Tunnel ram / plenum question

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Post  JBR-3 on September 1st 2019, 8:39 pm

What would be the effect if you divided the plenum in half, front to back ?

The front 4 cylinders, 1-2-5-6, would feed from one carb and the rear 4, 3-4-7-8, would
feed from the rear carb.

I know that when #1 intake valve opens, that port certainly does not feed from the secondaries on the rear carb.

If there is an advantage, I could see smaller plenums and possibly better carb transient reponse.

Maybe this has already been done and I don't know the term for it.

Note: firing order is 1-5-4-2-6-3-7-8.


Last edited by JBR-3 on September 1st 2019, 8:54 pm; edited 1 time in total (Reason for editing : more info)

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Post  BBFTorino on September 1st 2019, 11:07 pm

I don't know what it would mean as far as power production, but it would be an interesting comparison to see.
Consider that the old school stack type of mechanical injection, and even the Inglese, or Webber carbureted setup's use a carb venturi (or injector) for each cylinder, and when they are tuned in just right, they are very reactive and make great power.
The situation you are describing would be the halfway point between these two setups.

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Post  DILLIGASDAVE on September 2nd 2019, 2:08 am

It would probably be a given amount closer to acting like the old school IR tunnel ram setups they tried back in the early days of Pro Stock than an open plenum tunnel ram can/does (4 cylinders pulling on each carb venturi vs 8 cylinders pulling on each carb venturi). But it still wouldn't be the same as a true IR setup.
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Post  rmcomprandy on September 2nd 2019, 10:25 am

JBR-3 wrote:What would be the effect if you divided the plenum in half, front to back ?

The front 4 cylinders, 1-2-5-6, would feed from one carb and the rear 4, 3-4-7-8, would
feed from the rear carb.

I know that when #1 intake valve opens, that port certainly does not feed from the secondaries on the rear carb.

If there is an advantage, I could see smaller plenums and possibly better carb transient reponse.

Maybe this has already been done and I don't know the term for it.

Note: firing order is 1-5-4-2-6-3-7-8.

Some older Weiand "Hi-Ram" intakes had open spacers over the 4 bunched runners which did exactly that when using Dominator carbs. Just enough plenum to dampen the pulses so it was not actually individual runners.
They ran OK on some early Pro Stock engines but, nothing like today's options.

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Post  JBR-3 on September 2nd 2019, 1:20 pm

What I'm interested in more specifically is how would it reshape the torque curve,
would it flatten out the curve, possibly making less peak HP but more HP down a little lower ?
If it did not work for ProStock, what does that mean, less peak HP ?

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Post  blown473 on September 2nd 2019, 7:36 pm

If you tried that I'd use carbs that are one to one linkage style like the Holley # 4224 Center Squirter 660 CFM units. Or the 750 CFM version. More even flow across 4 runners, better balance.
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Post  Paul Kane on September 3rd 2019, 3:19 pm

JB, are you still playing with small block fords? And if so, might you have an Offenhauser tunnel ram? Because I have just the thing for a dyno comparo:

Offy Indy Runner Adapters.

Just unbolt the upper plenum from the tunnel ram base and bolt these on instead:

Tunnel ram / plenum question Offyin11

Tunnel ram / plenum question Offyin10


All your questions will be answered. Cool
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