H.Q. RALLY PREP

Hi, i have recently bought a version2 95 sti-ra, i have noticed that the bonnet vents are covered over on the inside, is there a reason for this as they would let a lot of the underbonnet heat out??? or would it affect the balance of air into the engine bay and flow through the intercooler??

cheers mark

harvey

iTrader: (48)

Assuming your under bonnet layout is normal, then removing the blanking plate(s) will actually increase under bonnet temps. Incredible but true.

scoobfan

I believe they are there for asthetics more than functionality.

If they were functional the newage would have them as well !



Rob

dazc

I've never seen a rally car with them still blanked off.
Mine are removed. Daz

scooby-woo

I have mine off, it helps to show of the shiny pipe work....

Hoppy

Rally-P, you've answered your own question, and so has Harvey.

Daz, I've never seen a rally car without a front-mount IC and custom bonnet vents, either

Airflow under the Scoob bonnet is complex. It's all part of the fun of owning a family saloon with a flat-four that's been turbo-charged to the hilt and with all the turbo-related bits in the wrong places. It's fun, but also a carefully balanced trick.

It's not just a case of letting cool air in, but also getting warm air out, down and under the car. Leave the vents alone!

Cheers,

Richard.

PS The STi inlet scoop is just an expensive styling mod (like my useless £450 STi carbon strut brace ) and does sweet FA for induction. If you want to see how to do it, see Floyd's car in Projects.

dazc

Richard, don't you bother watching Group N cars?

I have a FMIC, but would still remove them even with a top mounted IC. I would like a reasoned explanation as to how it gets warmer under the bonnet with them removed.

I can understand possible localised hot spots due to warm air being sucked out of the vents (due to a vacuum caused by faster air flowing over the bonnet), but the average underbonnet temperature IMO has to be cooler. They are simply not big enough to affect the mass of airflow which comes through the radiator and out beneath the car.

Also if you are standing in traffic they let a massive amount of heat out, reducing heatsoak which any TMIC suffers with.

I still stand by my original statement. Daz

harvey

iTrader: (48)

If you read my post you will see I am referring to a standard car
Some years ago I invested in several maximum and minimum thermometers and did a lot of work on the under bonnet air flow and temperatures. That is why I am talking from a position of knowledge.
Air obviously enters the front of the engine bay over the radiator and through the gap between the bonnet and headlights, it then flows over the engine and aided by the air flow through the scoop accelerates down the transmission tunnel.
Had I not conducted these experiments I would have expected that removing the blanking plate(s) would have been beneficial. The fact that it is not, I believe, is because the area on the top of the bonnet is low pressure and therefore air is sucked out through the open vents and this disrupts the naturally intended air flow and reduces the venturi effect down the transmission tunnel.
There are situations where there is a benefit from removing or partially removing the blanking plate. eg. When a divider is used to segregate a cold area along the chasis rail towards the inner wing and cold air is forced in through the inner wing by brake ducting or whatever to create a cold air breathing system.

dazc

Were your tests on a standard car? Probably not. The point I am making is that the most dangerous time for a standard car as you know is sitting in traffic giving the engine a good heat soaking, then blasting it away from the lights.

I agree that whilst an extra degree or two may be bad for ultimate performance whilst on the move, but the safety margin of reduced heatsoak and better cooling when air is not flowing through the car make it a worthwhile mod.

I disagree as a sweeping statement, that removing the cover plates increases underbonnet temperatures full stop, which seems to be the current train of thought.

Nice to see a reasoned point of view Harvey.
Daz

Jolly Green Monster

iTrader: (4)

Also done some experiments and came to the same conclusion as Harvey.

Sat in traffic the heat exits through the vent above your air filter like a chimney and heats the filter as it leaves..

The only time it is worth removing the vent is if you have a cold air box and need the hotter air in it to exit like Harvey says.

harvey

iTrader: (48)

The thermostatically controlled fan(s) add air flow when static.
Sitting at tick over is not a performance time.
Once on the move, induction temps drop quickly.
The engine spends more time on the move so this is a far more important area to optimise because higher temperatures, even a few degrees, rob power and economy.
The cooler your induction temps generally, the less likely you are to run det and hence the safer your engine.
This is the factual information but if you want to run vents off, its your engine.

dazc

The thermostatically controlled fan(s) add air flow when static.
Sitting at tick over is not a performance time.

1) Agreed, but the uncovered vents will let hot air out almost immediately on coming to a stop reducing heatsoak from the off. Fans will only come on after being stationary for a while once heatsoak has already occured.
Never said it was a performance mod, more of a precaution.

Once on the move, induction temps drop quickly.

2) Agreed, but if the underbonnet temperature is hotter to start with it will take longer to cool.

The engine spends more time on the move so this is a far more important area to optimise because higher temperatures, even a few degrees, rob power and economy.

3) Is a few degrees really that noticeable from a performance point of view? This is probably the difference between today and yesterday, not summer and winter. The issue here is keeping the engine safer and preventing heatsoak. Whilst on the move I would be happy that enough air is getting through to keep things cool.

The cooler your induction temps generally, the less likely you are to run det and hence the safer your engine.

4) Agreed, but are we still talking about a standard car...

This is the factual information but if you want to run vents off, its your engine.[/QUOTE]

5) Thanks

harvey

iTrader: (48)

Some of us have put time and effort into investigating underbonnet temperatures and purchased suitable measuring equipment.
Some of us monitor ambient, induction and throttlebody temperatures from permanently installed equipment.
You have an opinion without the benefit of this measuring equipment but your theories may change when you have hard information. I was surprised by my findings when I started investigating what was really happening.

jonny gav

iTrader: (1)

removing the vents actually increases heat soak at standstill.
remember the hottest part of an engine is the exhaust manifold.
the subaru manifold is located under the engine so the hot air is dragged up through the engine bay heating everything on its way to the open vents...........as harvey has said, its your engine!

dazc

Jonny, even with the vents covered the heat will still rise. What are you saying happens to the hot air around the manifold? Convection says it will displace any cooler air higher up irrespective. Back to square 1. The idea is to try and remove it rather than ignore it.

Harvey, I don't doubt your research, the theory just seems difficult to believe and goes against the grain. I concede that whilst on the move there may be some minimal performance loss, (which I wouldn't imagine is noticeable) but removing heat from the engine bay whilst stationary has got to be beneficial.

Sat in traffic are you saying that the temps will still be lower with the covers in place?

jonny gav

iTrader: (1)

.....but not at the same rate as it does with the vents removed.

if you doubt what is being said then do the experiments yourself and draw your own conclusions.

i too was sceptical, as you i thought removing the vents would cool the engine bay, the fact is it doesn't, it makes it worse.