hood vents really work!

[drshark]

Good morning all,

A lingering question in my mind has been whether the hood vents really work. My initial observation has been that at rest, the engine compartment cools quite quickly, just minutes. But that's so incredibly subjective.

Yet, there remained a lingering suspicion altered air flow makes the radiators and engine surface cooling less efficient/effective. So, here's a new observation now it's a little cooler, with highs in the 40's and 50's: I can drive ten miles or more in suburban traffic (some stop lights, mostly 25-40 mph) before the water temperature reaches 200 F. Much longer than it used to be--even last spring I was always surprised how short the warm-up cycle was.

As we move into real winter (not too cold here in Niagara Falls), I'll monitor to see what happens at 10 F. or lower. Beginning to wonder if I might block off some air like they do in really cold places!

Bottom line: the engine compartment is certainly cooler than it used to be...

Shawn

 

[Finglonga]

If they worked they would be there as standard.

 

[Tatanko]

As we move into real winter

the engine compartment is certainly cooler than it used to be...

FTFY. It's getting colder out, consequently your car is taking longer to warm up and doesn't get hot quite as easily. That will happen to any car as the seasons change

If they worked they would be there as standard.

This is kind of my thought too. They look cool, but I can't say they're accomplishing anything without some actual scientific data to back that up.

 

[The Fed]

I only need to drive about a mile and half and my water is 200*. I'll see how long it takes once the weather gets colder.

 

[The Fed]

Good morning all,

A lingering question in my mind has been whether the hood vents really work. My initial observation has been that at rest, the engine compartment cools quite quickly, just minutes. But that's so incredibly subjective.

Yet, there remained a lingering suspicion altered air flow makes the radiators and engine surface cooling less efficient/effective. So, here's a new observation now it's a little cooler, with highs in the 40's and 50's: I can drive ten miles or more in suburban traffic (some stop lights, mostly 25-40 mph) before the water temperature reaches 200 F. Much longer than it used to be--even last spring I was always surprised how short the warm-up cycle was.

As we move into real winter (not too cold here in Niagara Falls), I'll monitor to see what happens at 10 F. or lower. Beginning to wonder if I might block off some air like they do in really cold places!

Bottom line: the engine compartment is certainly cooler than it used to be...

Shawn

Just remove the rubber weatherstripping from the rain tray. Instant hood vent.

 

[drshark]

Sorry, didn't make myself clear... In comparison to the same temperature last spring (pre-new hood), the warm-up cycle is massively longer. That's my point, data-wise: I certainly couldn't drive the car for 15 minutes at 50 F and not hit the thermostat at 200 F.

I just didn't notice the difference in warm-up time all summer at 70 or 80 F. But now at 40-50 F, the unassisted air cooling is almost enough to never achieve 200 F.

As for why VW didn't do vents in the first place, it's no doubt a combination of tooling costs and the Q-car design aesthetic.

 

[greggles]

The way air is routed and ducted through the car is carefully engineered by VW. Adjusting the way air moves under the hood can have adverse effects that are impossible to see without access to slews of temp probes, and a wind tunnel.

You could probably get a good idea of what modifications to this venting does, if you had an infrared camera, and a dozen or so probes under the hood.

 

[nate704]

VW and other mass production auto manufactures do not have hood vents because:

1. It is not necessary
2. It does not appeal to more than 99% of potential buyers
3. Most aftermarket or custom add ins look "eBay ricer"
4. It will do more harm that good unless IF it's designed and developed by manufactures. for example: Decreased resale value, if it's functioning vents, your engine room will be a big air filter which will have full of nasty dust, road debris, build up just like your air filter.

 

[drshark]

Ok, I try not to lead with this, but for what it's worth... I hold a PE license; my Master's thesis was in fluid mechanics. I worked for a multi-national company in R&D, followed by a laboratory instrument manufacturer. So, I get engineering design theory and practice. I get measurement, and data vs. anecdotal evidence. Yes, vents can be a benefit or harmful, both thermally and aerodynamically. So for the good of our community, please allow me to re-state my observation and conclusion.

If a reader is considering the Revozport carbon fiber hood and is worried about the vents having a detrimental influence on cooling, my qualitative observations indicate that it's extremely unlikely. My bias would be a positive impact, but I can't tell by how much.

