Induction system - Improvements, initial tasters…
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 may 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 -
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 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 America since a majority of the companies that do this are based there!
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 fueling 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 traveling 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.