Plenum Volume For 300 Inline 6

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Aug 11, 2021 | 11:45 AM

old28

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Fleet Mechanic

Joined: May 2017

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From: Southern CA

Plenum Volume For 300 Inline 6

What is a good intake plenum volume for a stock rebuilt 300 4.9L inline 6? Truck is just a cruiser, used for mild street driving, no loads.

Motor has headers, electronic ignition, Clifford open 4V 6 cylinder intake with a 4x2 carb adapter and runs a Holley 2300 350 cfm or Autolite 2100 1.08 venturi 2bbl carb.

Want plenum volume as small as is practical for this intake, carb & my driving conditions.

Aug 11, 2021 | 01:05 PM

RichS2659

Cargo Master

Joined: Feb 2006

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From: North Pole, Alaska

I think that means having a flow bench to do that. Who has one in their back pocket? Clifford is still in business. Contact them.

Aug 11, 2021 | 05:14 PM

old28

Thread Starter

Fleet Mechanic

Joined: May 2017

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From: Southern CA

Rich -- Never used a flow bench to figure plenum volume. Just a lot of measuring & cubic inches. I am for sure an old school builder. Thanks

Aug 11, 2021 | 06:15 PM

HIO Silver

Fleet Owner

Joined: Dec 2009

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From: NorCal

Quote:

Originally Posted by old28

Rich -- Never used a flow bench to figure plenum volume. Just a lot of measuring & cubic inches. I am for sure an old school builder. Thanks

Consider using some plexiglass to seal off the ports and then pour rubbing alcohol into the intake until full. Then dump out the contents into a graduated beaker (think chemistry class) and measure in cubic centimeters (CC).

As far as a specific volume... hell if I know. However, I'd port-match (aka "gasket-match") the intake and radius (with a carbide bit on a die grinder) its innards for smoother flow.

Aug 11, 2021 | 07:44 PM

85e150

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How does it run now?

You might be overthinking this.....

Aug 11, 2021 | 10:56 PM

old28

Thread Starter

Fleet Mechanic

Joined: May 2017

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From: Southern CA

Have not tried the with the Clifford 4x4 adapter, the 4x2 adapter on my truck as medical issues are holding me up working on the truck.

As I have rotated the adapters 90 degrees & installed a plenum divider so 1 throttle bore feeds the 1-2-3 cylinders & other bore the 4-5-6 cylinders I will try the setup like I have it now the first time and get some new AFR gauge readings.

Then if my measurements indicate that the main plenum below the carb has to much volume & will rebuild the 2bbl carb adapter to a 1/2" plate with carb studs. I will bolt it directly to the Clifford Intake & remove the Clifford 4x4 & 4x2 adapters. This change will reduce the main plenum chamber volume.

Aug 12, 2021 | 11:55 PM

JEFFFAFA

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Joined: Mar 2013

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From: Phoenix, Az.

I don't understand why you want a smaller plenum. Smaller would not be better for the air/fuel flow. A shorter plenum would require the air/fuel to make a sharper "L" once in the intake. This is why a 1" or 2" spacer under the carb on our V8's brings more power to the table. I'd bet that Clifford did their homework.