Opel GT Forum banner

1 - 20 of 57 Posts

·
Member
Joined
·
792 Posts
Discussion Starter · #1 ·
Looking at the engine bay of any modern "rice rocket" tells you that a great deal of improvement has been made from days our Opels were made especially in the area of cold air intake.

I want to streamline my air intake on my FI 75 sportwagon and here are some thoughts:
1) Replace stock the air intake using 3" smooth tubing coming off the MAF and rerouting it back towards the firewall shelf that is underneath the plastic cowl vent that seals against the underside of the engine hood.

2) Cut a 3" hole in the underside of the vent to take advantage of the air that gets forced off the hood and windshield through the slots cut in the hood

3) Build a lightweight box that would rest on the shelf, house the air filter and permit the air to travel through the bottom of the "cut-out". into the filter without sucking in hot engine air.

QUESTIONS
If I cut this hole in the plastic vent and built a box to house the filter would I be introducing engine fumes into the cabin?

How might I be able to eliminate water from rain or car washing going into the filter area?
Your thoughts are appreciated
 

·
Banned
Joined
·
3,842 Posts
Cold air cowl induction is a very good idea. You need to find every air opening or outlet in the cowl to make it work. Custom filters for all openings and be absolute of your drain points. If it can be done with a GT a Manta should be easy.
 

·
Super Moderator
Joined
·
13,841 Posts
Paul, I've had good luck pulling cold air from the stock inlet location at the grill. Except with a few mods of course. First, I cut out the actual plastic grill area blocking the air inlet. Second, I toss the stock plastic 'scoop' and make a new steel one with a 3" tube attached. Very simple, almost looks stock, and you don't need to cut the body to do it.

Bob
 

·
Super Moderator
Joined
·
9,749 Posts
Bob;
How about that scoop as one of the items that you could "offer?" What about a box/fixture for the top of a 32/36 weber?
 

·
Member
Joined
·
792 Posts
Discussion Starter · #5 ·
RallyBob said:
Paul, I've had good luck pulling cold air from the stock inlet location at the grill. Except with a few mods of course. First, I cut out the actual plastic grill area blocking the air inlet. Second, I toss the stock plastic 'scoop' and make a new steel one with a 3" tube attached. Very simple, almost looks stock, and you don't need to cut the body to do it.

Bob
Do you keep the stock air filter box and the flexible rubber air tube or did you replace that with smoother plastic or metal hose.
 

·
Super Moderator
Joined
·
13,841 Posts
azopelnut said:
Do you keep the stock air filter box and the flexible rubber air tube or did you replace that with smoother plastic or metal hose.
You can do it either way.
 

·
Member
Joined
·
792 Posts
Discussion Starter · #7 ·
Cai

Bob
Does the fact that the stock 1975 FI set-up basically makes two 180 degree turns adversely effect air flow to the intake?
First
Air travels away from the entry point in the grill up through the air box and back towards the front of the car through the MAF
Second
Air leaves the MAF and makes a 180 through the ribbed rubber hose into the intake
 

·
Super Moderator
Joined
·
13,841 Posts
azopelnut said:
Bob
Does the fact that the stock 1975 FI set-up basically makes two 180 degree turns adversely effect air flow to the intake?
Of course. Just switching to a Megasquirt (or any programable) is worth HP because the air flow meter is eliminated, along with those nasty tight bends.
 

·
Your Noble Friend ;-)
Joined
·
4,400 Posts
Air inlet

As I understand the original question, you want to place the air intake in the area where the hood meets the windshield. From an aerodynamik standpoint, that's not a good place to put it. While you drive, the air "moves" over the hood, up the windshield, then back along the roof line. While the air moves faster (in the red area) the pressure sinks. This results in a low pressure area right in the area between the hood and the windshield. This is why hoods are vented on this location - to suck out hot air from the engine compartment, not to press cold air inside. The faster you go, the lower the pressure. It would be pulling against the "forces of nature" to have your air intake mounted there. Instead you better re-route it to the front of the car, where the oncoming air "presses" against the front grill. There is a lot of air turbulence, but the highest pressure of outside air on your car.

