Installing an AML Performance Secondary CAT Delete Kit in an Aston Martin DB9

If you are a member of any of the Aston Martin Owners FaceBook Group or online Forums (like PistonHeads or 6SpeedOnline) you will likely have seen some chatter talking about fitting a ‘Secondary Cat Delete Kit’ (SCDK) in your car to increase the power and improve the sound.  It’s not like the stock DB9 has a deficiency in either department already, but its promise is a simple change will give you more of each.  Intrigued by this I couldn’t resist researching how to tackle this.

There are several vendors that make SCDKs, and in this article I will be covering my personal experience with the kit from AML PerformanceAML Performance is a small company in the UK, and I met its parts manager Mark Holden in the Aston Martin Owners Facebook Group.   He is a DB9 owner himself.  Mark suggested I might want to try out a set of their SCDKs and I took him up on the offer.   Read on to learn about their kit and how to fit it.


What is a Secondary Cat Delete Kit?   I had to figure this out too.   The DB9 V12 engine is fitted with a total of six (6) Catalytic Converters.  Two Primaries and one Secondary for each bank of the engine.  So four primaries total, and two secondaries.

A Catalytic Converters job is to reach very high temperatures internally (as much as 800 °F) to chemically change the pollutants in the exhaust stream into less nasty things.  They contain very expensive exotic metals to do this, including platinum.  You can learn more about it over on Wikipedia here.   Basically an exhaust gas cleanser.

The Primaries are located right up in the engine bay and are integrated into the exhaust headers near the cylinder head.   Each primary serves three cylinders.

There are O2 sensors up and down stream of each primary catalytic converter.  O2 sensors tell the PCM (the computer that controls the engine) how much unburnt fuel is in the exhaust stream as it enters the converter and as it exits.  The PCM then adjusts the amount of fuel injected into those three cylinders to achieve the perfect balance of Air/Fuel ratio,  maximizing fuel economy, power and minimizing emissions.

Most cars only have a primary catalytic converter.  This begs the question what is the secondary catalytic converter installed in the DB9 and Vantage actually doing.   I don’t know.  There are no O2 sensors fitted around it, so it has zero influence on the cars control system.    I suspect it might be fitted to ‘polish’ the exhaust stream in cold start scenarios where for a very short period of time while the converters are getting up to temperature some pollutants are slipping through the primaries.  But, the secondaries will be colder than the primaries, and thus function even worse.  Hence, I don’t know.  If you work for Aston and really know, please leave a comment below or email me using the Contact page above.

The downside of this extra set of converters is additional restriction (back pressure) in the exhaust system flow.  For performance reasons we do everything we can to eliminate back pressure so we can let the exhaust breath easier.   More flow generally means more power.   The best scenario is big short straight pipes with no restrictions (just like a Top Fuel Dragster).  We can’t exactly have flames spitting straight out the side of the engine bay, so we are seeking a compromise.

A SCDK does just what it says, it removes the Secondary Catalytic Converter and replace it with a simple straight section of pipe.  Supposedly this accomplishes three things that you might find desirable in your car:

  • Improves power by reducing back pressure and improving exhaust flow
  • Improves the sound of the car since less of the exhaust noise will be stifled by the converter
  • Reduces the weight of the car by removing two big bulky converters with a simple and lighter straight section of pipe.

Is is legal?   In the US the answer is officially ‘No’.   Generally anything the modifies anything with respect to the emission controls system is not allowed.   As such, I am only doing this as a test of what a SCDK will do and then I will return my car to stock ;>)

The Plan

I know there will be some of you saying “Aston designed it into the system for some reason and it would be foolish to remove it”.   I get that.  But, I also believe manufacturers have to make cars that work in the world market, and perhaps this was designed in as a safety factor or mandated requirement for some country that doesn’t apply to where I live.   I couldn’t resist trying it out, so here is my plan:

OK, enough research and planning, lets get on with bolting some parts on the car!


We need to only order a couple of items to tackle this project.

