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MG MGB GT V8 Factory Originals Technical - Firebird hood vents

Guys, in the endless discussion of louvering hoods, using vents and so forth to get the hot air the hell outta there, some of you have mentioned putting Firebird vents in the rear corners of the hood. I think I've seen this on a couple of cars and it looks fine to me. __Does anyone know what year Firebird?__ I'm going to try to track down a set if someone can set forth this data point. Oh, also, is it clear that the rear corners of the hood are a negative pressure area (my car is a GT, if it matters)? Sure would be a bummer to put in the vents and discover that they weren't sucking out underhood air at speed.

Do the aforesaid vents not allow much rain into the engine bay?

My conclusions are (i) louvered hoods look too non-stock and let in all kinds of rain, and (ii) to really do a good job with vents in the inner fenders, you have to do it with the engine out, and that ain't happening any time soon, so (ii) the hood vents may be the best bet. But something's gotta give, it's a damned furnace in there (4.2 stuffed into a CB engine bay)!
Harry

Harry,
The vents that I have on my car are from a 1991 Pontiac Trans Am. (I Think)
Anyway go to the local Pontiac place and tell them you want HEAT EXTRACTORS. That is what they are called. Also be sure the small screens That guard the open area is with them.
Hope this helps. Bob McClain
Robert McClain

Bob, many thanks ... but a Trans Am is a Chevy, and a Firebird's a Pontiac! Anyhow, I'll give it a whirl.
Harry

Harry, the ones I used in the pass are the ones from a 85 Pontiac Trans Am. The hood has a set at front to extract hot air from the radiator. Also the Trans Am is equiped with a small set at each corner of the hood to extract hot air from the engine compartament, these worked in conjuction with the ones on the fenders. If you are planning to use them on your MG hood then choose the ones from the front of the hood and get the mesh wire backing and the body clips to install them. I have use them in various projects and they really worked well.

Pontiac has the best cooling system. Please note that the TA does not have a grill openning. These vents also come in other years of Pontiac TA's.

My TA never runs hotter than 200 degrees in a hot summer with a stroked 350 ci to 383 ci TPI FI.


r/ Bill

Bill Guzman

Bill, do you have a picture online by chance of your installation?
Harry

Harry,
Seems to me that Pontiac made the Tran Am.
The heat extractors do work. When the cooling fan comes on I can feel the heat from the eng. compartment being blown back on my arm, if my arm is hanging out the window.
A friend of mine put small cotton strings across his hood to see what the air flow was. At fairly low speed (30 to 40 mph)the strings went straight back. At a higher speed, about 50 to 60 mph the string that was taped to the heat extractor was being sucked into eng. compartment. This was not scientific, but that is what it did. Try it your self, maybe you can get different results.
Bob McClain
Robert McClain

Robert, of course you are right, a Trans Am is a Pontiac -- I pulled up behind one today in fact. Shows you how much I know about Detroit iron.

I'm going to try to get a set of these heat extractors. Just to confirm, you're saying to get a pair of what they use on the front of the TA hood and put them into the rear corners of the MG hood?

Thanks!
Harry

the standard front end pressures are a negative pressure (partial vacuum) at the front of the hood and a building positive pressure begining 2x the height of the windshield in front of the windshield and peaking at the base of the winshield. Ford GT40's have a big hole in the front end right behind the radiator which sucks the air out. Many hi performance cars have their engine air intake facing rearward right at the base of the windshield - the hi pressure spot. The heater intake is in the hi pressure area.
The general problem with the mg engine bay is getting the air out. - hi pressure builds internally. try opening the hole for the other side steering shaft. You will have hot air blasting under pressure into the passenger compartment.
If the strings are flowing into the "extractor" at hi speeds that means air is being forced into the engine compartment rather than being extracted. The goal is to reduce engine compartment pressure and allow more cooling air to flow thru. Louvres or rear facing vents at the very front of the hood just behind the radiator would appear to offer the best results.
Barry
Barry Parkinson

guys...My car is under construction right now but I'm going to use an MGC aluminum hood that I don't want to cut up as its tooooo valuable....what do I do to get the heat out?
Anthony Barnhill

Harry, I will dig for pictures, I am not very organized when it comes to pictures, my wife....will help me find them. The car was a GT with a high compression 215 Kenny Bell equipped, built in 1978.

Barry is right, the vents are used to extract the heat from the engine. The front vents of the Firebird (Pontiac) are used on the corners of the hood on the MG, they must be install in the same fashion as the Firebird, louvers facing rearward to create pressure above the louvers thus extracting heat out. This louvers would be more effective if used with an electric fan. The electric fan turns at higher speed at idle than a conventional fan. The idea of using them on an MG is primerly to extract the heat at idle and added bonus at road speed.
Ofcourse the best way is through the fenders wells.

Robert, if the experiment was conducted on a MG then you are absolutly correct. This is part of the design on an MG to get air to the heater and the passenger compartament, like b
Barry stated this area is the high pressure area. It is a good vent for a carb intake, I have used it for that reason alone, who needs a heater in Ca.

There are some negatives, the hot air will have to go somewhere and that is up and above the wishield and down again. But, this is reduce if they are install on the corners of the hood. A solution could be a deflector. This is install just behind the louvers, the deflector would be about the same thickness of the louvers to deflect the air higher and would also create a higher pressure area. I don't know how this would work, I have not experience this mod.
I forgot to mention that the Z cars, Nissan 240,260, and 280 also used the same system of louvers on the hood. Also open up the front area where the radiator is so that air will flow through the engine bay, this will force the hot air out through the vents.
I have not used them but they seem to be well design.

r/ Bill

Bill Guzman

This thread was discussed between 08/06/2000 and 12/06/2000

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