Custom bonnet idea

[scott.mohekey]

I've been playing with an idea for a bonnet in solid works. The key parts to this idea are that there is a pair of reversed vents to remove hot air from behind the radiator, and a smallish front and center scoop to force cold air down the back of the engine, removing the hot air around the rear turbo.

I couldn't figure out how to save pictures from solid works without them being horribly pixellated jpgs, so I've attached them as pdfs.

Comments are more than welcome.

[VR4WGN]

nice but you need to sit it on a picture of a car man,to see if the suage lines dont clash,very nice

[scott.mohekey]

I'm planning on using the existing bonnet lines. The side lips will come up from these existing lines.

[OnBoost!]

Yeah looks good..:thumbsup:

[MarkSanne]

Looking good!!

[Davezj]

looks great,
a bit like the manaro/VXR8 when the redid the styling, i think. just checked, the limmited edition super charged monaro/VXR8 transition model was a scoops up bonnet and not down.
i suppose it is more like the F60 (enzo) and lotus elise but more subtle

but from a purely practicle point of view if you make level of the bonnet the same os the original and the scoops go down wards, you will be very close to the engine bits under there. and if you raise the front of the bonnet up so the scoops only go back down to the original level of the bonnet it will look a but odd.
but that is just one persons opinion.

[scott.mohekey]

but from a purely practicle point of view if you make level of the bonnet the same os the original and the scoops go down wards, you will be very close to the engine bits under there. and if you raise the front of the bonnet up so the scoops only go back down to the original level of the bonnet it will look a but odd.
but that is just one persons opinion.

I'm planning on raising the center bulge of the bonnet.

[Davezj]

that would do it,

just like mitsi did it when going from a PFL to FL that should look great. so is you bonnet going to be a UFL (Ultimate Face Lift).

[scott.mohekey]

that would do it,

just like mitsi did it when going from a PFL to FL that should look great. so is you bonnet going to be a UFL (Ultimate Face Lift).

Well, I want it to look really good AND be as effective as possible.

[VR4WGN]

and a TNUK to lay one up from its mold id say.. the tight corners create problems lol,but will minimise if you roll the m nicely

[scott.mohekey]

A TNUK ? I'm going to be making this from steel. No plans as yet to mold for fibreglass/carbon fibre.

[giblet]

Looks rather nice!

[VR4WGN]

wow really? thtd be a good challenge lol,ll the best lol im sure itll be spiff lol

[Kenneth]

Looks good Scott.

I have a question though. What are you trying to achieve over-all? I know that technically, you want to get some cooling on the rear turbo but...

Under what conditions do you want the rear turbo cooled, and why? (I am ignoring the front one, as front reverse vent bonnets are easy enough to get)

My understanding of turbos is that it is the heat that makes them work. The hotter the gases, below the point things start to melt, the better. (We are assuming an exhaust which isn't backing up too much pressure)

This being the case, you don't really want to cool the turbo (unless you have conditions where your exhaust temperatures really are getting too high), you want to stop the heat affecting parts in proximity which don't like the heat. I would have thought that wrapping everything would be more effective in that regard, cheaper too.

Also, you might want to think about what happens when you stop. Heat rises, so the ducting to the rear turbo really needs to either point upwards (like the front) or you will need some way (say a fan) to move the air from the back to the front.

Seeing as the windscreen end of the bonnet is a high pressure zone (when you are moving at speed) wouldn't it be more effective to just have open vents which are sealed around the part(s) you want to cool and have an outlet in the under-tray? This way, when moving, you get pressure on top and suction at the bottom which will move cold air down. Then, when stopped, natural convection will disperse heat as the rear vent will act like a chimney.


You might have guessed by now, but I have actually spent quite a bit of time thinking on this exact same issue.
If I was going to amount of effort, I think I would use vented guards and find some way to vent the back of the engine bay out to the fender vents (and perhaps use some fan assistance) and just use the front reverse vents as in your model.
This would be in conjunction with decent, sealed on 3 sides, under tray which acts like a rear vent for the engine bay, drawing hot air out and under the car.

[scott.mohekey]

Looks good Scott.

I have a question though. What are you trying to achieve over-all? I know that technically, you want to get some cooling on the rear turbo but...

