I’ve been pondering on ways to improve the performance of my DB9. Is it better to make changes that add horsepower or to loose weight? The answer is to do both of course, but which will provide more seat of the pants perceptible performance for the least amount of dollars? Which will be easier to accomplish? I set about figuring it out below.
Power to Weight Ratio
One way of ranking two different cars performance is to compute their power to weight ratio. If you think about it, a 4,000 lbs. car with a 400 BHP engine should be just as quick as a 2,000 lbs. car with a 200 BHP engine.
The industry standard for computing the power to weight ratio appears to be either:
- Standard – Divide the official Curb Weight (in pounds) by the Engine Power (in brake horsepower) and get a value of lbs per bhp. A lower value is better.
- Metric – Divide the Engine Power (in horsepower) by the official Curb Weight (in metric tonnes) and get a value of bhp per tonne. A higher value is better.
The Curb Weight of a vehicle is the complete weight of the vehicle with all fluids and a 90% full fuel tank. This value does NOT include the weight of passengers or cargo. Do not mix this up with the GVWR.
For my 2005 DB9 Coupe I found the Curb Weight by simply looking it up in the Aston Martin DB9 Owners Manual Specifications Section 13.7 about Vehicle Weights. A coupe with an automatic transmission weighs 3,968 pounds (1,800Kgs). A manual coupe is the lightest, and the automatic Volante is the heaviest.
There could be a debate about whether horsepower should be measured at the crank (bhp) or actual rear wheel horsepower measured by a dyno that factors out all the mechanical losses getting the power to the road wheels. For this article we are going to use listed engine horsepower as this is readily published by manufacturers. And I know this article isn’t talking about ‘Torque’ either, which is just as valuable a measure as it really relates to that ‘push’ you feel of acceleration.
For my DB9 we can also get the power by referring to the Aston Martin DB9 Owners Manual Specification Section 13.2 about Performance. For my year the engine power is rated at 450 bhp.
Power to Weight Ratio for a 2005 DB9 Coupe
Using the numbers above we can then simply calculate the power to weight ratios:
- Standard = 3,986 pounds / 450 bhp = 8.82 lbs per bhp
- Metric = 450 bhp / (1,800Kgs /1,000 Kgs per tonne) = 250 bhp per tonne
How does that compare?
Now knowing those values I was interested to know how the DB9 compare to other cars. I found this article online that lists several other interesting vehicles along with a few I wanted to include. Remember my DB9 coupe is 8.82 lbs per bhp and a lower number is better. Ranked from worst to best:
- Porsche 911 Carrera (2005 997) – 9.75 lbs/bhp
- Aston Martin Vantage V8 (2006) – 9.52 lbs/bhp
- Aston Martin DB9 (2005 – my car) – 8.82 lbs/bhp
- Aston Martin Vantage V12 – 7.33 lbs/bhp
- Aston Martin DBS (2009) – 7.33 lbs/bhp
- Aston Martin Vanquish (2012) – 6.78 lbs/bhp
- Aston Martin DB11 V12 Coupe (2017) – 6.89 lbs/bhp
- Aston Martin DB11 Superleggera (2018) – 5.22 lbs/bhp
- Porsche 918 – 4.08 lbs/bhp
- Bugatti Veyron Super Sport – 3.46 lbs/bhp
- McLaren P1 – 3.35 lbs/bhp
- Ferrari La Ferrari – 3.32 lbs/hp
- Ariel Atom – 2.43 lbs/bhp
- Koenigsegg One:1 – 2.23 lbs/bhp
The Koenigsegg One:1 is the first production car to reach the mythical ratio of 1 bhp per Kg.
You can see both the base early DB9 and Vantage had better numbers of the base Porsche 911 of the era. The DB9 had the edge on the Vantage. The later Vantage V12 and DBS were exactly the same. The new DB11 Superleggera is the cream of the Aston crop.
What’s the Best Way to Improve Performance?
At this point I was wondering what would be the best way to improve my DB9’s performance. Find ways to reduce weight? Bolt on some additional performance? Would the options available make a perceivable ‘seat of the pants’ difference (a.k.a. is it worth the effort)?
To start with I crunched a few numbers [I gotta be me]. What I really wanted to know was how many pounds of weight loss would be equivalent to adding one horsepower.
The current power to weight ratio holds the answer. Reducing the weight by 8.82 pounds produces the same performance gain as adding one horsepower.
Now, if we think about this the other way if we leave the power untouched (450 bhp) and want to attain the same performance improvement as adding 20 bhp, we need to reduce the weight by ~169 pounds. That’s a lot of weight [like a whole me!]
Leads me to think about a few other things:
- What weighs about 8 or 9 pounds?
- A small human head
- A small bowling ball
- A gallon of milk
- What about the running with low fuel?
- A full tank of fuel in a DB9 is 80 liters.
- The official curb weight was at a 90% fuel level (72 liters).
- If we drive with just a quarter tank of fuel (20 liters), that’s 52 liters less. Fuel weighs about 1.7 lbs. per liter. This would be ~88 pounds lighter.
- That’s equivalent to adding about 10 bhp (460bhp total). So, your car is 2.3% quicker on low fuel vs. a full tank.
- Probably tough to feel this in the seat of the pants.
