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Induction system - Improvements, initial tasters… |
19 September
2001 |
At the end of every racing season the new/recurrent
champions are confirmed, and the rest of the field turning their attention
towards next year/season and ways of beating the crowned champ. Invariably this
involves that age-old quest for the illusive extra horsepower this year's
champion seemed to have over everybody else. So I thought it might be useful to
have a look at some stuff that is largely over-looked by many and to further
agitate those little grey cells.
Modern technology has seen dramatic improvements in
power outputs over very recent years. Just look at Formula One, where 3000cc
engines are now putting out the sort of power not even dreamed of not so long
ago at rpm levels that make motorcycle engines flinch! The rule makers keep
trying to slow them down, but they only go faster. However, the reasons for
this are not purely engine size/cam types/valve gear/cylinder head proficiency.
It was realised some time ago that the whole package was important, so every
aspect of a racing car's anatomy received the same amount of attention to
detail and development - each area demanding it's own team to investigate and
control it; the 'Management' team. The
technology available has distilled down over the years to the somewhat smaller
budgets of the club racer, and Mini owners are among them, yet few exercise
this holistic 'management' approach.
It was with this in mind that I did the recent series
on going racing on the cheap in Mini Magazine - looking at the package as a
whole, dealing with all those things that tend to come second for the
uninformed yet are the much more affordable options for going fast initially.
Now, blending a little of the 'holistic package' with budget tuning and the
management aspect, here's how to add real on track performance at a modest
outlay -
Thermal Management.
It has nothing to do whatsoever with keeping the
driver warm. Or cool for that matter. It has EVERYTHING to do with the engine's
operating conditions. Of particular importance - induction charge temperatures.
I have touched upon this subject in various articles
in various media mediums in the past. Some folk have ventured trying the
suggestions - and have been astonished at the results. Others have got the
wrong end of the stick, or got way too complicated in their application without
any real thought to what they're doing, and have been confounded by a myriad of
problems. So here's a quick trip through what it's all about to set the ball
rolling.
The power potential of any engine is wholly dependent
on how well it fills its cylinders and its volumetric efficiency - a known and
accepted fact. What is not often appreciated is that it isn't just the VOLUME
of fuel/air it can consume with each 'lung-full', but the MASS of that
lung-full. 'Mass' can be applied to any number of subjects, but generally has
one inference 'lots of'. What our engines need then is lots of fuel and air
crammed into each lung-full - more commonly called the 'charge weight' in
automotive circles - very ably demonstrated by what happens when strapping on a
supercharger or turbo, as this is EXACTLY what they do. The result is an
increase in the charge weight contained in each lung-full. But even they can be
improved upon in any given situation.
The critical factor here is the induction charge
temperature. It needs to be kept as low as possible commensurate with optimal
fuel atomisation. In other words, every effort should be made to ensure the
in-going induction charge is as cold as possible without literally freezing the
fuel out. If you do this the result will be more horsepower and - more
significantly - more torque.
Being pedantically repetitive to make sure you've got
your minds around the idea that reducing intake temperatures are beneficial to
power and torque output the crux of the deal is this - although engines may be bored
and stroked, and/or fitted with more carbs, free-flowing induction systems and
more efficient (modified) cylinder heads to increase its power potential by
increasing its capacity, increasing the charge weight (mass/density)
will greatly enhance these improvements. To do this we need to make sure the
ingredients going in that form the intake charge (air and fuel) are as cold as
possible without actually freezing out the fuel. Has it sunk in now? Good. So
how do you achieve this?
A really good start is to isolate the intake from the
exhaust manifold. A phenomenal amount of heat is emitted by the exhaust
manifold - not at all helped by the fact it's trapped in a very small area
between the bulkhead and a hot engine - that only exacerbates the problem. At
the very least some form of heat shield should be placed between the intake
manifold port runners and the exhaust. Use stainless steel in preference to
mild steel, as it is a very much poorer conductor of heat. And as that subject
has come up - a stainless steel exhaust manifold is not only more durable than
the mild steel versions, but also retains more heat in the exhaust due to
aforementioned heat conduction maladies. A further step forward is to stick
heat-shield material on at least one side of your shield if not both. Polishing
or chroming the outside of the intake manifold is the next step. Polishing
costs little but your time.
Better than this though is using a decent thermal wrap
on both exhaust and intake. I say 'decent' as there are some poor imitations of
the really good stuff on the market. I always use the original 'Thermotec Cool-it'
wrap and highly recommend this. An alternative to the heat wrap is to have
thermal barrier coatings applied to the inside of both exhaust and intake
manifolds. Even better, use it along with the wrap. Only hassle here is it's
very expensive to have done if you live anywhere else in the world except
On to cold air induction. Having trunking picking up
cold air from behind the grill and dumping it at the carb mouth performs
wonders in the intake temperature reduction stakes. Refrain from connecting
this directly to the carb mouth though. Use a decent, high-flowing air cleaner
and dump the cold air on that. Too much air disturbance at the carb mouth will
cause excessive turbulence, reducing power output by messing up airflow and
fuel metering.
That lot is all pretty straightforward and can be seen
in varying amounts, and occasionally all together, on a number of racecars in
club racing but very rarely on the road. Now for a very little used method of
reducing intake charge temperature at club level - fuel cooling.