The AML Performance Secondary Cat Delete kit is available for about £500 (about  $625 USD) by contacting Mark by email ( or by phone at +447870618007.   The £500 includes VAT and either Free fitment if you bring in your car, or Free shipping.  Nice deal.  They ship everywhere, but obviously this is a great choice in the UK and the EU.

It’s a very professional looking kit.  The pipes are made from stainless steel and highly polished.  The welds are beautiful.  The pipe diameter steps down right near the header flange and is a very smooth stretched section without any welds.  This smooth transition certainly will help the flow as its transitions to the smaller pipe size of the rear exhaust section.

The kit includes the two secondary Cat delete pipes only. It does not include new nuts, bolts or exhaust gaskets.

I generally don’t like to reuse gaskets as a rule.   They are Aston Martin p/n  4G43-5E281-AA-PK (2 pack).   You can it from your local Aston Dealer or online for about $36.57 USD here.   I actually picked mine up from Frank Marino who had an extra set he was selling off on the FaceBook group for Aston Martin Parts (which I suggest you join, lots of interesting stuff at good prices show up there, find it here).

Note:  As you’ll see in the video when I install the kit, the old gaskets I removed were in remarkably good condition after 15 years and 42,000 miles.   They easily could have been reused.   I live in corrosion free dry California.  You might opt to pick up the gaskets anyways since they aren’t a big investment and to have them in case yours aren’t in the same shape.

You can potentially reuse your existing bolts too, but that will depend on their condition when you removed them.   Mine were serviceable, but were showing signs of thread wear.  All those years of heat.  You may opt to locally source a set of six Stainless Steel bolts and nuts for your project.

One supply I ordered in to have on hand was Exhaust Paste.  The official Aston Martin Workshop Manual Section 9.03 on the Exhaust System Piping says to use some undefined Exhaust Paste during reassembly of the clamped end.  I ordered a small tube of Permatex Exhaust Paste online from Amazon for about $3.70 USD.   My plan was to review the joint when I disassembled it and if it had no paste on it from the factory, I was NOT going to apply any during the pipe install unless there was any pitting or corrosion on the downstream exhaust pipes.  As you’ll see mine are in near pristine condition, so I ended up not using this.  But, if you live in a more corrosion prone environment, you might have some on hand and use a dab during reassembly to make sure you get a gas tight seal.

Ok, we’ve got pipes, nuts, bolts and gaskets.   Next up….

Tools Required

You don’t need anything exotic, just some basic shop tools:

  • 13 mm combination wrench
  • 13 mm deep socket
  • T30 Torx Bit
  • Universal Joint for Socket
  • 6″ extension
  • 18″ extension, or multiple smaller extensions
  • Ratchet
  • Torque Wrench capable of 30 Nm
  • Inspection Light
  • Microfiber towel


You’ll need to tackle a few steps first to get to the point where its time to install the Secondary Cat Delete Kit.  I’ve made articles and videos on each of the steps to get you ready:


The only reference is about 11 words in the official Aston Martin Workshop Manual Section 9.03 on the Exhaust System Piping.   It makes reference to using the exhaust paste and provides the torque specs (which is good).   Below is my more detailed description with a few tips along the way.

The whole process will take only about 20-30 minutes.

