Hood venting

69427

69427

The Artist formerly known as Turbo84
Joined
Mar 30, 2008
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Clinging to my guns and religion in KCMO.
I liked what Chris (vette427sbc) did on his hood vent, opening up the area above the radiator on his L88 hood. I'm thinking on doing something similar, right after I round up a little more courage to cut into the fiberglass. In the mean time I'm welding up a piece to deflect a portion of the radiator exit air (there's obviously not enough hood opening space to try to exhaust all of the radiator air out the hood). This picture is during the fitment process. I've since welded up some reinforcements to it, as I'm not sure what the air/wind force against this piece is going to be.

IM002425_zps7459946a.jpg

Haven't decided whether to bend the panels in a vee shape to follow the contour of the scoop, or to leave it straight and just let the hot air dump into the first bulge in front of the vent slots.
 
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Like this (XP819) hood?

126953de153372a7a.jpg


Could be useful.
Cheers - Jim
Ahhh - had to do some more sleuthing to find your reference:
126953de18c079cbe.jpg
 
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If it is not a sealed duct it may not flow out too well. If the under car pressure is lower it may want to flow down.
 
This is a topic dear to my heart. My 81 and I spent three years doing high speed 40 mile jaunts across the Mojave desert. We did a lot of thinking about cooling and airflow during the summer months and the GM guys had it mostly right the whole time. My Brit cars....not so much:smash:

The racer is where I started studying and experimenting with the underhood pressures as I was getting a lousy delta across the rad. decades later I do not overheat racing in the desert heat. So by extension, I maintain that no car should overheat if properly built. OK, That damned yellow Jag is still F..... with me but I think I know why:thankyou:

Point is, the engineers did it right to start with and if you're suffering now it's because the basics have been ignored. Clear the debris, explore the delta and ensure things are working as designed.....:mobeer:
 
GT6Steve said:
This is a topic dear to my heart. My 81 and I spent three years doing high speed 40 mile jaunts across the Mojave desert. We did a lot of thinking about cooling and airflow during the summer months and the GM guys had it mostly right the whole time. My Brit cars....not so much:smash:

The racer is where I started studying and experimenting with the underhood pressures as I was getting a lousy delta across the rad. decades later I do not overheat racing in the desert heat. So by extension, I maintain that no car should overheat if properly built. OK, That damned yellow Jag is still F..... with me but I think I know why:thankyou:

Point is, the engineers did it right to start with and if you're suffering now it's because the basics have been ignored. Clear the debris, explore the delta and ensure things are working as designed.....:mobeer:
Click to expand...

The engine doesn't overheat. Even with a 195 thermostat, 90+ degree track days, and the parking light grills blocked off. No basics have been ignored.
 
rtj said:
If it is not a sealed duct it may not flow out too well. If the under car pressure is lower it may want to flow down.
Click to expand...

I agree. I've got a differential pressure gauge (the top smaller one) to measure the points under and over the hood.

IM002245_zpsa5452c7c.jpg

I'm planning on trying some tufts also to get a visual look at the airflow, but with the frontend shape of a C3 I honestly can't imagine it having less pressure (at speed) under the hood than on top of the hood.
 
There ya go! The magnahelic tells the tale. Having that tuirned my racer around and it also confirmed the 81 was working right. Folks seem to tyhink the cooling is some kind of mag
69427 said:
rtj said:
If it is not a sealed duct it may not flow out too well. If the under car pressure is lower it may want to flow down.
Click to expand...

I agree. I've got a differential pressure gauge (the top smaller one) to measure the points under and over the hood.

IM002245_zpsa5452c7c.jpg

I'm planning on trying some tufts also to get a visual look at the airflow, but with the frontend shape of a C3 I honestly can't imagine it having less pressure (at speed) under the hood than on top of the hood.
Click to expand...
 
Nice set up.
I wonder how many of us "aero-kooks" have a Magnehelic Gauge?
[should we ask for a show of hands?]​

Just a few questions, if I may:
Are you using a multi-tube setup, and pulling "taps" or reset/retape after each run?
Have a recorder with you to take data?
Or, using a camera?

