Intake manifold cross over tube

Sent: Saturday, May 05, 2007 1:24 PM
Subject: Re: [Fot] Intake Manifold Crossover

When I was doing radical MGTD engines in the early 50's I closed the balance pipe with material and welded in a fill and smoothed, it did provide better power and amazing it also smoothed out the idle on a big camshaft. But those engines had a single port for two combustion chambers. It so amusing that when I sold my MG special that had that manifold on it the guy thought I trying to pull something off on him cause the idle was pretty smooth for a big cam. I drilled a 1/2" hole in the balance pipe covers and the idle came right back rough as a cob and he was happier.

I was talking to Jimmy Coan just yesterday about this, and he remembers I did it on the TR-4 in the engine dyno and the mid range and idle characteristics were so bad we had a hard time stopping the engine from shaking out of the cradle. From this even though the flow bench said other, I passed on the mod. With the S.U.s or Stromberg it is important but of course on Webers each inlet port has it's own carb so no contest there. Can't explain it.

The cross over balance pipe contributes to the flow of the mixture when you have only two carbs for four cylinders.

Lets start with the mixture flow going to number one, then the flow must reverse itself to get to number three, this makes number three slightly leaner, but now the inertia of flow is going to three and the next open valve is number four so it gets a higher inertia of flow, then to number two and the reverse takes place and the flow is leaner as the inertia of flow was going in the opposite direction, but number one now benefits from the flow to number two and so it is slightly richer than number two. make sense? It really works that way and is the reason I always check the number two and three plugs on a full throttle clean shut off. Those cylinders are most prone to detonation because of this.

This being true ( and it is) you would think that plugging the balance pipe would better all around, but it's not. Weird!! Your power may differ.  I did replace the balance tube with clear tubing and you could see the mixture pulsing back and forth. Interesting , but of little value.

Never Be beaten by Equipment
Kas Kastner

Return to the page top