Does an engine really back pressure?? - Chevelle Tech
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post #1 of 17 (permalink) Old Dec 5th, 17, 8:33 AM Thread Starter
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Ron
 
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Does an engine really back pressure??

I have always heard that an engine NEEDS back pressure from the exhaust, for that reason you should not have an exhaust that is TOO free flowing. Is this BS or what? If so, why?
Yes I'm already into the cold weather questions

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post #2 of 17 (permalink) Old Dec 5th, 17, 9:02 AM
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Jon N.
 
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Re: Does an engine really back pressure??

Total BS. What works best is an exhaust that flows smoothly and freely, and sucks the burned gasses out of the cylinders, sort of like how water is siphoned through a hose. For further confirmation on the subject, research how the Ramchargers (Chrysler engineering guys actually) came up with the whole ram charging idea, and how it worked. Same ideas works on the exhaust, same as it did on the intake side.

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post #3 of 17 (permalink) Old Dec 5th, 17, 9:07 AM
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Re: Does an engine really back pressure??

I'll leave this here.

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post #4 of 17 (permalink) Old Dec 5th, 17, 9:09 AM
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Re: Does an engine really back pressure??

Quote:
Originally Posted by Hotwire View Post
They also did a header size video, guess which one made more power? Hint: It wasnt the smaller one.
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post #5 of 17 (permalink) Old Dec 5th, 17, 9:45 AM
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Re: Does an engine really back pressure??

That Engine Masters has a commercial for everything...
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post #6 of 17 (permalink) Old Dec 5th, 17, 9:53 AM
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Re: Does an engine really back pressure??

On an F.I. computered Harley I lost low end by pulling the mufflers out of a set of 2" Python pipes, I then put some type of restrictors in and it was good again, I'm not a tuner though so it might have been tuned to be fine with the open pipes, this bike was also tuned before hand on a chassis type dyno and has a dynajet programmer on it, so I think it only matters on something with a computer


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post #7 of 17 (permalink) Old Dec 5th, 17, 9:57 AM
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Re: Does an engine really back pressure??

This is a confusing topic that has nothing to do with maximum horsepower and everything to do with driveability of a street driven vehicle. In your horsepower equation there is velocity. Implicitly the higher the velocity the more horsepower. BUT on a street driven car you need maximum velocity at a lower RPM than maximum and you can make that work by closely matching the restricted intake and exhaust systems. When you open them up, they generally don't perform well on the bottom end. That's why most stock cars can easily make more peak power by increasing flow (removing restrictions).

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post #8 of 17 (permalink) Old Dec 5th, 17, 9:59 AM
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Re: Does an engine really back pressure??

Quote:
Originally Posted by shovelrick View Post
On an F.I. computered Harley I lost low end by pulling the mufflers out of a set of 2" Python pipes, I then put some type of restrictors in and it was good again, I'm not a tuner though so it might have been tuned to be fine with the open pipes, this bike was also tuned before hand on a chassis type dyno and has a dynajet programmer on it, so I think it only matters on something with a computer
I was going to ask if it was tuned for the open exhaust.

73 nova,3515#(all steel)Pump gas!(91octane)406ci N/A, best 1/4,1.45 '60,[email protected],10.6 @ 122.38, short shifting,NO tuning!


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post #9 of 17 (permalink) Old Dec 5th, 17, 12:15 PM
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Re: Does an engine really back pressure??

Quote:
Originally Posted by bh10 View Post
They also did a header size video, guess which one made more power? Hint: It wasnt the smaller one.
No, it was the right one...

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post #10 of 17 (permalink) Old Dec 5th, 17, 4:23 PM
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Re: Does an engine really back pressure??

The Harley was dyno tuned with the mufflers in the pipes, I ended up buying some type of restrictor baffle deals and installed them and it ran like it did after the tune but had the Harley sound, I cant have a quiet Harley, lol
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post #11 of 17 (permalink) Old Dec 5th, 17, 4:42 PM
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Re: Does an engine really back pressure??

Quote:
Originally Posted by steelcomp View Post
No, it was the right one...

The left one worked, too!

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post #12 of 17 (permalink) Old Dec 5th, 17, 4:57 PM
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Re: Does an engine really back pressure??

Quote:
Originally Posted by shovelrick View Post
The Harley was dyno tuned with the mufflers in the pipes, I ended up buying some type of restrictor baffle deals and installed them and it ran like it did after the tune but had the Harley sound, I cant have a quiet Harley, lol
My son and I put Vance Hines pipes on his 2003 Wide Glide (my Harley that resides in his garage 1-1/2 hours away) and it made no difference except for making it sound AWSOME!!! It has a carb with electronic ignition. Yes, I even bought the pipes for my Harley in his garage...something isn't right here...

On my first Harley, a used '75 XLCH Sportster, the first thing I did was go to the chopper shop and buy a set of 2" drag pipes. It ran fine.

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Engine parts: (Heads, cam & lifters from Lewis Racing Engines)
ZZ502 Short block
AFR 305 CNC 106cc Chambers 8002 springs
Bullet HR ADV Dur 295/297 238/242 @ .050 .680/.650-110 LSA-108 ICL Morel lifters
Holley Avenger MPFI
612HP/594TQ
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post #13 of 17 (permalink) Old Dec 5th, 17, 5:29 PM
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Re: Does an engine really back pressure??

On harleys like everything else i like the quietest system that doesnt hurt hp. It is so much nicer to ride especially when you lean on the throttle without having your brain rattle out of your ears. I try to tell those straight pipe guys how much power they give up compared to a well designed 2 into one collectored system but it falls on deaf ears. Pun intended. And......... back pressure is never ever ever good. Like already said it's about Scavenging and velocity and the intended RPM range
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post #14 of 17 (permalink) Old Dec 5th, 17, 9:06 PM
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Re: Does an engine really back pressure??

I have a 93 VMAX I bought this spring.
I can't get myself to take the 30lb muffler off it.
I have slip ons but kinda like the subtle rumble it makes with the stock one.
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73 nova,3515#(all steel)Pump gas!(91octane)406ci N/A, best 1/4,1.45 '60,[email protected],10.6 @ 122.38, short shifting,NO tuning!


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post #15 of 17 (permalink) Old Dec 5th, 17, 9:51 PM
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Re: Does an engine really back pressure??

It more or less comes down to the effect of the exhaust piping and mufflers on the gas flow leaving the cylinders. A well designed system will set up some waves that assist in scavenging the spent gas out of the cylinder. Theres a bunch of gas flow and fluid dynamics involved and a good bit of trial and error. On a HD there are only two cylinders, so they are somewhat more sensitive to the effect of pipe length, shape, etc, that say a V8. As noted, even exhaust sound can vary from annoying to wonderful on the same engine with just a muffler or pipe change.

Note that turbos do add some impediment to free flow, but do recover some otherwise wasted energy to make more power. This stuff is fascinating, but being an ME took another level of calculus over civil, so I settled for being a civil and doing roads not cars.

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