Intake performance question

[71dustar]

I recently heard through the grape vine that if I were to machine my dual plenum intake a 1/4" down on the bridge (to allow equalization between the two) there could be some HP gains. Has anyone experimented with this modification created by Smokey Yunick (I believe credit goes to him or at least popularized by him).

Ciao,
Jonathan

 

[70aarcuda]

you can add a spacer under the carb to pretty much do that same ...

 

[Abodybomber]

Depends on what manifold,and what heads,and if ported.Milling the divider down loses some torque,but the motor "sees" a bigger intake plenum at higher rpms .Usually done,picks up 300 to 500 rpm,on shift points.Considered,a "fine tuning" idea,on certain combinations.Given enough gear and converter,usually cheaper to install a single plane intake.

 

[rumblefish360]

In the MP performance books they have this mod listed and shown for the LD-340. I have the mod done on this intake as well as the trimming of the carb pad edge to fit the TQ carb.

I would not recommend this mod unless you want/need to be sneaky. The spacer is a better option unless hood clearance is an issue. But it does work. If you are just making a notch for balance between the two sides, that is one thing. If your going to mill the whole divider down. It is another thing.

The whole divider missing creates a large open plenum best suited for use with realitivly large cams. As said above, a single plane intake is normally used at this point. And it would be a better choice.

The main reason for milling the divider down a lot is to leagally cheat in class racing where a dual plane intake is demanded to run. The milling creats a large plenum area that sort of copies the single plane intakes while retaining the dual plane look on the outside.
It also shortens the runners length to the carb. The wall adds length.
Shorter runners equal higher rpm, less torque in most cases.

(MoPar was recommending this mod to be done with cams over .550 lift. You know must read between the lines and go back into history to know what kind of cam they were talking about. I'll spoil the homework part of this and let you know they were talking about a cam of at least 250* of duration @ .050.

This is mostly considered either a little big cam or a big little cam. )

The notch mod is good for balance from stock to mild cam upgrades. It allows both sides of the engine to be able to have access to both sides of the carbs primary's. once the engine speed picks up, the heavy draw from the engine will not see any real bennifits from the notch.

 

[AdamR]

every LD340 Ive had came with a notch in the plenum for the same reason.

 

[rumblefish360]

Oh, I would not recommend this mod on a RPM. It is an excellent intake and picks up where the LD-340 left off at. I also would not change a LD-340 for a RPM intake unless the price is worth a few ponies and every last bit of power is sought for your racing endeavor.
On the street or once in a while track run, you'll not know the difference.

Also. The plenum mod (if ya didn't catch it all ready) is basic ignoring power below 3,000 RPM's along with the size of the cam. Hence the mentions of a single plane use at this point.
If your ignoring the low RPM range, what would be the point of a dual plane?

 

[AdamR]

the shift in the power band would depend on how much of the plenum you take out. If you take it to the floor you are going to lose a ton of torque.