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MG Midget and Sprite Technical - Cooler damp weather. Performance

In another thread, about ethanol fuel, Nigel said.

"summer may be two weeks but we've had mostly sunny days here since SORN season started (1st October) ------ drove mine yesterday and thought how much better the engine seems to go in the cooler temperatures, --- "

Yup I agree with you Nigel, and that's an interesting observation you make about how much better the car goes in cool weather, especially damp cool weather.

I noticed that about 40 years ago, in a Consul Capri. Damp November weather seemed to make the engine more powerful, and I notice the same thing today with the Sprite. The whole car feels "tighter", but the engine definitely goes better. Feels as if it's suddenly gain a bit extra hp.

Do you remember those kits you could buy, to inject water mist into the inlet manifold? Claimed to give a power boost and improve mpg. I wonder if the cool and humid air being drawn into the carb, is doing the same thing?

As for the whole car feeling tighter, I put that down to the rubber components of the suspension being stiffer in the cold, and softer in the hot weather.

Lawrence Slater

No, it's just the rust brought on by the wet weather siezing things up!

Today should be good for high horsepower! It isn't good for putting rear axles back when your feet stick out beyond the garage door into the rain!
Graeme W

seem to remember reading many years ago that wartime Spitfires had a water mist injector fitted...
David Cox

Has no one ever read vizard or any other book about internal combustion engines?

Of course cooler air is denser and there for makes for a better mix (most power is always made slightly lean)and bigger bang.
The engine also runs cooler (again better for power) due to the bigger heat exchange.
Water injection is used to cool the charge and avoid detonation in blown engines.
I doubt moist air really makes a difference.

But the cool air does work
Onno K

isn't it to do with the density of the air (or is it the fuel or both?)

my mate once had an import Scobby that had a push button water mister, he used to fill it up quite regularly, I think the later ones were automatic(?) reminded me of sqeezing the platic bottle in front of the carb air intake :)

of course the overall performance of the car might not be better as the summer tyres, brakes and other bits wont be working at their best and the engine might not be running at its optimal temperature (what is the optimal running temperature do you think for a reasonable standard A-series in a Spridget?)
Nigel Atkins

Cooler air is more dense and therefore has a greater capacity to carry fuel vapour = more combustible charge in each cylinder = more hp. Its well documented and pretty standard. One of the reasons that many cars have the carb (or throttle body) fed from outside the engine bay, drawing cooler air. Add to this a certain level of water vapour which additionally helps the fuel to atomise and fully disperse in the air coming in, plus the oxygen component in the water helps too.
Guy W

there you go Onno you don't necessarily need to read those books :)

you posted whilst I was typing, so, >>The engine also runs cooler (again better for power)<< if it's better for power then it ought to be better for fuel economy but I thought others have it that the engine wants to run hotter for better fuel economy, so what is the optimal temperature, does it say in these books?
Nigel Atkins

Yup Guy, Onno, I know about the cooler air effect. I was talking speciffically about 'damp' cooler air. The increased humidty adds to the beneficial cooling effects.

Don't you remember the kits you could buy from the Exchange and Mart motor section? No, I guess not.

Anyway, I found this.

"Atmospheric Humidity;
Increasing the humidity can have advantageous effects against the engines susceptibility to detonate. First, increased humidity raises the charge density giving a less volatile mixture thus increasing the reaction time. Secondly, the increased humidity tends to cool the combustion chamber during the intake stroke lowering the internal temperatures. The easiest way to have a controlled increase in humidity is by use of a vapor injection system. There are two brands which I know of, Edelbrock and Spearco. I have recently installed the Spearco system on my Weber head TC. Installation takes about three hours and is very straight forward. It is worth noting that J.C. Whitney sells the Spearco system for a mere $47, less than half what you would pay locally."


"Getting to Know Water Injection
The benefits of Water Injection are substantial and well documented. A lot of people hear about the idea of injecting water into the air that enters the engine and wonder if putting water into the engine isn't a bad thing.

