This time on Road and Race we look into increasing
horsepower As you all know by now I own a 986 Boxster
S. On this episode I wanted to look into the options you have when trying to increase the
horsepower of the engine. Although these will be specific example for the Boxster the principles
can be applied to any combustion engine car. My main investigation focussed on this chart.
The later model 987 Boxster has the same 3.2 litre M96 engine but produces 28 bhp more
so what did they change to get more power out of the same engine?
I’ve 5 mods to discuss and I’ll list them in order of best value.
Just to let you know I’ve not done these modification myself so this is simply a show
to discuss what options are available from the research I’ve done and what gains could
be achieved. I do plan to give some of these a go in future episodes though and until I
do the horsepower gains listed here are simply a theory until we put them into practice.
So, up first…. 1. Airbox and intake. Cost £100. Bhp gain
– up to 8 bhp. Cost per bhp £12.50 Without going into massive detail a combustion
engine produces power by mixing air with fuel and igniting it. Any restriction on the volume
of air the engine gets will reduce the amount of power it can produce. From my research the airbox on the 986 Boxster
is quite restrictive. It was redesigned and used on the last 986 Boxster iteration the
“550 Spyder 50th Anniversary Edition”. I’ve not been able to find out if this is
the only performance related change but this edition has 266 bhp which is 14 more than
mine with the same 3.2 litre engine. This same airbox was used for the later 987
Boxster and Cayman models. You can pick one up second hand for about
£70. The disadvantage is that it is slightly larger
and needs some of the plastic cutting away to make it fit. You’ll also need to spend
a further £30 on a few other bits like a plastic welding kit and such. 2. underdrive pulley. Cost £130. Bhp gain
– up to 5 bhp. Cost per bhp £26 What is an underdrive pulley? Here’s a picture
of the Boxster engine and I’ve coloured the pulley in blue. It’s a circular disc
which is directly attached to the engine’s crankshaft and rotates when the engine turns.
This rotation drives a belt (shown in red) and powers the auxilary items such as the
air con, alternator, power steering pump and coolant pump. An underdrive pulley is a smaller version
which, as the name suggests, will spin slower (because it’s diameter is less) thus making
the auxiliary systems draw less power which is released back to the engine. But if the auxiliary items are running more
slowly will they still work ok? Well, that’s the question. From what I’ve read they should
all still work fine but I won’t know for sure until I try this myself. This is quite an easy mod and if you are able
swap an engine belt then you should be able to easily do this freeing up to 5 bhp. On
a 986 Boxster you will need to use a Dremel to cut away a small piece of metal to allow
the smaller pulley to fit. 3. Larger throttle body and plenum. Cost £160.
Bhp gain –up to 6 bhp. Cost per bhp £27 Here’s a simplified version of the Boxster
air intake system. Fresh air comes in though the air intake, travels though the airbox
and filter then hits the throttle body. This device opens and closes when you push the
accelerator pedal. After that the air hits the plenum which distributes the air to the
engine intakes. On the left of the picture is a stock 986
Boxster air intake, throttle body and plenum. Air comes in though the intake and hits a
76 mil tube where the Mass Air Flow sensor is housed. From there it narrows to a 68 mil
throttle body. Below is the larger 987 air intake matched
with a larger 74 mil throttle body and plenum from a Porsche 996 Carrera. The air flow is
now no longer restricted down to a 68 mil opening and this should give more power.
Second hand the parts and new tubing needed should come to about £160 and could give
up to 6 bhp.
4. New Exhaust. Cost £350 to £2000. bhp 6 to 22 bhp cost/bhp 33 to 90 If you’ve free’d up any restriction to
the air coming into the engine then you need to think about making sure it can leave just
as freely. An exhaust usually consists of 3 sections. The the exhaust gas from the engine flows
though the manifolds (here in blue) to the catalytic conveter (red) where the harmful
gases are reduced. From here the remaining gases travel out though the muffler (yellow)
where noise is reduced. Full sports exhaust systems which freer flowing
cats and mufflers cost as much as £1800 and claim to increase horsepower up to 22 bhp.
They would also produce a much sportier sound. As the catalytic converter is usually the
most expensive part of an exhaust a cheaper option is to replace just the muffler and
potentially the manifolds aswell. A no brand muffler and manifold would set you back about
£350 and give up to 9 bhp. Apparently they might not sound great and may drone on long
motorway journeys though so I guess you get what you pay for. 5. ECU remap. Cost £550 6bhp £92/bhp
The final option in this list is reprogram the car’s onboard computer. This allows
you take maximum advantage of all the mods you’ve made to the car by adjusting engine
parameters such as ignition timing and fuel injection. Lastly here’s a table summarising all we’ve
discussed so far. The new air intake is the best value mod costing £12.50 per bhp gained.
The underdrive pulley costs £26 per bhp and the 996 throttle body and plenum £27 per
bhp. A budget exhaust costs £33 per bhp and ecu remap is £92.
Excluding the last two items as they are the most expensive you could potentially have
a 19 bhp gain for £390. If you’ve found this video useful please
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