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Old 10-10-2004, 09:01 PM
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BBC rectangular port intake, oval port heads..

What is the mystery behind using a retangular port intake with oval port heads on BBC?

Is it best to use a dual plane of single plane, or is it a matter of what the application calls for?

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Old 10-10-2004, 09:57 PM
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No mystery, just use the gasket that fits the intake. The head surface will take care of itself. A dual plane intake is better for street use where some low-mid range response is required.
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Old 10-10-2004, 10:05 PM
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Hold on,Im mystified,do tell.

Why would you do this,other than a lack of resorce?
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Old 10-11-2004, 07:37 AM
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The practice was common on early Super/Stock drag cars. Also when a small port engine was used to replace a large port. The intake was retained for the "look". As the rectangle port intakes are usually aluminum Holley profile carb bases.
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Old 10-11-2004, 07:49 AM
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I thought it may be a matter of flow characteristics at high rpm, but was not sure.

I use a single plane intake, 850 Vac. Sec Holley, TUNED for my application. It pulls very scary from 3-6k, decent throttle response as well down low. Thanks.
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Old 10-11-2004, 07:54 PM
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Well,I have done that,I used to do that also on the ford 2.3 motors,newer heads had a d port,but just to save the trouble and money were the reasons,I figured you would loose some volosity though,hurting the low end? Hmmm I wounderd what my larger oval 454 would work like in my 90 gmc fuel inj truck,using the 90 intake w the 73 vet heads. again hmmm
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Old 10-12-2004, 11:07 AM
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As mentioned, you can certainly do this but unless you grind out your oval port heads to roughly align with the rectangular ports in the manifold, you will really mess up the flow characteristics as the incoming air/fuel is going to be hitting the corners created by the stock oval ports. You actually cannot grind out oval port heads enough to match a rectangular port manifold. At the top of the port there is not enough material to do this. Lay a rectangular port gasket on your oval port heads and you will quickly see what the problem is.

As a rule of thumb, unless you have a cam that provides good bottom end torque, a single plane manifold will sacrifice some bottom end to provide more top end. You probably won't see much difference between a single and dual plane unless you are below 3,000 rpm or so which is where the dual plane should be better, or above 5,000 where the single plane should be better. Another aspect to this is if the big block engine, which has a lot of torque to begin with, is in a light vehicle, the effect of a single plane manifold on low speed performance is going to be less noticeable.
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Old 10-12-2004, 11:11 AM
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I may be missing something, but why dont you just use a single plane oval port inake like the Victor Jr 454 O?

Adam
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Old 10-12-2004, 11:17 AM
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Your not alone.

I thought there may be some phenomena I was unaware of going on.
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Old 10-12-2004, 02:21 PM
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BB Oval Port Single Plane Manifold

Professional Products also makes a very inexpensive oval port single plane big block manifold. It is part #53036 (polished) and #53037 unpolished. It will be available some time in November or December.
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Old 10-12-2004, 04:40 PM
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My initial question was pertained as to why hotrodders used to install a square port intake on oval port heads, and what the results were.

As I stated earlier, I use a single plane Weiand Xcelerator intake, but it is an oval port model, and really works great. It has a pent up floor on it to negate the effects of the induction "bouncing or reflecting" back up the carb, and to smooth flow of the induction. I took it one step further, and placed a divider that I made similar to the small block model, and placed a cut out in the middle similar to the Performer RPM. I compared it with a new spare rpm that I have for another motor, and it beat the rpm hands down.
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Old 10-13-2004, 09:27 AM
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Rectangular port manifold on oval port heads...

It's possible that hotrodders were using rectangular port manifolds because there may not have been as good a selection of oval port manifolds available. I really don't know because I wasn't that up to speed on manifold availability back when oval port and rectangular port heads were first on the scene. Of course now you can pretty much get anything you want.

However, I laid a rectangular port gasket on an oval port head I have here and I would be hard pressed to see how you could get any kind of decent performance out of such an arrangement because the port mismatch is terrible. On the upper part of the ports the rectangular port is close to a half inch bigger than the oval port. At the bottom of the port and width-wise, it isn't a bad match and you could probably port the oval head to make an exact match in those areas. But there isn't much you can do about the top of the port. There isn't enough wall thickness on the oval port head to take out more than about an eighth of an inch of the half inch mis-match.
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Old 10-13-2004, 09:41 AM
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My thought is that guys would think of all kinds of reasons (rationalizations) to run the R port manifold the the O port heads simply 'cause they HAD one and were too cheap to get the RIGHT one. After all, that kind of mis-match would make anyone think twice about what they were doing.

tom
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Old 10-13-2004, 12:54 PM
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That would have been my thinking,but I got sucked into the word"mystery"
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Old 10-14-2004, 11:49 PM
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Re: BBC rectangular port intake, oval port heads..

Quote:
Originally posted by wildman926
What is the mystery behind using a retangular port intake with oval port heads on BBC?

Is it best to use a dual plane of single plane, or is it a matter of what the application calls for?
The only time I've heard of anyone mixing oval/rect. parts, was with rect. port heads and an oval port intake. The smaller oval ports on the intake act like an anti-reversion setup.

Reasoning being, keep the intake velocity up compared to a rect. port intake. I would probably only do this with a single plane intake. But, who knows.....the dual plane setup 'could' provide better throttle response on the street.

If a rect. port intake were installed with oval port heads - you'd have such a mismatched setup, it would run like a stone. The intake flow would slam into the egdes of the oval ports.
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