Dieter
 

Attachments

·
Member
Joined
·
792 Posts
Discussion Starter · #10 ·
Cai

I like your wheel caps with the blitz Dieter... where did you get those?

Thank you for the explanation
 

·
Super Moderator
Joined
·
5,920 Posts
heimue said:
As I understand the original question, you want to place the air intake in the area where the hood meets the windshield. From an aerodynamik standpoint, that's not a good place to put it. While you drive, the air "moves" over the hood, up the windshield, then back along the roof line. While the air moves faster (in the red area) the pressure sinks. This results in a low pressure area right in the area between the hood and the windshield. This is why hoods are vented on this location - to suck out hot air from the engine compartment, not to press cold air inside. The faster you go, the lower the pressure. It would be pulling against the "forces of nature" to have your air intake mounted there. Instead you better re-route it to the front of the car, where the oncoming air "presses" against the front grill. There is a lot of air turbulence, but the highest pressure of outside air on your car.
Dieter
Sorry to contradict you, but the "cowl" area (the trailing edge of the hood, up to the leading edge of the windshield), is often one of the BEST places for "forced induction". The change in angle creates turbulence, which in turn creates a high pressure area. That is why most cars use this area for the inlet of the interior ventilation system. In fact, few (if any) cars use this area to ventilate the under-hood area.

Many muscle cars in the 60's and '70's that had big hood scoops facing forward on top of the hood benefited from a cooler intake air charge, but did NOT benefit from the forced induction that a cowl induction system provides. An notable exception was the early '70's Chevelle, which advertised (correctly) that its reversed cowl induction system provided a slightly forced induction of intake air. Another example was the '70's Firebird, with the rear facing "shaker" hood scoop. The same aerodynamics applied to it as the Chevelle. However, I believe that the AMOUNT of actual induction-induced performance was minimal. The main benefit was the cooler air inlet temperature.

As for taking inlet air from the grill area, this can certainly be effective from the perspective of providing a cold air intake. As for a "high pressure, forced induction" system, it depends entirely on the specific aerodynamics of the car. In most cases this area IS a higher pressure area than, say, on top of the flat portion of the hood (where the GT, for example, places hood louvres to draw out hot under-hood air, which the higher velocity air stream is "attached" to the hood and creates a low pressure area). But vehicle aerodynamics can be quite complex, so it can be presumptuous to state that the highest aero pressure is found at the grill area.

JM2CW
 

·
Banned
Joined
·
3,842 Posts
On a GT the cowl was designed to draw cold air to the interior. As speed is increased the given temperature decreases and pressure increases in that area. That is the reason for that upward slant at the window side of the hood. It forces air into the cowl and as it crosses the edge it disapates heat. That is why so many car designers have chosen to use it. Not that I've researched this to death or spent alot of time designing a cold air cowl induction for twins. Oh wait, maybe that was me.
 

·
Registered
Joined
·
521 Posts
I'd still have to agree with Dieter on this one. I can't see how the air would be forced back towards oncoming air from inside the cowl (under the hood). Basic aerodynamics would suggest a low air pressure at the trailing edge of the hood creating a rolling turbulance right there. Air flows towards a low pressure point. The air under the hood would want to continue to flow up and out at that point. It's the same as when you open the window inside your car (hi pressure) to let the cigerette smoke outside (low pressure) because the air passing by keeps creating a void.

I guess as I re-read this the only way it would work I suppose is to have the area sealed from the edge of the hood to the intake to have continues low pressure from the carbs. But even then it would be fighting itself. It would be minimal positive induction.

I'd have an intake routed from the front of the car.
 

·
'NO,......O P E L..G T!'
Joined
·
585 Posts
ConreroGT said:
It's the same as when you open the window inside your car (hi pressure) to let the cigerette smoke outside (low pressure) because the air passing by keeps creating a void.
I can't beleive you smoke in your GT :p
 

·
Super Moderator
Joined
·
13,841 Posts
FWIW, in Nextel Cup cars they draw in the air to the carburetor from the base of the windshield, NOT from the front of the car. Those aerodynamicists all agree this is the best place to get the air from, at least at the speeds they travel at.