Just a few things I've been thinking about as I get closer.

Cheers - Jim
 
GT6Steve said:
There ya go! The magnahelic tells the tale. Having that tuirned my racer around and it also confirmed the 81 was working right. Folks seem to tyhink the cooling is some kind of mag
69427 said:
rtj said:
If it is not a sealed duct it may not flow out too well. If the under car pressure is lower it may want to flow down.
Click to expand...

I agree. I've got a differential pressure gauge (the top smaller one) to measure the points under and over the hood.

IM002245_zpsa5452c7c.jpg

I'm planning on trying some tufts also to get a visual look at the airflow, but with the frontend shape of a C3 I honestly can't imagine it having less pressure (at speed) under the hood than on top of the hood.
Click to expand...
Click to expand...

This isn't a cooling thread. This is an underbody pressure/lift (reduction) thread.
 
phantomjock said:
Nice set up.
I wonder how many of us "aero-kooks" have a Magnehelic Gauge?
[should we ask for a show of hands?]​

Just a few questions, if I may:
Are you using a multi-tube setup, and pulling "taps" or reset/retape after each run?
Have a recorder with you to take data?
Or, using a camera?

Just a few things I've been thinking about as I get closer.

Cheers - Jim
Click to expand...

I'm just a one man, low budget operation. I've got a couple of ten foot lengths of tubing (plugged into the Magnehelic) that I move around and tape onto body locations or plug into pressure/vacuum taps. No data recorder or anything, I just take a glance at the gauge(s) during the long straights on test & tune track days. Would be nice to have a printout of pressure versus speed or RPM, but I'm happy to just get whatever info I can however I can.
 
Just so were all on the same page here, this is my most recent cut to the hood vents:
FE7BD4A2-6A8F-48C8-B7C8-CAC27820FBC9_zpsxywtqxmt.jpg

43B14BD8-C4F5-4BFD-A7AC-36CD4042D678_zpsezqxnyod.jpg

B9A0A1EC-D0C4-45CC-9CC7-CB22D209F1C5_zps8cyejsdx.jpg

While my first cut that Jim pictured in post #2 probably had a greater low pressure zone at the vent, the newer opening should allow for more airflow, but likely has no low pressure zone.
 
vette427sbc said:
Just so were all on the same page here, this is my most recent cut to the hood vents:
FE7BD4A2-6A8F-48C8-B7C8-CAC27820FBC9_zpsxywtqxmt.jpg

43B14BD8-C4F5-4BFD-A7AC-36CD4042D678_zpsezqxnyod.jpg

B9A0A1EC-D0C4-45CC-9CC7-CB22D209F1C5_zps8cyejsdx.jpg

While my first cut that Jim pictured in post #2 probably had a greater low pressure zone at the vent, the newer opening should allow for more airflow, but likely has no low pressure zone.
Click to expand...

I like the design of the screen. What is it, and how are you attaching it?
 
vette427sbc said:
B9A0A1EC-D0C4-45CC-9CC7-CB22D209F1C5_zps8cyejsdx.jpg

While my first cut that Jim pictured in post #2 probably had a greater low pressure zone at the vent, the newer opening should allow for more airflow, but likely has no low pressure zone.
Click to expand...

Looks good.
 
so does the tube connect to the gauge at both ends and then as it gets squeezed produce a movement on the meter? I guess I have to do some reading up on this it sounds interesting.



69427 said:
phantomjock said:
Nice set up.
I wonder how many of us "aero-kooks" have a Magnehelic Gauge?
[should we ask for a show of hands?]​

Just a few questions, if I may:
Are you using a multi-tube setup, and pulling "taps" or reset/retape after each run?
Have a recorder with you to take data?
Or, using a camera?

Just a few things I've been thinking about as I get closer.

Cheers - Jim
Click to expand...

