Stop Saying Car Exhausts Need Back Pressure

Noobz347 · Jan 22, 2018

His initial analogy where he's varying atmospheric pressure, doesn't include the effect on the inlet but the rest of this is very good.

What he's describing is a "Tuned Exhaust". Something that a few of us have talked about at length but keeps still crops up a few times a year.

View: https://www.youtube.com/watch?v=jjPeP_Nn2B4

General karthief · Jan 22, 2018

I think in the past people have confused 'back pressure' with the scavenging wave that bounces back in the header tube.
I tried to explain this when some very young friends of mine attempted to run the engine with no exhaust manifold, of course it ran like crap, they insisted it was the lack of 'back pressure'.
He has a very good explanation.

Mustang5L5 · Jan 22, 2018

I remember when I was 18, a buddy had a 1989 GT that he ripped the entire exhaust off and just had open headers dumping right at your feet. Car was BONE STOCK, but it sounded like the world was coming to an end when he drove and revved it. Literally drove around town with everyone thinking it was a 10-second car.

Now this stock 89GT ran a 14.5 at the track when he first got it. He went back thinking he chopped half a second off the ET. Nope...car was stuck in the mid 15's all night. Needless to say an exhaust went back on, and the car was back into the 14's after that.

Hoytster · Jan 22, 2018

It's amazing how a well designed, tuned exhaust can increase the VE of an engine. You can really see the effects with a properly designed 2-stroke chamber and how utilizing the exhaust pulses improves power. Of course it's a little different with a 2-stroke, but the concept is the same.

Noobz347 · Jan 22, 2018

Hoytster said:
It's amazing how a well designed, tuned exhaust can increase the VE of an engine. You can really see the effects with a properly designed 2-stroke chamber and how utilizing the exhaust pulses improves power. Of course it's a little different with a 2-stroke, but the concept is the same.

Yes. The difference is that you're now tuning the exhaust for 2 phases with a pulse of 4. The placement and size of the crossover pipe is pretty important on our cars too.

The other thing that comes to mind as I think about this more are folks who spend money on large headers and bolt them up to an otherwise, stock exhaust. I would hazard to guess that the overall outcome would be less power than stock in most cases because scavenging just kicked in the balls.

Hoytster · Jan 22, 2018

Noobz347 said:
Yes. The difference is that you're now tuning the exhaust for 2 phases with a pulse of 4. The placement and size of the crossover pipe is pretty important on our cars too.

The other thing that comes to mind as I think about this more are folks who spend money on large headers and bolt them up to an otherwise, stock exhaust. I would hazard to guess that the overall outcome would be less power than stock in most cases because scavenging just kicked in the balls.

You're absolutely correct. That's why I would say that an exhaust has to be looked at as a whole system. And that system has to be designed around your engine goals, which is a whole other system. Everything has to work together to maximize power.

cleanLX · Jan 22, 2018

Well, they do benifit from back pressure, what folks cannot understand is that back pressure is not resistance, it's actually the exact opposite.
That, is the amazing thing... even tho thoroughly explained in the video, youtube comments kept using the term "back pressure" when they were actually talking about resistance.
There's no helping some.

Hoytster · Jan 23, 2018

I think terms might be interchanged a little here and there, but from everything I know, back pressure is a resistant pressure inside a system. Basically, a pressure force on the exhaust side that must be overcome by the expelling exhaust gases, which is not beneficial to making power. Some backpressure does increase velocity which can benefit low end torque on a poorly designed exhaust, but you lose any HP benefit. The beneficial aspect of a tuned exhaust is not the exhaust "drive pressure/back pressure", but the sound wave that bounces back that can be utilized to create a low pressure area by the open exhaust port to help suck in fresh fuel and air into the combustion chamber (increasing VE).

Or maybe I'm just stupid?

Noobz347 · Jan 23, 2018

Hoytster said:
I think terms might be interchanged a little here and there, but from everything I know, back pressure is a resistant pressure inside a system. Basically, a pressure force on the exhaust side that must be overcome by the expelling exhaust gases, which is not beneficial to making power. Some backpressure does increase velocity which can benefit low end torque on a poorly designed exhaust, but you lose any HP benefit. The beneficial aspect of a tuned exhaust is not the exhaust "drive pressure/back pressure", but the sound wave that bounces back that can be utilized to create a low pressure area by the open exhaust port to help suck in fresh fuel and air into the combustion chamber (increasing VE).

