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Lifting the hood for speed runs and track use - a good or bad thing?
I have wondered whether it is advantageous to lift the rear of the hood (Bonnet) on the Cobra to relieve under bonnet temperatures and pressure and hence draw cooler air into the carb and reduce nose lifting at speed.
Older threads on here discussed it but never really came to any conclusion - I quiet like the look if not lifted more than 2" and have noticed a huge amount of hot air coming out of there at lowish speeds (under 50 mph) but wonder if anybody has done any before and after testing at high speed. Would think it would get the forward facing scoop working better as well as reduce the high pressure area in front of the windscreen and possibly improve air flow over it and also keep engine temp down and help reduce heat soak at end of laps helping you pull a lot more heat out of the whole system on the cool down lap. With all these potential benefits why don't more people do this very simple mod on track days? |
Don`t have a definitive answer , but at a track weekend over Memorial weekend at VIR , the subject came up bewteen a FFR driver and me . He had a FFR spec racer and I had my ERA . He said his front end got light at about 130 mph , as did mine . We talked about adding a front spoiler and I suggested he try raising the rear of his hood some and see what happened ( he could do that a lot easier than I could ) . As luck would have it , he forgot to fasten his hood pins and lost his hood on the first lap , but decided to make lemonade instead by testing out if no hood would make the front more stable at speed . He said he couldn`t see any difference , but then , he had no hood at all . Maybe he screwed up his air flow totally with no hood .
I`ve wondered the same as you if it would make a difference as it seems logical that it would . |
Years ago I experimented and made some small 'L' brackets to secure the back edge in a raised position using the latch mechanisms there.
I was looking for cooling improvement, not aero improvement. As Snake said, it was effective at highway speeds. Above 100MPH, I noticed a strong lift at the brackets at the rear and decided I didn't want to risk eating the hood. So after a while, I abandoned the brackets rather than drive slower. |
I have left my hood unlatched on the track to see if it would lift up. If it did, that would tell me there's pressure under the hood trying to get out. Then I would fab some some brackets to lift it up. It doesn't lift. It bounces around a bit, but doesn't lift. I imagine if I didn't have a windshield it would be a different story.
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We had a fellow lose his hood on the interstate last year, or the year before, doing no more than 65 or 70. He forgot to fasten it and it flew off.
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My normally sealed air pan was defeated by eating hot air rushing over the carbs with my hood jacked up. The whole exercise had zero gain for me unless I drove under 60MPH to keep it cool. |
I never lock my hood down....and it also does'nt lift..one time I braked really hard and scraped the nose of the car on the tar.....the hood lifted momentarillay then......on the track i always locked it..........
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I think there is enough flow through the side vents to prevent any "air pressure" from building under the hood. If the hood lifted with speed, I would speculate it could be the result of low pressure on top of the hood pulling up in combo with the scoop channeling air down through the engine out to the side vents. If your hood flew off because you forgot to lock it down, I expect you won't do that again! Seems like the side vents would let more air out than would flow through the radiator. I'm sure the front cowling will allow a lot more air into the radiator than the radiator will allow to flow through. That is one of the reasons the Cobra has such a high drag coefficient and referred to as an aerodynamic brick.
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I built a tribute Cobra rather than a replica. it has a space frame and is running an LS6 engine. The frame design forms a VEE, the point of which is behind the radiator. I sealed the engine compartment. There is an aluminum cradle on top of the frame that seals the frame to the body. The frame is 16 inches tall and is aluminum skinned. I put 2 side vents per side. They are stacked. The radiator stands straight up and is only 18 inches wide and 12 inches tall. I have run it on the track at high speed with out any perceivable lift. And it runs cool. I've never had a temperature problem.
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Here is a picture of the double side vents and engine compartment. I have since routed the air filter to the front of the cradle, installed a longer throttle cable, and raised the rear ride height slightly. Next are vintage looking wheels. Always something to do.
http://i1143.photobucket.com/albums/...psa03ee7e0.jpg http://i1143.photobucket.com/albums/...psc232d5cc.jpg |
This is from the best build thread I have ever read or seen. It is page 9 of Morris's build and on page 10 Tom Kirkham has pictures of how he helped Morris determine just what was the best ways to get air out of the engine compartment. You will be surprised at how little the side vents do. Scroll down to post # 180 and you can go from there as far as it interests you. Page 10 has the pictures of Tom driving his Cobra with the tuffs taped to it to get an idea of the air flow. Notice the ones around the side vents really aren't moving much.
KMP 259 Phase 2 Ron :) |
Thanks for the info and the link Ron. I went to it and read it, but, didn't see the pictures.
I have seen Morris's car and it's a masterpiece. My car has a Cobra body but everything else was custom built by me. I was trying to duct the air passing through the radiator out the side. The engine compartment is sealed and the air is ducted to the side. I don't run inner fenders either. The frame is a space frame skinned with aluminum. It has a flat bottom with full aluminum under tray. The Trans tunnel is a stressed "Backbone". I sit in the frame, not on top of it. The headers are routed up and over the foot boxes. They pass through the cut outs in the aluminum body cradle. (I think the radiator is 14 inches tall not 12. I can't remember exactly.) I will have to try the yarn test. But, I do know that the car runs cool and is very stable at speed. I have never felt like the front end was light. I had a blast building the car. With a Tremec 6 speed and C5 Corvette suspension it's a blast to drive too. I built it to be a daily driver. It gets over 30mpg and is still fast. It was the fastest car on street tires at 2008 Run n Gun. http://i1143.photobucket.com/albums/...ler34Front.jpg |
One thing I will say..........the hood on my car does not lift........there is a fairly good size gap all around the sides of the hood which I believe allows the excess air to escape...if there is any.......the front of the hood does have a rubber seal....but only the front...... When the fans kick on in traffic you feel the hot air
coming out the side vents.... |
Funny years ago when dailing in a carb had overlooked latching the hood till out on the freeway running in the triple digits noticed the hood latches were turned wrong but no action since then never have latched them.
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I thought it is better to keep the rain out? JD |
Do not seal the bonnet......except the front and one little piece of rubber at each rear corner......just to stop rattles and squeeks and to line up the hood.....sorry bonnet to the fender......
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Standard procedure not to seal the hood per ERA manual. I just installed mine - 5 short strips to support the hood but will allow heat to escape.
http://www.clubcobra.com/forums/pict...ictureid=10174 |
...but very little escapes.
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These posts are confirming what I had come to believe about Cobra replicas...different models are just that - different. On my Lone Star if I drive with the hood unlatched it blows up at 60 to 70 mph and goes higher the faster you go. On a two lane road meeting a truck, it will blow up at 40 to 50 mph if it is unlatched. My front end has never gotten light at speed; however, the rear starts to raise up above 160 mph.
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