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MG MGB GT V8 Factory Originals Technical - Small 12V to exhaust air through wing?

I have a CB V8 conversion with the block-hugger style headers. As you can imagine, it gets pretty warm in there, especially going slowly. I don't want to do anything like hood louvers, but looking at the engine bay, the inner wing on the driver's side has a relatively flat area several inches square. I was thinking of buying a 5" 12V fan, cutting a hole in that wing, and mounting the fan so as to blow outward. Is this a good plan? Does it matter that this would only be on one side? Does it matter that my fan/hole would be a few inches forward of where you guys typically cut a hole in the wing for your RV8-style headers?
Sam Finn

Wouldn't the hot air then be trapped inside the wing as it would be dumped behind the splash panel? & if you dump it in front of the splash panel don't you stand a chance of getting road slush into your little 12v fan? Reason I'm asking is that I'm considering cutting a hole in my outer wings, using a set of AC Cobra louver vents & duxting air out through them.
Tony Barnhill

Tony, no, I'm talking about in front of the splash panel (there's not much room behind it anyhow). Basically where the RV8-style headers go. I don't imagine it would get gunk into the fan (as if that mattered, as Jerry Seinfeld would say) since the fan would be blowing into the fender well. I'd probably put a screen of some sort in front of the fan just to be on the safe side. But note that the guys running RV8-style headers, with no special fans or anything, rarely if ever seem to have problem with road gunk ... the theory, I guess, being that when the car's in motion, the fender well is a low pressure area.

I think I can pull off a very nice installation, but before I embark on the project I would like to hear what, if anything, the shortcomings of my sneaky little plan are.
Sam Finn

Thanks for the idea guys ! I was looking for an air exit from the engine bay ! I removed the bumpers from my '66 this weekend and used blocks to space the lower valance out to even up with the grill. I was thinking about cutting a coupled of 4" holes through the lower valance and ducting air into the engine compartment through aluminum dryer duct hoses... just didn't know exactly where to let the extra air out.

Keep the good ideas coming !
Sean Squires


The RV8 header arrangement does seem to
leave the engine bay cooler - in part perhaps because it moves a section of the heatsource outside the bay.

You might bear in mind that the inner wing provides strength - if you do cut holes in it, you would want to re-inforce it around the hole - as with the RV8.

Tony's thought about pushing the hot air behind the splash panel is worth some serious thought because it would take air from the top of the bay through the strength sections and transmit through the wing section where there is room enough for a fan. This would need to be set up before the wings are fitted or in the case of my 3.9 - when the wings are changed.

My temorary solution has been to lag the top of the headers so that heat is no longer radiated directly upwards. This seems to work reasonably well.

Good luck.

I have begun to make this modification to my CB. I cut a 4" hole in each side toward the firewall where it is flattish. There appears to be no interference with
this positioning. I am going to use small electronics muffin fans; they are cheap and if they don't last I'm not out much. SPAL makes a 5" that's probably more durable, but I want to have a proof of concept first. If it is a go, I'll either keep the muffin fans or move up to the SPAL.
In doing this, I thought I was a trailblazer, but I was talking to an old hot rodder about hood louvres and he told me he had come up with a better solution 12 years ago. He popped the hood on a 50's pickup with a BB Chevy installed and there they were - two muffin fans exhausting to the fenderwells. He says they work wonders.
Erich Hovley

The rout I'm taking is a little different & still in conception stages....have acquired a set of AC Cobra chrome fender louver vents ($110 for pair) & am cutting fenders below the stainless trim belt line to fit them aka the thinking a set of louvers cut in the inner fender will allow hot air to vent out into the area behind the splash panel w/o affecting strength...will get air to outer fender via some large flexible ducting attached to both inner fender & backs of Cobra, you've given me a second thing to think about--putting small 12v fans inside the ducting, hmmm...
Tony Barnhill

Keep us posted Tony


I think your idea, if I understand it, is very clever. This might just kill two birds with one stone -- having dry, hot air circulating inside the splash panels would probably give excellent rust prevention. The question I have is, then, why you need a tube between inner and outer: what's to stop you from just mounting a fan on the inner wing panel _inside_ the fender assembly -- that is, outboard of the inner wing -- having the fan blow air into the sealed area behind the splash panel, and having that same compartment vented with some form of louvers? In this way, the fans would be totally out of sight. Clearly, if the fans ever went kaput it would be something of a pain to get at them, but that seems pretty unlikely.

