Edelbrock RPM intake

[Joe-JDC]

I am with Jay on this one.  I have dyno tested several engines where the spacer made 35 hp difference, and where a spacer made absolutely no difference.  It came down to the engine heads, camshaft lift/duration, and compression needing more airflow to make the additional power.  The spacer on a higher horsepower engine helped give more plenum volume which the engine was needing.  On a lower 400-450hp engine the spacer did not help on a SP intake, but on a 600hp engine with same intake manifold it made 12 more horsepower.  Also, on every dual plane intake I have dyno tested with the divider cut down, there was a torque dip that could not be resolved with spacers until the horsepower reached near 600 hp level.  At some point, spacers, timing, carb jetting, valve lash adjustment made absolutely no improvement in power or torque.  Leaning the carb out or changing valve lash will cause the torque to pick up, but horsepower to go down or vice versa.  Just my dyno experiences, but I have to go with what worked or didn't work for me.  Joe-JDC

[blykins]

My thanks to everyone who took the time to reply and share valuable information. Chuck

This is one of those subjects that unless someone has a very specific combination or question, we can only speak in generalities. 

It's worth a forum search to see what different members (especially posters in this thread) have found to be successful involving plenum dividers and spacers. 

[Barry_R]

I guess I agree in generalities that you simply need to try the spacers and see what happens.  I used to think that an open one inch was "the ticket" on the Performer RPM packages I was building, until a customer just insisted I try his 4 hole 'cuz it worked so well in his truck.  I stuck it on and watched as it made a ton more torque below TQ peak and lost nothing upstairs.  Since that lesson it's been "bolt it on and try it", and I usually keep a couple on hand for testing.  Then you need to do that same thing on track because in that environment you will be transitioning through the RPM band at varied loads and speeds instead of the steady controlled rate of acceleration on dyno.  To Blair's point - I usually pull and looks at a few plugs as well because that averaging in the collector is a "real deal", and sometimes the reason that changes show "no gain" as one hole gets better while another gets worse.  Some EMC guys (Scott Clark... )were monitoring eight O2 sensors for individual cylinder tuning.

[1967 XR7 GT]

Has anyone tested the Edelbrock RPM intake with a notch in the plenum divider like the Blue Thunder intake? Thanks Chuck.

Are you talking about the 1" to 1.250" little notch towards the back of the plenum divider on Blue Thunder Intake ?  Which, there were some other aftermarket manifold companies also had cut them.

If so, the companies that did cut those notches, were so that the 3 Barrel Holley could be used on there manifolds.

[My427stang]

In some cases with EFI it's best to notch, as you should have the idle air control able to affect both sides.

With a carb, when Edelbrock first started notching the BBC and other manifolds I called and asked a guy I knew in their dyno shop (not even sure if the shop exists anymore) , and they said they saw no reason not to and most of the new releases would be notchied.  However, they told me leaving the ends as a flow director worked pretty well and of course, ensure a blunt edge.

I never did a back to back test, and as others said, other factors in play, but I continue to do it, along with a little rubbing on the plenum roof turn.

I attached a pic of a BT and an Edelbrock RPM for fun

The BT was for EFI, the RPM was on my Mustang and really did well on a 489 inch FE with a 1000 cfm carb, went on to sell the combo to a forum member and had great street feedback as well.

As far as spacers, same as others, some you expect will do great things, some you expect will do nothing, and in the end, the engine decides

[fryedaddy]

is it true that a duel plane intake will take a bigger carb than a single plane?

[Joe-JDC]

At WOT, check what the vacuum reading is on the intake manifold.  If it is more than 1.0" of vacuum, then the engine can use more carburetion.  Not that it needs more, but if you have 1.5", 2.0" then you definitely need a larger carburetor if you are trying to maximize the set-up.  I just got off the dyno Monday with a small engine that made 437hp with a 650 on dual plane, and installed a SP intake with 930 BG carb and made the same horsepower.  The dual plane had 1.1" of vacuum, and the BG 930 had .9".   When the engine reaches its potential, then go with the smallest carb that does the job and be happy.  JMO, but I can live with that.   Joe-JDC