Carb to hood spacing

NHRA1926 · 05-19-2021, 09:48 AM

How much room is needed between the carburetor vent tubes and the underside of the hood before problems can arise? I just installed a flat style hood on my 67 Camaro to be able to comply with a local race class which does not allow me to use my cowl hood. My vent tubes are beveled 45 degrees, but the flat top of the tube is about 1/4 inch from touching the hood. Luckily, there is a small crease in the center of the hood that is beveled up giving me a little bit of room. I'm also concerned with how much space is needed between the top of the opening of the carb and the hood. I don't like the idea of not using an air cleaner, which also does not allow the hood to close when installed. I do have a 1" spacer under the carb that is custom made to the opening of the intake carb mounting surface. I know could just remove this, but not sure how much it will affect the performance of the engine. I have moved the hood hinges up to the highest position possible. Looking for some input from you Stock & Super Stock racers.

Jim Caughlin · 05-19-2021, 12:46 PM

I recall someone doing some dyno testing using a Stock legal Holley (in other words, choke plate and horn intact) to see how close the underside of the hood in relation to the top of the choke horn would be. They mounted a chunk of plywood over the dyno and did pulls at various heights to see at what point there was a performance drop off. The magic number turned out to be 2".

Jim Caughlin
SS 6019

SSGN · 05-20-2021, 06:23 AM

Space the back of the hood,tell everyone the springs are weak

NHRA1926 · 05-20-2021, 07:52 AM

Someone told me to try actually lowering the front hood hinge bolt. The rear bolt hole is not adjustable, but the front one is. It might sound backwards, but try it. I guess it kind of makes sense. If you lower the front of the hinge, in theory it might push up on the rear of the hood. Going to try adjusting it today.

Keith 944 · 05-20-2021, 09:34 AM

OK here’s a scenario if you had the hood open and had an air cleaner on it say a low profile air cleaner, how much room would you have between the vents and the top of the air cleaner? probably not much. So it’s probably around the same as the hood to vent clearance now.
I would suggest making a pass as close as it is and then taking that spacer off and making another pass see if there’s any difference.
go back with which one you prefer. Forget about adjusting the hood and making that look all stupid!

NHRA1926 · 05-20-2021, 03:26 PM

Thanks Keith, that's exactly what I plan on doing, making back to back passes, 1 with and 1 without the spacer. Great perspective on original clearance of vent tubes under the air cleaner.

Robert Simpson · 05-21-2021, 05:51 PM

Quote:

Originally Posted by Keith 944

OK here’s a scenario if you had the hood open and had an air cleaner on it say a low profile air cleaner, how much room would you have between the vents and the top of the air cleaner? probably not much. So it’s probably around the same as the hood to vent clearance now.
I would suggest making a pass as close as it is and then taking that spacer off and making another pass see if there’s any difference.
go back with which one you prefer. Forget about adjusting the hood and making that look all stupid!

I completely agree and you can add a carriage bolt in place of the air cleaner stud. Adjust it so that it barely clears the hood when shut. This way if your hood moves or engine moves slightly the vents will not be covered.

Robert

Adger Smith · 05-21-2021, 11:32 PM

Make the tubes as short as possible and cut at an angle.

Bob Mulry · 05-22-2021, 04:35 PM

You can also lower the engine by redrilling the mounts and resectioning the frame mount if you have enough oil pan clearance....

Just make sure that you reset the pinion angles

SSDiv6 · 05-22-2021, 08:14 PM

It depends with the type of carburetor, choke tower/no choke tower, hood scoop/no hood scoop.

For a flat hood car, minimum 2" and if allowed, use a lower pan drop down air cleaner pan. With a big block car, if possible, a minimum of 3".

Due to the air flow path, turbulence and design, with an active air scoop, a minimum of 4" clearance.

05-19-2021, 09:48 AM	#1
NHRA1926 Member Join Date: Dec 2015 Location: Bowling green, Ky Posts: 208 Likes: 11 Liked 161 Times in 51 Posts	Carb to hood spacing How much room is needed between the carburetor vent tubes and the underside of the hood before problems can arise? I just installed a flat style hood on my 67 Camaro to be able to comply with a local race class which does not allow me to use my cowl hood. My vent tubes are beveled 45 degrees, but the flat top of the tube is about 1/4 inch from touching the hood. Luckily, there is a small crease in the center of the hood that is beveled up giving me a little bit of room. I'm also concerned with how much space is needed between the top of the opening of the carb and the hood. I don't like the idea of not using an air cleaner, which also does not allow the hood to close when installed. I do have a 1" spacer under the carb that is custom made to the opening of the intake carb mounting surface. I know could just remove this, but not sure how much it will affect the performance of the engine. I have moved the hood hinges up to the highest position possible. Looking for some input from you Stock & Super Stock racers.

