Bonnet lifting

[AlanDITD]

Just been chatting to a mate about how to keep charge temps down on his corsa as its cutting boost when the inlet temps run to high. We are going to lift the bonnet at the rear to increas airflow in theory. DOES THIS ACTUALLY WORK?

Im thinking trying a simlaer thing on the legnum (if i end up keeping it) we all know inlet temps are very high and the cars suffer massivly with heat soak.

How much benefit could you see with it, i have seen it done for "coolness" personally im not interested in the look more the functionality.

Might give it a whirl anywaywith a few washers to space it out just see if anyones already tried it.

Cheers

[pitslayer]

Stand offs, I am not sure on front engined cars, but rear engined yes it does work. Wouldnt see why it wouldnt work on a front engine vehicle, as the heat should find its way out with air going in, and the rear opened up.

Might even work like the rear of a car, air travelling over might hit the void there and swirl around, and head into the engine bay

Not sure how you would work a stand off onto a Galant bonnet, as you usually need new hinges

[AlanDITD]

Metal braket mate, as a spacer where the hinge bolts to the bonnet i would think. or a block of metal.

Havent looked at the layout of the bonnet tbh im sure what ever way its done it can be done, just wondering if its worth it. Will be fully reversable anyway.

do you need to worry about water getting in where the gaps are?

[WildCards]

LI'd be inclined to leave the bonnet as it is and get an enclosed cold air feed to the air filter if you're worried about heat soak. Perhaps get an additional oil cooler aswell. I very much doubt you'd get enough air into the engine bay to have much of an effect on the engine running temperature and as you say, you may have a potential issue with rain when the car is standing.

[pitslayer]

With the VWs (yes I know they arent the same, but the principle being used here is) You have decklid standoffs, which are brackets attaching to the hinges, but they use a massively different hinge system to most cars but they use two types.

What I would guess the easiest way to do it would be a spreader plate of sorts attached to the bonnet side of things and some extra long bolts, you would have to re-allign the bonnet aswell, as you have taken it off to fit them

Also, I would really consider how much electrics are in there, and how much water you want on them

ETA: Also the only real reason for them is to fit bigger things under the bonnet, with the Beetles you would do it so you could fit a set of 40 or 45 carbs on them with filters

[Turbo_Steve]

I'd look at trying to get some air to that rear turbo if you want to get the temperatures down. It's a big hotspot of static air.

[AlanDITD]

Yeah i gree steve, i was however wondering, i need to look mind it there would be any way of running some form of duct from the brake vents in the bumper into the engine bay not sure if theres room at all im just bored at work and passing the time,

But if i can force cold air into the bottom of the engine it should force the hot air to rise quicker and out the back of the bonnet where its lifted a touch. Possibly under the car and up passed the turbo. I dunno.

Even with a coldair feed the overall engine temps heat everything up especially when your running increased boost. I have seen huge powered turbo cars where the inlet pipe is ice cold to touch after a good ragging. The legnum one practically melts my skin off.

Maybe im barking up the wrong tree completely, i have never been happy with how hot the car runs really so would like to try somthing to help a bit.

[AlanDITD]

LI'd be inclined to leave the bonnet as it is and get an enclosed cold air feed to the air filter if you're worried about heat soak. Perhaps get an additional oil cooler aswell. I very much doubt you'd get enough air into the engine bay to have much of an effect on the engine running temperature and as you say, you may have a potential issue with rain when the car is standing.

Im not trying to get air in there by lifting the bonnet, more to let the hot air escape.

[I-S]

I think it would not help.

At speed, the base of the windscreen is a high-pressure area, from air flowing over the bonnet and also air above that that now hits the windscreen. As such, I'd imagine that it's one of the highest pressure areas when on the move.

Under the bonnet is a low-pressure area - you have the restrictions of the grille and the radiator preventing massive amounts of air from entering.

By opening a gap between high and low pressure areas, air will flow from high to low. Thus you will now have higher pressure air flowing into the back of the engine bay, looking for the next lowest pressure place - that is underneath the car. So the air will flow down the back of the engine bay and out past the exhaust to underneath the car. Overall, the pressure in the engine bay will increase somewhat.

That means that there is now less differential in pressure between the two sides of the radiator - and thus less air will flow through the radiator.