Or maybe I'm just stupid?

This in addition to:

Creating an exhaust that is too large will not create the venturi effect necessary to scavenge the next exhaust pulse, and so-on.

krazedstang · Jan 23, 2018

This was good, I am one of those who confused "back pressure" with "scavenging". Now I know better.

grumpy150 · Jan 27, 2018

One thing I noticed is when I looked up what to run on a 350 when I was a kid I found 21/8 Was the answer I got the most now that I've grown up (lol) I always wondered why 21/2 is the normal size everyone uses on the 5.0

General karthief · Jan 27, 2018

2 1/8 is not available in exhaust pipe?
I've noticed exhaust pipe is thinner now than in the 70's.
Technology, go figure.

grumpy150 · Jan 27, 2018

grumpy150 said:
One thing I noticed is when I looked up what to run on a 350 when I was a kid I found 21/8 Was the answer I got the most now that I've grown up (lol) I always wondered why 21/2 is the normal size everyone uses on the 5.0

Lol i know that 350 part is going to start some crap. But that' so not the point. And I'm not real brand specific in a perfect world there would be all blue ovals in my driveway.but when they give stuff

karthief said:
2 1/8 is not available in exhaust pipe?
I've noticed exhaust pipe is thinner now than in the 70's.
Technology, go figure.

grumpy150 · Jan 27, 2018

I fixed it once and still got it wrong 21/4. Now was that just a different time and technology tells us bigger is better or is this a pretty good question?

General karthief · Jan 28, 2018

Technology has changed so much, precise science allows for better metal and computers let us see what things will do when we add this or turn that, kinda makes us feel old don't it grump?
Sometimes I actually like it, sometimes not.

grumpy150 · Jan 28, 2018

karthief said:
Technology has changed so much, precise science allows for better metal and computers let us see what things will do when we add this or turn that, kinda makes us feel old don't it grump?
Sometimes I actually like it, sometimes not.

Oh ya it sure does. So really a
350 should have ran 2 1/2 ?or bigger? I remember my cosin had a iroc that had 3" single and a 305 tuned port. He claimed that was a big power adding exhaust. But we were smoking him in everyway posibbile in my brothers 5.0 convertible. My cousin rode in my last ford truck. He's all about the blue ovals now. But seriousy. There most be some serious learning to do to optimize your exhaust. Lots of chooses too. I also new a old guy that ran a 1 1/2 pipe from the center of the cross over pipe to the rear. It was kinda a automatic dump. Looked weird seeing three pipes out the rear. Car was real well known at the salt lake city dragway. Grumpys toy was the cars name. He had been running that set up since the 70's. That man was a true street racer. We took his money though. Lol There's a old timer story for ya

General karthief · Jan 28, 2018

That iroc was heavy and had less power than the 5.0s, specially if it had a slush box in it.
I've waxed a couple irocs around here with my junk.
Another thing technology has given us on the exhaust side is mandrel bends, no more crushing the pipe to bend it!

grumpy150 · Jan 28, 2018

Ya i knew about mandrel bending and that is a great thing. Just always wondered what the real best size diameter pipe for a small block and that would probably get into cubes and a bunch of other stuff. Like how much air and fuel you are putting threw it or want to anyway. I could go on about this all day.

90sickfox · Jan 28, 2018

" Everyone knows... B cam....open headers...50 shot...75mm TB = 10s quarter all day...ebry day !!!! "

This is what I go through #realconvo #justnow with the neighbors kid.

General karthief · Jan 28, 2018

That link in at the top pretty much explains it all

Stop Saying Car Exhausts Need Back Pressure

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

General karthief

wonder how much it would cost to ship you a pair

Mustang5L5

That is…until I whipped out my Bissell

Hoytster

I don't dare do that to my Knob

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

Hoytster

I don't dare do that to my Knob

cleanLX

Hoytster

I don't dare do that to my Knob

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

krazedstang

grumpy150

Active Member

General karthief

wonder how much it would cost to ship you a pair

grumpy150

Active Member

grumpy150

Active Member

General karthief

wonder how much it would cost to ship you a pair

grumpy150

Active Member

General karthief

wonder how much it would cost to ship you a pair

grumpy150

Active Member

90sickfox

Wasn't a pretty sight...and I've got big hands

General karthief

wonder how much it would cost to ship you a pair

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