1G hood modification (Support frame removal)

[daarkness92]

I have a 1gb and i want to lose some weight off the hood and i don't have money to buy a cool carbon fiber one so i thought i would just cut off most of the inner bracing and i want to know if anyone has done this before and what were the results? also im sure people have done it before but if not i will post pictures and what not if it works out!

 

[98mitsuGSX-SD]

Just removing the bracing from the interior of the hood would have no gains at all. It takes a considerable amount of weight reduction to result in a change in your 1/4 time. Plus if someone ever decided to put any weight on your hood it would collapse.

 

[90gsx16g]

Just removing the bracing from the interior of the hood would have no gains at all. It takes a considerable amount of weight reduction to result in a change in your 1/4 time. Plus if someone ever decided to put any weight on your hood it would collapse.

Hes absolutely right, i just finished reading a thread about weight reduction. Someone posted ( they are not entirely sure if its true) that 100 lbs of weight takin off equals .1 second decrease in quarter mile time. so from 12.5-12.4( with 100 lbs takin off) thats theroretical speculation though. and its not worth it to cut the braces out ### if some fat a** kid sits on your hood it will collapse

 

[bpestilence]

If you don't want to save up for a CF hood, get a fiberglass one. Not as strong, but they are still light, and are actually easier to paint to match the car.

Or learn how to do composites.

 

[99gst_racer]

I was curious to see how much weight could be removed from the stock hood. I cut that bracing out last month to see if it would make a difference. It's wasn't much. The bracing only weighed about 4-5 lbs.

Most of the hood's weight is in the bracing along the perimeter of the underside - that bracing is along the hood's main support, so it's not something that you could safely remove. removing the bracing in the middle didn't really compromise the hood structural integrity, but you definitely can't use it an a table anymore.


FYI: My hood is for an "off-road only" 1G. I don't drive it on public roads. Also, 5 lbs may not sound like much, but if you can pull 5 lbs from ten different places, then you're making progress. It all adds up.

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[daarkness92]

cool thanks for the info guys!

 

[EDM95]

Now that you got the hood off you should do that hood vent's mod.
Get some of that cool air in there.

 

[99gst_racer]

Now that you got the hood off you should do that hood vent's mod.
Get some of that cool air in there.

I did plan to vent the hood. But not to get cool air in - Because of the pressure differential, my goal will be to get hot air out.

 

[EDM95]

I did plan to vent the cool. But not to get cool air in - Because of the pressure differential, my goal will be to get hot air out.

What if you cut out some vents add some fiberglass hood scoops faceing with the open end towards the window?
Maybe you can modiefy the scoops with some kind of small fans blowing out the hot air.

 

[Locke]

What if you cut out some vents add some fiberglass hood scoops faceing with the open end towards the window?
Maybe you can modiefy the scoops with some kind of small fans blowing out the hot air.

Won't need fans, or anything extravagant. Just a hole, with perhaps a small lip on the leading edge or something to direct the air over the opening, rather than into the opening. The draft will pull the air out.

 

[mitch26753]

I had to remove the bracing on my hood due to the tial wastegate bouncing off the braces when shifting

 

[gixrman]

LOCKE is right you need something to create a pressure difference or a vortex to draw out the hot air.

 

[Talesin]

The problem is that the entire hood is a high pressure area, with the trailing edge (toward the windshield) being highest-pressure. The reason that people who shim the back edge of their hood up see lower underhood temps is not because air is being pulled out, it's because high-pressure, cold ambient air is doing a quick 180 and being pulled down inside. The pressure in the engine bay will nearly ALWAYS be lower than the pressure going over the hood, especially with the open path to the vacuum system underneath the car while at speed.
Vents on top of the hood will not pull air out. They'll suck air in, even 'pointed' backward... just the ones pointing back will create a lot more inertial drag from the high-pressure air's kinetic mass being forced to come up to speed. If you want to see this in action, shim up the back edge of your hood and tuft the car. You'll see that any tufts on the back edge will not follow the windshield up, but instead be pulled down toward the engine bay.

 

[architechnik]

I've put a large vent toward the front and a small vent toward the back of my hood, all the way across between the two center braces, just in front of the rear brace. The amount of air that flows out of that little slit is amazing, giving testament to the amount of under-hood pressure.

The strange thing is that this has actually caused a heat channel that I need to solve before next summer. The hood gets REALLY hot right in front of the vent as all the air rushes from the front to this hole. It is almost a direct 'V' shape across from each corner of the vent.

 

[FonZ454]

My hood is very similar to 99gst_racer's, but with more material out on the sides. I don't know it's actual wieght, but when lifting it open, it is for sure a lot lighter than a stock 1g's hood. The thing is, it's very flimsy. While open you can twist it quite a bit... but it still has plenty of rigity to not fold and be permantly damaged. It also has hood pins which I think you should definately run if you do any hood modification... just to be safe. Also , at idle it vibrates a lot and if quite a bi*** to adjust everything to stop rattles.

