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MG MGB GT V8 Factory Originals Technical - Got some hood vents ... now where to they go?

My CB BGT conversion runs mighty hot when the ambient temperature's high. To hopefully provide a place for evacuating hot air from under the hood, I got a set of small hood louvers, these from a 1984 Pontiac Grand Prix GTS. They're smaller than the Ford Cosworth louvers that I understand some people have used with success, and to my eye they look more appropriate. Can anyone help me locate them on the hood? I take it they shouldn't be placed too far aft as that is a positive pressure area. A few inches behind the rad, on either edge fo the hood, maybe? The body shop says, no sweat putting them in, but where do you want 'em? Any suggestions welcome.

(No, I don't think mucking up the hood is all that great visually, but I don't like some of the vents into fenderwells that I've seen -- much too much road spray, only effective at speed, and in any case pretty hard to put in while the engine's in place.)

Sorry, I meant 1994 Pontiac. You can get them right at the parts dep't of a Pont dealer.

Darren, If you are going to use them to cool the radiator, then install them just behind the radiator with the louvers facing rearward. If you are going to use them to extract hot air from the engine compartament, then install them at each corner at the rear of the hood, louvedrs facing the wind screen. These combine with a good electric fan should bring the temp down. Both should be use; behind the radiator and at each extreme of the rear of the hood.

Look in this thread, we were discusing Pontiac Trans Am Vents.
r/Bill G.
Bill Guzma

Ah so. You're saying I really need two pairs of these vents. I checked out the other thread. Would you have a picture of your car on-line? This is one of those "measure twice, cut once" scenarios -- I wanna make sure I go in there with a plan!

It all depends on when you want the maximum cooling and whether you want the natural movement of air and changing pressures to do the work, or whether you need a fan to assist.

If your looking for static cooling assistence then the vents at the rear of the engine bay will allow considerable heat energy to escape. However on the move the area above this vent position starts to see an increase in air pressure that will rise to above that which is inside the engine bay. At this time there will be no air movement outwards and, depending on vehicle speed, pressurised air forced into the engine bay.

For effective and unassisted cooling of the engine bay during driving then the vents should be positioned about a third the way along the bonnet (hood), which equates to just behind the radiator position. What you need to do is measure the height of the windscreen from the base of the metal surround to top edge and then add 50% to that measurement. Whatever that measurement is (I don't know as I don't have a GT) now measure that distance from the base of the screen forward along the bonnet.

From that point towards the screen will become a high pressure area when the car is driven, due to the physical block that the screen presents to the air. This will create the conditions I mentioned in the second para. Forward of this remains a low pressure area and with the high pressure underbonnet area you have a very positive movement of air out through the vents. Being as this will be in the immediate area behind the rad this will create a very positive cooling effect.

The positions of the vents of the RS Fords was not done for show and was the result of wind tunnel work, lets all benefit from this expensive testing.

You can go a stage further and do the same as John Bourke's V8, where John has created specific vents in the bonnet above the K&N filters for his SU's. These are not simple holes in the hood, but have understructures that emcompass the filters, so effectively insulating them from engine bay direct heat and feeding them cooler air. There are even water drain holes to allow easy escape of rain water without soaking the filters.

Roger Parker

A lot of people mention "yuck" getting in through the holes cut into the inner fenders.
Due to a previous owner/inept hot hodder, the holes through the inner fenders of my '71 MGBGT with a Buick V6 are larger than I would like/they need to be.
I'm really feeling left out here, the only thing that gets any "yuck" are the header tubes that pass behind the front tires (fenderwell outlet style headers). What am I doing wrong? Should I make some rubber flaps to help get some "yuck" into the engine compartment?
It is some consolation though, that I usually forget to plug the forward facing hood scoop when I'm driving in the rain. I wonder how much richer the mixture is when the air filter is soaking wet.
Randy Forbes

I've put a 300zx v6 in a 79 roadster and am about to do the headers. It looks to me like there is not enough room to turn the wheel full lock and clear the external headers. I'm using 6" wide 15" diameter wheels with 195/55 tires. The offset is such that I just rub the stock front sway bar at full lock. At opposite lock it looks to me to be too close. - Or am I looking at putting the header too far to the rear? I could get wheels with a wider offset but even then it looks like an interference fit. On the Yuck stuff It looks to me like a lot of yuck stuff could be tossed through that fender well opening in wet going.
Barry Parkinson

I'll bet your V6 will beat my V6. So there!
I don't know how much you can get away with. I'm using the same suspension as you (from an 80 B), but the stock RO wheels and 185/65 X 14 tires.
I have my tubes as rearward as they can go behind the front tires. A "little dressing with a calibration hammer" was required on the front most tubes at full lock/full bounce. The shiny metal will tell you where to aim the hammer!
There is a link to where there are some pictures of my car at the "302 conversion..." thread of this BBS. You can get a pretty good idea of the headers from there.
Also check the thread "Curtis ..." for some tips on making headers.
You will be greatly satisfied after turning out a pair of your own design headers. When you tell (knowledgable) people that you made them yourself, they will never look at you the same again.
Let me know if I can help.
Randy Forbes

I'm using the cb cross member and 1 inch off the cb coil. I'm also putting the fiberglass kit on that makes it look like a pre 289 AC ace. I figure I should come in around 1900 lbs. Compared to stock wheels I am about 5/8" inward. - lots of negative offset. I have some 1/4" spacers, but I think even with them I'll be too close. I think I'll try and squeeze the tubes next to the engine.
Barry Parkinson

Darren, I sold that car long time ago. My wife is looking for the pictures, as soon as they are found I will post on web. Please look at a Pontiac Trans Am.
My Mod 85 runs cool. These car have no grills at front and have a very efficient cooling system. Just duplicate. I am not a very fast typist to explain low and high pressures. Please note that the Pontiac Trans Am also has side vents for extracting hot air while in motion. Also the Nissan 280ZX has this vents.
My GT has a scoop with cold air box. My headers exit thru the fender wells and I am using 15X8 wheels with 202X55X15 with no problems of clearence.
on this web page is my GT see 2.8 Ford and under
Bill Guzman.
r/ Bill
Bill Guzman

This thread was discussed between 19/06/2000 and 20/06/2000

MG MGB GT V8 Factory Originals Technical index

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