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Intake manifold runner leak between cylinders (Theoretical question)

Practical Engine Building

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Hey all, wanted to get some professional opinions on the side effects of the following:

Mazda loves using this Tumble control mechanism (VTCS) for cold starts (I imagine its for emissions and faster warm up). Problem is, it is supposedly quite restrictive for forced induction.

First pic is of the system in the closed position (cold start). Even in the open position, the shaft holding the flaps is considerable restrictive for performance applications.

Removing them is a common mod on many Mazda models.

The second picture is the exposed cavities after the supporting shaft is removed, and are now left opened between runners. Most people seal these holes.

My question: If those cavities in the second picture are only bleeding to other runners, would there be a performance impact? What would the symptoms be?

NOTE: This is only a "leak" between runners in a common plenum, Any cavities protruding to the atmosphere are sealed.



Thanks in advance!


hi phil on the flow bench the go is to bung the holes. i have not tested on the dyno back to back to see what the difference is

Are the injectors upstream of the holes? if so, I could see a lot of issues with the fuel transporting across the different ports as the inlet valve opens and draws air in. This could cause you to be chasing fueling issues and also having inconsistent fueling in each cylinder.

From experience of removing the TGV's from a Subaru engine, and only sealing the exterior ports and not the internal channels, the engine didn't idle nicely and had poor throttle response behavior, and that was with only two joined cylinders.

What will happen is the cylinders will be drawing from their adjacent cylinders - the outers' will only be drawing from one runner, but the two centre runners will be drawing from two, when the throttle is closed, or near to it. This may cause an irregular idle, especially as the injectors will be out of phase, as mentioned above, but should have less affect at full throttle.

My personal view would be to plug them all, so there isn't any question about fuelling.

I appreciate the replies!

Ross, out of curiosity, do you guys bung the holes to eliminate the possibility of influence to performance, as in common practice?

Stephen, this engine is Direct injection, no port injection on the intake manifold. But couldn't the same concept apply to air travelling between cylinders as well?

Most definitely.

Thanks for the replies.

Gord, you posted within seconds of my last post!

Thank you for all the replies. Although there is no port fueling, my idle is inconsistent among other possible symptoms. I will be replacing the intake with another one that has the emissions system in place, I will report my findings.



Going back to an OEM setup is a good idea for a datum. With the owner's permission, I would then remove the blades/butterflies but leave the shaft in place and see what affect that has as the shaft will be blocking the interconnecting holes, then refit the manifold with them removed to double check your results.

I see the manifold has been re-worked - something that may affect high end performance is turbulence due to air being pulled into the port, at a minimum the disruption in the port wall will do this.

Some of my previous thinking was biased towards an ITB setup, but is this an engine with a single TB and plenum?

Hey Gord,

This is my vehicle, So I work on it when I have the energy haha.

That was an example manifold I pulled off the web, not the one I am using, just took the picture for explanation as the design is similar.

Yes, this is single TB going into a plenum.

I am not confident this is the cause of my inconsistent idle, but like you said, reverting to OEM is a easy way to find out. I am also dealing with a crankcase pressure issue that I am trying to remedy as well.

Hey guys,

Just an update, I reinstalled the intake with the emissions in place and the slight miss-fire at cold idle is gone.

I realized this is inconclusive since the ECU is tuned to run with the emissions system. A real test would involve me sealing up those holes and reinstalling that intake.

Thanks for the info guys!


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