rear vents on original GT's

David York · 08-19-05, 10:51 AM

Having a look around at original GT's I have seen some vents on the rear that I have not noticed before. Can anyone point out what they are for? Releiving pressure from the rear clam?

Mark Clapp · 08-19-05, 12:25 PM

Dave, yes, most MKIs have the vents like that picture. Some cars like P1075 left them out completely to let out as much air/heat as possible. MKIIs had screens.

Chris P. · 08-19-05, 12:45 PM

Interesting... I'm in the middle of aerodynamic design of my scratch built car's bodywork. With the slope on the rear vertical valance (where the exhausts poke through) of the GT40, it's hard to say, but in most cases there is actually reversed flow back there, at least closer to the top of the rear face - and, free stream pressure to boot. With the aluminum (or stainless whatever those happen to be) vents as low as they are in that car, perhaps there is actually a slight pressure drop and some air extraction from inside the engine compartment. I have not included internal flows yet in my analyses (CFD) and when I do, I will also be including heat. The heat alone will make a difference, increasing the pressure inside the bodywork a bit, and needing those vents as a bleed.

It would be interesting for one of the current GT40 owners to do a simple test - mount a taught, thin wire - vertically and set back from the rear face about 6 inches, set just off to the side from the exhaust outlets. Tie some very light strings at 3 inch intervals up the wire, and go for a drive with someone in another car alongside so they can take pictures. It would be really interesting to see what the air is doing back there on a 40.

On that note, does anyone here who is designing their own GT40 have a good 3D CAD model (solid model or maybe even surfaces - but not a simple AutoCAD line drawing) of the GT40 body? I could run some CFD analyses on the base body, then try front splitters, rear spoilers, etc... Anyone interested?

Ian Anderson · 08-19-05, 12:59 PM

Dave

Are you talking about the stainless louvered covers either side of the exhaust or the 3 holes at number plate height (or where a plate would be on a road car)?

I must say I had not noticed the openings at the sides of the rear but would think that it could help remove heat / pressure from the rear corners of the engine bay.

Ian

BenL · 08-19-05, 01:07 PM

[ QUOTE ]
Having a look around at original GT's I have seen some vents on the rear that I have not noticed before. Can anyone point out what they are for? Releiving pressure from the rear clam?

[/ QUOTE ]
While your question is a good one, that picture is a bad example to use for "original" vents. When the car was new the only vents on the back were the aluminum ones on either side of the exhaust. The three holes along the top were probably added when the car was restored.

If you want to see what the tail of 1036 looked like when it was new, take a look at John Allen's "GT40: The Legend Lives On", page 80 (it's the only black and white picture in the book). If you look closely at the picture of the original car, you'll see that the louvers get wider as the aluminum panel gets wider, something that they didn't bother to do when they restored the car.

Lynn · 08-19-05, 02:06 PM

Dave, Chris, et al.

What is missing in the picture in Dave's post that nearly all cars with these high wheel well vents had is the adjustable spoiler that goes over them. You will see this adjustable spoiler on a lot of the Mk IIs as well. The spoilers have slots in the ends that match those in the fibreglass so that as it is raised or lowered the vents remain unimpeeded.

I don'k this know for an absolute fact, but I believe that this is one of the many tips, tricks and parts Ford passed on to JW for his campaign of MK Is in '68 and '69. As originally configured the early MK Is got real squirrly at speeds well under 200mph. The one other thing that was changed was the ride height of the car, which was not symmetrical front to back. The changes were primarily developed by Ford in wind tunnel testing. If you've seen the interview of JW, he talks about all the goodies Ford gave him when they pulled the plug on MK II and IV cars after the FIA effectively outlawed the big blocks. Those "goodies" included a lot of setup information in addition to hard parts for the cars.

Regards,
Lynn

David York · 08-20-05, 05:25 AM

I found this link http://www.racingicons.com/gt/1076.htm that shows the spoiler Lynn mentions on 1076. I also shows a detail shot of the corner vent I was talking about. THe text says it was a added to relieve pressure from the Wheel Well.

Has anyone replicated this spoiler for their Kit Car? It looks fairly simple in design but it would be great to know what the material used was. How high it could be adjusted into the airstream etc.

Since we are discussing aerodynamics who can give me details on the front spoilers used. I have seen everything from winglets just forward of the wheel well to a full chin spoiler. WHich was more effective which one is better?

Finally on the subject is it worth while to extend the floor pan to enclose the engine compartment? Will this help to cut the lift experienced by the MKI cars?

Thanks for the help.

Lynn · 08-20-05, 02:43 PM

Dave,

Good question on what the adjustable spoiler was fabricated from. My gut is telling me aluminum, but that is more of a guess than anything.

The only chin splitter that I have ever noticed is the one that Mark La Vea makes and I will let him describe its effectiveness. As to the dive planes, I have been told that they really did very little to help keep the nose planted at speed. But, I have seen no documented data on down force with and without them.

Regards,
Lynn

Chris Martino · 08-20-05, 10:06 PM

The front canards and wheel arch spats appear to be used on some of the Gulf cars for the most part. I have no idea if they proved to be effective but the Mirage cars were fitted so who knows? The rear spoiler looks to be left off 1036, maybe stainless steel?

