Cost effective ways to reduce under bonnet temps

[davekmoore]

Thanks Col. The radiator has been flushed and refilled including a proper air bleed since these pics. The tube now always has a circular cross section. I reckon the system had a small amount of air in it previously, leaving room for the tube to go flat under a vacuum.

[speed]

Does the problem still exist after the flush/bleed?


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[davekmoore]

The tube now always has a circular cross section.

Coolant temps were always ok and are still ok.

Still got high under bonnet temps though and want to get some vents in the bonnet to reduce these. Anyone had any experience of having an expert cut louvres directly into the bonnet? What cost?

[Magpie]

davekmoore I'm going to try and do some diferential air pressure testing when I'm home next and will be happy to post the results. I just need to decide if I put the reference air pressure inside the cab or on the bonnet badge. I'll post in a few weeks.

Plan is to do the car front to back, but happy to just do the bonnet.

[sailaholic]

Dave. You never told us which air temp sensor your using or why it's an issue apart from idle problems.


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[davekmoore]

The sensor is the one nearest the blue oil filter in the first pic on page 2 of this thread. I'm told lower underbonnet temps will make it easier to get consistent idle and that lower intake temps = more power.

[gslender]

I'm told lower underbonnet temps will make it easier to get consistent idle and that lower intake temps = more power.

1) Not 100% correct. Any under bonnet temp that remains consistent will make it easier to get a consistent idle.

Temperature of the air itself isn't a limitation to a consistent idle - it is the changing air temp that is.

You'll never fix this if the tuner is thinking they can only produce a consistent idle if the air temps stay stable. In winter air temps will be close to zero and will be nothing like middle of summer when crawling along a freeway where outside temps will be closer to 40 deg higher! That in itself won't be consistent and how would it accommodate for that?

The solution is having the idle set in such a way that it can adapt to the air temps (either by the correct air sensor) and ECU that is capable of working and adapting air, fuel and spark to accommodate it. When you do this, it doesn't matter so much the under bonnet temps. This is how OEMs do it and this is fact because the MX5 already sucks air in from the hot side of the engine bay, and they run fine when in snow or in the desert in traffic. No idle problems at all. How do they do it????

2) lower intake temps = more power is valid, but obviously that's more a function of the intercooler than anything else. There is no way that under bonnet temp is making any significant difference to the air temp seen after the turbo has heated it to 150deg+ and then the intercooler cools it to 50 deg. There is nothing under the bonnet that will change that outcome. That's like saying cold nights make the fire harder to burn wood - sure it has a minor impact to the delay for wood to ignite, but largely the burning of new wood is a function of the temperature of the existing fire, not the prevailing initial temperature of the wood before burning. You would be better served installing a bigger intercooler and ducting more air to it than adding bonnet cooling vents.

Unsure who is feeding you this stuff, but honestly they're wrong and probably aren't thinking this through or lack the basic understanding of what they are doing.

Finding good tuners is hard because many of them are uneducated in engineering and/or lack the background basics in the field they are working in - after all, there is no regulated body or study needed to sell an ECU. Unlike being a medical Dr, you can setup "Bob's Race Engines" tomorrow and sell Adaptronic gear without any clue on what you are doing. That's why so many people travel across the states to get to a good tuner.

[sailaholic]

Agreed. Your chasing the pot of gold at the end of the rainbow.


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[Magpie]

I agree with the last 2 posts. Just installing vents without understanding the physics is chasing the pot of gold. Like indicator vents most people believe that they allow air in and cannot fathom that they in fact could be sucking air out and increasing drag. How can that happen, physics...

Why is it so...

Reducing under bonnet temps you need to undertsand Newton's Law of Cooling that is a convective heat transfer processes. It would not be possible to cool the engine bay down to the ambient temperature.

You need to know the:
Surface area m²
surface temperature (°C)
air temperature (°C)
convective heat transfer coefficient

The convective heat transfer coefficient is a product of air velocity.
at 70kmh (20 m/s) it is about 35 and around 25 kmh (7 m/s) it is 30.

Therefore the greater the velocity of the air the geater the heat transfer, to a limit, that is above about 20 m/s there is diminishing returns.

So what is the air velocity in the engine bay?

[davekmoore]

Thanks again for all the input.

What confuses me generally, and particularly based on the last few posts, is why then do so many cars have bonnet vents/louvres/whatever you want to call them? Are they just for looks?

[speed]

I'm curious about this too. Has it got to do with the type of Motorsport. Ie drifting vs circuit.
Drifters don't do the speed that circuit racers do.
Factory rally cars have had vents for years but not the entire bonnet like some drift cars. Is this a velocity thing or are the big vents just for toss factor?


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[rascal]

What confuses me generally, and particularly based on the last few posts, is why then do so many cars have bonnet vents/louvres/whatever you want to call them? Are they just for looks?

The bonnet louvres/vents true function is removing high pressure air from engine bay, which improves flow through the radiator (for more effective cooling) and improves aerodynamics. Removing some underbonnet heat at the same time is just a free and usually minor byproduct of that.

This heavily relies on them being in the right spot. In the wrong spot they can make things worse..

[sailaholic]

Vents do work in the right spot Dave such as sucking air through a radiator AT SPEED.

But your problem isn't high temperature it's a poorly set up tune. You said the car is stable as a rock engine and air temp wise on the track but it's sitting around air temp gets hot.

When your sitting around your airflow is from the fan. Also high intake temps don't really matter that much when your not really moving and trying to make power.




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[gslender]

Factory rally cars have had vents for years but not the entire bonnet like some drift cars. Is this a velocity thing or are the big vents just for toss factor?

Heaps of cars have wings on the boot from factory that do nothing but look cool too!

Many factory cars also have 20" wheels that actually reduce the handling of the car rather than enhance.

G

[greenltd]

Can I add my two cents worth? Apologies if I double up as I've not read the posts here.
High under bonnet temps? How and whe r has this been measured? Is it generally under the bonnet or in the intake pipe? Do you really have the problem you think you have?
Why does your car have this problem when it appears other cars don't? Is it a bad tuner or some other problem?
Just trying to provoke some back to basics thinking.