The truth of the matter is that it's a very good thing if done properly. Only a small amount of water is injected as a fine mist. This mist is evaporated quickly in the hot intake air. As the water evaporates, it cools the intake charge significantly.

Cooler intake temperatures make the engine less likely to knock. This allows the ECU to fully advance timing and extract more power. This strong cooling effect also makes the air more dense so the air mass in the cylinder can increase, which allows the fuel delivery amount to increase, which increases the power released for each turn of the engine.

It's often said that water injection will allow your car on a hot day to run as well as it does on a very cold day. For turbocharged cars, this also means that the engine can be run at a higher level of boost for a given grade of gas and ambient temperature. Often, non-turbocharged or supercharged vehicles will not see much of a benefit. However, it can allow high compression normally-aspirated cars to possibly run on a lower grade of gas."

Lawrence Slater

I thought it was to do with the O in H2O.

As with nitrous-oxide injection, it's the oxygen which is the important element.
Dave O'Neill2

Lawrence, this is nothing new. I had water injection fitted to my 750 Cambridge special in 1968
Guy W

I want to install a cryogenic frozen fuel system on my car...really cool stuff...several companies make the kits

Prop and the Blackhole Midget

Hi Guy.
I know it's not new, that's what I said in my first post. I was asking if anyone remembered the kits you could buy way back when.

What's a 750 Cambridge special?

Dave, "I thought it was to do with the O in H2O."
Well I was actually wondering that too. But does water vapour/mist in the combustion chamber actually give up it's O content? If so, what happens to the H, that's pretty volatile.

Lawrence Slater

Not sure if the pressure, or temperature inside the combustion chamber is capable of breaking very many of the molecules, as the chemical bond between H and O2 is quite strong, to separate them from each other, usually requires 800 ~ 1200C, and/or 1740 ~ 2900psi.

As Lawrence's source above states, the main benefit from using water injection is the very large amount of heat energy necessary to convert water vapor into steam, this cools the combustion charge very effectively, allowing the ignition to be advanced a lot before detonation will occur, and on engines designed for it, the compression to be increased without detonation (or, on a supercharged or turbocharged engine, the intake pressure to be increased while avoiding detonation).

The downside is that one must remember to keep the water jar filled, is all.

It is an interesting experiment, and easy to do, as you can make your own for "injection system" for pennies, then advance the timing as far as your engine will allow. This was a popular thing to do in the mid '70s when gas prices were skyrocketing (I still have the pump card from the first day of my first job, in 1978, $0.59/gallon, that seemed high, at the time, but it never went back down to that, ever again).


Norm Kerr

Interesting that Norm.

On a foggy november(cold) day then, there's an extra boost from the moisture in the air, that you don't get on a dry but same temperature day? I definitley notice the difference.
Lawrence Slater

Id think using a water vapor mist system would be 2 set backs

1. Washing down the cylinders so yiu would also need an oiler to keeo cly lubed and not breaking down likr fine gkass

2. Valves ... Namely you would need to regrind the valves and seats on a common basis, id think the heat would eat them alive
Prop and the Blackhole Midget

Actually, Prop, that was not found to be the case, because the amount of water used is very small. It was used to great effect during WWII in aircraft engines. A water bottle of only a pint or so, is plenty, per tank full.

Taking it to an extreme, one shop practice was to "Steam clean the top end" by essentially using as much water as the engine could manage to consume (without stalling) to produce a very large quantity of steam in the combustion chamber, and, hopefully, clean off and blow out the carbon that was built up inside. More of a "days of yore" than of much use today, as modern fuels leave behind so little carbon that it is not as big a deal as it had once been (back then, you had to remove the head every few years to chip off the masses of carbon built up). Anyway, I've never seen a true side x side test done of it, but I remember hearing about it and imagining that it might work (or, it might clog the muffler with wet carbon suddenly liberated from the combustion chambers!).

Regarding improved running on a foggy day, they always used to say, "never buy a used car on a rainy day" (because engines tend to run better), and "never buy a used car at night" (because paint tends to look better in artificial light, instead of sunlight).

