Archived Message
MGC Bonnet vs Cooling
I've just managed to pick up a ratty MGC bonnet for the princely sum of £70 that I'd like to experiment with on the car.

My car is a rubber bumper model and has a superb cooling system - even when the fan relay fried a couple of weekends back the car didn't overheat in heavy traffic with the heater running I just about managed to keep the needle out of the red zone.

If I fit a C bonnet to the car then I figure there is going to be a large void between the top of the radiator and the "hump" in the C bonnet. Is it logical to assume air flowing through the grille will take the path of least resistance over the top of the radiator, down through the engine bay and out of the bottom of the car?
This in turn potentially causing the fans to have to kick in more often to force the air through the radiator.

So really, the question is this: Have those of you who have fitted C bonnets found it neccesary to fit a length of foam to the top of the radiator to bridge the gap between rad and bonnet?

nb. I bought a starter relay to replace my fried fan relay - I figure if the original starter relay goes pop I can swap the wires over to get the car started and plug the heavey green power feed to the fans straight onto the fuse box as a "get me home" measure.
The fans will run constantly but one trip home won't shorten their life too much. When I bought my first MG BGT as a student and was largely ignorant to how cars worked, the dopey P.O. had hard wired the fan to run when the ignition was on. I did 80,000 miles in that car without a fan failure.

JC Moulds
01/06/2012 @  11:29
Under topic: V8
Archived Reply(ies) received for this Message
MGC Bonnet vs Cooling

Interesting thought, Jon. One of the problems with the V8 is the temperature around the carburettors. This may be worse with those, like me, who have K&Ns and so are sucking air from high up near the underside of the bonnet. So when going along I would have thought that the C bonnet might result in a stream of cool air running just under the bonnet giving cooler air for the K&Ns (more power...) and keeping the carbs cooler. However, when stationary or in slow moving traffic you would still have the problem of heat build up under the back of the bonnet. I did have a cheapo electronic thermometer on my float bowl and regularly reported above 70C in Italy (39C ambient)in very slow traffic and was amazed that the SUs worked at all and forgave them for being a bit skittish. Louvres might help with this. But those near the back of the bonnet are in a positive pressure zone when travelling so will cause air to be pushed into the engine compartment (if this were not so your dash board fresh air vents wouldn't work). This will still cool the carbs but maybe cause less flow thru the rad. However, in my experience cooling the water is not a problem if you have the fans suitably close to the rad matrix and use one of Clive's high efficiency radiators. So I've just persuaded myself that the ideal is a C bonnet with louvres near the screen and a CW radiator!
Altogether a very interesting area to investigate and I look forward to hearing others' opinions and then your actual real life findings!
Bob Owen
02/06/2012 @  03:09
MGC Bonnet vs Cooling

Jon, there may be a problem with using a starter relay for the fans. The starter relay has a low coil resistance so has very strong action. However, it expects to be active for only short periods.If used for the fans it can be on for long periods and may well over-heat. Since I'm not a concourse man I replaced mine many years ago with a modern cube relay specd for 12V (continuous) operation.
If you have a by-pass switch for the fans, as I do, then you also have an easy fall back position in the unlikely event that the modern relay fails.
I should add that with the CW rad and fans positioned with blades 2mm from the matrix (rad lifted a bit on its fixings so that the header is clear of the blades) I never actually need to use the over-ride.
Bob Owen
02/06/2012 @  03:19
MGC Bonnet vs Cooling
Bob is correct in raising the point about differences in the relays. Many people just say "6R4" without realising that there were about 24 different relays in the 6R4 series. Differences include continuous or intermittent use, simple on/off or changeover, multiple connectors, multiple switches, not forgetting 12V or 24V versions.

So the starter and fan relays have different Lucas identification numbers ( and BL part numbers ).

As Bob says the starter relay is an intermittent use one, the fan relay is specified for continuous use.