I'm just a one man, low budget operation. I've got a couple of ten foot lengths of tubing (plugged into the Magnehelic) that I move around and tape onto body locations or plug into pressure/vacuum taps. No data recorder or anything, I just take a glance at the gauge(s) during the long straights on test & tune track days. Would be nice to have a printout of pressure versus speed or RPM, but I'm happy to just get whatever info I can however I can.
Click to expand...
 
bobs77vet said:
so does the tube connect to the gauge at both ends and then as it gets squeezed produce a movement on the meter? I guess I have to do some reading up on this it sounds interesting.
............................
Click to expand...

I'm not sure I understand your question (but I'll throw out a response). The Magnehelic gauge is a differential pressure gauge. It just gives you a reading of the difference in pressure between the two ports in the back of the gauge. I hook up a couple hoses to the two ports (and say, tape one hose to the top of the hood, and the other hose to the bottom of the hood)and I can then measure the difference in air pressure between the top and bottom surface of the hood. I can also do this to check the pressure drop across an air filter, or the pressure drop across the carburetor at WOT, or any of a number of other things.
If you leave one gauge port open (to atmospheric pressure), then the gauge just acts like a common gauge where the reading is gauge pressure (as opposed to differential pressure or absolute pressure).
 
Here is another question, if I may:
Are you using/getting full swing on the 4.0 gauge- or way less? Haven't tried mine yet - its a 1.0 full scale.

I note there are a lot of digital versions on eBay for about 30 bucks. Some with data hold. Could be nice for a one man operation - or use a "voice" recorder to "announce your data". I'm sure Pappy will recall that "method."

I'm just a one man, low budget operation. I've got a couple of ten foot lengths of tubing (plugged into the Magnehelic) that I move around and tape onto body locations or plug into pressure/vacuum taps. No data recorder or anything, I just take a glance at the gauge(s) during the long straights on test & tune track days. Would be nice to have a printout of pressure versus speed or RPM, but I'm happy to just get whatever info I can however I can.
Click to expand...

BTW - I've added a few more of Simon McBeath articles in the Chassis/Suspension Downloads section for those interested. These are from "Aerobytes" in Racecar Engineering. More to follow.

Cheers - Jim
 
Magnahelic gauges

Quick point on your Magnahelic; these gauges are designed to operate in the vertical position. With yours in the passenger footwell at nearly horizontal the operation and calibration may be affected.
Very good gauges though. Being a pesky European, I like mine in Pascals.
 
Blue Juice said:
Quick point on your Magnahelic; these gauges are designed to operate in the vertical position. With yours in the passenger footwell at nearly horizontal the operation and calibration may be affected.
Very good gauges though. Being a pesky European, I like mine in Pascals.
Click to expand...

Thanks for the advice. I greatly appreciate it. :thumbs: I had it angled so that I could see it while I was driving, but I'll make a change to the bracket.

In most of my career I was using Pascals (kPa) for engine test load points and calibrations, but now that I'm retired, for some reason, I tend to gravitate towards the old English and American units of measure.
 
phantomjock said:
Here is another question, if I may:
Are you using/getting full swing on the 4.0 gauge- or way less? Haven't tried mine yet - its a 1.0 full scale.

I note there are a lot of digital versions on eBay for about 30 bucks. Some with data hold. Could be nice for a one man operation - or use a "voice" recorder to "announce your data". I'm sure Pappy will recall that "method."

I'm just a one man, low budget operation. I've got a couple of ten foot lengths of tubing (plugged into the Magnehelic) that I move around and tape onto body locations or plug into pressure/vacuum taps. No data recorder or anything, I just take a glance at the gauge(s) during the long straights on test & tune track days. Would be nice to have a printout of pressure versus speed or RPM, but I'm happy to just get whatever info I can however I can.
Click to expand...

BTW - I've added a few more of Simon McBeath articles in the Chassis/Suspension Downloads section for those interested. These are from "Aerobytes" in Racecar Engineering. More to follow.

Cheers - Jim
Click to expand...

This Magnelic gauge is a 0-40 inches H2O (about 1.5 psi). Measuring different exhaust system backpressures will move the dial significantly, while measuring air filter pressure drops don't cause much movement at all. I still need to measure my carb pressure drop at WOT to see what numbers I get.

I gotta check out those digital meters you mentioned. They sound rather interesting. :thumbs:
 
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