I'll leave the design implications of vents with just the observation that a Golf is a car built to a budget, but sports car designers with more money use them liberally.

 

[Al_in_Philly]

If they worked they would be there as standard.

Not always.

One thing you have to think about is where does the heated air from the radiator/intercooler/AC condenser go, and does the stock exit venting (beneath the engine compartment) fully evacuate it. At lower speeds, venting above the engine, instead of below it, will allow the hottest air to leave the engine compartment better. Depending on the placement of the vents, this added cooling might or might not continue at higher speeds, due to the air pressure build-up from the windshield (remember the rear facing hood scoop on the 1969 Z28, its placement forced air into the engine).

However, air vents over the engine have some disadvantages as well. First, they can let water fall onto the engine and its multitude of electrical components. Obviously, this is not a good thing, unless there is adequate shielding of those components. Second, and probably most critical for VW, making a special hood for a small percentage of their high performance models raises the price considerably. Also, there is the question of styling: would the average customer be more or less likely to buy a car with a vented hood?

So just like most mods, whether or not they make it to a car is often just left to the owners of the cars and an aftermarket which is willing to make those modifications easily doable.

 

[Tatanko]

Ok, I try not to lead with this, but for what it's worth... I hold a PE license; my Master's thesis was in fluid mechanics. I worked for a multi-national company in R&D, followed by a laboratory instrument manufacturer. So, I get engineering design theory and practice. I get measurement, and data vs. anecdotal evidence. Yes, vents can be a benefit or harmful, both thermally and aerodynamically. So for the good of our community, please allow me to re-state my observation and conclusion.

I eat my words

 

[drshark]

No sweat, man. We all cross the snarky barrier at times. I find I have to ratchet down all my own emails, texts, posting, tweets (back when I did that) by two or three levels...

 

[drshark]

I only need to drive about a mile and half and my water is 200*. I'll see how long it takes once the weather gets colder.

I just drove to work and really paid attention. Two miles at about 25-40 mph, a couple stops, at 50 F.

The water temperature gauge just budged off its resting place as I pulled into park.

 

[italynstylion]

As an RX7 owner, I'm intimately familiar with cooling both at speed and at rest in a vehicle. Here's my two cents (as a non-Golf owner....not yet anyway )

Cooling a vehicle in motion vs at rest are two totally different things. You either rely on a fan to PULL air over the radiators or heatsinks or you must be moving quickly for air to FLOW naturally over them. Typical fans only simulate about 40mph of airflow (depends on the car) but if you're moving quicker than that you can cool way better.

The problem with judging the effectiveness of hood vents by how long it takes the car to heat up is that you're measuring the time PRIOR to the thermostat opening. This means the coolant isn't even passing through the radiator yet. Thus, the engine is going to take the same amount of time to warm up regardless. The main factor determining how long the engine warms up will be initial temperature base on the specific heat of water and the materials forming the engine. The hood vents won't even come into play until there's enough heat to radiate from the engine into the engine bay.

All of that being said....I think hood vents might help for sure once the engine is up to operating temperature. Although the cooling system cools the engine, nothing else in the engine bay is cooled by it. Arguably, the cooling system radiator HEATS UP the rest of the engine bay by transferring it's heat away from the engine to the surrounding air. This means the rest of your engine bay components are getting hotter and hotter. Letting that air out via vents in the hood is effective for protecting the rest of those components but again; only at speed. This won't really work while the car is standing still. Not well anyway. You also need to look at the placement of the vents based on high and low pressure zones but that's an entirely different can of worms.

 

[drshark]

Good points, all. I would only disagree in the fine details. But the real bottom line is that I suspect we're discussing the number of angels dancing on the head of a pin. Very, very few of us push the thermal envelope--it's more about ensuring an aesthetic change (vents and creases) doesn't screw up normal vehicle operation. Anyway...

Pre-thermostat cooling: certainly some direct air-engine convection is occurring as air flows through the compartment. Thus, an increase in airflow volume causes a delay in coolant temperature rise.

Static cooling: have to admit that because I wasn't really focusing on at-rest conditions, I don't have more organized observations. But I will say I've been shocked by the velocity of air coming through the vents after I turn off the engine. Maybe several feet/second?

Thanks for the coherent analysis!