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MG Midget and Sprite Technical - Water Heated Manifold

Hi - Has anybody tried disconnecting the water heating to the intake manifold of a 1500 midget.
From what I've read, there may be a power bonus by running with the manifold cold.
Steve

I was under the impression that the water pipe that passes through the inlet manifold is there to COOL it, in order to prevent fuel evaporation?
Rich Amos (1330cc Blaze Red '72)

Hm... I thought that the idea was to heat the manifold so that fuel wouldn't condense out of the mix onto the inside of the mani when ambient temps are cold.

-:G:-
Gryf Ketcherside

I think it does two things:

Aids warm up time

Keeps things cool when in traffic or movin slowly by keepin the manifold at constant temp regardless of conditions and the amount of heat comin off the exhaust below.

Seems to make sense to me.
Bob T

I've read that for maximum power, the manifold should be kept cool, to get the most dense intake charge. However, by heating the manifold it keeps more fuel in suspension and increases warm up and economy.
Apparently, for every 10 degrees the intake is cooler, you gain 1 bhp, so water jacket at 90 degrees manifold running at 60 degrees = 3 bhp if disconected.
It seems to defeat the point of feeding cool air into the carburettor, then water heating the manifold?
Steve

Steve

It all depends on whether the water cools the manifold or not. If the manifold is absorbing a great deal of heat from the ex manifold and from the head, then coolant through the manifold might just cool it.

I left the water jacket disconnected for my tuned 1293, so clearly believed this was the better config.

KMidget uses a plastic manifold / plenum. Above 30/40 mph, the inlet air temp (measured on the runner just before #4 inlet to the head) runs 2-3C hotter than ambient; not bad!!

A
Anthony

It's the intake air temperature that the power increase relates to, not the temperature of the manifold. An increase of 30 deg on the manifold won't translate directly to a similar increase in intake air temperature.

Just how much will depend on a number of factors, principal amongst which is the amount of air flowing through the manifold.

At low throttle openings, air quantity through the manifold is small and the speed through the manifold is relatively low. This will favour heat transfer to the intake air, so there will be some power loss. However, note that this is at low throttle openings - when not much power is required, if you wanted more you'd have put your foot down further! On the other hand, these are also the conditions when you want good fuel evaporation so the warmth helps.

At full throttle, the air density is high and the speed is fast - both these mitigate against significant temperature increase in the intake air.
At 6000 RPM, the air speed in the manifold is of the order of 20-30 metres/sec, or about 50mph in real numbers. Its residency in the manifold at this speed is a mere few milliseconds. That's not enough time to increase the air temperature by much. I'd suggest it's probably less than a degree.

A final point to note is that the heating is an integral part of the set-up. Without it, the big 44mm carb will not behave as well at low speed/light throttle and hence driveability and economy will be noticeably worse. There are two reasons:

1. The evaporation process acts as a chiller (it is after all how a refrigerator works) and cools the manifold. This causes fuel to precipitate out of the air stream, requiring much bigger jetting to get the right mixture at the plug.

2. The poor evaporation conditions mean bigger fuel droplets rather than finer atomisation. This too demands a richer mixture to get it right at the plug, even then it will be tetchy in its characteristics.

Smaller carbs overcome these problems because of their higher gas speeds which stir things up. However, as soon as you go big in ports or carbs these two problems kick in at low speeds/throttle openings.

Heating the manifold overcomes this, yet the temperature effect on full power is minimal.

I stick with the water heating.
Paul Walbran

Ant - is your temp measurement above 30-40mph on full throttle or cruise conditions? (Note also how much longet the K manifold arms are than the HIF44 set-up for the A series!)
Paul Walbran

Hi Paul

Temp measured at cruise, so light throttle openings. When idling/slow moving in traffic, I've seen 75C for inlet air temps, reflecting the under-bonnet temp increase. I use the fresh air ('heater') hose to let fresh air into the engine compartment - and have made an identical one on the other side, so it seems there's a good flow into the compartment >30/40 mph. (Of course some of the air coming in goes through the rad as well.) Generally the air inlet temp settles back after 5 mins or so of 40mph.

I have to report that Lesley didn't enjoy reading the params off the laptop quite as much as I did hearing them(!)

A

Anthony

Paul --

Thanks for the info! That was really interesting. There's more to that system than I'd originally suspected, and your explanation makes perfect sense.

-:G:-
Gryf Ketcherside

Interesting point Paul -I hadn't considered air velocity relative to the heating of the mixture.
Why do you think the 1275 manifold is unheated?
The intake ports of the 1275 manifold are at least as large as the 1500 and I assume the air velocity has a good chance of being lower on the smaller engine?
Some people have the 1275 intake manifold ceramic coated to cut heat transference from the exhaust, so is this relative only to the 1500 engine?
Steve

IIRC the original Midget manifolds were heated - by coolant, not by being part of the ex manifold as on earlier variants.

I don't think it needs to be heated in the UK; but for colder climes, could be beneficial. It's a case of 'one size for all'.

A
Anthony

This thread was discussed between 27/03/2008 and 31/03/2008

MG Midget and Sprite Technical index

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