Intake ports slightly wider than head ports

[boogie]

I have an Edelbrock RPM Q-Jet manifold that I would like to run on a 383 SBC that has Dart S/S 165 heads. The manifold measures 1.170 at the width and the heads measure 1.120.

Bad deal?

[bigjoe1]

Dont worry about it


JOE SHERMAN RACING

[Tuner]

Could be an advantage, particularly if there is a step at the floor, head higher than manifold floor.

[Truckedup]

What is is the story behind the mismatch being an advantage? ...

[mag2555]

I would neck it down. On the runners that approach the head with a curve I would neck down the long side of the runner as that's where must of the air at high flow rates travels , and on the runners that meet the head more straight on I would neck down the common wall of the manifold.

If you use something like JB weld you can wet your finger with soapy dish water and shape very closely to what's needed by hand and then use a 80 grit cartridge roll to do the final shaping , just put in more fill than you need and sand it out.
You should go back into the runner at least 1.5 inches to reblend things well.

[Mark Workman]

Dont worry about it


JOE SHERMAN RACING

X2

[Rizzle]

That .050 difference is fairly negligible, especially for what I'm assuming is not a max effort build (165 heads on a 383).
If the long block is together, bolt it on and enjoy it.

[bigfoot584]

Don't try and reinvent the wheel.
That's .050 thou or .025 per side, unless I was
class racing trying for every fraction of horsepower
I'd just leave it.

Like Joe said X4

[Tuner]

What is is the story behind the mismatch being an advantage? ...

There are more than a few examples of a step facing into the flow that indicate it can be beneficial, particularly a mismatch at the floor.

For starters, it is well known that, in spite of flying in the face of “conventional wisdom - common sense - it just doesn’t look right”, a square-port intake on an oval-port BBC head runs great, gets mileage and makes good power, better power than some supposed to be correct oval-port intakes.

Anybody who was paying attention and is old enough to have played with the engines when they were new has seen it.

I had an experience with a 225 V-6 Buick in a Jeep that had a near 1/8” step from the stock intake manifold floor to the head; the head floor was higher than the manifold. The manifold was off to do a valve grind so porting the floor to match the heads seemed the thing to do.

This engine had a small Isky hyd. cam, headers and a 500 Holley 2bbl that had been tuned and fiddled with so it would 4-WD good at crazy angles and climb steep hills without stalling, it got 25+ MPG.

After the valve grind and porting job, which included a bowl job in the heads and matching the heads and manifold to the intake gasket, the formerly snappy engine with a good torque and tractor-like low speed lugging abilities and the aforementioned good mileage was sluggish junk and did not respond to carb or ignition tuning attempts to return it to its former good performance.

Efforts to tune outside the engine were obviously getting nowhere, and having had a previous experience with a step facing into the flow on a 289 Ford carb spacer that was extremely beneficial (is an understatement), we pulled the intake and ported the floor down to mimic the original mismatch and the original performance was restored.

Go figure.

The ’66 220 HP 289 has a sharp step in the carb spacer bores at the casting parting line, the lower half is smaller than the upper half with an irregular 1/16”-3/32” step. On a carb overhaul/tune-up, removing the step on a ’66 Mustang that got 25 MPG and ran 9:50’s in the eighth caused it to fall to 15 MPG and 10:70’s. After much anguish and fiddling with the carb and timing, etc, replacing the spacer with the ugliest one we could find in the wrecking yard put it right back to its original performance.

Go figure.

Those two incidents are the scariest thing about carb tuning I know.

Another example is a dyno test done by Joe Mondello back in the late '70s with the 460 Ford marine engine. One of magazines, Power Boat I think, had numerous Joe Mondello tech articles about modifications of the 455 Olds and 460 Ford jet boat engines. The high performance 460 that was married to Berkeley pumps in the popular little jet boats had standard small port heads with a large port Cobra Jet intake. The intake mismatch is about 1/4" all around the port, about half as "bad" as the BBC oval port head with a square port intake. One of the tests Mondello did was to port the head to match the CJ intake. Eliminating that step mismatch at the gasket flange killed the power bad, 40-50 HP as I recall,

I think the most important areas to match are the port roof and outside radius wall.

[Truckedup]

I reason I asked about the port mismatch advantage....A few years ago me and my grandson built a 454 Chevy for his mud truck...Stock motor home 454 short block with swap meet Edelbrock heads and small GMC type blower on a Holley intake. The ports on the intake were mismatched with a bottom step going into the port.. The engine runs good for what it is used for...

[Tuner]

I reason I asked about the port mismatch advantage....A few years ago me and my grandson built a 454 Chevy for his mud truck...Stock motor home 454 short block with swap meet Edelbrock heads and small GMC type blower on a Holley intake. The ports on the intake were mismatched with a bottom step going into the port.. The engine runs good for what it is used for...

Do you mean the head floor is higher than the manifold floor, so it is a step up going into the head?

That step up with a sharp edge was a make it or break it crazy important detail in the V6 Buick Jeep example.

[The Ry Guy]

What about the reverse situation of the OP where the manifold port is narrower than the cylinder head? Will the flow slow down at that transition enough to cause fuel to drop out of suspension?

My guess is it's not really a big deal on a little 302 Ford that has the Mustang GT cam (for now) with pocket ported GTP heads, and a Edelbrock Performer 302 intake? I'd say the port in the head is about an 1/8" wider than the manifold on both sides. The floor and roof match up pretty close though. (I did not port it this way, the stock GTP port I'm guessing is wider than the older style heads). I haven't run this engine yet,so I don't know how it runs but I am curious. From my understanding the RPM Air-Gap manifolds ports are bigger, but I would like to use what I have to start with.

[boogie]

This is a low RPM torque build. The port height in the heads is slightly larger than the intake port height.

I am going to try it. Thanks for the replies!

[mag2555]

The more expanding and contracting you make the air stream go thru the narrower the motors power band will be , it's a simple fact, HP could be the same either way, but not the power band!

[Truckedup]

The more expanding and contracting you make the air stream go thru the narrower the motors power band will be , it's a simple fact, HP could be the same either way, but not the power band!

Interesting..can you explain this in more detail ?