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BBC intake mods

General engine tech -- Drag Racing to Circle Track

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Machtuck
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BBC intake mods

Post by Machtuck » Fri Nov 01, 2019 7:27 am

Hi everyone, i have a pancake, low rise 1971 bbc cast iron intake. Are there any known mods that could be performed other than extrude honing? Plan on running a 3/4” quadrajet spacer. I’m assuming that it is a smog intake but maybe not as smoggy as the later model intakes that have the EGR. This one is devoid of the EGR.
Thanks

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Re: BBC intake mods

Post by mag2555 » Fri Nov 01, 2019 7:35 am

Other then porting or sending it out to Brezinsky for rework the next best thing to do is to run a large Carb .
The larger then needed Carb will slow down the excessive runner air speed above 4500 rpm and allow rising power numbers to be made and a slower nose over of such numbers.

If your running a Q jet era Manifold then this is where a 1000 cfm Carter themoquad will be the way to go if not restricted by some rules.

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Re: BBC intake mods

Post by MadBill » Fri Nov 01, 2019 5:59 pm

mag2555 wrote:
Fri Nov 01, 2019 7:35 am
Other then porting or sending it out to Brezinsky for rework the next best thing to do is to run a large Carb .
The larger then needed Carb will slow down the excessive runner air speed above 4500 rpm and allow rising power numbers to be made and a slower nose over of such numbers.

If your running a Q jet era Manifold then this is where a 1000 cfm Carter themoquad will be the way to go if not restricted by some rules.
Ummm.. I'm sure a big Carter carb will help, but how does a higher-flowing carb slow down runner velocity and why would you want to? :-k
Felix, qui potuit rerum cognscere causas.

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Re: BBC intake mods

Post by mag2555 » Sat Nov 02, 2019 6:37 am

I say this because back in the late 90s I had a .030" over 455 cid street strip motor of mine one a dyno and the Q jet was giving us issues so we got that worked out and with a 1 inch spacer ended up making 406 hp at only 4400 rpm, this was with a Performer Intake.

Air consumption was only 645 cfm so the 750 Q jet had a good amount of cushion I would say , yet just for S & G we slapped on the a Holley 800 and picked up 12 hp, so draw your conclusions from that.

My money was on better average port air speeds more inline with what that cid motor could tolerate thru those size Intake runners and the fact that the bigger Carb pressurized the Plenum better.

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Re: BBC intake mods

Post by MadBill » Sat Nov 02, 2019 6:19 pm

The bigger carb would indeed pressurise the plenum better, aka reduce manifold vacuum.

The standard CFM rating of American 4 bbl. carbs is at an arbitrary 1.5" Hg. pressure drop; considered in the fifties when 4 bbls. became common, to be typical of then-current passenger car engines and not coincidentally, measurable with the same capacity flow benches that tested 2 bbl. carbs at 3.0" Hg.

The only magic in having a carb whose flow rating matches your engine's peak air consumption on the dyno is that it should in theory result in exactly 1.5" of manifold vacuum at that RPM. Unrestricted race engines commonly use much higher CFM-rated carbs in order to increase the density of the inducted air. For example, as long ago as the late sixties, some Ford factory Trans Am Series Mustangs used a pair of 1,000 CFM plus Dominator carbs on their 305 c.i. engines whose theoretical air consumption at 8,000 RPM would have been ~ 700 CFM. This would have resulted in a manifold vacuum of ~ 0.6"Hg at peak revs. and probably 35 HP more than a single 700 CFM carb.
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Re: BBC intake mods

Post by F-BIRD'88 » Sat Nov 02, 2019 6:43 pm

Why not just use a better high rise dual plane intake on this?

What car?

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Re: BBC intake mods

Post by Blakeslee » Sat Nov 02, 2019 7:23 pm

X 2 ^^^^^^

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Re: BBC intake mods

Post by tenxal » Sun Nov 03, 2019 7:49 am

Machtuck wrote:
Fri Nov 01, 2019 7:27 am
Hi everyone, i have a pancake, low rise 1971 bbc cast iron intake. Are there any known mods that could be performed other than extrude honing?
There are several shops that can internally port the intake for some serious power gains. Brzezinski is the most prominent among them. Their full out intake work requires machining 4 holes into the intake and then plugging them after the work has been done. If you want the intake work to be absolutely undetectable, they offer a couple of different options...acid porting being one of them.

There are also shops that specialize in 'prepped' intakes for NHRA Stock Eliminator class that have as much internal work done as the cut apart Brzezinski intake but they don't cut the access holes. All the work is covered up and the internals and externals appear stock. It's time consuming and expensive. But they will pass any NHRA tear down. These guys are true cast iron artists. :)

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Re: BBC intake mods

Post by Coloradoracer » Sun Nov 03, 2019 9:11 am

Unless you plan on running this in a class where that intake is REQUIRED, your money will better be spent on a newer, better intake. Lots of choices here.....
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Re: BBC intake mods

Post by Machtuck » Mon Nov 04, 2019 7:08 pm

Thanks for the replies. Yes, going to bolt on the performer RPM. So much for trying to make a sleeper out of this. The cam is a solid flat tappet and makes power from 2500-6500. Don’t believe the low rise cast iron is going to cut the mustard.
Thanks again all.

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Re: BBC intake mods

Post by Machtuck » Mon Nov 04, 2019 7:15 pm

Thanks for the info tenexal. I’ll look into it.

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Re: BBC intake mods

Post by F-BIRD'88 » Mon Nov 04, 2019 8:38 pm

Consider the DuaL QuaD Performer RPM AIR GAP
Intake manifold.
Its powerfull.

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Re: BBC intake mods

Post by turbo camino » Tue Nov 05, 2019 12:19 am

Paint everything black and put 'Goodwrench 305' stickers on the valve covers.
DON'T PANIC

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Re: BBC intake mods

Post by Machtuck » Wed Nov 06, 2019 9:18 pm

Bought a performer rpm. Interesting info on the late 60’s trans am era.

I like that goodwrench sticker idea. 😂😂😂😂😂

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Re: BBC intake mods

Post by turbo camino » Wed Nov 06, 2019 10:42 pm

In my experience the black paint, stickers, bench seat & column shifter were enough to cancel out any complaint about the single stage of nitrous. "Oh come on, how fast could it really be? Look! It's even still got the EGR valve on it!" <snicker>

(EGR ports went nowhere in the manifold, and the nitrous jet was somewhere north of #100, but hey...)
DON'T PANIC

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