Chris

Canuk40 · 08-23-05, 02:08 PM

Hi Guys,

The GT40s went through almost as many variations as the Cobras, so goes the discussion of what's correct/authentic etc. but I'm going to leave that can of worms closed

The vents above the tailights were thought to improve rear brake cooling by reducing the air pressure in the rear wheel arches, therefore more air would feed through the ducts suppying air the the rear discs.

Im not sure if that was ever proven but they sure look cool.

The lisence plate recess was openned on race cars under the same logic, although to simply increase airflow and heat escaping the engine compartment.

The chin fins and adjustable rear spoiler were used on short tracks to increase downforce (decrease lift?) as the cars did get light at high speeds.

The new front nose introduced in 1966 and used on virtually all GT40 replicas solved most of the high speed lift problems, so much so that I can say at speeds well above 175mph these cars drive straight as an arrow.

Interesting how much technology was hoarded by different teams fielding cars for the same company with the same objective in mind...

David York · 09-04-05, 04:21 AM

Cheers Ian,

This is exactly the type of info I was looking for. I am sure that with the knowledge of this list we can put together a top ten of modifications that improve the cars on track performance.

I am certainly not worried about authenticity it is after all a space frame car I am building.

Guys before the poison darts come out, this is not a dig at those that build detail perfect replicas, the time and passion put in to these is considerable.

I chose to build the RF40 as I remember the impact of the gt40 and seeing them as a kid. They stir my soul. I chose the RF as IMPO it was engineered as a improvement to the breed with true racing potential. My goal is to build a good running car that is as safe, competitive and that takes into account all the development and ideas that worked to improve the car.

Obviously the GT is limited by its original design but simple to implement modifications like venting the rear wheel wells and adding a ducktail spoiler are cool to me.

So how about it, What are the top ten performance enhancements that can be made to improve this car.

Howard Jones · 09-04-05, 05:05 AM

mine, I have no idea how well it works. It does make a very good license plate mount.

Canuk40 · 09-06-05, 01:14 AM

Way cool Howard!

Cheers

Ian

Kalun D · 09-08-05, 01:35 PM

The GT40 is a very aerodynamically complex car. On a lot of the replicas the engine air intake area is not sealed off with a cold air box. This means the intake air coming in at the upper vents (behind the door) pressurizes the rear clip. Also in the front many replicas don't seal the air path where it goes through the radiator. All air coming in the front rad opening needs to go out the nostrils (spare tire cover) This can lead to the front clip being pressurized and contributing to lift at speed.

Canuk40 · 09-08-05, 07:49 PM

Kalun, Interesting concept. I'd like to instrument one of our cars to find out what goes on regarding air pressure in the nose and under the rear clip. Areo is very unpredictable but ultimately makes sense when the data is analized.

On the CAVs the radiator is significantly wider than the original so it takes all the air coming in the nose. Additionally the hood vents, both daul nostril and single outlet are full depth and go right down to the bottom of the rad. That probably contributes to the excellent high speed maners of the CAV cars. I know Roy Sales of VIP Classics was clocked on radar at over 175mph.

All the body opennings have a maximum flow rate, once that's been attained the air just flows right on by, also you're at maximum drag from the ducts.

I suspect when you're really going, the engine cannot take all the air available from the rear window plenum (checkout vents in rear window of the Mirage) so air would only come in as fast as the engine draws in the case of a close fitting cold air box, surplus would spill off under the clip. The rear vents beside the tailpipes are several times larger than the total air entering the roof and side vents, take away the engines' cfm and let the air out the back into a low pressure area and I wonder if there would be any pressurizing of the engine compartment. hmmmmm...

Ron Earp · 09-08-05, 08:16 PM

Kalun,

I might be mistaken but I think I saw in the GTD Club mags from a good while ago a study of air through the radiator at speed and nostril size (including Le Mans panel). Standard size nostrils/panel was not enough to get the air out and created a positive pressure on the clip. Solution was to make a bigger opening, which helped, and even better was to allow some of the air to escape from the wheel arch area on the space frames which can be open. Using these techniques the positive pressure was eliminated, but as I recall if the front clip area was sealed and standard molded parts were used there was a good deal of positive pressure. I can't remember if this was a GTD Mag article or a study someone emailed me but it was well done, maybe a GTD person will chime in.

Ron

Tim Kay · 09-09-05, 01:03 AM

In my Tornado, I have experienced my single nostril cover lifting at speeds over 90 mph [img]/ubbthreads/images/graemlins/shocked.gif[/img] It was not an issue until I paneled the space frame in the wheel arch area (as mentioned by Ron) in addition, my nostril does not taper all the way down to the base of the radiator. I may just louver the panels to relieve some pressure and fabricate my nostril to the base of the radiator.

David York · 10-10-05, 05:43 AM

Hi Calun and Ian,

Help me as I do not understand fluid dynamics in detail. [img]/ubbthreads/images/graemlins/confused.gif[/img]

If I seal the air plenum with a cold air box. In effect sealing the engine bay so no air spills from around the throttle bodies. Then do I have to vent the area above the cold air box? Or will it presurise to the point that external air will just not enter the side intakes behind the door? Thus providing a presurised cold air intake. [img]/ubbthreads/images/graemlins/grin.gif[/img] Or will it eventually blow the rear window out or create so much drag that it is not worth it? [img]/ubbthreads/images/graemlins/mad.gif[/img] Will sealing it off effect the overal cooling around the engine and headers or are the side scoops enough to handle this need? [img]/ubbthreads/images/graemlins/confused.gif[/img]

As usual thanks in advance of the replies.