Backpressure on 4cyls.

mitchell35758 · Nov 29th, 2005, 02:36 PM

I know that ideally you don't want backpressure, but, you need a condition to keep you engine from being damaged that creates some backpressure just as a side effect.

In the articles that I have read about back pressure, they talk mostly about V8's and now I am beginning to wonder if my "universal magnaflow" is not that safe for my 4cyl. Going by the fact that I have half the cylinders as a V8, there will not be as much exhaust passing through the system. With the universal magnaflow, I think that it might by too unrestrictive to allow for the needed condition that creates the "un-wanted" back pressure as its side effect.

Am I just being paranoid, or might I be on to something?

avenger · Nov 29th, 2005, 03:36 PM

Backup...

First, the "Damage" has to do with the fact that you need a header, or headers, to keep the outside air away from your hot exhaust valves, otherwise you'll warp them...

Now. read this.

Quote:

"THE MYTH OF BACKPRESSURE"

…is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most impotant ones you need to be aware of to understand the things I'm about to say:

BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.

One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is it’s pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth, just that it makes sense in my eyes.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.

These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).

This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.

It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace.

mitchell35758 · Nov 29th, 2005, 03:48 PM

Quote:

Originally Posted by avenger

Backup...

First, the "Damage" has to do with the fact that you need a header, or headers, to keep the outside air away from your hot exhaust valves, otherwise you'll warp them...

Now. read this.

Not based off of what you included, just off what you said, to keep from screwing my exhaust valves up with my magnaflow, I'll need to install a header? I just don't know much about exhaust, so bear with me.

05 NISMO 4X4 · Nov 29th, 2005, 04:07 PM

Quote:

Originally Posted by mitchell35758

Not based off of what you included, just off what you said, to keep from screwing my exhaust valves up with my magnaflow, I'll need to install a header? I just don't know much about exhaust, so bear with me.

I think what he meant was you don't want the exhaust outlet of the engine block exposed to the outside air. Your stock exhaust manifold accomplishes this the same as headers. All headers will do for you is make the exhaust gases flow freer than the stock manifold. The combination of the stock exhaust pipes and your magnaflow muffler are adequate, and you shouldn't worry about damage to your engine. Believe me, the stock exhaust is restrictive enough to balance the pressure in the system...

avenger · Nov 29th, 2005, 04:28 PM

Cliff Notes: You're fine. The stock manifold and piping to the muffler will keep the delta pressure so you're not losing power.

jerryp58 · Nov 29th, 2005, 04:48 PM

Quote:

Originally Posted by 05 NISMO 4X4

I think what he meant was you don't want the exhaust outlet of the engine block exposed to the outside air. Your stock exhaust manifold accomplishes this the same as headers. All headers will do for you is make the exhaust gases flow freer than the stock manifold. The combination of the stock exhaust pipes and your magnaflow muffler are adequate, and you shouldn't worry about damage to your engine. Believe me, the stock exhaust is restrictive enough to balance the pressure in the system...

Sucking in cool air from too short an exhaust "outlet" (e.g., no manifolds, open manifolds, or maybe open shorty headers) is the only thing I've heard can damage the engine as far as exhaust is concerned (aside from a plugged/clogged exhaust). And I agree with 05 NISMO 4X4, you have way too much exhaust for that to ever happen. You can have too free-flowing an exhaust and reduce your bottom end torque, but again, I agree with 05 NISMO 4X4 that an aftermarket muffler will not have that much of an affect on an otherwise stock exhaust.

avenger · Nov 29th, 2005, 06:34 PM

Quote:

Originally Posted by jerryp58

Sucking in cool air from too short an exhaust "outlet" (e.g., no manifolds, open manifolds, or maybe open shorty headers) is the only thing I've heard can damage the engine as far as exhaust is concerned (aside from a plugged/clogged exhaust). And I agree with 05 NISMO 4X4, you have way too much exhaust for that to ever happen. You can have too free-flowing an exhaust and reduce your bottom end torque, but again, I agree with 05 NISMO 4X4 that an aftermarket muffler will not have that much of an affect on an otherwise stock exhaust.

re: too free flowing... this is never bad and won't reduce bottom end torque, what hurts is when the piping is so large that the hot gas, upon leaving the cylinder, expands too much and thus cools down. this slows the gas and actually raises "back pressure", reducing power.

free flowing narrow pipes to keep the optimal delta pressure is the ticket

mitchell35758 · Nov 30th, 2005, 12:36 AM

Thanks guys

Like I said, I don't know too much about this topic and I was beginning to wonder if the "universal" magnaflow was ok for my small engine. Thanks again.

johnnyhammers · Nov 30th, 2005, 12:50 AM

you're truck is just fine. I don't want to get into the details because I'll be here till dawn if I do, but your engine will be totally fine. you can feel free to add a header if you want, or just stay with the stock manifold, either way will be OK.

I can dig up some old but very good articles on this topic if you want. Just ask, but I warn you, they're very boring, but fascinating at the same time.

mitchell35758 · Nov 30th, 2005, 12:56 AM

Quote:

Originally Posted by johnnyhammers

I can dig up some old but very good articles on this topic if you want. Just ask, but I warn you, they're very boring, but fascinating at the same time.

I think I have had enough reading on back pressure. I been reading articles and other bits of info all over the web. Thanks anyways

I might have to upgrade the manifold to a header some day. I think that will be a while though, I have spent enough money on this damn truck for a while. Tell you what though, it sure is fun doing it.

johnnyhammers · Nov 30th, 2005, 01:27 AM

spending money is always fun if it's not required by law. souping up cars is fun whether you've got the cash or not. You're right Mitchell, this is fun!

Johnny

mitchell35758 · Nov 30th, 2005, 02:25 AM

Quote:

Originally Posted by johnnyhammers

spending money is always fun if it's not required by law. souping up cars is fun whether you've got the cash or not. You're right Mitchell, this is fun!

Johnny

hell yeah!