I second Bob's suggestion to use a modern cube relay. The original fan relay took both the control voltage and the power feed from the same fuse ( supply switched by the ignition switch ). One with separate connections enables the power feed to be taken directly from the permanent live, through a fuse of course, reducing the load on the ignition switch. Relays are available with a built in fuse which simplifies the wiring.
Jim Gibson
03/06/2012 @  04:20
MGC Bonnet vs Cooling
Thank you for the feedback, gents. I know youíre pretty hot in the field of electronics, Bob, so Iíll heed your advice and get a new fan relay ordered. I suppose it wonít hurt to toss the other relay into my box of ďjust in caseĒ spares that I carry around?

I know a lot of folk like to fit fan override switches to their V8ís, but I couldnít bring myself to drill a hole into the dash when itís survived unmodified for 37 years and is one of the few cars that still have an almost flawless factory applied crackle finish? Whilst Iím not a concourse guy either, the modifications I have carried out in the past have been done in such a way that they are fully reversible back to stock specifications with basic hand tools when I eventually decide Iíve finished trying to make the car look like a reject from 70ís ďKustom CarĒ magazine and want to tone it back down a little bit.

Whilst Iím aware etiquette suggests itís bad form to big up your own car on a forum, in all seriousness my rubber bumper model runs just as cool as all the chrome V8ís Iíve owned so Iím pretty anxious not to upset things mechanically. I once proved this to myself with a laser thermometer after Nicola and I had been out for a run in the pair of V8ís. Surprisingly the RB car was running a couple of degrees cooler than the Chrome nosed car after the same journey?
Iíve got a real Ďbee in my bonnetí (if youíll pardon the pun) over the popular opinion that RB cars are prone to overheating. These cars were designed to run in countries much hotter than the UK so I passionately believe that if the cooling system is properly maintained and in fine fettle they should be fine in the UK? Certainly, I feel the experiences Iíve had with the car serve to reinforce that opinion?

You do make an interesting point about the temp of the float bowls, though, Bob. This problem has been negated on my car to a degree by the fitment of a ĎWebberbrockí 500 carb & Edelbrock performer inlet. (The air filter/bonnet modification Iíve got in mind may well end up swinging the issue the other way by inducing carb icing but weíll see what happens) However, I can relate to your thoughts on K+Nís. When I first bought my car it too had K&Nís fitted. I then switched back to a full factory inlet system and noted the car had a bit more pep, yet had a more muted inlet roar. I accept that may well be a moot point as I do not believe the previous owner had the carbs re-jetted to suit the K+Nís?
A pal of mine built a little device for his log burning fire using some steel ducting and a fan cannibalised from an old computer. Imagine a ďUĒ shaped length of steel conduit with a computer fan on one end going under the grate, turning through two 90 degree bends and then the other end of the conduit pointing back into the room. The fan draws air from the room into the pipe which is heated by the fire above and warm air gets blown back into the room. A variation on the same theme could be to use a computer fan and some ducting running from either the grille area or from down the side of the gearbox to the back of the engine bay to provide a source of cool air to chill the float bowls down when youíre touring abroad?
As an aside, you might consider re-fitting the original air cans again as recently there was a note on the website stating that there was now a K+N filter element suitable for fitment inside the lobster pots, although I failed to track it down on a brief search of the archive?

The only other issue I can think of with a C bonnet on a rubber bumper car is going to be the absence of the little blister on the nose of the bonnet compared with the stock B item. To get the lip of the bonnet to marry up completely with the bumper centre section Iím hoping to re-create the bulge by making a couple of careful cuts in the bonnet skin with a dremel and to weld in a couple of in-fill pieces to tidy it up.
I digress a little, but If it all goes to plan Iím hoping itíll at least raise a few smiles on the first of July at the retro show @ Santapod drag strip. There was a pretty good turn out from the MG Car Club last year Ė there was 10 cars in the little show and shine area from T types to midgets to MGAís plus we had 4 x factory v8ís, 1 x v8 conversion, 2 x v8 roadsters and 1 x genuine Costello GTV8 on the track. Iím hoping for a turn out of 10+ V8ís this year, not to mention that this year, former competitors from the now long defunct Rover V8 challenge will be using our little pit area for their first reunion - there should hopefully be a couple of the old race cars on display too, fingers crossed!
JC Moulds
04/06/2012 @  00:37
MGC Bonnet vs Cooling