Bob
 

·
Banned
Joined
·
3,842 Posts
On a GT with twins it works great. My 11:1 2.0 is fed this way. I don't know if anbody else has done it, but believe me it does work well.
 

·
Member 1000 Post Club
Joined
·
1,520 Posts
Remember, to consider the pressure differential. To get air to flow thru the radiator, the engine compartment will be at a lower pressure than the area below front bumper. The area just below the windshield is considered to be a high pressure zone.

Visualize the air coming over the hood and piling up at the base of the windshield. Some of the air completely changes direction and bleeds off to the size of the car, around the A-pillars, mirrors and such. While the rest of the air travels over the top/roof of the car. Decreasing the angle between the hood line and the winshield serves to reduce the high pressure zone and improve aerodynamics.

Keep in mind that velocity and pressure are inversely proportional. As the velocity of the air increases, the pressure in that area decreases. And, if the velocity decreases the pressure increases. Just as when dealing with an airfoil/wing. The air traveling over the top of the wing speeds up therefore the pressure drops. The air at the base of the windshield slows down, essentially it piles up looking for someplace to go, therefore the pressure in this area increases. the more straight up the angle of the windshield, like a Manta vs a GT, the greater the pressure increase.

Relative to each other, the engine compartment is at a lower pressure that the cowl area at the base of the windshield. If given a path, the air will flow quite easily from the cowl area to the engine compartment.

HTH
 

·
Registered
Joined
·
6,344 Posts
Way back when, the NASCAR folks started using aerodynamics to get the cars to go faster on the super tracks. Around the mid 60s, early 70s they found in wind tunnel tests the air pressure at the base of the windshield was significant enough and in sufficient volume that they could turn the filter box on the carbs 180 degrees and attach the air inlet to the cowl chamber and get a quasi charged intake system. This positive charging vice vacuum charging of the cylinders occurred up in the 100 MPH + range and the faster they went the more positive flow through the carbs. All this occurred with the 7 Liter engines and because of the speeds attained the restrictor plates and then smaller engines became the rule. To give you and idea of volume and pressure, last weekend, the Tide NASCAR Chevy that was campaigned last year was at the local Commissary, and the grill was totally duct taped except for an area of about 2" X 4" cut from the tape. That's all the area needed to keep the radiator cool during a race at speeds approaching and exceeding 200 MPH. A couple of notes on this after talking to the mech. By taping almost all of the grill area a smoother air flow is attained which means more speed and the high velocity air going into that small hole and dumping into a low pressure area significantly drops the air temp in front of the radiator. And the reason I got to talk to him, there was a $60K 350 under the hood, Margaret told him the same size engine I had in the monza, he didn't believe it, he said he was working Chevys for years and there was never a 350 in the monza. She called me and told me to bring it over. He's a believer now. :D
 

·
Registered
Joined
·
4 Posts
Performance for the 1900 wagon

Hey Bob RacrX here where can I get performance parts and a catalog of performance parts for my 74 1900 wagon. also can I go to a larger diameter rim and do you know the bolt patteren size the is standard for Opels...and are there after market rims that will fit this model? :cool:
 

·
Registered
Joined
·
2,733 Posts
Hey Ron, I see you've done some research on "cowl induction". Whether it's true or not, I heard long ago that Chevy's version as seen on '70 Chevelle, '69 Z28, etc. only worked at speeds over 80 mph.
So, what can be done, (anything?) to get some intake boost at lesser speeds, like 60 mph? Any ideas?
See what happened to Nextel Cup car #17 last night at Daytona? Too much duct tape on grille is worse than not enough! Incredible what a fine line that is...
And the Monza 350 was stock, wasn't it? I think that's the only small block they made then, other than the 400 (my favorite).
 
1 - 20 of 57 Posts
Top