Backpressure - Why it's good/bad
 

It seems a lot of people are talking about exhaust backpressure and how and why it causes a vehicle to loose power. In a nutshell I thought I'd try to clear it up a little.


BASICS:

Air enters and exits your combustion chamber through opening and closing valves. There are intake valves and exhaust valves. A common misconception is that they're not ever open at the same time. The truth is that there is a short period of time when both intake and exhaust valves are open... this is called valve overlap. The intake valve opens just before the exhaust valve closes 100%. This is good and bad. GOOD because the incoming fresh air/fuel helps push out the burned up air/fuel into the exhaust, but BAD because some of the unburned air/fuel sneaks right through and never gets burned; obviously burning this air/fuel is how the engine makes power.

BACKPRESSURE:

Backpressure controls how hard/easy it is for burned air/fuel to flow out of the combustion chamber into the exhaust.

At low RPM you want more backpressure to prevent the intake air/fuel from flowing through without getting burned. Reason being is because since the engine RPM is lower the valve overlap is longer so there is plenty of time for the burned air/fuel to move into the exhaust, but there is also more time for the fresh unburned incoming air/fuel to sneak through to the exhaust without ever getting burned.

At high RPM you want less backpressure. Since the RPM is higher the valve overlap time is much shorter so you want to make it easier for the burned exhaust air/fuel to move into the exhaust.

SUMMARY:

So "more" backpressure helps make more low end power, but hurts high end power and "less" backpressure helps make more high end power, but hurts low end power.

The idea is to have an exhaust with the perfect amount of backpressure to make the most power at the RPM range you want the power in. I myself would want the most power in the low to mid RPM range because that is the RPM range I am normally in. Now if I drove a Nascar I would want very little backpressure because a Nascar normally constantly operates at very high RPM.

Auto manufacturers try to make our OEM exhaust systems with the "perfect" amount of backpressure, but they're bound by one factor... SOUND. They have to make our OEM exhausts quiet and all this muffling adds too much backpressure which causes us to loose overall power.

MORE ADVANCED:

Another thing to consider is that new vehicles including our new F-150's have something called VCT (Variable Cam Timing) which changes the valve opening/closing and valve overlap which was not possible before VCT. The problem with this is that the VCT is controlled by the vehicles computer and is programmed to function with the OEM exhaust backpressure. This is why you can sometimes loose power by adding an aftermarket exhaust in a VCT equipped vehicle. Not to fear... the SuperChips programmer will address this issue.

I hope this helps. If you have any ?'s just ask.

Thanks,
Rich

Re: Backpressure - Why it's good/bad
 

This is why i have the 4.6. :D

You noted backpressure, and valve overlap. You might want to look into the EGR of an engine. But nice post, pretty well put! :thumbsup:

There are way of "tricking" the engine into thinking there is more backpressure or creating backpressure at lower RPM's.

1. My old '96 Cobra has Intake Manifold Runner Control Plates (IMRC Plates for short) which actually close off one of the two exhaust ports when engine speed is below 3500 RPM's. This increases "backpressure" and power. When RPM's go above 3500 RPM's, the servo-controlled plates slide, and open the second exhaust port to allow more flow.

2. My CBR929RR has a Honda Titanium Exhaust Valve (HTEV for short), which is also servo-controlled. At engine speeds of less than 7,500 rpm, this specially designed exhaust valve acts as a 360-degree exhaust collector to help boost low- and mid-range power. In the upper levels of the powerband it turns into a 180-degree collector in an effort to achieve maximum power output while retaining a high-degree of efficiency and rideability. The unit is machined out of titanium and is, all by itself, a beautiful work of craftsmanship.

It is odd that I happened to own two vehicles that use methods of increasing backpressure at low RPM's.

I thought EGR (Exhaust Gas Recirculation) was a little off topic. It's main purpose is to reduce nitrogen oxides for emissions by lowering combustion temperatures.

Plus, it doesn't effect power when we need it becuase in nearly all vehicles EGR is shut off during wide open throttle.

... and on top of that I'm tired of typing.

That's what I thought too. The EGR has nothing to do with backpressure...

I'm not too familliar with the IMRC system, but I'm pretty sure it controls the secondary butterflys inside the intake manifold. I didn't think it had anything to do with the exhaust. Could you explain?

The Honda exhaust valve sounds pretty trick though... must be tough to hold up to the heat.

I was editing... sorry.

Here's a picture of the exhaust valve...it is REALLY trick!

https://www.f150online.com/galleries...6484-98447.jpg

I will quote myself.

There is no such thing as backpressure. There is no air that goes back into the engine.

The only thing that matters is exhaust velocity. Design an exhaust system that keeps the speed of the exhaust gases high, and you'll have good torque at any engine speed.

Not really sure what this has to do with 2004 F-150's though.

Bottom line...put in a bad a$$ exhuast and a superchips programmer to keep your computer happy which will inturn keep your seat of your pants power happy.

My head hurts now... thanks a lot, guys! :mad:

Honestly.

I read every bit of that and only some of it I understood.

Lol, I read every one of them and only a few were correct!

Grim

Begs a question
 

A lot of good info here. Now I understand a little bit better about backpressure but all this begs the question: which of the aftermarket exhausts optimally provides more backpressure for low-end power and less backpressure for hi-end power?

:thumbsup: :thumbsup: good post. Nice to get a refresher on things. Even learn some new stuff.

Sean...check the exhaust vendor's dyno charts (is it naivete' to trust their charts?) or the charts posted by those on the forum who have made dyno runs.

Maybe I'm just a skeptic, but it's difficult for the common folk like us to cut through all the vendor's smoke and get the facts albeit some of the trade mags offer credible charts.

This topic takes research to get FACTS...some opinions about exhaust products on this forum are more 'seat of the pants' and urban myth than objective contributions.