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Ford Raptor Exhaust Theory

I’m writing this article today to help Ford Raptor Enthusiasts understand their trucks a little better, and to understand how their exhaust systems work. I’ve been a Ford Motor Company enthusiast for over 20 years. The Ford Raptor is certainly one of Ford’s greatest creations!! The downside to all of it’s Luxury, Power, and Off Road Capability is that it comes with a price.

 

Given it’s price, buyer demographics of the Ford Raptor tend to be guys old enough to remember the original 5.0 Mustang. Matter of fact it’s very possible that a Ford Raptor owner was once upon a time a 5.0 Mustang owner. After all, Ford made millions of 5.0 Mustangs! Both my wife and I owned 5.0 Mustangs!! She had a White 93 notch, and I had a Dark Blue 89 Notch.

 

I bring up the 5.0 Mustang and Demographics, etc. Because it’s very important to take everything that you know about 5.0 Mustang exhaust and completely forget about it!! Back in those days just about anything would make the exhaust flow better and more power was the by-product.

 

Our modern day Ford Raptor engines need an exhaust that works with its variable valve timing. Precise dimensions must be maintained or proper scavenging will not occur and the engine will waste HP pushing out the exhaust instead of turning the tires.

 

Ford engineers spent countless hours to figure out the exact dimensions needed to maximize the Variable Valve Timing system that is installed on every Ford Raptor.

 

When I first began my quest to build the perfect exhaust system I visited every shop in North County (San Diego). Everywhere I went I heard guys telling me, “you need back pressure or you will lose low end torque”. After countless hours of research, and consulting with my neighbor (retired Thermal Dynamics Engineer with Lockhead Martin, his biggest project was Apollo), I have learned that you don’t want any backpressure at all!! Backpressure kills power.

 

We want scavenging, not backpressure!! The problem with our 3 dimension world is that there is no such thing as exhaust tubing which gets bigger in diameter as the engines rpm increases and smaller in diameter as the engines rpm decreases. At the time of this writing this is impossible;) we have to settle.

 

If we size the tubing too small it will build good low end torque, but top end will suffer. If we size the tubing too big it will kill low end, but there will be NOTHING gained at the top end! This is because even though a restriction is removed (the muffler), not all of the restrictions are removed, specifically the stock exhaust manifolds. A good analogy would be, “why dig a bigger ditch if the chute in the dam is stuck only partially open”.

 

Bigger exhaust tubing gives the potential to flow more exhaust. But if we’re not removing the second biggest restriction in the system, the exhaust manifolds, then why are we making the tubing bigger in diameter and losing all that low end torque???

 

We have to look at where Ford engineers had their hands tied. The muffler and the exhaust manifolds. You can bet your bottom dollar each and every Ford engineer is a motorsports enthusiast!! Why would they spend all that time and $$ learning to become an engineer if they didn’t absolutely love motorsports. If they had their way EVERY Raptor would have long tube headers and a free flow muffler. If that was the case I wouldn’t be here writing this article 😉

 

Ford engineers’ ‘big brother’ is Ford accountants, and the EPA. The stock Raptor muffler certainly is not a Ford engineers greatest creation when it comes to power. It’s a huge compromise. It has to meet a certain sound level, and it has to meet a certain price point. The stock muffler is THE MOST RESTRICTIVE part of the stock exhaust system. It is DESIGNED to be restrictive, on purpose.

 

There are 3 ways to quiet down an exhaust system. Here they are in order of effectiveness: Restriction. Hands down the easiest, most cost effective way to quiet down a truck. Reflection. A reflection style muffler actually becomes a restriction as the rpm increases, so it’s basically 2 mufflers in 1. And absorption.

 

Restriction is what it sounds like, stick a huge restriction into the flow of exhaust and the sound stays inside the system. Reflection does basically the same thing, but it attempts to give the exhaust a path around the ‘wall’ that is placed in the way of the hot, fast moving exhaust gases. Absorption gives the sound an area to bounce around in so that the sound stays inside the muffler.

 

A properly built absorption muffler is hands down the best type of muffler for making power. Now I’m not talking about a Cherry Bomb Glass pack. I’m talking about sizing a chamber correctly so that the sound waves have a big enough space to bounce around in and absorb into the packing. Much like a box for a bass speaker. You can make a small box act like a big box by simply adding packing into the box. An absorption style muffler will do exactly the same thing.