Jon, details of my cube relay are lost in the mists of time. You need a 12V 30A or 40A SPNO automotive relay with a mounting tab and spade(fast-on) connectors. SPNO = Single Pole Normally Open, sometimes SPST = Single Pole Single Throw. Maplin do a suitable but unbranded one ref NO2AW at £2.99. I see on Ebay a claimed new and boxed Tyco relay for £1.10 (OK, P&P is £2,10) from

There's a strange standard for relay pin referencing so the coil connections are 85 and 86, the common is 30 and normally open is 87. A normally closed contact (not reqd here) has suffix "a".

if unrefd the two coil contacts will read typically about 100ohms on your DMM. The other two spades will be the contacts.

As Jim says, this is an opportunity to add a separate feed and fuse for the fans. I added a Bulgin FX0326 chassis mount 1 1/4" fuse holder (blade connectors) below the main fuse block and added rudimentary physical protection by screwing an insulating extension to the existing fuse box cover (I used some blank PCB material - always plenty lying around here!). Ebay has one of these at just over £3 inc carriage -
In line fuse holders can be used but I'm not a fan of these for higher currents. Use a standard 25A cartridge fuse element as for the other V8 fuses.

If a motor becomes obstructed or seizes or goes tight due to the bearing drying up then you may blow the fuse. Your temp gauge will give you the indication. On the original wiring various other functions also failed if the fuse blew, eg fuel gauge, tacho, indicators...
Bob Owen
05/06/2012 @  01:45
MGC Bonnet vs Cooling

Jon, back to cooling.
In tha past I've had cooling problems with my (chrome bumper)V8. However it transpires that there was more than one problem - always a serious complication for diagnosis. The actual water temperature problems were cured by changing the rad to one of Clive's hi-efficiency ones. For the extra money it seemed not worth replacing like with like. But the car still got lumpy in traffic even with water temp reading reasonable and would refuse to start if stalled. With all the fuel vaporisation publicity I suspected carburettion and had a spell running with a digital thermometer on the dash which showed me the float bowl temperature. Sure enough, if this got high (nearing or above 70C which it did in traffic) then i had problems. So I built aluminium housings for the K&Ns with 4" fans in line with 4" aluminium ducts from 4" holes cut in the rad flitches (coil, expansion tank etc etc moved, dipstick and assy cranked to allow 4" ducts). So we confidently set off to Italy on a "proving run". In traffic in central Italy it stopped. Italian drivers aren't very patient waiting at green lights... mmm, my new hi-performance cooling mod doesn't seem to work. Problem gets worse.Finally decide ignition is at fault and at the roadside change everything except the coil (forgot to include one in the spares). I managed to reach Terni where Piero Fusaroli took me to his local garage. I asked for "Uno bobbino per una Rover V otto, per favore". He dug in his scrap bin and found an old LandRover coil which he gave me. Fitted it to the V8 - perfect. The problem - old Lucas coils had rivetted contacts. When hot the rivets expand and the contact is not so tight so resistance goes up. There's still a spark but the energy is reduced. When the coil gets hot, spark energy is reduced by the copper resistance increasing but much more so if connections are also dodgy, just when it needs to be high to ignite mixtures no longer optimum because of over hot carbs...

With "new" coil no problems even crawling along in baking hot Italian villages. I dismantled my lovely aluminium boxing and air ducting when I got home. Just to keep the coil cool, as belt and braces, I re-mounted it in the now redundant hole in the rad flitch...
A bad case of engineer's hubris.
Bob Owen
05/06/2012 @  02:16
MGC Bonnet vs Cooling
Bob, thank you for your info. That's clarified things neatly as I've been a bit thick.
Initially when I considered what you had written it threw me off guard a bit. Why would there need to be a separate feed for a by-pass switch?
Then I realised rather than fitting the bypass as a piggyback to the switching side of the relay you are actually advocating fitting a fused separate feed to the orange supply wire to the fans.
Makes perfect sense actually - this would mean the fans could be triggered without the need for a functional relay.

