Is there a formula i can use to figure out how much plenum will be need for a 640ci engine that will be running dual rons toilets? The intake was used for gas before and i was told i will need to create a spacer to create more volume needed for the alchy. Any way to know exactly that spacer size i will need?

Not much help but, tunnel ram plenums on race gas fall between
the volume of one cylinder to the volume of all 8. I hear bigger
plenums will produce higher peak HP on the dyno but, are pigs on
the track.......but, then again how much of a pig can 640 inches
on methanol really be?

Here's a

I have seen some formulas for doing it BUT it sure
wasnt small and it was quite long... I looked at
Pipe Max to see what it said... for my engine it shows
about 1/5 of the engine volume MIN... doesnt say anything
about max or what would be optimum ... sorry I cant
help any more

Back in the day, Bill Jenkins said that plenum volume could be worked out, more or less, on a dyno but not with math since it varies too much with intake CFM, port flow etc. Big carb need less volume etc.
Increase the volume in small stages (at the time, by starting with a big plenum and stuffer plates) by removing some stuffers until you just get peak power.
Now add stuffers back until the power just begins to fade.
Subtract 10%.

Does it have a flat surface you can cut away to form a flange and add a spacer?

I did my BB tunnel ram so I could add different sizes
of inverted V plates to change the volume... I started
out at a little better than 3/4 of the engine volume
with no plates

I calculated my plenum volume in the mid 600's inch square. I know my math isnt perfect but its a ball park. It is easy for me to make a spacer to get the volume bigger but im sure to big is bad too. The two toilets are suppose to flow 3600cfm. The heads flow in the 480cfm range @900.

Jut dont know what to do. Add a spacer or not hmmmmm

James Monroe is the man you need to talk to. He told me on my tunnel ram it was more important that the runners be equal than worring about the size. I have the Indy tunnel ram & I had it ported before sending it to Ron's and James said it was fine,but I don't have as many cu. in. as you.
But I have learned this,the Mopar engines want more fuel than then chevy engines with equal HP.
James is sometimes hard to reach,but Jermey @ Ron's is aso very helpful.

They used wood blocks, sanded to round the edges and coated with varnish.
A V is a convenient shape, but it does change the amount of cross-breathing between banks slightly. You can also hang a V etc. from the top (upside-down) as long as it doesn't mask the entries.
If the top is much wider than the entries, add full-length cylinder sliced lengthwise with 1/2 on left and right. They don't have to be solid, or even completely sealed, as long as the joint is pretty small, like 1/8" maximum.
Best shape depends on what the interior looks like, a wide flat floor is easy, runners almost intersecting in the middle is bad.

Talked to someone as rons. They took a bunch of info from the engine and ended up telling me to make sure i have three inches above the runner of plenum. So by that i can use a good one inch spacer all around. When i asked them if they had a number a should shoot for he just said make sure its 3 or more inches above the runner.
Then he said i could even add some sort of spacer just under the terminators themselves.

So ill be adding a 1 inch spacer all around and see how that goes.