 

But, if properly built, an absorption style muffler will work like a RESTRICTION style muffler at low rpm, and like straight pipe at higher rpm. Here’s how: At lower rpm each cylinder fires a high pressure pulse into the exhaust system. This high pressure pulse has to speed up to exit the exhaust system. But, when the high pressure pulse enters the muffler can, since the can is so much bigger in diameter than the exhaust tubing, the pulse expands into the can and slows down. The following high pressure pulse hits the first slower pulse and it slows down.Then a chain reaction happens and each new pulse collides into the previous pulse. This reaction causes the exhaust tubing to fill up at a lower rpm than it would on its own and scavenging occurs siphoning out the exhaust from the next firing cylinder.

 

As the rpm increases this series of high pressure pulses acts more like a steady high pressure flow. This steady high pressure flow actually fills the muffler can and holds it under pressure. If the can is properly sized the pressure in the can will start to equalize with the pressure in the rest of the exhaust system right at the engines peak volumetric efficiency and instead of entering the can and slowing, it will actually blow straight thru the can as if it was straight pipe.

 

To sum it up, a properly built absorption muffler will act like a restriction style muffler at lower rpm and help build low end torque, but it will act like straight pipe at higher rpm so as not to sacrifice any top end hp.

 

The last few paragraphs above are very important when considering exhaust for a mostly stock Raptor (as far as engine configuration is concerned) The stock muffler truly is the most restrictive part of the exhaust system. This restriction is the only way Ford engineers were able meet the sound requirements that they had to meet to satisfy the Federal Government and the average Ford Raptor buyer, while still satisfying a team of Ford Bean Counters 🙂

 

Another huge factor to consider when designing a muffler for the Ford Raptor is drone. Ford uses a team of NHV (noise, harshness, vibration) engineers to identify annoying sounds (among other things) and to eliminate them. Most OEM mufflers use 1/4 wave or 1/2 wave resonator physics to cancel out annoying, dominating frequencies. Take a look at most OEM intake tubes. Ever notice how there is a small box, or tube hanging off the side? This is not your average ‘muffler’. The size and shape of this box, or tube is carefully thought out so that it specifically cancels out certain frequencies so that you don’t have to hear it every time you step on the throttle.

 

Your OEM muffler uses the same sound canceling physics. The second you remove your stock muffler in favor of a more aggressive, better flowing muffler, you’re also removing this sound canceling phenomenon built into that muffler. If you don’t do anything to cancel out these particular frequencies you’re stuck listening to the dang truck everywhere you go!! But by targeting the annoying frequencies, you can enjoy a great sounding V8 truck when you’re on the throttle, yet a stock quiet truck when just cruising.

 

There are 2 ways that I’ve seen the aftermarket approach this problem. The most common way is to attempt to cover up the drone. They will build a muffler big enough (or almost big enough) to absorb out the annoying frequency, or they will cancel out specific frequencies (like Corsa).

 

When you attempt to cover up the drone you end up with a very mild, almost stock sounding truck. Covering up the drone requires a really big muffler since the droning frequency has a really long wave length. And since the higher frequencies, the ones that we want to hear, have a really short wavelength, we end up absorbing the sounds we want to hear, and we have no highs or lows left in the sound of the exhaust to cancel out the low that we’re trying to get rid of. This makes for a quiet exhaust which you only hear as drone.

 

Then there is the approach where you make the muffler just big enough to accomplish the sound level goal, and use a tuned J pipe (aka Helmholtz Resonator, aka 1/4 wave acoustical stubbed resonator) to target the specific frequency that we’re trying to get rid of.

 

My approach to making that race car sound on the throttle, yet quiet on the highway is to absorb enough of the sound that it’s not too loud with a properly sized muffler, then whatever annoying frequency that is left over gets cancelled out thru physics using a 1/4 wave acoustical stubbed resonator. This approach is the most simple design I’ve found that is actually effective.

 

I’ve built hundreds of 1 off systems and thought to myself, this system has so much drone that there is no way you could ever reduce it to the point that anyone would want to daily drive the vehicle. To my surprise, I’ve been able to ‘fix’ the drone on a pretty good percentage of these prototype configurations. (Most still drone badly and I never put them into production;) But the ones that are Rowdy on the throttle, yet Quiet on the highway are released into production so that everyone may enjoy them!!!