Back Pressure

[TC]

So I'm sitting here thinking and with all this talk about mufflers, I was wondering how much back pressure is to much??

And at that how much is to little or is there such a thing as to little back pressure??

[yellomalibu]

Re: Back Pressure

Whether working on the car, doing floor tile, or some other project that forces me to bend over for long periods of time, I get terrible back pressure. ;D


But seriously folks...
I wonder if "back pressure" is a real term used by real performance gurus. Is "back pressure" just a different term for flow impedence? Is "back pressure" a trade off between lower rpm torque and higher rpm horsepower?

[DanStokes]

Re: Back Pressure

In the circles I revolved in, "back pressure" was used to refer to the positive (usually) pressure exerted on the flow path out of the exhaust ports - however it got there.

Should not be confused with differential pressure in the exhaust system, such as across the cat or muffler. This is measured as a difference in pressure from the exhaust system at the point of measurement (pre muffler, post muffler, etc.) referenced to atmosphere.

There can be WAY too much BP and under some circumstances, too little. I don't remember the details but I'm thinking you need a certain amount to keep the combustion chamber up to temp and to keep the exhaust from purging the fresh intake charge out of the chamber. I haven't kicked this around with anyone for a long time and may have a slightly scrambled memory on this one. Dynoroom, anyone....... Please - INPUT!

Dan

[JeffMcKC]

Re: Back Pressure

I don't know anyone elses thinking on this. Mine is the least amount as possible, I prefer the header to do its work independently of the exhaust, the primary tube size and length being right and the right size of collector. The pulse wave being right will help with the signal. With the total length of the exhaust added its all thrown off. The camshaft and tune of the motor should take care of the "heat" in the exhaust. I count exhaust as whats after my header. I have yet to not drop the mufflers and pick up .20 and 3-4 MPH in a quarter.

[urwurznitmahre]

Re: Back Pressure

I don't know anyone elses thinking on this. Mine is the least amount as possible, I prefer the header to do its work independently of the exhaust, the primary tube size and length being right and the right size of collector. The pulse wave being right will help with the signal. With the total length of the exhaust added its all thrown off. The camshaft and tune of the motor should take care of the "heat" in the exhaust. I count exhaust as whats after my header. I have yet to not drop the mufflers and pick up .20 and 3-4 MPH in a quarter.

BUT IN THE WORLD OF MPH AND Exhaust emissions..
you want some. so
1) the fuel doesn't end up in the exhaust and
2) it's a internal egr system.. that keeps the nox gasses in check and temps down..
in a em mission app. fuel and unused oxygen will screwwith the fuel maps

[JeffMcKC]

Re: Back Pressure

I don't know anyone Else's thinking on this. Mine is the least amount as possible, I prefer the header to do its work independently of the exhaust, the primary tube size and length being right and the right size of collector. The pulse wave being right will help with the signal. With the total length of the exhaust added its all thrown off. The camshaft and tune of the motor should take care of the "heat" in the exhaust. I count exhaust as whats after my header. I have yet to not drop the mufflers and pick up .20 and 3-4 MPH in a quarter.

BUT IN THE WORLD OF MPH AND Exhaust emissions..
you want some. so
1) the fuel doesn't end up in the exhaust and
2) it's A internal egr system.. that keeps the nox gasses in check and temps down..
in a em mission app. fuel and unused oxygen will screwwith the fuel maps

no its not a EGR and a complete burn happens before the valve is opened.
This is where the way I tune seems to be different than a lot of people and camshaft pick is too. I believe that Geek will tell you my engine that idles with 2-3 inches of vaccum at 1300 RPM will be pretty close to passing emmissions I do not tune with AFR it leaves a lot on the table.

[Bamfster]

Re: Back Pressure

I have never lost et & mph when dropping the mufflers, even when its tuned to run with mufflers. I tend to run alot of overlap so I can get away with more compression on pump gas, so that isn't ideal from an emissions stand point.

[MadmanMark]

Re: Back Pressure

I've had the same 3" x-pipe system with cut-outs for two different engines, both tuned for running open. At the track with the 383, there was no difference running open or closed. With the new 427 sbc, the extra backpressure costs me a few tenths and a few mph.

One thing that seemed a little odd to me, was the 383 would get incredibily better mpg on the highway (14-15 vs 10-11) with the exhaust open vs closed. (I haven't done a similar test with the other motor).

[JeffMcKC]

Re: Back Pressure

Most people run their motors way to rich and cold making a lot of unburned fuel.

I am not trying to say mine will pass a test just its not as far off as you would think.

[urwurznitmahre]

Re: Back Pressure

I don't know anyone Else's thinking on this. Mine is the least amount as possible, I prefer the header to do its work independently of the exhaust, the primary tube size and length being right and the right size of collector. The pulse wave being right will help with the signal. With the total length of the exhaust added its all thrown off. The camshaft and tune of the motor should take care of the "heat" in the exhaust. I count exhaust as whats after my header. I have yet to not drop the mufflers and pick up .20 and 3-4 MPH in a quarter.

