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 Post subject: Plenum?
 Post Posted: Sun Jan 08, 2012 8:16 am 
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How does one size a one??? I know that my car pickup et and mph when I removed the 1" 4 hole
tapered spacer and replaced it with a open 1" spacer.
And the EGT's were better balanced with the open spacer..
I was going to try a another 1/2 open on top of of the 1 " open spacer to see it it liked
that. Or even just go with the 1/2 open spacer...
oh and then there is the hole E85 or alky thing :-

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 Post subject: Re: Plenum?
 Post Posted: Sun Jan 08, 2012 10:51 am 
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 Post subject: Re: Plenum?
 Post Posted: Sun Jan 08, 2012 10:58 am 
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You also have to consider to, the open spacer makes the turn from the carb into the runners more gradual.

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 Post subject: Re: Plenum?
 Post Posted: Sun Jan 08, 2012 11:21 am 
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There is a formula for calculating Plenum size.

Give Chad a Call.

http://www.speierracingheads.com/


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 Post subject: Re: Plenum?
 Post Posted: Sun Jan 08, 2012 2:20 pm 
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There are a lot of variables involved.
On a typical single plane intake the carb and distance above the floor have a lot to do with it. If the carb is small the airspeed will be fast entering the plenum and it can't turn into the runners easily so it will want a bigger plenum.

On high rpm all-out race engines I've been told by record setting engine builders that 60-70% of engine displacement is a good starting point. For a 540ci that would be 324CI (note this is cubic inches not cc). Pretty big and not what your 540 bracket engine would want.

I have the Engine Pro engine design software by Patrick Hale and it recommends plenum size, but it is really for all out builds and the plenum size it recommends is more for sheet-metal intakes. With cast single plane intakes you don't have much to work with except adding spacers and isn't the same as adding volume by moving the sides out.

Nobody can calculate how much spacer/plenum you need on a single plane engine. I don't think there is going to be a huge gain with any spacers. On the engines I've dyno'd the change with spacer testing has been small.

Rick


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 Post subject: Re: Plenum?
 Post Posted: Sun Jan 08, 2012 4:12 pm 
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It's closer to 10% of the total cubic inches.


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 Post subject: Re: Plenum?
 Post Posted: Sun Jan 08, 2012 5:36 pm 
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Scientific design of exhaust and Intake systems by Smith and Morrison, is the book that explains it all. Anyone who knows the right answers got it from that book.
Plenum volume is not the only consideration. A tall skinny plenum is different to a low wide one. You have to consider the volume has one benefit but the wall surface area has a negative. There's also runner entrance interaction.


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 Post subject: Re: Plenum?
 Post Posted: Sun Jan 08, 2012 6:03 pm 
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Scott Smith wrote:
It's closer to 10% of the total cubic inches.

Some simple math will show that it is much bigger than that.

Take a 500ci P/S engine

500ci x 10% = 50ci
the cube root of 50ci=3.684"
So a cube measuring 3.684" on each side is 50ci.
How many of those could you fit in a P/S plenum?

Rick


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 Post subject: Re: Plenum?
 Post Posted: Mon Jan 09, 2012 4:44 am 
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The short and curly's from that book--- the plenum has to be 8 to 10 times the volume of one cylinder. Ive built manifolds to that spec and they work well .


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 Post subject: Re: Plenum?
 Post Posted: Mon Jan 09, 2012 5:09 am 
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shrinker wrote:
The short and curly's from that book--- the plenum has to be 8 to 10 times the volume of one cylinder. Ive built manifolds to that spec and they work well .



That's it. I knew there was a 10 in the formula. I'm getting old. O:)


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 Post subject: Re: Plenum?
 Post Posted: Mon Jan 09, 2012 6:06 am 
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Ok then how does one figure the volume of the plenum then #-o

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 Post subject: Re: Plenum?
 Post Posted: Mon Jan 09, 2012 12:34 pm 
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There are a couple of ways.

The hard way is to cc each runner then cc the entire intake and subtract the sum of the runners from the total and that is your plenum volume. Takes a bit of time but gives very accurate results.

Then the easy, lazy man's way. On a single plane you can approximate by measuring the height, width, and length of the plenum area. In each direction measure as close as possible to an average of where the plenum ends in that direction. On sheet metal intakes I've done measurements and used geometry to calculate from the dimensions of the shapes.

Volume(cubic inches) = H x W x L (all in inches)

Rick


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