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Dominator carb to hood clearance

General engine tech -- Drag Racing to Circle Track

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gnicholson
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Dominator carb to hood clearance

Post by gnicholson » Thu Jul 30, 2020 3:43 pm

I've just finished installing a new 532 in BBC in my 70 Chevelle. With a 4in fiberglass cowl induction hood the carburetor has 2 inches of clearance at the front from the flange to the hood and 3 in in the back. I've always heard the general consensus is you need four in clearance minimum to get the air to turn into the carburetor and not cause turbulence problems. This is a drag car and I've tried mocking up I carbon fiber K&N scoop but it just does not look right on the car and I don't like the visibility problems it creates I guess my only option is to use one of those Sunoco style hoods or a Cowl Hood if this is a problem

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Re: Dominator carb to hood clearance

Post by F-BIRD'88 » Thu Jul 30, 2020 4:00 pm

You can find drop base aircleaner systems and parts
here https://www.ramairbox.com/ Including Ram Air for your Chevelle with a Dominator carb.

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Re: Dominator carb to hood clearance

Post by gnicholson » Thu Jul 30, 2020 4:20 pm

I guess the big question is if 2inches is enough clearance

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Re: Dominator carb to hood clearance

Post by Steve.k » Thu Jul 30, 2020 4:54 pm

Two inches should be enough. The biggest problem is the vent tubes for bowls. I had one that was just below hoodscoop. The incoming air underscoop just above the vent tube pressured up the bowls and the car went way rich. The Holley tec said minimum of 2” from top of vent.

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Re: Dominator carb to hood clearance

Post by gnicholson » Thu Jul 30, 2020 5:09 pm

I've been running 2 spark plug boots with a plastic piece of tubing with holes drilled for the vents per tuner and that fixed the erratic performance and afr with the old setup which was using a 1 in spacer and about as close. seems to run fine after that but beside the vent problem I would think the 2 in of Clearance wouldn't allow the air enough area to make the turn and cause airflow and turbulence probs

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Re: Dominator carb to hood clearance

Post by Steve.k » Thu Jul 30, 2020 6:03 pm

It should. You see those plexiglass scoops guys are using like a ram are in that 2-3 inch range above venturi. However vent tubes im not sure if above or not.

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Re: Dominator carb to hood clearance

Post by F-BIRD'88 » Thu Jul 30, 2020 7:54 pm

The shape of the drop base and its lid help direct the air into the carb from all around as designed.
The ram air aids air pressure @or above ambient and the air cleaner smooths and straightens the airflow.

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Re: Dominator carb to hood clearance

Post by MadBill » Fri Jul 31, 2020 12:07 am

The original "L88"-style bases flow very well, but there are many terrible copies that don't. (e.g. all spun aluminum ones)
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Dominator carb to hood clearance

Post by BILL-C » Fri Jul 31, 2020 7:05 am

I have done multiple tests regarding this issue on both engine dyno and chassis dyno. Too many times we have run across situations where jetting on engine dyno did not end up being even close to correct in car. Almost always the culprit was air cleaner lids or hoods being too close to carb boosters. The quickest and easiest way to do test is on chassis dyno with hood or air cleaner lid on and off on back to back tests. To see EXACTLY where that sweet spot is for booster to lid or hood clearance is, we made an adjustable " hood" out of plywood and threaded rod so were could raise and lower it in relation to carb boosters and run engine on engine dyno. Mule engine was a 500 cid bb mopar bracket race engine making approx 750 hp. Lowering plywood to 2.5 inches away from air cleaner flange lost 75 + hp compared to previous "open" run. Carb went very rich . After dropping jet 10 sizes we got afr back to where it was, but still had major power loss. At 3.5 inches we could get power back with jet change. At 4.0 inches the power and jetting was back to "open" numbers.
Carlquist Competition Engines

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Re: Dominator carb to hood clearance