05-19-2021, 12:46 PM	#2
Jim Caughlin Senior Member Join Date: Mar 2002 Location: Woodburn, Or Posts: 635 Likes: 70 Liked 693 Times in 208 Posts	Re: Carb to hood spacing I recall someone doing some dyno testing using a Stock legal Holley (in other words, choke plate and horn intact) to see how close the underside of the hood in relation to the top of the choke horn would be. They mounted a chunk of plywood over the dyno and did pulls at various heights to see at what point there was a performance drop off. The magic number turned out to be 2". Jim Caughlin SS 6019

05-20-2021, 06:23 AM	#3
SSGN Senior Member Join Date: Feb 2006 Posts: 625 Likes: 110 Liked 130 Times in 46 Posts	Re: Carb to hood spacing Space the back of the hood,tell everyone the springs are weak __________________ The more people I meet the more I like my dog Kevin

05-20-2021, 07:52 AM	#4
NHRA1926 Member Join Date: Dec 2015 Location: Bowling green, Ky Posts: 208 Likes: 11 Liked 161 Times in 51 Posts	Re: Carb to hood spacing Someone told me to try actually lowering the front hood hinge bolt. The rear bolt hole is not adjustable, but the front one is. It might sound backwards, but try it. I guess it kind of makes sense. If you lower the front of the hinge, in theory it might push up on the rear of the hood. Going to try adjusting it today.

05-20-2021, 09:34 AM	#5
Keith 944 VIP Member Join Date: Feb 2006 Location: Dela-where? Posts: 2,116 Likes: 1,004 Liked 1,649 Times in 521 Posts	Re: Carb to hood spacing OK here’s a scenario if you had the hood open and had an air cleaner on it say a low profile air cleaner, how much room would you have between the vents and the top of the air cleaner? probably not much. So it’s probably around the same as the hood to vent clearance now. I would suggest making a pass as close as it is and then taking that spacer off and making another pass see if there’s any difference. go back with which one you prefer. Forget about adjusting the hood and making that look all stupid! __________________ KEITH MAYERS 2 X somebody many X nobody Last edited by Keith 944; 05-20-2021 at 09:39 AM.

05-20-2021, 03:26 PM	#6
NHRA1926 Member Join Date: Dec 2015 Location: Bowling green, Ky Posts: 208 Likes: 11 Liked 161 Times in 51 Posts	Re: Carb to hood spacing Thanks Keith, that's exactly what I plan on doing, making back to back passes, 1 with and 1 without the spacer. Great perspective on original clearance of vent tubes under the air cleaner.

05-21-2021, 11:32 PM	#8
Adger Smith VIP Member Join Date: Dec 2001 Location: Texarkana Ark/TX Posts: 2,348 Likes: 434 Liked 677 Times in 261 Posts	Re: Carb to hood spacing Make the tubes as short as possible and cut at an angle. __________________ Adger Smith (Former SS)

05-22-2021, 04:35 PM	#9
Bob Mulry VIP Member Join Date: Apr 2002 Location: Coarsegold, CA Posts: 1,017 Likes: 57 Liked 321 Times in 101 Posts	Re: Carb to hood spacing You can also lower the engine by redrilling the mounts and resectioning the frame mount if you have enough oil pan clearance.... Just make sure that you reset the pinion angles __________________ Bob Mulry 7516 STK A & M Motorsports

05-22-2021, 08:14 PM	#10
SSDiv6 VIP Member Join Date: Feb 2003 Location: Glendale, Arizona Posts: 2,991 Likes: 693 Liked 1,457 Times in 543 Posts	Re: Carb to hood spacing It depends with the type of carburetor, choke tower/no choke tower, hood scoop/no hood scoop. For a flat hood car, minimum 2" and if allowed, use a lower pan drop down air cleaner pan. With a big block car, if possible, a minimum of 3". Due to the air flow path, turbulence and design, with an active air scoop, a minimum of 4" clearance.