If you really want to improve the cooling, try fitting a deeper front spoiler - as low to the ground as you practically can. This will result in less air flowing under the car, lowering the pressure beneath the car with three benefits: Reduction in drag coefficient (as the bottom of a galant/legnum is not smooth) for a slight increase in frontal area, Reduced high-speed lift (lower pressure under car prevents lift) and the reduced under-car pressure will pull more air out of the engine bay (as that is the primary low-pressure exit for air in the engine bay).

Now, someone is probably going to mention Evo bonnet vents. The important thing to appreciate about how they work is that they are not just a simple hole in the bonnet. The shaping around the vent, which is recessed behind a lump, results in a low pressure area being formed.

[AlanDITD]

Ok cool, well have you seen how low my car is ;) were talking 2" max off the floor so there is barely any area atall under the engine.

I do see what you mean about pressure areas, but i have seen quiet a few high powered cars doing this, speccys tigra being one of them and thats running 11.2 1/4 miles road legal FWD.

So im not saying the science is wrong, but it does seem to work on some cars. Does the air not just flow in the direction the car is moving ie out and away from the car?

secondly, what about when the car isnt moving, when it suffers the most heatsoak. i.e your queuing for the 1/4 mile. waiting in traffic etc will the heat just rise up and out then?

[I-S]

When static then yes, bonnet lifting may help matters because said pressures don't come into play (hence radiator fans - to artificially create pressure to move air through the rad). As the hot air wants to rise, it will come out at the lifted edge.

Air will flow in the direction that is easiest to flow - high pressure to low pressure.

When you're thinking about air flow you're thinking about air moving over a static car - obviously not the case. The air is the static thing that the car is trying to get through, and the air is just being pushed out of the way. Your thought about air flowing one way is ascribing significant momentum to the air - however, since it has little mass it has little momentum.

Think about it from the point of view of the air - say a molecule that is a little above the level of the headlamps. First of all, it gets pushed a little in the direction of movement of the car because there's more air behind it being pushed. Then when the car gets to that point the air is pushed upwards by the slope of the bonnet, and the car continues moving. As the car passes, the air molecule gets pushed further upward by the bonnet, then the windscreen until it gets up to the level of the roof. The roof then passes underneath and suddenly the car ends, and the air molecule can fall again. So, overall the air molecule was pushed aside (upwards) and some way in the direction of travel of the car.

If it had a reason to (ie low pressure exit from high-pressure area) it would instead have gone down the firewall and under the car (and probably in the process have got taken rather further in the direction of travel of the car than ones that went over the roof).

[Turbo_Steve]

However -

I was thinking in terms of a "sunken" vent in the bonnet:
A Facelift bulge all the way up the bonnet, which suddenly stops just above the rear of the engine, resulting in a rigid edge. Behind this is a curved surface which eventually matches the normal bonnet line at the base of the windscreen, but that curves downwards into the engine bay. Airflow across this vent will result in -ve pressure. The question would be: how much. As Isaac points out, this is a high pressure area, and as such even using this method, there may simply be very limited -ve pressure if the air isn't moving fast enough.

[MarkSanne]

I suggest to read all parts of this interesting article:
http://autospeed.com/A_2159/cms/article.html

[Davezj]

this has been colered before and i some have alread tried this. not sure whwere the thread went so i will go look for it.

here is one thread but not the one i was thinking of.
http://www.clubvr4.com/forum/showthread.php?t=30620&highlight=bonnet+lift

goes away to check again.
http://www.clubvr4.com/forum/showthread.php?t=13041&page=2&highlight=heat+rear+turbo

still not the one i was look for but that is the power of search!!!

[AlanDITD]

Nice cheers guys god enough, a mod for the week then i would say.

Was speaking to a few folk tonight that redcon this +ve pressure things isnt right and that theres actually a vacumm created there drawing air out from the engine.

Any thoughts?

[Confused]

The cabin air feed is taking from the area at the back end of the bonnet/bottom of windscreen, and that can force air into the cabin - therefore = high pressure.

[bradc]

What Isaac said above. It may work quite well at low speeds however.

[bradc]

I'd also like to point out that so far you've mentioned Corsa's and Tigra's. If they actually knew anything at all, they'd be driving better cars.

[AlanDITD]

I'd also like to point out that so far you've mentioned Corsa's and Tigra's. If they actually knew anything at all, they'd be driving better cars.

tbh mate if you could make a FWD do a 1/4 mile in 11.2s thats road legal id be amazed? There cars they serve a purpose nothing more.

plus bitching aside, the theory is the same, you put a GT35R turbo in there it will build a lot of heat that needs to escape.