FYI I bought the car like this, and If I repaint it I may consider getting a fiberglass hood... This gets the job done, but it does loose a good deal of it's structural integrity.

 

[Talesin]

I've put a large vent toward the front and a small vent toward the back of my hood, all the way across between the two center braces, just in front of the rear brace. The amount of air that flows out of that little slit is amazing, giving testament to the amount of under-hood pressure.

The strange thing is that this has actually caused a heat channel that I need to solve before next summer. The hood gets REALLY hot right in front of the vent as all the air rushes from the front to this hole. It is almost a direct 'V' shape across from each corner of the vent.

I'm just curious how you tested how much air is coming out of the rear vent? At idle, the pressure system that I'd been talking about is non-existant, as the car isn't in motion. At that point, standard convection (hot air rises, cool air sinks) takes over, and obviously the hood-vents will allow hot air to escape as cool air is pulled up from under the car into the engine bay as the hot air is released. But I have to stress that this is happening while the car is stationary, or at best low-speed, unless the front vent is considerably larger than the rear.. enough to significantly disrupt the dynamic high pressure system I'd described before. Even with a front vent, the air pressure under the hood is going to be at a lower pressure than the air moving up the hood and over the top of the car.

The engine bay air pressure is fed by two possible sources in the stock configuration; air coming through the center aperture for the radiator, and air passing underneath the front bumper. Air passing underneath the front bumper tends to follow Newtonian law... since there's very little to pull it up into the engine bay (along the lines of a scoop or other upward deflector, which people have made to get more air to the radiator after a particularly large FMIC install) and the path under the car is, for the most part, unimpeded, it can largely be written off. The air coming through the radiator has the radiator itself as a fairly major restriction, potentially along with a FMIC for those so equipped. Also, our central ducts (especially on 2Gbs) are not particularly large... even the 1Ga GSX 'widemouth' bumper only allows the central section to pass into the engine bay. Perhaps three or four feet wide (at best), by seven inches tall. Compared to the full width of the car (at least five to six feet), and the entire slope of just the front hood, COMPLETELY unimpeded... the air system above the hood WILL be higher pressure than that below the hood. Any vents or holes will allow high pressure air to move into the low pressure area, not the other way around.

With a front and a rear vent, a slipstream could be created, essentially allowing the air passing over the hood a 'secondary' path, at the same time picking up heat as it passes through the engine bay. The only down side to this is that the air exiting the rear vent (unless the vent is specifically made) will cause extra turbulence in the formerly-unified airflow passing over the car.. creating additional drag, and worse fuel economy. It will also raise the underhood air pressure... at particularly high speeds (triple digits) this could interfere with the natural vacuum that forms under the car, and make the handling get twitchy if the vehicle's design relied on aerodynamic effect to act as a stabilization effect (a spoiler is an add-on aero stabilizer, as an example).

 

[mavisky]

You may want to read this decently put together article before expounding anymore of your aerodynamic knowledge.

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The Alnor Velometer Jnr | AutoSpeed Blog

Here's another good example. The Lou Gigliotti Motorsports team ran for years without a hood vent, then did some wind tunnel testing with GM and came back with these hood vents to help lower temps during their races.

new vents

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That's why I chose my VIS Invader

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[Talesin]

Unfortunately, from reading those first two articles that you had listed, Mavisky, they were not using their manometer to measure pressure drop *in front of* the intercooler/radiator as compared to inside the engine bay, but rather either comparing the pressure inside the engine bay to that inside the cabin (ram-air vs. a static pressure system), or the pressure immediately behind the intercooler to a point further back inside the engine bay, which will NOT tell you how much air is transitioning through the system, simply the rough 'shape' of the underhood system itself. The pressure in front of the bumper is SIGNIFICANTLY higher than ambient pressure, even at standard freeway cruising speeds.

For building an under-tray, they did the testing properly (and were rewarded with a smoother transition to the under-car vacuum system and more powerful engine bay air exhaust in that direction), but they were mislead by their manometer readings insofar as intercooler flow, as they were measuring the wrong thing.
You start out by blocking off the intercooler intake area and testing the pressure immediately in front of the intercooler blockoff versus static (in-cabin) pressure to get a rough baseline of the actual pressure that the intercooler is under at a given moment. Then, you unblock the IC intake area and compare the pressure in front of versus the pressure behind the intercooler to get a rough ambient flow efficiency for the core. You can also measure the un-blocked intercooler intake-side pressure... to work with what those articles claim, the pressure will be the same, no matter if the intercooler intake is blocked or unblocked. In fact, if their claims that air will go through the intercooler 'backward' were true, the unblocked pressure would be HIGHER than the blocked pressure! Short version, they need to learn how to use a manometer properly, or at least think about what they're trying to measure.