Under what conditions do you want the rear turbo cooled, and why? (I am ignoring the front one, as front reverse vent bonnets are easy enough to get)

My understanding of turbos is that it is the heat that makes them work. The hotter the gases, below the point things start to melt, the better. (We are assuming an exhaust which isn't backing up too much pressure)

This being the case, you don't really want to cool the turbo (unless you have conditions where your exhaust temperatures really are getting too high), you want to stop the heat affecting parts in proximity which don't like the heat. I would have thought that wrapping everything would be more effective in that regard, cheaper too.

Also, you might want to think about what happens when you stop. Heat rises, so the ducting to the rear turbo really needs to either point upwards (like the front) or you will need some way (say a fan) to move the air from the back to the front.

Seeing as the windscreen end of the bonnet is a high pressure zone (when you are moving at speed) wouldn't it be more effective to just have open vents which are sealed around the part(s) you want to cool and have an outlet in the under-tray? This way, when moving, you get pressure on top and suction at the bottom which will move cold air down. Then, when stopped, natural convection will disperse heat as the rear vent will act like a chimney.


You might have guessed by now, but I have actually spent quite a bit of time thinking on this exact same issue.
If I was going to amount of effort, I think I would use vented guards and find some way to vent the back of the engine bay out to the fender vents (and perhaps use some fan assistance) and just use the front reverse vents as in your model.
This would be in conjunction with decent, sealed on 3 sides, under tray which acts like a rear vent for the engine bay, drawing hot air out and under the car.

I was thinking that I would put a blanket over the rear turbo, to try and reduce heat exchange between the turbo and the rest of the components behind the rear of the engine. The front scoop is mostly due to aesthetics. If its not going to be overly useful for cooling of the rear engine, then I could probably put the inlet filter behind it and route to the MAF somehow.

I don't really want to have open vents at the back of the bonnet. I don't particularly like them.

[KiwiTT]

isolid works. /thread hijack ... We use that at work ... We have problem with SW09 x64 on W7 x64 constantly crashing.

[scott.mohekey]

/thread hijack ... We use that at work ... We have problem with SW09 x64 on W7 x64 constantly crashing.

That's the same setup as I am using. I've had the odd crash, but not as often as you suggest.

[Turbo_Steve]

Ken - whilst you're completely correct, both Subaru, and mitsubishi on the Evo X have a bonnet scoop (or part of the bonnet scoop) directing a flow of cool air over the turbocharger.

I would you're underestimating just how much airflow you'd need to cool the turbo itself to a point where it started to affect EGTs - just think in terms of the amount of airflow the downpipes are exposed to, and the sort of temperature gradients they see: we're talking a HUGE volume of air to affect a temperature differnce of more than 10deg at 700deg...IIRC from my school days it's something in the region of 100,000CF for a 500g piece of steel.

The sort of airflow though a scoop is enough to offset some of the radiated / convected heat coming off the turbocharger, but I doubt it's going to impact EGTs in any significant way, especially as the EGTs from the rear bank are going to be hotter than the front anyway (which is, in itself, a non-ideal design).

[John TheAntique]

Before the Scooby and Evo's the mark 1 Starions had a vented bonnet that channelled air under the bonnett and out over the turbo (which was not otherwise cooled).

[scott.mohekey]

Ken - whilst you're completely correct, both Subaru, and mitsubishi on the Evo X have a bonnet scoop (or part of the bonnet scoop) directing a flow of cool air over the turbocharger.

I would you're underestimating just how much airflow you'd need to cool the turbo itself to a point where it started to affect EGTs - just think in terms of the amount of airflow the downpipes are exposed to, and the sort of temperature gradients they see: we're talking a HUGE volume of air to affect a temperature differnce of more than 10deg at 700deg...IIRC from my school days it's something in the region of 100,000CF for a 500g piece of steel.

The sort of airflow though a scoop is enough to offset some of the radiated / convected heat coming off the turbocharger, but I doubt it's going to impact EGTs in any significant way, especially as the EGTs from the rear bank are going to be hotter than the front anyway (which is, in itself, a non-ideal design).

I think Ken still has a point in regards to the lack of flow at stand still. Would putting a simple vent at the center rear of the bonnet for this cause problems with flow from the scoop when moving?

[Turbo_Steve]

I'd say it's impossible to know what will happen with vents without a wind tunnel, however the hygrometer is probably a good way of checking the pressure at a given location.

