RPM manifold

[jayb]

Rory, I am baffled to this day by the low end torque performance of the Performer RPM on those two engines.  I remember playing all kinds of games with the carb trying to get those numbers up where they should be, without success.  It seems strange in the face of a lot of reports about engines in vehicles, where that manifold seems to have a lot of low end torque.  I have no explanation - Jay

[My427stang]

Jay, think it could be the low vacuum at WOT during dyno testing, vs in use at higher vacuum levels (part throttle?)  especially with a vac sec.

Regardless, the RPM ports are big, so you'd have to expect some slowing of movement, but I also haven't seen anything in use act like the curve on the dyno.  Even in some overcammed, under-compressed, uncer-inched applications it seems to do OK, or at least can be tuned to do OK with a little timing

[jayb]

Well, if that were the case you'd expect to see the same performance from some of the other manifolds, right?  Like the Blue Thunder intake, which has even bigger ports and runners?  Yet it wasn't there on the other intakes.  Strange...

[machoneman]

Well Jay is that not something you could test the next time around?

IIRC, you wire up the secondaries on any vac. secondary carb when on the dyno. If so, maybe don't do so and see if low rpm torque picks up. Just a thought......

On edit: 'Course you've probably already tried this, eh?

[Qikbbstang]

When you go to Mc Donald's they better not screw up a hamburger conversely when you bolt on an Edelbrock Intake manifold they better not screw up on that. Manifolds are what put Edelbrock on the map they know exactly what their doing.  Blair metioned the RPM's intake is like two single planes but personally I disagree the lower section is indeed like a single plane but the upper plane much more closely resembles a common dual plane.
 

Bravo on Blair providing some details on how he modified the RPM. I've long wondered how the pro "builders" handle enlarging the runners of an RPM because the RPM has extremely uniform runner dimensions from essentially the plenum to the flanges. If one ports an RPM it's a given the runners will end up sort of hourglass shaped somewhere because there are areas a mortal can not get to. Typically runners on intakes are tapered and "porting" a common intake is simplified by only needing to do the area adjacent to the flange. Not so the RPM to enlarge a runner the full length is pretty much impossible to do unless you happen to have a 4" tall little buddy you can send into the runners to get to the "blind" areas/sections that you can't get to or even see with a die grinder and burr.  Realistically short of cutting apart a RPM the only way to truly open up the runners from plenum to flange on an RPM is perhaps Extrude Hone.  Making the big power with an RPM sure says it's not necessary to have large runners
   One of the wackiest manifolds you'll ever see is the Edelbrock F427 which almost uniquely*  has enormous funnel shaped runners. To port match a F427 only requires an inch if that at the absolute most since there is so much taper.   
* The FoMoCo Sidewinder also has highly tapered cavernous runners

[My427stang]

Well, if that were the case you'd expect to see the same performance from some of the other manifolds, right?  Like the Blue Thunder intake, which has even bigger ports and runners?  Yet it wasn't there on the other intakes.  Strange...

Good point, dunno LOL

[Joe-JDC]

Well, if that were the case you'd expect to see the same performance from some of the other manifolds, right?  Like the Blue Thunder intake, which has even bigger ports and runners?  Yet it wasn't there on the other intakes.  Strange...

A quick look at the camshaft you were using, and its lobe centers, and installed position would be a good indicator of why the torque was low on the RPM intake.  Also, if I remember correctly you only used a 1/2" open spacer for the dyno testing, and the spacer can be part of the problem with the RPM, and not other intakes.  I know the cam was the same for all tests, but there again, it proves that different intakes like different camshaft specifications, and one "cam fits all" does not work.  The more I dyno test intakes and spacers, the more I find that each engine combination will like something a little different to get the maximum torque and horsepower being sought after.  When you throw in a mix of different headers, the game completely changes.   You can test every conceivable combination of heads, intakes, camshafts, headers on a given shortblock, and many will be similar, but one will finally prove the best combination of all of the tests, and it will not be the one you expected.  Joe-JDC

[Qikbbstang]

All I can say is WOW on JDC pointing out: "if I remember correctly you only used a 1/2" open spacer for the dyno testing, and the spacer can be part of the problem with the RPM, and not other intakes."

With the Perf RPM having the unique (2) single plane arrangements it's obvious there's some serious Edelbrock voodoo going on. What get's me is I'm pretty sure Jay was once posting info on his making a removable plenum spacer on I'm thinking it was on a Perf RPM?....

It's always interested me on people removing part of the plenum divider which essentially turns a dual plane into a single plane with funky long runners. I think I recall more than one of Barry R's EMC Motor's ran a highly modified Perf RPM with major surgery on the divider   

The current SHELBY FE Intake is obviously an exceptionally close copy of the PERF RPM with the exception the SHELBY has a significant cut down of said plenum divider.

Talk about an interesting dyno comparo a PERF RPM vs a SHELBY

https://www.shelbyengines.com/products/shelby-dual-plane-intake-manifold

[FElony]

https://www.shelbyengines.com/products/shelby-dual-plane-intake-manifold

The Chelbee intake has those square bumpsies on the plenum floor. Do they really do something, and what can be done to replicate them on a smooth floor?

[cammerfe]

There's been a bunch of work done on what are often referred to as 'turtles' or 'stuffers'. Some are installed using epoxy and others are made to be bolted to an otherwise smooth plenum floor.

KS

[FElony]

There's been a bunch of work done on what are often referred to as 'turtles' or 'stuffers'. Some are installed using epoxy and others are made to be bolted to an otherwise smooth plenum floor.

KS

I'm well aware of those. But what's the scoop on these?

[My427stang]

The picture of that Shelby bothers me as-is.  Sharp edges have no place in that area and will likely shear fuel (I hope that is the right term) as it passes over those edges.

Although at first glance that looks exactly like how I do FE RPMs, I'd knock those edges down on the divider and in the roof of the plenum.  All sharp turns need to be rolled back a bit.  We reached pretty deep into the ports as well, but in my opinion, the big money areas are all the sharp edges at every turn point. 

[MeanMofakee]

Not trying to change the subject, but I guess I kind of am. Have anyone of you engine builders used the Edelbrock 7505 2x4 air gap manifold. I know there is the issue with carb choice, but was wanting to go with a dual EFI setup that will fit on that manifold. Seems like a BT 2x4 MR is hard to come by nowdays.

[Joe-JDC]

The Edelbrock RPM Air Gap 8V intake needs a little work on the #8 port down below the carburetor area to help with the turn towards the port, but overall, it flows well with simple gasket match.  Worked well enough to break a C-6 bellhousing on a customer's roadster.  I have been looking for one for a while now, but they seem to be non-existent.  Joe-JDC

[jayb]

I had good luck with mine on the dyno, despite the Edelbrock carbs.  I'm surprised they are not available; maybe they flopped in the market or something?