Hood low pressure area question.

I had a cold air intake that wrapped around the front of my radiator until this year. It was complex and this year I ditched it in favor of a simple elbow off the intake into a 9 inch round open filter element through the hood. It is shielded from engine heat. So it sits at the front of the hood just behind the radiator. I am debating a shroud that would have the engine pull air from the base of the windshield. I am just not sure how effective that would be. Is there enough pressure to give a noticable ram air effect at 80 mph? Thats all the speed I make on a quarter mile track. Don't want to make a pointless change. I'd show a photo but I cant figure that out on my new phone yet.

Base of the windshield will have the highest pressure short of being in front of the radiator. Enough pressure to give some noticable ram air effect - no. Ducting cool air to it is probably more beneficial than snaking a hose to the windscreen.

at multiple hundred miles per hour the pressure rise at the base of a typical car windshield is only a few tenths of a psi over atmospheric, so you won't see any massive increase in power, but you may see less debris in your filter.

This thread is relevant to my interests.

So, let's say you were more interested in venting trapped hot air from under a hood. How would you locate the lowest pressure areas on the hood before you broke out the Sawzall?

Bonus points for being something I could photograph and write about.

Make yourself a ghetto manometer, a U-shaped plastic tube with colored water in it, then run the ends to two points you're thinking of putting the vents. The point with lower pressure will draw the water higher. To answer your question though, the lowest pressure will be just aft of the radiator part of the hood.

You have my permission to screen-shot this :P

tape bits of yarn to various areas of the hood, strap go-pro to windshield pointed at bits of yarn, record yarn as you drive at consistent speed.

Watch video and note which bits of yarn lift (low pressure) and which flatten out (high pressure). Regroup yarn to the area where you notice the most low pressure to fine tune the overall area, retest, etc. until satisfied. Mark and then cut out the area, add necessary ducting and drain holes for water.

JG, generally the lowest pressure is at the most curvy parts of the body. The front of the hood (transition from the grille) and the sides of the hood (back near the A-post). The pressure gets higher as it runs along the center of the hood from front to back, but lowers as it rolls off the sides of the hood. Keith Tanner did a study of hood pressures for the Miata and that is generally true for all cars with a windshield.
The problem with venting the hood for temperature is you usually vent downforce as well.

stafford1500 wrote: The problem with venting the hood for temperature is you usually vent downforce as well.

Add a proper splitter to the front and you should help improve both situations.

stafford1500 wrote: The problem with venting the hood for temperature is you usually vent downforce as well.

It's the other way around. That is, adding vents is typically done to let hot high-pressure air out from under the hood into the low pressure area above the hood. So not only does it keep the car cooler, it also helps fill the vacuum that's trying to suck the front of the car upward.

Unless of course the vents are placed in the wrong areas, then it does the exact opposite!

Also ducting under car to the hood spot. Relevant to my 1uz swap. Studied fluid and aerodynamics. Front of the hood is best, then duct radiator to that spot for max cooling flow. Also keeps hot air out of the bay.

Well my car does not overheat so underhood venting not required for me. It doesn't sound like the low pressure area would help so I will just leave things the way they are.

Blobs of paint will show the airflow quit well. If it worked for Boeing, it can work for you.

I read a good example of ram air effect from the evaluation of the Mistubishi Zero. It has a knob that opens direct un-filtered ram air to the engine, probably not super optimized, but the opening is right at the front of the plane on the cowling.

Effect at 300 mph? Around 5 hp for a 1000hp motor.

In reply to aircooled:

Ya know, it's really too bad mitsu can't put out a sports car called zero. I think enough time has past, most of the old timers from that war are dead.

Trackmouse wrote: In reply to aircooled: Ya know, it's really too bad mitsu can't put out a sports car called zero. I think enough time has past, most of the old timers from that war are dead.

interesting to note.... although many of the old timers are dead... Mitsu has been SERIOUSLY lying to the Japanese govt (fuel mileage) and they will be forced to stop selling 8 models!! So Zero may be their profits.....