BPG_Austin

I"m wondering about airflow on hoodvents. On a thread DrNick started a whlie back he said after installing his hood vents hot air was sucked into the AC intake. I'm wondering if there is a solution to this problem. In the second picture, the right vent has a small raised deflector on the back. Will this 'bump' be enough to disrupt the airflow so hot air won't be pulled in the rear of the hood? Would those with experience please comment on what's required to keep vented airflow away from the AC intake. Thanks

Ben

hacker-pschorr

IMO people seam to forget about ducting. In almost every factory car with hood vents, there is some sort of ducting to direct whatever heat they are trying to remove.

The newest car I've seen with hood vents are in the Aston Martin that seal up to some type of duct work designed to draw the heat away from the exhaust manifolds. Looks like aluminum foil.

EDIT - found a picture - the four silver rectangle things on either side of the motor:

atb

Ben, wouldn't you need more of spoiler to redirect the air? It seems that the bump would just create a low pressure area behind itself and pull the air flow back down?
BTW, there's a '68 Chevelle SS that wants its hood scoops back. (I used to own a '68 Camino)

BPG_Austin

lol atb. Just playing around here. I'm thinking Rush hour, 100 plus degree temps here in Texas. Car has headers... Trying to make it look decent. No big scoops here. Looking for functional, compact, low profile ventilation. hacker, thanks for the ducting picture. Food for thought.

BPG_Austin

Ha, Right on.. Just got that chevelle comment.

mark kibort

ive done the tests with the pressure gauge on the hood. If follows the pressure graph that has been passsed around. putting the vent on the hood aligned with the front of the rim to the center of the wheel bearing, will yeild a flow upward due to that being the lowest pressure on the hood. a vent futher back or up front near the nose, will be exposed to higher pressure , where this air would then be pressed into the engine bay, or cabin if it is at the base of the windshield. (the other high pressure zone)

venting on race cars in these areas are for two distinct purposes. routing air from the nose inlets out the low pressure hood zones to enhance downforce, or to use mild ram effect to enhance engine power.

if you review the presssure zones of the 928, you can determine what is best for you . without them, you are shooting in the dark as some have been.

below shows some tufts testing on the hood vent verifying flow. they stand on end at 100mph at a 45 degree angle ! Ive also found that aero chart.

mk

mark kibort

notice on the Austin Martin, the vents are at the extreme sides of the hood. we dont know what the air flow characteristics of the Austin Martin are , but if they resemble the 928's, then there would be some air ejected there, and it might be far enough away from the cabin inlet as to not bring hot air into the vents. anything in the center will add to the air that is available to the heating and AC vents. no "bump" is going to change that.

mk

BPG_Austin

Thank you for the good information Mark. Pertaining to your comment about function: I'm not attempting to achieve higher engine Hp or create more downforce. I'm looking to dispell heat from the engine bay during stop and go traffic. At speed engine temps will drop to center of the guage, idling in heat the temp guage will creap 2/3rds to the red mark. My understanding is that this is within normal ranges for these cars. I have never overheated even in the hottest weather, but thought it might be fun to try and drop temps a bit. does your hood vent have any effect in this regard?

Jim bailey - 928 International

The engine temperature is being controlled by the fan speed computer next to the passenger seat to the extent that it can. It reads radiator coolant temperature , auto trans temperature , intake manifoild temperature , if the A/C is running and it decides to open the louvers (if still operating) and run one fan two fans and at what speeds to maintain the desired operating temperature. However if the louvers are blocked open you will get more airflow at speed and the temperature will often run lower than the desired operating temperature even though the fans are turned off. What I am saying is possibly the higher temperature is the normal designed operating temperature and the car is running a bit cool on the highway .

mark kibort

I think Jim has it right. i havent noticed any real differnce in temp readings, and i really wouldnt expect to. in my case the air that would come through the radiator, would just all travel out to the bottom of the engine, now, some of it comes up the hood (downforce).
will vents really lower engine bay temps? hard to say. i bet a couple of side vents might lower the overall temps in the engine bay, but i dont think it will effect engine temps in traffic. it might however, over the long haul, be easier on the rubber components in the engine bay by a slight amount.
But, as Jim says, the temps are really controlled by the fan effectiveness.

mk

BPG_Austin

Thanks guys. It sounds like a case of not needing to tamper with good engineering as long as the car is functioning as is should. I might make a set of vents anyhow if I can find a hood to cut. A fun experiment. I appreciate everyones input.

Ben

mark kibort

tape the proposed cuts with blue painters tape and use a metal cutting blade on a hand hold jigsaw. just be very careful . i would stay flush like the austin martin set up, even go to the extent of trying to find them. I did a search and even called some dealerships, and those vents were very expensive. so, i settled for some custom aluminum screens from a shop in LA.


mk

928SS

fwiw, in socal w/100+ temps in traffic and on the track I"ve had no issues w/header heat causing problems even with the stroker lump thingy.

I did jet hot coat them... also have a pair of devek 1710cfm spall fans behind the radiatior that seem to push a lot of air through there. when on a dirt road, it creates a pretty large dust storm if the fans come on... I suppose header wrap would help a bit more, but there are issues w/that sometimes.

Mako 928

On the subject of header wrap. It is suposed to lower engine temp and improve scavenging. Anybody use this and what would be the negative effect of using it?

mark kibort

good question. I was thinking of wrapping my headers. supposed to keep the heat in the pipes for better flow. also lower engine bay temps. what are the downsides? (besides they look ugly)

Mk