A vent forward of the rear of the bonnet that is forward facing should see positive pressure however, and an Evo X style bonnet on the VR4 (based on the FL) would be an awesome awesome proposition: the Evo X bonnet is actually fairly subtle, and I think a subtle interpretation of it would work very well on the VR4 : especially if you could work a stealthy air intake into the front edge.

[VR4Kaos]

personly like " caishanvr4 " bonnet thinnk its the evo 7 very nice with the lil side scoop aiming straight at the air intake

[scott.mohekey]

I'd say it's impossible to know what will happen with vents without a wind tunnel, however the hygrometer is probably a good way of checking the pressure at a given location.

A vent forward of the rear of the bonnet that is forward facing should see positive pressure however, and an Evo X style bonnet on the VR4 (based on the FL) would be an awesome awesome proposition: the Evo X bonnet is actually fairly subtle, and I think a subtle interpretation of it would work very well on the VR4 : especially if you could work a stealthy air intake into the front edge.

I was originally planning on putting a scoop at the center rear of the bonnet, but was worried it would just get all the hot air coming out the front vents.

[VR4WGN]

evo10 has it,iv got the bonnet here with me and it has a vent in the midle near mid/back

[scott.mohekey]

I've had a look on google images, and I don't really like the look of them. They're a bit too subtle!

What if I were to have a small vent at the center rear of the bonnet that is only open when the vehicle isn't moving?

[Turbo_Steve]

you COULD just put a grille there. It'll pull air through when it's moving, at least in theory.

And if the Evo X one is too subtle....copy it but unsubtle it a bit!! :D

[scott.mohekey]

I think what I've got is just about the right level of subtleness. /pan

I just need to sort out the vents, and yes, I was meaning a grille at the back, maybe at the trailing edge.

[mattnz]

personly like " caishanvr4 " bonnet thinnk its the evo 7 very nice with the lil side scoop aiming straight at the air intake

I don't know where you think the air intake is, but it's not there :P

[scott.mohekey]

My intake will be fed from a cold air box where the battery is (see my other thread about moving the battery to below the boot), with some ducting to the front bumper.

[Kenneth]

Over the turbo charger, or over the parts in close proximity?
Do you know for a fact that it isn't just to keep the air moving around that area so it doesn't soak into and destroy plastic parts (cables, connectors, wires etc) in the vicinity?
If so, I am honestly keen to know as it clarifies a point I have spent much time considering.


Ken - whilst you're completely correct, both Subaru, and mitsubishi on the Evo X have a bonnet scoop (or part of the bonnet scoop) directing a flow of cool air over the turbocharger.

[scott.mohekey]

As far as I know, on the Subaru I've had a good look at (2000 WRX), the scoop is ducted to only the intercooler. However there is a pair of grilles on either side of the bonnet, but they're towards the middle, not the rear.

[Turbo_Steve]

I believe it's something they've reduced and eventually removed as the car has evolved: the old legacies had a bonnet scoop which ONLY cooled the turbo (the intercooler was an air/water type): it had dedicated pieces of tin to funnel the air over this component only: the turbo even had a cowling which mated with the "collector" under the bonnet scoop.

The 96-98ish designs use the left-most (as you face the car) section of the scoop as a seperate collector. This travels through a mesh and points down onto the turbo.

In 1999ish the P1/STI v5 era, they relocated the turbo slightly: further back, and with different pipework (under the inlet manifold) and at this point they started fitting a piece of louvered tin under the left-most scoop, which makes the incoming airflow a lot more vague.

I beleive they stuck with this design until the Bugeye was realeased (circa 2001).

I don't know for a fact why they uses this system, but I think the fact that they dropped it when they revised the turbo orientation and pipework is probably significant: as you say, I suspect it's to reduce heatsoak inside the engine bay.

Interestingly, Cylinder 3 (which is slap bang next to the turbo IIRC) tends to be the most det-prone on the subarus. The later models seem worse for this than the WRX engine I had in the single turbo'd legacy, with it's nice turbo-cooling cowl. I had a factory fuel rail and headers, as I did when mapping later models. Co-incidence?

[scott.mohekey]

I'm going to do a revised model tomorrow, taking everything here on board, and see what we come up with.

[tw5jim]

dont know if this is any good to yer but plymouth/chrysler made an air grabber scoop on their roadrunner models check on wikipedia

[scott.mohekey]

Interesting.