Is idling with hood up cooler for engine than with hood down? - PeachParts Mercedes-Benz Forum

Chris W. · #1 07-06-2012, 11:30 AM

Reading the thread about idling your diesel for long periods to generate power during a blackout reminded me of an opinion I've long held. Someone mentioned keeping the hood up to run cooler. (let's assume this is a hot weather discussion!)

And having lived in Manhattan for a few years back in the day, I can vouch for the fact that a lot of taxis, Crown Vics in this case, used to pop the hoods (not all the way up, but opened against the secondary catch) during hot days while running around the city.

I maintain, however, that this is counter productive. When you open the hood, you allow air to bypass the radiator, thereby lessening the effectiveness of the radiator. With the hood closed, you are forcing the (relatively) cool air from outside to pass through the radiator rather than around it.

What say y'all?

Rgds,
Chris W.
'95 E300D, 372K

Jeremy5848 · #2 07-06-2012, 12:06 PM

"Keeping the engine cool" is not necessarily what you want to do. One of the reasons for the belly pans and other shrouding is to keep the engine warm in cold weather. In warm/hot weather this is not so important but you're right, the hood and shrouding do affect the air flow through the radiator and around the engine. So long as the engine doesn't get too warm, which you can see on the coolant temp gauge, the hood can stay closed. A closed hood also reduces the noise footprint and attracts no nosy neighbors.

Jeremy

Zulfiqar · #3 07-06-2012, 02:07 PM

hood open with car idling would probably help as heat rises and will want to rise quick, however a moving car exhausts hot air from the air pressure differential under the car relative to the engine room.

Air&Road · #4 07-06-2012, 02:09 PM

An engine relies on airflow over the engine itself more than most people think. We learned this in the days of Model A's and 32's running around showing off our fancy hot rod engines with no hood on a hot day.

Zulfiqar · #5 07-06-2012, 02:22 PM

Quote:

Originally Posted by Air&Road

An engine relies on airflow over the engine itself more than most people think. We learned this in the days of Model A's and 32's running around showing off our fancy hot rod engines with no hood on a hot day.

man - how old are you?? you were suping up model As

vstech · #6 07-06-2012, 03:19 PM

Quote:

Originally Posted by Zulfiqar

man - how old are you?? you were suping up model As

My father souped up his A plenty in the early 60's it was about a 30 year old car then... so... sounds about the same as most of the 123's and 126's we all work on today...

tbomachines · #7 07-06-2012, 03:28 PM

While idling, in generall it is helpful to pop the hood to vent the rising hot air. While in motion, depends on the aerodynamics of the vehicle and the specific cooling airflow patterns I would think.

connerm · #8 07-06-2012, 04:24 PM

If you've popped your hood, please remember to close it fully before you go driving around. I'd hate to read that someone bent their hood, roof, hinges and their windshield experimenting with heat dispersal. MBs ain't no NYC taxis.

Zulfiqar · #9 07-06-2012, 04:36 PM

Quote:

Originally Posted by vstech

My father souped up his A plenty in the early 60's it was about a 30 year old car then... so... sounds about the same as most of the 123's and 126's we all work on today...

yep - I think we'll be telling such stories to our grandchildren, most probably cars would resemble computers and we would say that we worked on cast iron block diesel engines with no on board computers controlling them.

vstech · #10 07-06-2012, 04:39 PM

first off, don't idle for extended periods with a diesel! it's bad for the car.

second, there is not much heat generated by a diesel at idle, so overheating should not be an issue.

thirdly, IF you have gone for a long spirited drive, and you have to leave the car idle for much time it could help to vent the hood temps if you open the hood for a short time. I'd not just pop the hood, I'd recommend actually opening it up.
the radiator fan shroud will pull air THROUGH the radiator, and the aux fan will blow air through the condenser, and the excess can go around the rad, or through it. with the hood open, the aux fan can easier vent the a/c temps.
our motors need air moving around the engine, but with the hood closed, little air is flowing around the motor anyway, so opening the hood will aid in getting airflow both around the motor, and through the condenser!

but it's just my opinion. I'm no expert on automotive design or anything. just feels right to me.

Geeky1 · #11 07-06-2012, 07:46 PM

I experimented with this on my car (while stationary only) before I replaced the radiator last summer (it was original and starting to plug up).

With the car parked, fully opening the hood was worth about 5*C versus closed. Having it popped and resting on the catch didn't make a difference to anything.

Keep in mind that this is with a silted up radiator that was only managing about a 15*F drop across the radiator core, in about 95*F weather. Coolant temperatures were about 100*C with the hood closed-and the aux. fan cycling to keep it there-versus about 95ish with it open.

I've actually got a spare hood that I'm going to put some vents in when I can manage to finish the work that needs to be done on the fiance's Volvo and the Z06. Ricey, I know, but I think it can be done without messing up the lines of the car, and dropping the underhood temperature is never a bad thing. Particularly since it's supposed to get a GTX3071R at some point.

compu_85 · #12 07-06-2012, 08:04 PM

I would think hood up would make it easier for air to exit the rad fan.

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