Landracing Forum
Tech Information => Technical Discussion => Topic started by: Leadfoot on July 28, 2014, 01:00:50 PM
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Just wondering if anyone out there has had problems with carburetor's leaning out at speed ( 250 ) with a fwd facing Pro stock style hood scoop ? I keep richening the jetting and a couple cylinders (5-7 ) stay running Lean. I have plenty of room above the carbs and running 7 1/2 psi fuel pressure.
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I am not experienced with a Pro-Stock hood scoop but I would try it with an air cleaner of some type and see if it goes away-- I had a car that would pull hard at lower speeds and fell on it face as speeds increased---put an air cleaner on it and problem went away---yep---I changed the scoop.
-5-7 was the reason for a 4-7 swap on BBCs
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I do have a 4-7 swap cam. Has anybody tried putting a hose in between the vent tubes ??
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I suspect you are setting up turbulence or a swirl inside the base of the scoop at speed.
Back in the 1960's - 70's Chrysler published plans for their ram air hood scoop used on the cars like Sox and Martin
http://www.gilchristbnw.com/GPC/DR/PS/S%26M7201.jpg
The scoop was called a Bowman scoop as I recall. It was important to have the right dimensions above the carburetors to avoid the air flow turbulence screwing up the air pressures in the float bowl air bleed tubes and the throats of the carburetor. As I recall the bottom of the "duck bill" on the scoop needed to be about 1-2 inches above the top of the carb so the air would settle down and dissipate turbulence.
I assume you have a block off plate on the bottom of the scoop that only exposes the carb bores and air bleed tubes to the pressurized air in the plenum of the scoop.
If not, you could be getting hot air coming up from in the engine compartment due to pressure changes in the engine compartment at speed. This could cause some bores to see cold ram air from above and others pulling in hot air from the engine area on other screwing up the mixture once your speed gets fast enough to cause major pressure differences under the hood and in the hood scoop.
If it is turbulence a couple layers of screen or as mentioned above an air filter box could break up any air turbulence and equalize air pressure over all the carb throats and float bowl air bleed tubes.
If I can find my old direct connection book that has those plans I will try to post the info they had on that scoop design I am sure the modern scoops need to address some of the same issues.
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Don't re-engineer the wheel by condeming the firing order, find the real problem before you torch a hole. Connecting the bowl vents won't fix this.
5&7 are going to be the first to lean out if you have a fuel supply problem.
It's impossible to diagnose from here with no details on the engine but if jetting up didn't help, you might have a weak signal issue with a bad intake gasket or maybe a tight intake valve adjustment or a valve problem.
Leak it down to make sure those two holes are good & then pull the intake & eyeball the gasket.
Never assume anything, check all the basics first.
Sid.
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Tom Bryan did a lot of research on scoops, maybe he will jump in and answer some of your questions.
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Ok there is some good points made in the above threads. The heads are fresh new gaskets seems to be a on going problem. How much pressure could I be seeing at 250 mph ?
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http://www.wallaceracing.com/ram-air-calc.php
Theoretical about 1 psi, real hood scoops only recover about 60% -90% of the theoretical, so you are looking at maybe 3% boost in performance or about .3 - .4 psi in a real hood scoop, but keep in mind if you are making 2000 hp 3% is a free 60 hp. (minus airdrag from the scoop)
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Check out the TECH&FAQs tab on the home page for some scoop on scoops! :-o
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What method are you using to select your jet sizes. Are you doing it on a dyno?
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I am jetting to reading the spark plugs and best mph. And the " the Great White Dyno ". Going to try some diffrent things this year , I have allready changed high speed jets from 33 to 28 and mains from 93 to 96. I also took out the 45 -90 degree fittings from fuel system.
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Are you seeing these issues only at speed or on a dynamometer as well?
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At speed.
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You don't have your foot in it for a minute & a half on the dyno so confirm fuel supply with a gauge during a run. Put a pyro in #7 so you know whats really happening. I don't see how a possible scoop issue is only going to affect two cyls.
Sid.
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Gary, aren't you running Nitrous? If so, I am assuming your not running out and seeing a pressure drop at the top end, and as Kiwi mentioned, those would be the most likely to lean out. Bob
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First, let me explain that I have spent many hours on the flow bench testing air flow with Holley carbs.
These are some FACTS I have found and proved many times.
The scoop is only a part of the function in flowing air into the carbs.
The space above the carbs after 2 inches is wasted.
Holley carbs draw their air from the sides, not straight down.
The more area around the perimeter of the carbs the better.
The largest area air filter is best to calm and slow the air around the carbs.
The only trick I have ever found that did work was the carb-hat that helps smooth and direct the air into the carb.
( note; I have ruined more than one Holley trying the latest trickshit modification.)
After the air enters the carb and is mixed with fuel it still has to flow through the plenum, intake runner, intake valve etc. Many places that can affect the charge in each cylinder differently.
Doug Odom in big Ditch
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Yes Bob I do run nitrous on occasion . The problem seems to go away on nitrous it evens out ALL the cylinders. The problem is mostly just on the gas no spray. Are you coming to Speedweek this year ? Hope to get a chance to talk with you.
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I have a flat pan that I got from RJ race cars that is radius around the carbs. There is about 3 and half inches above the carbs and the pan is sealed to the scoop. A carb hat ? is the like a KN snub stack ? Also I'm using a Dart tunnel ram that has been cleaned up in the plenum area.
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Gary, I will be there minus race car. I will for sure visit you and everyone else I haven't had time to chat with over the last sixteen years!
Are you using a GM block or a Big M block? Filled water jackets? I wonder if maybe your block might have a core shift problem in the block or head?
It might be worth sticking a camera in both next time its apart.
Look forward to visiting and stealing all your speed secrets!
"One Run" Bob Drury
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Ok it will be good to see you Bob. I'm running a Merlin 3 block. There has been some great posts and a lot of ideas to think about. Thanks to everyone.
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Gary, Yes, Carb-hat was the first ones made. Not sure they are in business anymore. K & N stub stack same thing.
Doug
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.... I will for sure visit you and everyone else I haven't had time to chat with over the last sixteen years!...
Yes, let's make a visit happen this time.
Helped a guy you know with his '36 Buick about a week ago here. He was on a long cruise with a bunch of other guys and was having carb problems. Neat car that was mostly original. One of the few '36 cars that could do highway speeds today stock (well sort of, he runs an overdrive manual trans.),
Sum
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I will be there minus race car. I will for sure visit you and everyone else I haven't had time to chat with over the last sixteen years!
Look forward to visiting and stealing all your speed secrets!
"One Run" Bob Drury
Hey Bob, you can have one of mine for free.... Bring a car and make at least 2 runs... :| :roll:
:cheers:
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I am bringing my firesuit and helmet just in case a "Car Fairy" decides He is too old to drive................. hint, hint..............
One Run, out..................... :roll:
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It was a long time ago when we did this so my memory is fuzzy. The best place for the ram air inlet was in the very front of the vehicle where it was not influenced by the various ways that air can travel around body work. This was a bike so the intake was where the headlight normally was.
A duct ran back to a metal box. Another duct exited this box and ran back to the carb. A big flap valve hung on one side of the box. It would open when the air pressure in the box got too high, like when running into a headwind. This limited the ram air effect so the engine would not run lean. It sort of worked.
Jetting was done by trial and error. The jets were richer for ram air than without.
It was hard to tune. Optimum main jet size was influenced by the speed the bike was traveling and to some degree, the direction the wind was blowing. The experiment was abandoned.