Yessiree - reducing fuel temperature contributes
handsomely to reducing intake charge temperature and dramatically increases
charge weight. All that's needed is to run the fuel through what the racing
fraternity call a 'cool can'. Simply a container with a removable sealing lid
through which a length of coiled copper fuel line runs round/through it and
into which is put a mixture of pure alcohol and ice or dry ice. To maximise its
effect it needs to be as near the carb as possible without being in the engine
bay. Definitely an inexpensive way to more
power and especially torque, but needs combining with all the other options to
reduce induction temperature, otherwise the cooled fuel will be heated up again
by the time it gets into the cylinders and much of the possible advantage will
be lost.
NOTE - the engine will need to have its fuelling
re-set when any of these methods are used. Cooling the fuel and intake air will
increase fuel droplet size.
Now, before the aforementioned tasters on reducing induction
charge temperatures agitates folk into beating me over the head with the
benefits of ram-charging using cold air picked up from behind the grille and
'forced' into the carb/induction system/engine by means of a sealed section of
trunking linking a large panel filter at the grille end to a purpose built
plenum bolted to the back of the carb/intake body. Sorry guys and gals - nice
thought, but it ain't so. Let me explain a few things…
There seems to be two main sources for the reason why
many folk rush out and slap a pressure ram-charging system onto their Minis -
the 'stick-your-hand-out-the-car-window-and-feel-the-pressure' one, and the
'look-what-they're-doing-with-motorcycle-engines-these-days' ones. Both are
fairly plausible when considered briefly, but a little investigation proves
otherwise.
Anybody that has stuck his or her hand out the window
of a moving car has felt the seemingly immense pressure it creates against your
hand. Essentially there are two factors that affect this 'ram-air' pressure -
air speed and density. The pressure changes in proportion to density, but
changes at the SQUARE of speed. This means that when a ram-induction equipped
car doubles it's speed, the ram-pressure generated increase by a factor of
four. Intense! Now the downer. There is a formula to illustrate/assess this,
but take it from me - a vehicle travelling at 100mph under ideal conditions
only generates 0.177psi of ram effect pressure. That’s 1.22% more than normal
atmospheric pressure. You therefore need to be doing well in excess of 200mph
to achieve 1psi of pressure. Not the territory of your common or garden Mini.
Nor even your super-tuned one, come to that (as, brick, a, aero-dynamic -
re-arrange to form a common phrase associated with Minis). So that's the
'hand-out-the-window' syndrome explained.
Pressurised air boxes create havoc with fuelling and
air/fuel mixtures. Not so much of a problem on fuel injected cars where the
programming can be sorted to deal with it. Carbs, on the other hand, tend to do
weird things. A carb is little more than a self-powered, low-pressure fuel
injection system. Where a pukka injection system works on typically 40-ish psi,
a carb uses around 1psi. Quite a difference. To achieve maximum power under
most instances, the air/fuel ratio needs to operate within fairly tight limits
- say 5 to 6 percent. Outside this, power will drop off quite a lot. Without
getting into major carb 'modus operandi', carbs generally have two stages of
venturi. The secondary venturi is there to amplify the pressure drop across the
main venturi. This creates a stronger draw on the fuel main jet hole. This
system is very susceptible to outside influences - and why running a decent,
high-flow air filter will improve fuel metering and therefore power output by
stabilizing the air around the carb entry.
Then there's the problem of what it does to the carb
itself. Increasing the pressure at the carb mouth may not automatically cause a
suitable rise in pressure within the crab's float bowl. If an imbalance occurs,
it will lean out the carb mixture - and that can have potentially disastrous
results. What is required to circumvent this problem is to make sure the area
in the float bowl and above the fuel level is linked to the ram-air sealed box. Fortunately, Webers have just such a facility
- those holes just above the intake mouths. These obviously must be sealed off
from the out-side air, within the pressurized air box to enable them to work.
It's worth noting that if you go this route, the jet access cover should be
sealed too.
By way of a comparison of how much gain can be had by
either cooling the incoming charge or from ram charging, consider this -
Density increases as temperature drops - roughly
speaking 3.3 deg C will see a change of around 1% in density. Production engine
intake charge temperatures are regulated to about 80-85 deg C for maximum all
round efficiency. Feeding cooler air in from outside the under-bonnet
environment can see that dropped to around half that. That's an increase of air
density in excess of 10%, which gives a similar effect to around nearly 2psi of
super-charging pressure.
I largely suspect that the gains achieved by using
sealed air intake systems such as seen on Mini Miglias and the like are made
through reduced induction temperatures, not ram-charging. Ram-charging is a
real science. Consider the ram induction hoods/snorkels used on those monster
V8 motors, and indeed the design of those air boxes used on motorcycles. Notice
anything similar? They both have much smaller intake mouths/nozzles than the
plenum area. Want to know why? It has to do with causing a pressure drop across
the intake area - not feasible unless there is a precisely calculated, designed
and manufactured intake air box/plenum used that follows the basic details
above. And then they will only work when exhaustive dyno and on-track/road
tuning has been carried out. Not the sort of thing I'd recommend a club racer
to get into unless a very serious budget was involved!
Have a think about this lot. My experiences have shown
considerable on-track gains can be made with limited budget but a bit of
careful thought and planning.