Post by Fatman » Fri Jul 31, 2020 9:23 am

BILL-C wrote:
Fri Jul 31, 2020 7:05 am
I have done multiple tests regarding this issue on both engine dyno and chassis dyno. Too many times we have run across situations where jetting on engine dyno did not end up being even close to correct in car. Almost always the culprit was air cleaner lids or hoods being too close to carb boosters. The quickest and easiest way to do test is on chassis dyno with hood or air cleaner lid on and off on back to back tests. To see EXACTLY where that sweet spot is for booster to lid or hood clearance is, we made an adjustable " hood" out of plywood and threaded rod so were could raise and lower it in relation to carb boosters and run engine on engine dyno. Mule engine was a 500 cid bb mopar bracket race engine making approx 750 hp. Lowering plywood to 2.5 inches away from air cleaner flange lost 75 + hp compared to previous "open" run. Carb went very rich . After dropping jet 10 sizes we got afr back to where it was, but still had major power loss. At 3.5 inches we could get power back with jet change. At 4.0 inches the power and jetting was back to "open" numbers.
How much clearance do you feel would therfore be needed with a dominator carb at 1000 hp and at 1200hp?

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Re: Dominator carb to hood clearance

Post by gnicholson » Fri Jul 31, 2020 11:57 am

BILL-C wrote:
Fri Jul 31, 2020 7:05 am
I have done multiple tests regarding this issue on both engine dyno and chassis dyno. Too many times we have run across situations where jetting on engine dyno did not end up being even close to correct in car. Almost always the culprit was air cleaner lids or hoods being too close to carb boosters. The quickest and easiest way to do test is on chassis dyno with hood or air cleaner lid on and off on back to back tests. To see EXACTLY where that sweet spot is for booster to lid or hood clearance is, we made an adjustable " hood" out of plywood and threaded rod so were could raise and lower it in relation to carb boosters and run engine on engine dyno. Mule engine was a 500 cid bb mopar bracket race engine making approx 750 hp. Lowering plywood to 2.5 inches away from air cleaner flange lost 75 + hp compared to previous "open" run. Carb went very rich . After dropping jet 10 sizes we got afr back to where it was, but still had major power loss. At 3.5 inches we could get power back with jet change. At 4.0 inches the power and jetting was back to "open" numbers.
thankyou that's exactly the info I wanted. The carb was to close with the last engine too. Allways thought it should have been faster with what it made on the dyno. Prob a forward facing hood scoop is the way to go as bad as I hate to mess with the cars appearance. This 532 made 944 at 7700 on the dyno

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Re: Dominator carb to hood clearance

Post by BILL-C » Fri Jul 31, 2020 3:01 pm

Fatman wrote:
Fri Jul 31, 2020 9:23 am
BILL-C wrote:
Fri Jul 31, 2020 7:05 am
I have done multiple tests regarding this issue on both engine dyno and chassis dyno. Too many times we have run across situations where jetting on engine dyno did not end up being even close to correct in car. Almost always the culprit was air cleaner lids or hoods being too close to carb boosters. The quickest and easiest way to do test is on chassis dyno with hood or air cleaner lid on and off on back to back tests. To see EXACTLY where that sweet spot is for booster to lid or hood clearance is, we made an adjustable " hood" out of plywood and threaded rod so were could raise and lower it in relation to carb boosters and run engine on engine dyno. Mule engine was a 500 cid bb mopar bracket race engine making approx 750 hp. Lowering plywood to 2.5 inches away from air cleaner flange lost 75 + hp compared to previous "open" run. Carb went very rich . After dropping jet 10 sizes we got afr back to where it was, but still had major power loss. At 3.5 inches we could get power back with jet change. At 4.0 inches the power and jetting was back to "open" numbers.
How much clearance do you feel would therfore be needed with a dominator carb at 1000 hp and at 1200hp?
I believe we are dealing with wave reflection issues here more than air restriction issues. My best "somewhat educated" guess is that the 4 inches from carb to scoop or hood clearance is ok. More probably better.
Carlquist Competition Engines

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