  • Work on one side at a time.  Install the entire left pipe or the right pipe first, then move onto the other.  This works out better with wrenching for the first one you do.
  • Clean the header flange of any gasket residue or carbon build up.  I merely used a shop towel to wipe mine down.  If yours has more residue or corrosion, use some emery paper or whatever is necessary to get a nice smooth surface for the gaskets.
  • Cleaner the down stream piper flared pipe connection.  Again, I was able just to wipe mine out and make sure it was smooth.
  • Offer the pipe up into position.
  • I loosely fitted one of the bolts to keep it from dropping on my head while I fiddled with the clamp end.
  • I moved the clamp over the flared ends to just loosely hold it up in place.  I deliberately did NOT tighten this joint yet.
  • Fit your gasket into place.
  • Loose fit the three bolts through the flange and gaskets.  The bolts point to the back of the car, and pass through from the front same as you removed the old ones.   Fit the nuts as tight as you can make them finger tight.
  • Snug up all three nuts:
    • Uppermost – I used the 13 mm box end of the combination wrench and could just barely get it onto head of the bolt, but it will fit.  Hold it in place with one hand.   I used the 6 or 18″ extension, universal joint, 13 mm socket and ratchet to get on the nut (see photo).  Snug the nut up tight, but leave the final torquing for later.
    • Lower outer – Getting the wrench on requires stuffing your arm in above the aerodynamic under tray.  Not hard, just tug it down and put your arm in.  Getting the wrench on isn’t hard.  Use the 13 mm socket, universal joint, 6″ extension and ratchet to get on and snug this one up.
    • Lower inner – easiest of the three.  Follow the same process to snug it up.
  • Torque up all three nuts to 30 Nm (22 ft-lbs).  Similar to snugging them up, I follow the same process to get my torque wrench in place and properly tighten them.
  • With the flanged end all tight, we can now move to the back end of the pipe and worry about mating it up with the flared exhaust pipe.
  • If your joint needs to have exhaust paste to seal it up, now is the time to fit it.  As noted, mine is in great condition so I omitted this step.
  • I went to the back of the car, grabbed the rear section of the exhaust pipe and gave it a few mighty shoves forward to butt it up tight to the new pipe.  While I was doing this I went back and wiggled the joint each time to help align the two surfaces between pushes.   Spend the time on this to get them flush fit to each other.  This is important.  The clamp will NOT suck them together that much.
  • Using the 13 mm deep socket and ratchet, run the nut up just until the clamp begins to snug up, but you can still move it around a little bit.
  • With them butt flush, center the clamp over the joint and get it squared up into position.  It can wobble around a fair bit.  Pay close attention to the position of the threaded bolt bit.  You want it in the original position mostly straight up and down towards the center, but slightly rotated up so that it doesn’t stick down too far where it will interfere with the shear plate when you reinstall it.  See the photo.
  • With it aligned, snug it up.
  • Use your torque wrench and do a final torque to 20 Nm (15 ft-lbs).
  • The pipe should be fully installed now.  Give it a solid tug/wag to be sure it feels 100% solid in the car.  Check your clamped joint with the inspection light and make sure it looks fully butted and sealed up.
  • Now move on and repeat this process on the other side.
  • When both pipes are installed, give them a wipe down with the micro fiber towel to leave them shiny and remove any greasy hand prints you left on them.  You want to get this grease off before you start the car so it doesn’t smoke off when it gets hot (and it will get really hot).  Don’t be surprise by a tiny bit of odor the first time you run the car.

Buttoning Up

With the pipes fitted you can get on with putting the rest of it back together, including the proper way to refit the shear panel.  I’ve got a few articles and videos to help:

Weight Savings

While I had the old secondary cats removed, and the new pipes still on the bench I stole my sweetie’s kitchen scale and weighed the difference.

The original secondary Cats weighed about 3.67 Kgs each, for a total of 7.334 kgs (16.17 lbs).

The AML Performance pipes weight just 3.333 kgs (7.35 lbs) TOTAL for both of them.  That’s a 4.001 kgs (8.82 lbs) weight saving.   

If you’ve read my other article on the Power to Weight Ratio of an Aston Martin DB9 (check it out here) you’ll see that a weight savings of 8.82 lbs is equivalent to adding 1 bhp of engine performance.   Therefore our 8.82 lbs of weight savings here (fluke that its an exact match) is equivalent to adding about 1.0 bhp just on its own.  Small, but every bit helps.

Sound, Power, and SMOG testing

In my next articles I will cover the change in sound (which is immediately notable – check out the article here), is there an increase in power by Dyno testing her again (rumor has that it adds 10+ bhp – check out the article here), and show the results of the California Emissions SMOG testing (check out how it did on the test here).


Here is a video where I follow the procedure to get them fitted.  Check it out, seeing how its done is always helpful.

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