BUT IN THE WORLD OF MPH AND Exhaust emissions..
you want some. so
1) the fuel doesn't end up in the exhaust and
2) it's A internal egr system.. that keeps the nox gasses in check and temps down..
in a em mission app. fuel and unused oxygen will screwwith the fuel maps

no its not a EGR and a complete burn happens before the valve is opened.
This is where the way I tune seems to be different than a lot of people and camshaft pick is too. I believe that Geek will tell you my engine that idles with 2-3 inches of vaccum at 1300 RPM will be pretty close to passing emmissions I do not tune with AFR it leaves a lot on the table.

and if when your under blowdown with exhaust valve open, and you have some backpressure, the cyl doesn't completely empty, leaving burt gasses , intake opens and try's to full the cyl. but there is unburnable gasses taking up space.
how is that eny different to an open egr valve , that lets burt gasses into the intake stream and into the cyl. taking up space that burnable mix would be otherwise???????????????/

[JeffMcKC]

Re: Back Pressure

I don't know anyone Else's thinking on this. Mine is the least amount as possible, I prefer the header to do its work independently of the exhaust, the primary tube size and length being right and the right size of collector. The pulse wave being right will help with the signal. With the total length of the exhaust added its all thrown off. The camshaft and tune of the motor should take care of the "heat" in the exhaust. I count exhaust as whats after my header. I have yet to not drop the mufflers and pick up .20 and 3-4 MPH in a quarter.

BUT IN THE WORLD OF MPH AND Exhaust emissions..
you want some. so
1) the fuel doesn't end up in the exhaust and
2) it's A internal egr system.. that keeps the nox gasses in check and temps down..
in a em mission app. fuel and unused oxygen will screwwith the fuel maps

no its not a EGR and a complete burn happens before the valve is opened.
This is where the way I tune seems to be different than a lot of people and camshaft pick is too. I believe that Geek will tell you my engine that idles with 2-3 inches of vaccum at 1300 RPM will be pretty close to passing emmissions I do not tune with AFR it leaves a lot on the table.

and if when your under blowdown with exhaust valve open, and you have some backpressure, the cyl doesn't completely empty, leaving burt gasses , intake opens and try's to full the cyl. but there is unburnable gasses taking up space.
how is that eny different to an open egr valve , that lets burt gasses into the intake stream and into the cyl. taking up space that burnable mix would be otherwise???????????????/

Your fogetting camshaft timing, and the headers function, proper runner length and plenum size . My Drag Week motors have no ehaust stains in the intake port, meaning no reversion of exhaust into the intake to delute the charge. Why do you want unburned gas in the cylinder taking up room for power to be made? (I know what the EGR does) But what your looking for when making power is no reversion and no exhaust left or as little as possible.

[urwurznitmahre]

Re: Back Pressure

I don't know anyone Else's thinking on this. Mine is the least amount as possible, I prefer the header to do its work independently of the exhaust, the primary tube size and length being right and the right size of collector. The pulse wave being right will help with the signal. With the total length of the exhaust added its all thrown off. The camshaft and tune of the motor should take care of the "heat" in the exhaust. I count exhaust as whats after my header. I have yet to not drop the mufflers and pick up .20 and 3-4 MPH in a quarter.

BUT IN THE WORLD OF MPH AND Exhaust emissions..
you want some. so
1) the fuel doesn't end up in the exhaust and
2) it's A internal egr system.. that keeps the nox gasses in check and temps down..
in a em mission app. fuel and unused oxygen will screwwith the fuel maps

no its not a EGR and a complete burn happens before the valve is opened.
This is where the way I tune seems to be different than a lot of people and camshaft pick is too. I believe that Geek will tell you my engine that idles with 2-3 inches of vaccum at 1300 RPM will be pretty close to passing emmissions I do not tune with AFR it leaves a lot on the table.

and if when your under blowdown with exhaust valve open, and you have some backpressure, the cyl doesn't completely empty, leaving burt gasses , intake opens and try's to full the cyl. but there is unburnable gasses taking up space.
how is that eny different to an open egr valve , that lets burt gasses into the intake stream and into the cyl. taking up space that burnable mix would be otherwise???????????????/

Your fogetting camshaft timing, and the headers function, proper runner length and plenum size . My Drag Week motors have no ehaust stains in the intake port, meaning no reversion of exhaust into the intake to delute the charge. Why do you want unburned gas in the cylinder taking up room for power to be made? (I know what the EGR does) But what your looking for when making power is no reversion and no exhaust left or as little as possible.

YOUR DRAGWEEK ENGINES GOT NOTHING TO DO WITH A EMISSION COMPLIANT engine..

[TheSilverBuick]

Re: Back Pressure

YOUR DRAGWEEK ENGINES GOT NOTHING TO DO WITH A EMISSION COMPLIANT engine..

Some how I don't think TC was wondering about an emission compliant engine either.

[urwurznitmahre]

Re: Back Pressure

YOUR DRAGWEEK ENGINES GOT NOTHING TO DO WITH A EMISSION COMPLIANT engine..

Some how I don't think TC was wondering about an emission compliant engine either.

Ha. I doubt it also.. but thats where the thread led..

[Eagle Kammback]

Re: Back Pressure

Here TC, this may help you

http://kalecoauto.com/index.php?main_page=product_info&products_id=2