Norm Kerr

"750 Cambridge special" - based on chassis of a pre war Austin 750. In the 1950s and 1960s the 750 Motor Club was the leading motor racing club in the UK, originally restricted to Austin 750 engined cars and then expanded to cover all sorts of clubmans classes for tack events. Cambridge was a particular body style similar to an Austin Ulster, but with a slab fuel tank at the back like on an MG.

Mine had twin Amal carbs and I installed water injection on it. Top speed was around 95 which was somewhat terrifying on such a vehicle. But then in those days I was young and had no fear!
Guy W


I can assure you 100% that water injection into the head will de-coke an engine, i had a cracked head between the valves on my old toyota engine and i was running straight water without anti... When i pulled the head the piston top, cumbustion chamber and valves where shinny clean

Id think you might need very good fing seal to avoid water, passing into the crank case


Prop and the Blackhole Midget


Instead of water.... What about this idea

Pure octan booster or racing fuel...rig it so its only injected at high sustained rpms....the engine would be hot at the top of the rpm range where you want the cooler temp, so the Oct.Booster would come on at say around 5500 rpm and that slows down the speed of the combustion burn and would reduce pinging and reduce coke build up thus allowing more longer, slower dissy advance at the top of the rpm range

In fact you might even be able to use a 1lb bottle of camp stove propane or mapp gas and use a nos set up to control the gas flow mechanics

Prop and the Blackhole Midget

From JC wintney....water/methonail injector kit

Prop and the Blackhole Midget

You can get fancy with pumps and meters and stuff, but here is the simplest type:

simply air drawn from a vessel with water, or rubbing alcohol, which has air bubbling through it to create a vapor

just make sure the hose that connects to your vacuum port is above the level of the fluid, and you are in business

Norm Kerr

...and here is a link to how to DIY the more sophisticated approach:

Norm Kerr

Guy, that sounds like a cracking car. What do they look like?

Prop, I think you meant Norm when you said about decoking the engine.

Norm, thanks for those links. I might just have a go at that. There's definitely an effect on a cool misty day. So I figure all you need is to deliver a mist to the insides of the air filter. That's how the damp air get's there without any gadgets anyway, so I reckon all that's needed is a way to create a very fine fog, and deliver it to the indsides of the air filter. Just as Pat Goodman does.

My memory is coming back now too. I remember sending off for information on the system I saw in the exchang and mart all those years ago. I was going to fit it to the 1962 For Consul Capri i had. I don't remember why
I didn't persure it, but it might be because I got banned from driving -- 18 months . Whoops.

Anyway, it sounds like a fun idea to try.
Lawrence Slater

Water injection has been around for ages
I ran it on a mate's turbo v8 years ago, we used a 50/50 mix of water and methanol and used heaps of it, usually with angry driving it would use 2 litres ( it was a quart bottle ) to around half a tank of petrol
At rebuild time which was quite often it would be as clean as a whistle inside, hardly any carbon at all
Best thing was no sign of detonation/pinging at 14lb boost and 8.8 comp.
William Revit

If it really does work, why was it never employed in the mass market I wonder?
Lawrence Slater

because you have to keep filling it, is why

if you really make use of it, you are running a very high CR, with a very advanced timing, so if you ran out you'd quickly ruin the engine

with modern electronic controls it might be easier to prevent that risk, but production cars are always tuned to a far more "nominal" range, for protection against tolerances (in both owner maintenance and in vehicle build) anyway


Norm Kerr

Ah right, that makes sense.

Interesting experiment though. As I definitely notice a difference on a misty cool day, even with my current tuning, I think I'll do it -- just for amusement -- when I get round to it. If I don't alter the CR or tuning to take extra advantage of it, and then run out of water, it'll make little difference. 2 Litres of water isn't much to carry around and refill when filling the petrol tank.
Lawrence Slater

This thread was discussed between 04/11/2012 and 11/11/2012

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