Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Street Tunnel Ram
View Single Post
  #21 (permalink)  
Old 02-06-2007, 11:38 PM's Avatar is offline Moderator
Last wiki edit: How to rebuild a Rochester Quadrajet 4MV carbureto...
Last journal entry: How to change auto shift timing on 200R4
Last photo:
Join Date: May 2002
Location: Bakersfield
Posts: 8,455
Wiki Edits: 21

Thanks: 6
Thanked 45 Times in 33 Posts
Originally Posted by xntrik
Single-plane and dual-plane intakes operate differently, as you know.

All new SFI cars ARE long single-plane tunnel-rams with small plenums, layed over sideways because they are dry flow, so they can lay them over. That is also why they require such large LSA cam angles to idle.

The primary purpose of a tunnel ram is not to shorten the runner, but to give the air a straight shot into the intake valve, which improves fuel atomization because the fuel does not get slung out by centrifugal force and run down the port walls. (New cars with SFI eliminates the fuel distribution problems.)

The true tunnel rams can be short, or long. Plenum big or small.

Look at the new Weiands for good tunnel rams. Lots of things have changed over the years, including tunnel rams. I just put a new one together last week.

Properly done tunnel rams will make more torque and power across the board, except at the very lowest rpms where all single plane intakes suck, literally, figuratively, and poorly, pulling exhaust into the intake manifold.

You are probably like most of us. We have always seen big carbs on big plenums on overcammed engines with crappy ancient heads running for 7500 rpm. Ya, they really suck, in every sense of the word.
I was using the single plane comment sarcastically. TPIs can use long inlet runners to take advantage of organ pipe theory and get a tuned boost without the danger of liquid dropout since the gas is injected near the intake valve. Detroit TPIs have the x-over long tubes on production cars since they are designed for low to mid range speed boost. The lower the design boost speed the longer the tubes need to be.
Tunnel rams are designed for organ pipe boost at the tuned length (short runners @ high rpm). There is no way to get the uniform short length in a dual plane or single plane manifold, that is what they designed the tunnel in the first place. What you gain in high speed organ pipe boost you lose in low speed loss of velocity in the relatively huge plenum. Tunnels as opposed to TPIs are hampered by the fact that the fuel is introduced at the inlet not the outlet. Once atomized gasoline ends up condensing and puddling resulting in bad distribution in a tunnel at low speed. It is unavoidable, regardless of the size of the carb. And the number of turns has zero impact on organ pipe boost, the only critical parameters are flow area and runner length. Think of a tunnel as injector stacks (tuned for high speed organ pipe boost) with a carb on top instead of injectors at the base.

Tunnels are great for drag racing for which they are intended and for looking mean on the street. However, if used in the latter application they will perform poorly at the rpms encountered 99.999% of the time.

Here is a photo if me in mid construction of the tunnel I built circa 1968 when I was in college. I used it as a physics project as well as a cool looking street manifold. This was before commercial tunnels had hit the market. I designed the runners for 3000rpm primary boost and as you know there is a harmonic secondary boost @ 1/2 design rpm or 1500rpm in my case. Worked great when I put my foot in it but regardless of the size carb I used (I tried several including a Rochester 400cfm 4-jet clear up to my current Holley 950cfm 3-bbl) it was terrible at normal driving speeds. These things just are not intended for the street.

Reply With Quote