I think it would be best for my application to make a little ally plate that could be bolted to the cross brace under the dash for the switch. This would at least negate drilling holes in the dash. It should only be used in emergencies really. The normal operation of the "otter switch" should negate the need for going anywhere near a bypass switch.

Have you got a picture of your ally air boxes? It does sound like a very interesting mod, even if you did solve the problems via other means! It'd certainly be interesting to put the car on a rolling road and see if there was any appreciable difference in power?

My C bonnet arrived on Tuesday. It's a little beat up but a lot better than I was expecting. I decided to take a couple of days off work and moonlight as a body guy. Whatever it has previously been fitted to was obviously involved in a smash as the frame is wrecked and there are ripples in the skin. It'll do for experimenting with in the short term until I splash out on a brand new one.

So far so good - I'm about 3/4 of the way there with it now. Although it's taken a lot of work getting the right shape to the power bulge - what a boring messy process! Hoping to prime it by the end of the week if the weather is kind to me?
JC Moulds
07/06/2012 @  07:22
MGC Bonnet vs Cooling
Some interesting stuff. The volatility of fuel is adjusted to be lower in summer than it is in winter so that vapour lock is less likely to occur in high ambient temps. Even at 70degC the max that can boil off is 5% by volume so I would imagine that a sound electric pump would easily keep up with the engine. Pleased to see that Bob's Italian adventure was solved with a new coil. My factory V8 has never displayed vapour lock symptoms which I think reflects the fact that the present RVP (Reid Vapour Pressure) values used by the fuel companies to adjust for seasonal ambients is OK for our V8s.
French drivers in Amboise were no more sympathetic to English car drivers than Italians and particularly MGs when things go wrong at traffic lights. I had clutch hydraulic probs last week in the Loire that involved starting the engine in gear, lots of double declutching and an infuriating inability to find neutral when I really needed it. New clutch master and slave cylinders going in now plus a new brake master, after 170K miles all well past their useful life.
Tony Lake
07/06/2012 @  22:35
MGC Bonnet vs Cooling
Hi Tony,

dare I be picky and say should that be 170k miles ? But as the mile is not an SI unit should we even use kilo to signify 1000 ?

As you know, capital K is used in technology, to signify 1024 - arising from powers of 2 in binary arithmetic etc.

Actually the accuracy of our odometers is probably such that either k or K is close enough.

Now that I've lightened a slow afternoon, you can all give me a kicking for typing 6R4 when I meant 6RA for the Lucas relays. Maybe I've seen too many naff personalised number plates, or I was just thinking of my lottery list of cars.
Jim Gibson
07/06/2012 @  23:27
MGC Bonnet vs Cooling
Some progress: We're now in primer!

The frame of this bonnet is wrecked - it's held together by lengths of steel rivited on. So, this means the frame interferes with the rad support. I'll have to notch the frame on the bonnet as I'm certainly not cutting the car about!
Like I said from the outset this is just an experiment to see what can be achieved. I'm not planning on leaving it on the car for any real length of time.

There may not be too much of an issue with the radiator as I first thought. The front structural section where the catch mounts to the bonnet extends backwards towards the front of the rad on the C bonnet.
So, the narrowest point between the radiator and the bonnet is about 10mm accross the front of the top tank on the radiator. This is actually a similar fit to the V8 radiator so in reality I don't think any foam strips will be required since they are not used for the same sort of gap on the original instalation.
So, it looks like we're makin' bacon at this point! I'll know for certain once I've notched the bonnet frame and tried it back on the car.
JC Moulds
13/06/2012 @  19:14
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