[Turbo_Steve]

Brad, funny, but Alan has a point: these guys are up there with the masters on the tuning front, so whilst it's funny, it doesn't hold much water: past a certain point, it becomes a chassis and nothing more.

I would imagine they have a good laugh at the heavy, lardy 4wd estates that can't get past the 12s.

[bradc]

Yeah I was just being a knob :)

Remember that on the new EVO X and on all wrx's, the hole at the back of the bonnet is designed to be an intake. I'm willing to bet that Mitsi and Subaru know more than some guys with a Corsa.

[AlanDITD]

Brad, funny, but Alan has a point: these guys are up there with the masters on the tuning front, so whilst it's funny, it doesn't hold much water: past a certain point, it becomes a chassis and nothing more.

I would imagine they have a good laugh at the heavy, lardy 4wd estates that can't get past the 12s.

I wouldnt say masters of tunning, but we have alot of people willing to try differnt things. Everything on a freinds corsa was built on the driveway not a garage in site. Other than the throttle bodies and the mapping.

Although the legnum is very well respected given its size... most people love it.

[ritch_w]

This is an old skool mod, popleeur on MK2 escorts etc back in the day and Lancer EX 2000s too.

dont know about thermodynamics of it all tho.

gratitous lancer pic
http://www.zhaust.com/forum/index.php?t=getfile&id=42806&private=0
(withour raised bonnet!)

[Turbo_Steve]

The thing about bonnet lift is that it doesn't JUST make space at the back of the bonnet. It also lifts the sides: There is air flowing over a flat surface with a sudden lip: this generates -ve pressure at the rear sides of the bonnet.

[stuey]

There was a whole thread about 6 months back about raised section of the rear bonnet.
If I remember it was more to do with trying to find a cooler high pressure intake point, and options were redesigning a facelift style bonnet with the larger bulge that carried up to the windscreen, but with a gap to take advantage of the high pressure air at the base of the windscreen.
A bit like the muscle car style 'cowl induction' style bonnets..

[Davezj]

stuey how about a link!

[HPRULZ]

just out of interest what effect does the full engine undertray have on flow and are cars with one hotter or cooler that ones without? I know cold spec cars have them fitted because I have one on mine but do they have any influence on the engine bay temp because they must affect the airflow to quite a large extent. anyone?

Kev

[scott.mohekey]

I seem to remember some of the guys on ozvr4 removing the rubber seal from the rear end of the bonnet.. meaning there is no holes on the sides and air is free to be pushed into the engine bay from the rear of the bonnet. I imagine it would help in forcing more air flow past the rear turbo?

[Davezj]

That is what i have done.

but you should add some kind of block to support the rear of the bonnet as it vibrates quite a bit a speed due to the fact you have just removed all the support from there.

only draw back it at low speed the in car heater is sucking in the hot air which is coming out of the bonnet due to the heater inlet vent being right at the base of the windscreen. you also get fuel and hot engine smell form under the bonet as well.

[shaun1978]

its probably been covered many times before but what about stopping the heat in the first place and thermal wrapping the manifolds etc?

[Davezj]

yep covered before,

i offically say this kettle of fish is "OPEN"

enjoy!

[taupodrifta]

i have lifted mine as when i got the car it was running hot under the bonnet and since lifting it has made a huge difference

[Gowf]

Mine although now carbon, is raised, as it was when it was steel. The rubber seal was also removed. The only vibration that i found was the scuttle, but that was nothing that ducktape couldnt solve.

Now..... I will say this, yes it does work to lower temps, as i had my AIT sensor sitting around a mushroom filter(which i used to have) (couldnt be arsed putting it in the charge air, but thats another story anyway), and if i remember correctly it managed to lower underbonnet temps by around 8 degrees i think.... please remember it was a while ago.

Now what has been said is true, there is a high pressure area at the base of the windscreen, as shown here:

uploaded/3977/1250034566.jpg

now by raising the bonnet you will actually create a low pressure region below the streamline, which will aid to remove unwanted heat.

This is further emphasised due to turbulence effects created by trailing vorticies which occur when the incident angle is changed dramatically, ie at the base of the windscreen:
uploaded/3977/1250034817.jpg

The effect of these phenomena combine to again aid an airflow from the stagnation point through to the top of the bonnet.

Although ground effect plays a big role with regards to undertray design, a similar design to the bonnet raise underneath will also allow extra engine cooling, but there are a lot more variables to consider there........ But to put in in very simplistic terms its better to actually have an undertray than not.

[bradc]

mmm XJ220!