Streetmaster intake on C1AE heads

[JimNolan]

Guys,
   I'd like to know if having a mis-match at the bottom of the intake and head runners would cause a problem. I'm running a cast iron 63 intake on D2TE heads now. That seems to work OK even though you could trip over the step it causes with the low rise intake and the medium rise heads. Now, I'm wondering if I reverse the step with the smaller intake opening will this cause a problem. My 57 Fairlane is getting a new intake, cam and headers and I'm wondering if I shouldn't use C8AE-H heads or something like that. Jim PS. I use the car mainly for trips and shows so I don't have to go with my hair on fire all the time.

[drdano]

It wont be a problem.  I have the same heads, but an RPM intake and pondered the same question here when I was porting it.  The consensus was there is no issue leaving it with the LR heads.  If the intake runner ran into a smaller head port, that would be a major problem.

[jayb]

Jim, when you get my book look up the miscellaneous testing chapter and you will see that I tested port mismatches in both directions, bigger intake ports going to smaller head ports, and smaller intake ports going into bigger head ports.  For the engines I tested, around the 400-500 HP range, there was no differences between them.  In other words, low riser manifolds on medium riser heads gave the same results as medium riser manifolds to low riser heads.  The port mismatch you are considering is not a problem.

I think for the most part port mismatch is overrated as a consideration for an engine in the 500 HP power range.  I'd be concerned at the 600+ HP level, but at 500 and below the engine won't really know the difference - Jay

[cammerfe]

Conventional wisdom would certainly suggest that a mismatch AGAINST the direction of flow would cause problems. But I had the experience, several years ago that seemed to show that it isn't always the problem you might think. I had a 460-equipped Town Car that needed to be available and running, and terminal carb difficulties that were being a pain in the neck. A bench-racing session resulted in the removal of the entire factory intake system and the installation of an aftermarket manifold and Holley carb that were otherwise decorating a shelf. The major problem was that the manifold was intended for one of the 'CJ' iterations of the 385-series and there was a decided mismatch---in the 'wrong' direction. Since it logistically made sense, I went ahead anyway.

It worked like a charm---even to the point of providing both a significantly more powerful 'feel' and slightly better mileage. I won't attempt to explain it because it goes against what I otherwise think I know. But it was altogether successful and I ran it for over a year in that configuration.

KS

[JimNolan]

Thanks guys,
     I appreciate the information. Jim

[jayb]

Conventional wisdom would certainly suggest that a mismatch AGAINST the direction of flow would cause problems. But I had the experience, several years ago that seemed to show that it isn't always the problem you might think. I had a 460-equipped Town Car that needed to be available and running, and terminal carb difficulties that were being a pain in the neck. A bench-racing session resulted in the removal of the entire factory intake system and the installation of an aftermarket manifold and Holley carb that were otherwise decorating a shelf. The major problem was that the manifold was intended for one of the 'CJ' iterations of the 385-series and there was a decided mismatch---in the 'wrong' direction. Since it logistically made sense, I went ahead anyway.

It worked like a charm---even to the point of providing both a significantly more powerful 'feel' and slightly better mileage. I won't attempt to explain it because it goes against what I otherwise think I know. But it was altogether successful and I ran it for over a year in that configuration.

KS

Ken, that's a well know example in the 429-460 engines.  Putting the Cobra Jet intake on a standard 429 motor gives a big boost in power, despite the fact that it seems like the intake flow will hit a wall when it runs into the smaller head port.  Apparently the motor doesn't care about that, and FEs behave the same way for the most part.  Again it is not something you would want to do on a max effort engine, but for a typical street engine this kind of a mismatch is perfectly acceptable.

I should point out that most of the flow in the port goes along the upper half of the port, so a match at the roof of the port is pretty important in any application.  In the post topic question, matching a LR manifold port to a MR head works because the roofs of the two ports line up pretty well; likewise a MR manifold port to LR head has a good match at the roof, and in fact 428CJ engines came with MR intake ports and LR head ports.  If you somehow had the situation where there was a mismatch at the roof of the port, I think you would see a serious performance degradation.  But, I can't say for sure, because I've never tested anything with that kind of a mismatch.

[Barry_R]

Not only is the CJ/nonCJ port mismatch a known thing in 460 circles - it was a FACTORY production package on many marine applications.  I've done several of these and used to fill the bottom of the intake with epoxy.  After not doing it on a rush deal and seeing no difference I stopped.  Just dyno'd a customer low riser 6 pack on an Edelbrock head a couple weeks ago and it ran great with power right around expectations and no issues whatsoever.

The perception is that of a flow path, the reality (Ballinger HATES this abuse of terminology - he's technically "right" but the analogy still works   ) is that the depression in the cylinder sucks against the port and that depression sees that mismatch as just a poorly shaped funnel - a funky shaped bigger plenum.