As I mentioned before and as confirmed by "007 Vantage": The only time otherwise that an engine/tranny should be used as a brake is on steep declines coming down a mountain to help lighten the strain on the braking system.

When I select a lower gear in my 280HP V-6 '03 Acura MDX automatic, it works very well to keep my speed down without any application of my brakes all the way down an 8-mile grade--HWY 431 at Lake Tahoe, NV. I have often been at the bottom of the grade and noticed the strong smell of overheated brakes from drivers who use their brakes coming down the long grade, without using engine braking. I'm sorry this "engine braking" does not work well on my V-8 vantage because I am convinced the ECU feeds fuel to the engine, increasing the fuel when I select even lower gears--this keeps engine braking to a minimum and causes to me occasionally use my brakes, which I don't have to do in my other cars.

 

Hey dicktahoe,

Valid point ... Another problem on the vantage (and the reason why its not the best at engine braking) is the fact that there is so much rotational mass in the drivetrain that it just wants to keep on spinning. The decel takes significantly longer than would normally otherwise in an avg car, this also detracts from the engine-braking effect. On my lambo its the exact opposite, I have never felt such a strong engine braking effect and thats b/c there is very little rotational mass throughout the entire drivetrain by comparison do the aston (mid ending, 2WD rear transaxle, rear wheel drive, all superlight weight components etc). Engine compression also does play a bit of a factor as well (lambo is pretty high at 12.5:1).

18 wheelers have been using this concept for decade, otherwise known as a "Jake-brake" or "compression-brake" ... that's when you hear a really loud nose come out of exhaust stacks of an 18 wheeler, what you are hearing is the actual compression & decompression of the pistons to actually slow the car down as it dumps the "exhaust" air directly into the atmosphere instead of going through the standard exhaust route (almost like a waste gate, but purely air)

 

The MDX is an auto trans which uses a torque converter, your V8V uses a clutch system attached to a manual tranny via the torque tube, weither you have a stick or asm in your vantage, you still have a clutch and "manual type" trans..the engine will need more fueling to feed the motor due to the rise in rpm when dowsnshifting, ...simple differences between an auto trans with converter and manual trans with clutch...the rate at which the vehicle slows when downshifting on a simple manaul trans all rely on engine compression and driveline gearing (trans and axle gearing ratio's)

 

Can't really compare the Lambo and V8V on deceleration, Lambo is v10 and Aston is V8, even if they had the same compresion the V10 will still be harder to turn, also the Lambo uses a 3.08:1 final drive and the V8V uses 3.91:1 final drive..the 3.91:1 is a lot easier to turn thus turns the motor easier and faster.
The V8V was fitted with the tall gearing to get the best torque down low versus up high, the V8VS is even taller at 4.182:1...

 

Let me see if I can be more clear. Some moden cars have a fuel cut-off when coasting--this provides for maximum engine braking. If you have an "Instant MPG" display, on most cars it will show a maximum MPG--usually 99.9 (when coasting in any gear while your foot is fully off the accelerator). If I leave the Aston in 6th gear and take my foot off the throttle, it reads 99.9, but when I downshift to 5th, it reads about 67mpg, and in 4th around 45mpg, and in 3rd about 32mpg. That tells me the Aston ECU is feeding fuel to the engine, which will not let the engine produce maximum braking--its as if I'm keeping my foot slightly on the accelerator.

 

Ok, not exactly how this MPG display works. First your engine will keep firing the injectors by looking at crank postion and cam position, thus firing of the spark plugs and injectors can be timed correctly. When you downshift your rasing RPM so yes the motor needs more fuel because the injectors are firing faster due to the increase in RPM...

..The MPG display has a lot of readings to calculate it's MPG average or instant reading. Basically the best way and simple way to explain this is, The distance you can travel with throttle pedal position, RPM, and speed will affect the MPG/instant readings. If you look at the MPG/Average there will be no change unless there is change in your driving behavior or status(change from city driving to highway)..

Speed up to 100MPH and put the car in N, coasting and no throttle load will cause the RPM to drop near 800RPM, then your display will show 99.9MPG/Instant...Now put the car in gear, your RPM's will raise which will require fuel..it's not a Hybrid so the fuel will not cut off. The "Fuel-Cut-Off" is incase of rollover, impact, over RPM redline, or engine faults flagged that require to shut off the fuel for safety..If you can simulate a fuel cut off when downshifting it's because you hit the RPM redline and shouldn't have downshifted in the first place