How best to cut the bonnet?

[BudSlater]

After the last track day, I'd like to try reduce under bonnet temperatures a little if I can. From speaking to folks at the day, general consensus was that holes needed to be cut in the bonnet.

 

It's not a hard core racer so I'm not after circular holes by the bulk head, but as you can see from the picture below, there are 3 narrow bits which could be cut out and help. Worth doing and if so, how, dremel, jigsaw, angle grinder?

 

Just to be clear, I wouldn't be making 3 large rectangles, more just cutting the "bottom" of each of the visible U shapes you can see below

 

[pickmaster Andy Lowe]

A tip with a hole saw is to file or grind the teeth flush with the outer radius

They are normally angled inward then outward by filling the outward teeth flush you get a cleaner cut in fiberglass

And a block of wood clamped under helps keep the lead stable

[Rory's Dad]

Those shapes are there to stiffen the bonnet from excessive flexing.  I think I'd start by shrouding the radiator so more air goes through (and not round) and the engine runs cooler.  Obviously you can rig up some aluminium sheet but you can also add some spacers so the radiator is nearer the front of the nose.

[BudSlater]

Those shapes are there to stiffen the bonnet from excessive flexing.  I think I'd start by shrouding the radiator so more air goes through (and not round) and the engine runs cooler.  Obviously you can rig up some aluminium sheet but you can also add some spacers so the radiator is nearer the front of the nose.

 

It's already got a ducted nose in it, I think it might be time for a thicker alloy rad though, was going to do this in combination with cutting the bonnet. Maybe I should just get a v8 bonnet

[jim_l]

You need to clarify whether you want a cooler running engine or cooler under bonnet, they are different things.  I ducted my rad and the engine now runs cooler, but under bonnet, particularly exhaust side, is noticeably hotter.  

 

I am contemplating letting more air around the rad to cool that area.

 

            Jim

[BudSlater]

You need to clarify whether you want a cooler running engine or cooler under bonnet, they are different things.  I ducted my rad and the engine now runs cooler, but under bonnet, particularly exhaust side, is noticeably hotter.  

 

I am contemplating letting more air around the rad to cool that area.

 

            Jim

 

I will be running ceramic coated exhaust manifolds post-winter, so I'll focus on cooler bonnet. From what you've said, it sounds like gaps in the bonnet will not achieve this so I'll leave my pristine bonnet alone haha

[pickmaster Andy Lowe]

How about flaring the sides out as some do to allow air to escape?

I have my manifold wrapped and I think this keeps the heat down under the bonnet

[Dave Eastwood (Gadgetman) - Club Chairman]

Before cutting holes, you need to establish where they'll do any good. For instance, you look to have a screen in the photos, so conventional wisdom suggests that the slots nearest the screen at least, will be in a high pressure area at speed. Ie air will be trying to get in. (This is precisely why, for years, manufacturers put the intakes for heater systems there, and why in those older cars, you'd have good airflow through the heater at speed, even without the heater fan going.

 

It's possible to shape the rear edge of the bonnet in such a way that air gets drawn out. Though in a Westfield without an FW bonnet, this is tricky because of the bonnets return edge.

 

The much more straightforward way of drawing air out, as Andy said, is to cut and flair the engine bay sides.

[Rory's Dad]

You need to clarify whether you want a cooler running engine or cooler under bonnet, they are different things.  I ducted my rad and the engine now runs cooler, but under bonnet, particularly exhaust side, is noticeably hotter.  

 

How come??

[jim_l]

I am assuming that one of the consequences of ducting the rad has been to restrict the air that used to flow unhindered down the side of the rad, into the engine bay and out under the car.

 

I noticed that the plastic of some spade connectors had gone brittle, having previously been fine.

 

Forcing air through the rad and running your engine coolant 5 or 10 degrees cooler doesn't change the fact that your exhaust valves and primaries are probably between 500 and 750 degrees © and those areas of the under bonnet need their air flow.

 

Jim