I was thinking about the flex tubing before this thread gave me the idea of a little're right, I can just mount the fan on the outside of the bulkhead & dump the air into the fender opening...would make it a lot easier to get to the fan if I ever needed to replace it...believe the Cobra louvered vents would pull air on out of the inner fender area....Guzman, where are you when we need your engineering expertise?
Tony Barnhill

Problem: to much heat to few escape routes. Before I inserted the engine into my 1979 B conv. body I recessed the inner fenders even more, around the header area. I also dressed the area at the fire wall therefore increasing the escape area around the top of the bell housing. This has really helped get the hot air out by increasing airflow. I to tried the Cobra vents before I recessed the fender, to be honest although they looked good they did nothing to reduce the heat problem, you also have the disadsvantage of water getting into the body work
via the vents and flexi pipe. I replaced the vented fenders and started hammering. Sometimes simple is better

Mike Cook

Mike...Is that your new V8 you're talking about or the red one? Is your new one in shape that I can come over & steal some of your great ideas?
Tony Barnhill

This might seem a little expensive, but some of you might want to get your headers jet-hot coated. This keeps underhood temps. lower, and it also preserves headers. There are a few companies that do this. A few people have done this and it looks & performs great. fwiw, mike
mike childress

I have the Jet-Hot coated headers (regular block hugger style) and I must say, it's an amazing product because you can practically hold onto the headers while the motor's operating. That being said, I don't know if it's such a great idea, for one thing I worry about all that heat going through the exhaust system. Even so, the whole thing is touch and go: I have (i) an engine-driven fan in a cowling, plus the two electric fans, (ii) an extra 4" height rad (goes to the bottom of the x-member), (iii) the Jet-Hot coating, and (iv) the ST air dam, which should help vacuum air out from under the car; the freaking thing _still_ runs hot around town, albeit right on N at speed. I am sure that what is called for is some kind of active purging of the under-hood hot air. I think the next thing I may try is hooking up a big electric fan horizontally under part of the engine bay, and if that doesn't work I'll try this muffin-fan-in-fenders thing may be worth trying.

Ron, what motor do you have and what rad. type are you using?Mike
mike childress

Mike, Rover 4.2 carbureted; heads and cam have been significantly massaged; don't know brand of rad, looks like any other V8 rad except a few extra inches have been added vertically.

For the muffin-fan-blowing-through-inner-wing setup, how many CFM would suffice to purge a significant amount of the hot air? It seems to me that this idea is, in essence, a winner. At speed, air would automatically extract itself due to the low pressure in the fender wells, and as long as the CFM displaced was sufficient it seems to me you'd have the problem licked at low speeds as well. If I had the car all apart, I'd be inclined to follow Tony's lead, but I think this can be elegantly done without going inside the fender structures.

After thinking about it, wouldn't the optimal position for the fans be fairly high up on the inner wings? ... after all, the hot air isn't going downward. In most V8s, the one empty space in the engine bay is on the driver's side a few inches above and behind the road wheel, angled about 45 degrees to the ground. You could surely situate a 5-1/4" fan there, which I think would extract in the neighborhood of 225 cfm. I can't quite tell if one could get in another fan on the passenger side, just aft of the oil cooler, at least with the engine in place ... you probably could if need be. What'd be the best tool to use to make the holes, I wonder?
Harry car is totally disassembled while I'm doing the V8 conversion...took the fenders (oops, wings!) back off & studied the area behind the splash panel..believe I can fit the fans outside the engine compartment in the inner fender area near the top & pull air out through a set of louvers placed right about the height of where the fuze box usually sits (moving it inside car anyway) & at same height on opposite side..then, can fit the Cobra louvered vents a little lower below the ducting will be going to try it & will put photos on my web site as I get to this, somebody tell me where I can buy the little fans.
Tony Barnhill

Tony - try Grainger Supply in Huntsville, or, your local Radio Shack.

Sean Squires

Better yet Tony, go here

click on the "shop our online store" button and scroll down until you see "fans" they have quite a few interesting items there. the largest is a 120mm (5") but they have some for high velocity low clearance applications that may work.
Sean Squires

Sean, I don't think those little fans will have nearly enough oomph. I suggest that Tony try

... and Tony, please keep us apprised of your efforts.


Just 2 things solved my heat troubles.....switching to RV8 stlye headers and the highest cfm/rpm cooling fan from Hayden.These cars only get hot at idle, so the key is really the cooling fan. I have had 4 different pusher fans,none doing the job until I cut and squeezed the above mentioned fan in. It was the answer!
The RV8 headers make sense especially for those of us in the south.Also anyone with hotter cams etc. My own engine is a 3.5 with slightly higher compression pistons and hotter cam.Radiator is standard dimension MGB but 4 core.Wish it had been made 2-3 inches taller.

I made the cutouts exactly where you describe. Started with a 4" hole saw and abandoned that quickly to go with a sheet metal nibbler followed by filing and grinding. The hole saw kept catching and was going to do a lot of damage; the nibbler worked perfectly.
Erich Hovley

Hi Tony
I like you fan in the wing idea, I already have RV8 headers so its not critical for me. I was thinking more in terms of rust prevention than heat exhaustion. I may put small 4 inch in the inner wing to slightly pressurise and dry this area. With small capacity fans and the fact that its almost impossible to seal this area properly I could get away without exhaust vents.

