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Discussion Starter · #1 ·
I was told I need to block off at least one of my butterflies on my bird catcher or my car will not run or idle well for a street car. This is my first EFI 8-71 setup before so I would like some opinions and insight on this. It's a stage 3 billet / anodized 8-71 and going on top of my 540 bbc. It is fuel injected and will be custom dyno tuned once it's all together.

the thing I don't understand is why I was recommended a bird catcher if I need to block 1/3 of it off? However on my camaro I made 1150 Wheel horse through a single 90MM TB so I guess it's understandable having 3 - 110MM TB's is overkill.

My question is how do I figure out exactly what I need to block off? this is a 90% street car & 10% track car. According to the CFM calculators my motor will pull in around 1800 CFM at peak power, How do I relate this to the blower hat? Do most people just block off one and call it a day? or should I block off both on the outside and just keep the middle one operational? what do you guys suggest? I have been looking for answers for days now and I can't seem to find any good sources. Any help will be greatly appreciated. Thanks guys.
 

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I wouldn't have an answer for you, but is it possible to get some dyno time and experiment with having all three open vs blocking one?
 

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Discussion Starter · #3 · (Edited)
I wouldn't have an answer for you, but is it possible to get some dyno time and experiment with having all three open vs blocking one?
I thought the same thing but a couple guys said it will work perfectly on the dyno without any plates because you are stationary and the motor will only pull in the air it needs.. they said the reason you need to block them for street use is because you are going down the road at 30-50-70 mph and the big hat sucks in way more air then the engine can use which causes the problems ( its a huge wind block and it's cramming tons of air in the hat at all times, air it doesn't need ) . if this was a drag only car it wouldn't matter since you are WOT the entire time during the 1/4 mile race but I guess on the street you must block it or the hat will cram too much air in the motor from the velocity and air movement while driving.

I just have no idea how to calculate how much of the hat I should actually block off? That's where I am totally confused. One side and leave two open? block the two outter TBs and only use the center one? .. I really don't know how to calculate it which is why I am confused. a few people said I MUST block the hat but they are not sure to what extent.. Now you can see my dilemma, it's a tough one.
 

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If you can forgive me for trying - I'm an artist, not an engineer - but it sounds like in interesting challenge (and the folks who probably know are asleep now :)

If each of the three ports is 110mm across (do I read that correctly?), that's 3.14 sq inches each for a total of 9.424sq in. or .785333~ sq ft.
At 60mph you pass through 5,280 ft.
.785333 x 5,280 = 4146.56 cubic feet per minute trying to enter that birdcatcher at highway speeds.
That's ignoring the effects on the airflow from the car's hood and intake itself.
That gives you about 1382 CFM on the highway, for each of the three openings- again, in a perfect situation, ignoring air pressure change due to aerodynamics.
 

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if you were running around with no blades in your hat maybe that might be an issue but I've never heard of anyone blocking a hole off.
 

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The problem that you might have is that the throttle pedal position may be a "hair trigger" at low throttle openings.

By this I mean that you may have so much throttle blade area that idle and low speed may all be in a very short travel on the pedal. In other words, when you open the throttle, it may open too fast.

The only way you're going to know if this is really a problem is to go ahead and put it together and see how it acts. Then you might consider mods.
 

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I tend to agree with Rick L on this. I think you can tune it to work as is . but try to set up your throttle linkage for as long a throw as you can stand . that will allow you to feather the throttle easier. Also have you considered going to a bug catcher? I have a friend with a 8-71/bug catcher on a small block and it works fine. remember the engine can only accelerate if the butterflies are opening/open. you can usually relieve the " hair trigger" with tuning as well . just m 2 cents :anim_25:
 

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Discussion Starter · #8 ·
thanks for the replies guys.. I am still trying to decide what to do but frmo what I have read the bug catcher is for engines up to 425 cubic inch and the bird catcher is for 426- 590 cubic inch.. then the buzzard is the big boy for 590+ cubic inch. From everything I can tell I have the right hat, I will just need to block one port off still.. I have yet to find someone that runs a bird/bug on the street without blocking one port. Does your buddys bug catcher have a block off on it? Everyone I am coming across online says you need to block one off for a street car and you want to leave them open for a track car. I still have lots to learn about this though
 

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I have run birdcatches with no noticeable problems, currently run open scoop with screen in bottom with no problems. Had a frien who had hillborne injection with bird catcher without problems. Curious what are the problems experienced by not blocking off ?
 

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I talked to Andy at Hilborn last week and he told me the reasoning for blocking off one inlet on the inside of the catcher with a block off plate was to limit the rush of air as you open the throttle blades. On a track car, you're always running WOT so it's never an issue. On a street driven car, much of your driving is part throttle and that's where you want to balance air flow.

He said the difference is night and day with respect to the motors part throttle habits and it will be more responsive and have much better control on the street without affecting WOT when you want to take it to the track. It truth and science so I am told. Call Andy in PA and talk to him. Guys right on the money.

Running the custom Enderle air filter from BDS is also supposed to help it out while filtering out unwanted bugs and debris at the same time.
 

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Discussion Starter · #11 ·
I talked to Andy at Hilborn last week and he told me the reasoning for blocking off one inlet on the inside of the catcher with a block off plate was to limit the rush of air as you open the throttle blades. On a track car, you're always running WOT so it's never an issue. On a street driven car, much of your driving is part throttle and that's where you want to balance air flow.

He said the difference is night and day with respect to the motors part throttle habits and it will be more responsive and have much better control on the street without affecting WOT when you want to take it to the track. It truth and science so I am told. Call Andy in PA and talk to him. Guys right on the money.

Running the custom Enderle air filter from BDS is also supposed to help it out while filtering out unwanted bugs and debris at the same time.


That's what I was told as well. I decided to run a block off plate in one of the ports and tune it from there. If these bird catchers are rated at 4600 cfm I will still have plenty of air flow available even after deleting one throttle body. Thank you for the reply and I will keep you all updated with my progress.
 
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