plenum sizing

[Anonymous]

Hi. I have aquired a head that has been modified to suit tripple sidedraft carburettors. however, i want to run a single 350 holley. I am planning on making plenum out of 60x60 mild steel and mounting the 350 holley in the middle. comments?

 

[Thad]

Rule of thumb, volume equal to one cylinder volume for torque.

 

[6bangerbill]

I dug out some plenum information by David Vizard. For a four cylinders use 50 to 60% of volume of all four cylinders, for a three or six cylinders use 65 to 80% of volume of three cylinders. This is for an engine speed of 5000 to 6000 rpm. Increase this plenum volume by 30% for engine speeds of 3000 rpm decrease volume by 12% for 7500 rpm. Just a note,Build it small and vary plenum volume with carburator spacers for final tuning. Also, split the six cylinder manifold into two plenums and make two 3 cylinder manifolds. Kind of weird, huh!

 

[100_2023]

If you're going to sell the current intake shown in the picture, I'd be interested in buying it from you. You can PM me if you're interested. Thank you.
Darrell

 

[Stubby]

When this was first posted, I thought it was possibly a 2V head but the angles didn't look right. After I posted the inlets I found on ebay and Adam pointed out they required furnace brazing and seeing the amount of portwork done to the exhaust, it would be interesting to know the history of this head and how well it works.

 

[addo]

I found another fellow who was doing similar conversions to the Lynx effort, on a changeover basis. He was in Sydney, and traded under his own name "Bob Blattmann". Lynx used to have their own machine shop, and build motors. So, there were several hats in the ring, from the sound of it.

 

[Fiorelli]

I dug out some plenum information by David Vizard. For a four cylinders use 50 to 60% of volume of all four cylinders, for a three or six cylinders use 65 to 80% of volume of three cylinders. This is for an engine speed of 5000 to 6000 rpm. Increase this plenum volume by 30% for engine speeds of 3000 rpm decrease volume by 12% for 7500 rpm. Just a note,Build it small and vary plenum volume with carburator spacers for final tuning. Also, split the six cylinder manifold into two plenums and make two 3 cylinder manifolds. Kind of weird, huh!

how much of a plenum do you need for 2000 RPM and below? if you split the 6cyl manifold into two manifolds do you need 80% of the volume for each or do you split it in two?

Jeremy

 

[Anonymous]

I see no one is listening.

 

[6bangerbill]

This particular Vizard quotation is a real quandry to me. It is well known my most thinkers that increased rpm requires a larger plenum volume for correct resonation. THe exact opposite is emplied by his words quoted in the article above. I can only conclude that an error was made by the printer or stenographer recording his statements.

The volume of air in the plenum and the volume of air in a single cylinder are supposed to react or "resonate" as air is moving from one conainer to the other. Vizard discribes it as analygus to a weight suspended between two springs. The the weight (air in the intake runners) is being pushed back and forth thru the port at a frequency determined by the two springs ( the two volumes of air in the cylinder and plenum). The idea is to time the system by sizing its elements to a point that air is being pushed thru the ports toward the cylinder at an rpm of your chosing.

Vizard gives the following intake runner diameter formula:

Dia in inches=Sqrt((Liters x VE x rpm)/(V x 18.5))

Where V= maximum intake velocity (he uses 180 ft/sec)

VE=volumetric efficiency

No formula is given for plunum volume in the article.

As for plenum volume for 2000 rpm, we can only guess. It would have to be less than 65%, say maybe 26% of the cylinder volume served by the plenum. On a 300 cubic in. engine with a divided plenum, this would be 26% of 150 cubic inches, or 39 Cu. in. for plenum volume. Anybody measured the volume of a stock 4.9 efi plenum? I think its tuned for close to 3000 rpm. Anybody know if the stock efi plenum is divide? It does have two inlet openings in the throttle body.

A friend of mine who builds sheet metal manifolds for Mitsubichi 4 cylinder racing engines uses 50% for Naturally aspirated and 100% for turbo charged engines. His engines make nearly 1000 hp on turbo and nitrous configurations, he must know something.