Nick Smallwood

Nick...I can't take credit for the fan idea..this string has had some great ideas....I was toying w/venting through the fender..& the fan is a great addition...I'm going to go w/Cobra louvered vents just because they look good but you could probably get by w/o an external vent...air escapes on its on anyway.
Tony Barnhill


Keep us posted on the project's success! Do you have a way of posting a couple of photos online?

I just skimmed this thread. I am fairly familiar with most 12V fans like those used in computers, only different sizes.

Someone above brought up a good point about the water/mud issue. This will definitely be a problem, the debris coming off that wheel will be travelling at high speed, probably within 10% of cars speed. Don't take this the wrong way, I am just trying to help everyone not fall into a potential problem. Thinking that a "mere" fan will stop water or debris from entering is not realistic. That debris has already overcome a 30mph wind resitance and is still going, so the extra 2mph resistance of the fan will not slow it down. Now if we can get the fan "out of harms way" I would be very interested.

My plan is to have a front Dam installed and use the 2 openings for running lights as scoops and put small duct running up to the side of the front wheel wells and pointing down at the headers. I know racers use thi system for cooling brakes, so this will just be a alteration of that plan. That should blow hot air down and out. Maybe I will incoporate your fan ideas as well so in traffic I can still force air our.. hmmm ideas..
Larry Embrey

I will try to post some pictures. Unfortunately my project has been derailed by an addition to my house (the wife won't go for living in a larger garage...).
Based on the placement of the holes, I really think this is the way to go for the clean original look. No scoops, vents, bulges, etc.
Erich Hovley

One of the Future projects I have is a V8 Ford.
I have been studying the heat problem. Without vents on the hood the other solution that I can add is the Electric cooling fan with openings as it has been suggested do not need an extra fun to suck the hot air out. I once had a XJ12 Jaguar that use to melt vacuum hoses when driven in traffic on a hot day. The solution was to cut vents on the inner wings and replace the engine driven fan with a high speed electric, this fan would operate at high speed while in traffic thus exiting the hot air through the vents. The temps were lower, I lower the temp control SW to 160 degrees and the temps went even lower. The inside temp was measure with a temp probe. The electric fan has plenty of power to exhaust the hot air. The vents were fabricated with household vents as use in garages and the likes on houses, any hardware store carry them. They come in different sizes.

I found that the best place to exit hot air is as far back as possible. I like the DB4 Aston Martin side vents style; Tony the Cobra vents would look great on a MG. The Factory racers Alpine Tigers were modified with similar vents to keep them cool.

Bill Guzman

Bill...If I could find (& afford) DB4 side vents, they would be my first choice.
Tony Barnhill

Since the underbonnet area becomes a high pressure area once the vehicle is moving the issue of moving air out through additional exit holes isn't a problem. As has been mentioned only when the cars speed has reduced to less than say 20mph, is there an equalisation of pressures so air has to be physically moved by the engine cooling fan and any additional fan(s) installed.

May I suggest that with the addition of these extra fans you control their operation automatically by installing a thermostatic switch, such as is the case with the MGF's mid engine layout. Sensor installation should be near the carbs as this is one of the first areas to suffer from excess heat, and is in the 'hot end' of the engine bay. Such a set up should be very efficient.

Roger Parker

Has anyone actually run their MG with a temp probe under the hood to see what the temp is while driving or sitting still. I know it doesn't take a genius to figure out when your feet are almost on fire where the heat is coming from. I was just wondering what the actul is before and after this type of modification.

Sean Squires

Perhaps add fender flares so as to add front wings, this will leave large gaps and allow plenty of hot-air out. (ala Viper, Porsche GT1, etc) Since the wheel wells are low pressure area's, the hot air doesn't have anywere else to go other than out of the large voids.

I was looking at a page today, (Byron's), then thought about how great his idea was. I am referring not only to the twin turbo modification but to the fact that he also added a duct on his hood to allow hot air from behind the radiator a place to escape. It won't pass as a sleeper but the engine and driver will be cooler.
James ?


Ive tried two different ways the check the temperature under the hood. First a cooking thermometer, it must have been broken because it didnt register. Next I tried one of those inside-outside battery powered led temperature indicators. The instructions suggest locating the remote sensor behind the front bumper, but I put it near the air-cleaner.

In one non-MG vehicle I tried it in it registered 130 F. In my MG V8, it exceeded the 158 F (70 C) limit of the device.

George Champion

I think when I get back to my V8 project (after getting a '79 ready for a cross-country road trip), I'll go w/original plan...Cobra chrome vents in fender below beltline & put the little fan inside the fender behind the splash panel to pull air out of the engine compartment....will also look at coating my headers..
Tony Barnhill


I've run underbonnet temperature sensors on the car on a race track, the result suggest a 15 to 20 degree C increase on ambient at the carbs which seems to be 50% radiator heat and 50% exhaust/engine.