Yes, the professionally wind-tunnel tested vents tend to work quite nicely thanks to the Bernoulli effect. Notice how their vents do not interrupt the airflow of the vehicle, while the back end is inset, and the vent is pointed backward (oddly enough, similar to how the stock Evo vents are oriented). This uses the unified airflow to draw air along. However, notice how all of them are placed toward the front of the hood (and few to none toward the trailing edge of the hood). Suffice it to say that the areas where the Bernoulli vents are placed during wind tunnel testing are the areas with greatest effect... high-pressure air moving very fast in a unified form.

The previous posters were talking about doing the 'hood vent mod', which doesn't generally involve the fabrication and design needed to properly integrate a Bernoulli-effect vent and actually have it work, as (potentially) with your purpose-designed CF hood. To test *your* vent, you would simply use a manometer to measure inside the engine bay versus immediately outside and below the vent, with the vent blocked and unblocked to measure the differential you're getting at-speed.

Once you start dealing with induced vacuum and speed of pressurized air and its effect on static air masses though, it gets more difficult to explain so that anyone wandering into the thread who doesn't have a workable understanding of aerodynamic physics can just look at it and understand. Omission for simplicity, rather than to mislead.

 

[mavisky]

In the third link they do test a vent they placed on the hood aft of the radiator I believe. I would agree that simply cutting a hole in your hood or raising the rear of the hood will not pull air out, but rather pull air in.

 

[Talesin]

Yes, they tested the hood vent at a dead stop.
I never said that when the car is stopped that standard convection would not allow hot air to exit that type of vents (after all, with no other factors of course hot air will rise, pulling cold air up from the ground under the car to fill the void), I was arguing that while driving at speed, they do not let hot air out, but rather allow higher-pressure cold air in, resulting in the lower underhood temperatures that those with that type of vent experience. Once you get the car moving at a respectable clip, the rules change and basic convection is overcome by dynamic pressure differentials.
The whole post was oriented toward 99gst_racer's statement that:

I did plan to vent the hood. But not to get cool air in - Because of the pressure differential, my goal will be to get hot air out.

I'd meant to point out that the pressure differential is positive *above* the hood, so without a properly purpose-designed Bernoulli effect vent, any hood vents will just allow cold air to flow in. That the pressure differential is working against him at this point.
Not to even get into what that high-pressure air from above the hood will do to disrupt the vacuum system that forms underneath the car at high speed, potentially greatly affecting the high-speed handling characteristics. After all, that's just a comparatively low pressure area, and the high pressure air flowing in from the hood vents will rush to fill it, crossing through the engine bay and carrying heat away under the car in the process, but potentially making the car's handling get 'twitchy' at freeway speeds and above.

 

[Icarus]

I'm a little hesitant to chime on what seems to be a point of contention, but a little clarification of the effects at play seems to be needed. The way Bernoulli's principle works is simple terms relative to car's hood:

When fast moving air travles over the slow moving air inside the engine bay a "vacuum" effect is created. Essentially the slow moving air tries catch up with the fast moving air. A great example of this that most are familiar with, is a bottom feed spraygun. As the fast moving air passes over the resevoir the difference in air speed "sucks" paint into nozzle. Carburetors work on the same principle.

There is a lot being left out here for the sake of simplicity. But as long as your hood vent is not redirecting air into the engine bay it will draw air out. The faster you go, greater the effect. There are numerous examples in high performance autos and aviation. I hope this clears things up without using more complicated terms and going into conservation of mass and energy

 

[123212321]

What if you cut out some vents add some fiberglass hood scoops faceing with the open end towards the window?
Maybe you can modiefy the scoops with some kind of small fans blowing out the hot air.

You don't understand. Hood vents work the same way that airplanes fly. It will suck air OUT of the engine bay.

 

[mavisky]

Just buy the carbontrix vent in that case. Much easier than making your own.

 

[mikh338]

I was curious to see how much weight could be removed from the stock hood. I cut that bracing out last month to see if it would make a difference. It's wasn't much. The bracing only weighed about 4-5 lbs.

Most of the hood's weight is in the bracing along the perimeter of the underside - that bracing is along the hood's main support, so it's not something that you could safely remove. removing the bracing in the middle didn't really compromise the hood structural integrity, but you definitely can't use it an a table anymore.


FYI: My hood is for an "off-road only" 1G. I don't drive it on public roads. Also, 5 lbs may not sound like much, but if you can pull 5 lbs from ten different places, then you're making progress. It all adds up.

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that is some good info i was wondering about removing the bracing as well, thanks for the great post with pics

 

[gixrman]

Just buy the carbontrix vent in that case. Much easier than making your own.

But were's the fun it that? Not being able to drive my car for over 3 mnths is what its all about!!