- Loosing some weight.
- Get rid of those few extra pounds around your waist. I could stand to loose about 15 pounds, and that would the same as 2 extra bhp.
- Get rid of that spouse that’s been bugging you [just kidding – but that could be worth 20 bhp]
- Empty the trunk of your DB9. Hauling around a bunch of crap in the trunk just slows the car down.
Reducing weight where it matters most
Reducing weight in a car is a great performance booster because it has secondary effects. A lighter car:
- Brakes better
- Handles better
- Gets better fuel economy
Adding more horsepower to the engine generally won’t help any of these things.
If you are going to eliminate weight in a car there are some places where its better
- The best possible spot is to reduce un-sprung weight (weight that isn’t supported by the suspension springs). The Brakes, Wheels, Tires and the Suspension components. Reducing weight here helps handling and acceleration.
- Reducing weight up high helps handling. The higher the position the weight is in a car, the more it acts like a pendulum in cornering and braking [so get a haircut]. We want weight to be as low to the ground as possible.
- Reducing weight ahead of the front axle or behind the rear axle helps with handling. A lightweight grille, or empty trunk, etc.
And what really got me started on this article then was evaluating what performance options would have the most benefit. Here are a few I looked at and the ‘Cost per horsepower’ I worked out. Read on, there is a no-brainer option.
- Lightweight Battery Kit – the battery in the DB9 is a massive beast (read my article on this). Redpants.lol offers a lightweight battery kit (click here to learn more about it) for about $530 USD that includes a nifty bracket and smaller battery. It claims to reduce the weight by 30 pounds, the equivalent of adding 3.55 bhp. This works out to a about $150 USD per BHP.
- Lighter Tires – the original tires on the DB9 are Bridgestone Potenzas. In another article I described alternative tire options, and my tire of choice is the Michelin Pilot Sport 4S. Another upside of these tires is that they weigh about 3 pounds less each than their Bridgestone counterparts. That’s 12 pounds less of coveted un-sprung weight savings, equivalent to about 1.5 hbp. Just make the choice the next time you get tires.
- High Flow Air Filters – the stock paper element air filters on the DB9 are fairly restrictive for breathing. Velocity AP offers a set of high flow air filters similar to K&N filters that you clean and re-oil when serviced (click here to learn more about it). They claim these will add 7 bhp. Wow! All this for just $120 USD. That works out to be just about $17 USD per BHP. Easy to install during your 2 year annual service. This seems like a no brainer.
- Secondary Cat Delete – the DB9 has two sets of Catalytic convertors in the exhaust system. These were fitted to meet air emission rules in various countries. The problem is that these cause a restriction in the exhaust flow, choking back some engine performance. Several companies offer a set of replacement exhaust pipes that replace the second set of catalytic convertors, thus improving exhaust flow and engine power. Simple enough install, just unbolt the secondary cats, and install these new straight bits of pipe. Velocity AP offers a solution (click here to learn more about it) for $795 that claims to add 11 bhp while reducing weight by 10 pounds (that’s another 1.2 bhp). That works out to about $65 per BHP.
- Upgrade the ECU – cars controlled by computers (like the DB9) often have a fairly conservative set of software parameters in them to give the car a nice balance of performance, reliability and fuel economy. People often talk about ‘Chipping’ a car to insert a more aggressive program into the computer. In a DB9 this is actually more simply done by uploading a new software program into the existing ECU. What’s neat about that is that you can have one program for more power, and restore the original program anytime you need to (for MOT or emissions tests, etc.). Several company offer ECU software upgrades. Velocity AP offers one (click here to learn more about it) for $1,395 that include the programming tool. They claim it adds a whopping 25 bhp. That works out to be about $56 per BHP.
- The Full Monty – Velocity AP offers a performance package (click here to learn more about it) that includes all the goodies noted above, plus one other major improvement – custom exhaust headers. Getting the exhaust breathing better is a big deal, and Velocity designed the ideal exhaust header with equal length tubes. These are very expensive, but they combine them in a package that includes:
- High flow headers
- 200 cell high flow catalytic convertors (rather than straight pipes)
- Some special wiring to convince the car its OK to be missing one set of catalytic convertors
- High performance air filters
- ECU upgrades
- The package price is $7,695 USD and claims to add 49 bhp total. This would bring you up to nearly 500 bhp total (down to 7.9 lbs per bhp), similar to the DBS. This works out to about $157 per BHP. Getting every last ounce of performance starts to get costly.
This list certainly doesn’t describe every performance upgrade out there, and I’d love to hear from you about others you’ve discovered and tried. Please leave me a comment down below and hopefully I can update this article with more options.
What am I going to do to Princess Piddles? Probably going to go for the air filters, secondary cat delete and the ECU upgrade. It might be good for about 40 bhp total and I suspect I can feel that in the seat of my pants. Plus I can undo all the changes easily if I want to. If I can get a sweet deal on the full monty I might go for it.
If you work for a company that sells performance upgrades and wants me to try one out and write about it, please contact me below by leaving a comment.
Whatever I do, I’ll be sure to post articles and videos on it. I expect that I will get my car dyno tested to measure the rear wheel horsepower before and after each upgrade so I can report the data back to you!