However this was at track speeds - temperatures could easily go higher under road conditions at lower speeds, the startline temperatures were about 60 degree C above ambient.


Dave Brooke

Thanks guys definately shows the merits of the project !
Sean Squires

Hi all,

I've been reading this thread and am wondering if anyone has tried what I am thinking of doing with our midget. What if you put blocks, say 19-25mm thick on top of the hinges between the hinge and bonnet. That would lift the rear end of the bonnet, which should help hot air exhaust out the back. Would be a subtle mod if it worked. Haven't looked at the kinematics of it (might have to use wedge shaped blocks) but it would be easy to try and involves no cutting of the car. Just a thought. Our Midget has a pretty hot 1275 in it so removing heat is an issue for it, also, although nothing like a V-8. Must admit, those V-8 conversions in B's are starting to look mighty interesting, would be a great car for road trips in the states (although would be a bit wasted on our little 28 mile long island)

Regards, David Walworth
David Walworth

hey Tony,
how about some MG side vents to get your air out ?
I'm not sure of the exchange rate, but they look sharp.

or try this one and hit "stainless steel grills"
Sean Squires

David, one problem with raising the rear edge of any bonnet (hood) is aerodynamics. The windscreen, if fitted, is a block to airflow and creates a high pressure area in front of it. The rule of thumb is that the height of the screen from the bonnet, plus 50%, is the distance that this high pressure area extends forward from the base of the screen. The effect is that once the car is moving at say more than 30mph the raised rear edge is now seeing air being forced into the engine bay. This will have the effect of blocking airflow from the engine bay.

At lower speeds the venting works very well so you have to assess where the heat issue has most effect before deciding which route you want to follow.

As a perfect guide the bonnet vents on the various Cosworth Fords of recent years shows the optimum vent position for heat removal. Just behind the leading edge of the bonnet and slightly rearwards of the radiator. In this position you benefit from the low pressure area created by the bonnet edge which sees massive airflow from inside the engine bay which is a high pressure area. Then there is the advantage that being behind the radiator it is the hot air that has passed through the rad that is removed so well.

Roger Parker

More ideas:

Myself, I am going to cheat. I want a 302 ford. I will do a tube frame and run without inner fenders. Mostly to be a sunny day/track day car.

Original thought was to actually remove the inner fender panels and haul them down to the local hot-rod louvre guy. Get as many in there and facing back so as to keep road muck out. Keep in mind though that the hot air is under the hood. Me I would go with louvered hood, but not to everyone's taste. At least this will allow the fan(s) to do their work at low or stoped speeds.

The ideal place for hot air elimiation would be through the triangular boxes at the upper scuttle. There is one on each side of the car. You know, the ones that are always rotted out when you remove the fenders. The box section runs into the closed space comprised of the door pillat, fender and splash panel. I think it is the area that Mr Barnhill is
considering. You could bolt the fan to the space between the fender and the outside of the footwell.

The question is, Why do you need a fan. You probably have good fans in the front of the rad. The problem is getting the hot air out (both because it is hot and because, since it has no where to go, makes your existing fans job impossible. If the engine compartment were to be ventilated via the strengthening box section, hot air could be pushed out (witht the existing electrical rad fans ). Once in this space you have two options; either cobra style vents as Mr Barnhill, or simply leave the splashguard off. Leaving the splashguard off has the benefit of using the low preassure of the wheel well (while moving) to draw more air out at high speed(Can't hurt can it!)

One issue that need to be looked at is that the footwell inside the car is going to get @#$%&* hot. Some kind of insulation will be needed. JM2$W to a long thread.

Pete Plouf


Thank you for the information. That is good to know. As I think about it, attempting to exhaust the air from the rear of the engine compartment would mean moving hot (less dense = less power) air right by the carbys. I like the louvers just behind the radiator, it makes sense. I don't think that I could make myself cut louvers in my steel bonnet but a glass fibre one, no problem. Done well would be both subtle and good looking. Of course, louvers at the forward end of the bonnet doesn't help much for the exhaust header heat. It's always a compromise, isn't it? Are the louvers on a Cosworth Ford similar to a Jaguar XKE bonnet or are they more of a small reverse scoop?

Thanks, Dave
David Walworth

V8 conversion project just took a new twist..the engine in my my daily driver-a '79-just went tits, everything I've prepped for my conversion body will be fitted to that car...will be on the road w/V8 in less than half the time....only problem w/MGF vents is contour won't fit MGB fender....
Tony Barnhill

This thread was discussed between 16/08/2000 and 21/09/2000

MG MGB GT V8 Factory Originals Technical index

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