- My model of MX-5 is: mk 3.5 Kendo
- I’m based near: Scottish Borders
- I’m looking for technical help or recommendations on: __
I recall reading some years (decades?) ago that the internal combustion engine performed more efficiently in damp weather - something to do with the raised humidity increasing the “density” of the air drawn into the engine and aiding performance. I have occasionally thought about this and my hypothesis - untested by any form of scientific experiment, I hasten to add - is that it is the case. I had a run in the MX5 the other evening when it was “showery” and it did seem to be going extremely well. When I originally read about this, it was in the age of carburettors, before electronic control of mix, timing etc, became common, so it may no longer be applicable. In short, is there any sense to this, or is is it the product of an over-active imagination?
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Back in the 1960s and 1970s you could buy water or steam injection systems to apply to the inlet manifold. The idea was that the superheated steam gave a bit more bang while reducing risk of detonation (pinking), and at the same time cleaned the engine and gave a few more mpg. Pie in the sky? Probably.
HOWEVER, it required full nerd-mode on the driver’s part, because this could only be used when the engine was at full temperature and at full power, needed to be stopped during idling and also long enough before the engine was switched off to prevent water settling in the bores and exhaust. And over-doing it was disastrous.
Bearing in mind it was being drawn in by manifold vacuum, controlling it was tricky, to say the least. Two of my work colleagues tried it on their old Mk1 Cortinas as pressurised steam injection, heated by the exhaust manifold, and claimed it “really did work.” But knowing who they were and remembering their other claims, the rest of us took it with a pinch of salt and decided to pass on the idea.
However 2. Although not with modern cars, I did notice an improvement in “go” with my old normal unmodified cars on wet weather, but then this was also the case in cold dry weather. Or maybe it was the more risky driving conditions making everything seem faster…
Maybe related in some small way, but many years ago, my 1970’s vintage MG Midget, felt quicker when I used an old hosepipe to channel fresh air from the grille, directly onto the twin carb air filters. Whatever the truth of the matter, it felt ‘sweeter’ when accelerating, and my next door neighbour, who also drove a Midget, felt this to be the case too! Maybe.
My Mk1 Cavalier had a cunning flap in the inlet duct leading to the air cleaner, such that warm air could be taken from over the exhaust manifold in winter and cool from behind headlamp in summer.
The problem the warm air fixed was icing up in the carburettor during winter. As the petrol atomises it cools down, and will freeze any water vapour in the air, possibly on that sub-zero jet nozzle.
Fuel injection avoids the problem by simple brute force.
The later Classic Minis have a similar system operated by a bimetallic strip. All to do with fuel economy not performance.
When testing Octane values on the CFR engine the moisture content of the air entering the engine has to be tightly controlled as variations ca affect the final Octane value
I would say that generally speaking, it is an accepted fact that if you have moist, cool air, you will get better power. This is due to more oxygen being taken in, for a better/more efficient burn in the cylinder. Therefore if it is cooler and there is moisture in the air, you should get better performance. However the engine will not automatically put in additional fuel to compensate, so you will not get as much power as you could. Conversely if the conditions are dry and warm, there is less oxygen in the air taken into the engine and performance will drop. As you go higher (although not so much in this country), the air also loses oxygen, so performance also suffers. At Pikes Peak race track in the US, as you go to something like 14,000 ft, the engine at the top is losing a good amount of its BHP compared to sea level…
I know that BMW with their last M3/4 GTS had a water injection system to improve performance, but the engine mapping took this into consideration and would apply additional fuel too, thus increasing performance even more…
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The engine will put in additional fuel if it has a maf sensor. It measures the mass and therefore density of the air which will be higher if it is cooler and more moist. Thats why dyno results have a correction factor that standardises temperature,atmospheric pressure and humidity.
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Snelly is right, on race bikes I used t increase jet size in cooler weather due to the air being denser therefore more air enters the cylinders so more fuel required, the opposite is the case in warmer weather/higher altitude, therefore in the cold weather more air and fuel = more power ! Carburettors obviously. Dave
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Actually the cooler the air is the less moisture it can hold.Think of humidity in the tropics and then in the arctic regions
Correct!
Also, density decreases with increasing altitude, which is what counts.
Most piston engined aircraft using petrol have a mixture control, which the pilot is required to use to reduce the fuel flow as the aircraft climbs, to keep the fuel/air ratio correct. Power output decreases as a result of the lower air density. Eventually, the aircraft runs out of enough power to continue climbing.
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What a lot of interesting responses. At least no one has said “bollocks” - or not yet anyway…
Thanks all for an interesting exchange which has sort of validated my hypothesis (I did wonder if I’d dreamt it). I am relatively new to this (cleverly bought my MX5 (my first) 2 weeks before lockdown) but already addicted to driving top-down, so don’t really go out in the rain. Now I’ll be nipping out after showers. Cheers!
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Ditto I bought mine 3 weeks before lockdown and had to sorn it, great little cars I’m glad I took the plunge, great club with plenty of knowledge and help, have fun,
Regards Dave
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Engines are mass flow devices, so air density affects the mass of air being consumed by the engine for a given volume. When air density is high, more kg/s of air are inducted into the engine which requires more kg/s of fuel to maintain the correct air:fuel ratio. Result is more power.
According to the Bosch ‘little blue book’, a rule of thumb is engine power drops about 1% for every 100m of elevation change.
Humidity affects oxygen content. As humidity increases, oxygen content per kg reduces and so power reduces too.
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Elegantly put!
I lived for many years at a bit over 5500ft altitude, so became a member of the mile-high club.
With atmospheric pressure at about only 81% of sea level, water boiled at 81.5C for insipid tea (and O-Level Physics practical needed an invigilators note about the altitude), and correctly adjusted cars had much less power, reputedly at the time about 82%.
This tallies well with the Bosch 1% drop per 100m up (-17% at +1,700m).
After driving to the car park at about 12,000 feet I remember staggering the final 500ft or so to the top of a peak in Rocky Mountain National Park. Literally a breathtaking experience, with oxygen content in the atmosphere estimated at 1/3 less than at sea level. I do actually have the teeshirt. But we digress…
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