed for the lower rpm range. That's were they do their most damage as far as racing goes. I'd rather have more power in the lower range any day than the higher rpm range. To me mustangs are dead in the higher range. They just are not made for very high speed racing. To me power starts decreasing after approx. 100mph. Kept climbing very quickly without mufflers well pass 100mph but it just didn't feel right.

FordManVT · Post by **FordManVT** » Mon Jul 14, 2008 12:11 am

I would say you did lose a bit of low end torque, but gained more than you lost at higher RPMs. Your 3v engine will love the high rpms. Think of it this way: if you make the same torque as before, but at a higher RPM, it means more power. Keep working for that top end power, and when your low-end gets poor, stick some shorter gears in it. I run 4.56 ratio gears in my modular.

about 100mph is where aerodynamics becomes a noticeably bigger factor, and the 05+ stangs don't exactly have a great front end in that respect.

Post by **ClassKing** » Mon Jul 14, 2008 2:07 am

MadBill wrote:No engine ever needs backpressure. Sometimes however retuning is required to take advantage of a reduction.
True story: My first summer job in a gas station, I noticed the mechanic crimping the tailpipe nearly shut on his '48 Chrysler. He explained that he figured if the gas couldn't get out, then it couldn't get in, and his mileage should improve. Even as a green 16 year old, I was pretty sure that wasn't right and told him so, but what, as he pointed out, did I know? Next morning, as he was beating it back into a semblance of round, I asked him how it had worked out. "%@# thing won't go over 25 MPH..."

Not to hijack the thread, but it loks like you've got some good answers and ya'll might like these stories. I hope. An EX advertising Pontiac engine builder came to me at a car show and said he'd "been thinking" that, why wouldn't an engine run better if you inverted the basic step header design. Start big and go smaller to the collector. His uneducated theory was, "wouldn't that increase velocity?"

In the Past 13 years Pontiac magazines got into this stupid thing about "velocity" - Gotta have more port velocity. I tried to explain that the word restriction goes hand in hand with velocity. Not a surprise, the two men who introduced this ignorance to the world, neither have had impressive performance results.

Second story. One of my engine combinations had the distinction of being the fastest known Pontiac powered street car for three years straight. Obviously the owner was very happy. Nice guy, but all too typical, his knowledge was magazine bred.

After enjoying a year or two of success, he asked me one day if he should change his exhaust to have more backpressure. "It'll make more power with backpressure right?"

mom!!!!

You know......there's some people you just can't reach.

exhausted · Post by **exhausted** » Mon Jul 14, 2008 9:01 am

The larger you make your exhaust system the easier it is for flow to go both ways. At lower rpm's there is more time for atmospheric pressure to get to the engines cylinders and a larger system just aids that process. You can adjust things to get back the torque if you want it like shorten cam timing but that will hurt your ability to make more horsepower which was why you put bigger tubes on right? You can not have both. If you just want to make it sound better you do not have to go larger, just smaller mufflers!

enkeivette · Post by **enkeivette** » Mon Jul 14, 2008 11:20 pm

Engines will always make more HP with less back pressure. Assuming the engine is tuned properly.

Dodge Freak · Post by **Dodge Freak** » Mon Jul 14, 2008 11:43 pm

FordManVT wrote:. Keep working for that top end power, and when your low-end gets poor, stick some shorter gears in it. I run 4.56 ratio gears in my modular.
.

I know zip about these new cars but I would agree, gear it up. You have overdrive trans. I don't and run a 3.91 gear in back, not sure what you have in there now but find out and take it up. You be surprise how much it helps out--and hurts gas mileage.

Not sure if theres a way to tune the A/F ratio and spark advance but if so, I get that too.

BTW... we have no safety or smog tests in Michigan but you may have were you live. You don't want to spend a few thousand dollars and then can't drive it legally on the road so check that out first.

chevy_power · Post by **chevy_power** » Tue Sep 01, 2009 11:35 pm

where do these people get the idea that an engine NEEDS backpressure? I don't get it.. I got into an argument with a friend and my instructor at my aviation tech school about this same subject. The instructor who has an A/P license and teaches powerplant for a living says yeah an engine NEEDS backpressure. I told him that i will install a grapefruit in his tailpipe of his turbo saab and drill a 1/4" hole through it, then he might understand what backpressure does for your engine.

SWB · Post by **SWB** » Tue Sep 01, 2009 11:46 pm

chevy_power wrote:where do these people get the idea that an engine NEEDS backpressure? I don't get it.. I got into an argument with a friend and my instructor at my aviation tech school about this same subject. The instructor who has an A/P license and teaches powerplant for a living says yeah an engine NEEDS backpressure. I told him that i will install a grapefruit in his tailpipe of his turbo saab and drill a 1/4" hole through it, then he might understand what backpressure does for your engine.

Not to get off topic, but I worked on an ex record holding competition aerobatics Pitts that had an equal length collected header dumping right under the cowling on the belly. Not only was the header designed to minimize back pressure but it's exit location would have been in a low pressure area about 90% of the time. I wonder what your teacher would think of that? FWIW, all of these types of airplanes use this configuration as far as I know...

DCK · Post by **DCK** » Wed Sep 02, 2009 12:04 am

chevy_power wrote:where do these people get the idea that an engine NEEDS backpressure? I don't get it.. I got into an argument with a friend and my instructor at my aviation tech school about this same subject. The instructor who has an A/P license and teaches powerplant for a living says yeah an engine NEEDS backpressure. I told him that i will install a grapefruit in his tailpipe of his turbo saab and drill a 1/4" hole through it, then he might understand what backpressure does for your engine.

It's internet education.

I'm not an "expert" but I never saw a reason for backpressure to be good back before I knew anything. I only understand a decently novice amount now compared to some real knowledgeable professionals here. At least I know that I have more to learn. Some of these car forums on the net are garbage in, garbage out.

BrazilianZ28Camaro · Post by **BrazilianZ28Camaro** » Wed Sep 02, 2009 12:37 am

Maybe this back pressure idea started back in the day where some two stroke motorcycle engine mechanics stated that these engines need exaust backpressure to increase the crankcase pressure to fill the cylinders...