engine dress up - Pontiac GTO Forum
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post #1 of 10 (permalink) Old 01-21-2014, 12:48 PM Thread Starter
 
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engine dress up

I already have a thread going on painting the engine. In the middle of that process right now. Wanted to also get some advice on some other things I'm thinking about. The engine had just been rebuilt before I bought the car last year. Completely stock. I want to install an edelbrock manifold and carb. Mostly to dress it up, but wouldn't mind boosting performance some also. Edelbrock has several options available. Wondering what the right manifold would be, plus what cfm carb would work best for this engine. Already looked into tri powers, but I don't see anything out there less that $2000. If I can find one.
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post #2 of 10 (permalink) Old 01-21-2014, 01:42 PM
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I know it's counter-intuitive, but for street engines none of the aftermarket manifolds will perform as well as the factory cast iron, especially if it's been "cleaned up" and port matched to the heads.
Also, the Carter AFB on your 65 was a square bore carb - all 4 holes are the same size. In 67 when Pontiac introduced the 400 and with it, the Rochester Quadrajet - those carbs were all spread bore - secondaries larger than the primaries. The Edelbrock Performer and Performer RPM are both spread-bore manifolds, with the "extra feature" that the RPM sits higher than the stock manifold and can cause clearance problems in some applications. Aftermarket square-bore Pontiac manifolds (Victor Jr, Torker II, Northwind) are usually single plane (higher rpm, more oriented towards racing at the expense of low end torque) and also taller. There may be exceptions, I'm just going off of what I'm familiar with.

Soooo... for 100% stock appearance on the 65 and also with good performance, keep what you've got. If you want to step up in performance a little and still be able to pass it off as mostly stock to the casual observer (especially if you don't remove the air cleaner), then go with a later model iron intake with a QJet. (And you could always paint the factory manifold aluminum to change the look if you wanted).

If you'e more concerned about appearance than performance, then it boils down to deciding which format carb you want to run (square or spread) then choosing a manifold to match, and deal with whatever clearance problems might arise (might mean you've got to switch to a thinner air cleaner).

In terms of CFM, on a mostly stock 389 700-750 ought to be plenty - and that's exactly what the "base" QJet delivers.

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post #3 of 10 (permalink) Old 01-21-2014, 05:49 PM Thread Starter
 
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Thanks Bear,,, it still has the old stock carter on it. I really am looking mostly to make it look good, and I've always liked the looks of an aluminum intake. You gave me a lot of good info.
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post #4 of 10 (permalink) Old 01-21-2014, 05:59 PM
 
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Bear, Great idea to paint your stock manifold silver for a "performance"look.
Never thought of that before..

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post #5 of 10 (permalink) Old 01-21-2014, 06:51 PM Thread Starter
 
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Mentioning painting made me remember a friend of mine talking about getting his intake and exhaust manifolds ceramic coated. I think that's what I may do,,,

Thanks again for the suggestions.
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post #6 of 10 (permalink) Old 01-22-2014, 12:03 AM Thread Starter
 
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One more question. If I stay with the stock manifold, what carburetor upgrades are available?
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post #7 of 10 (permalink) Old 01-22-2014, 10:25 AM
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If you're looking for more cfm than the AFB is capable of, then you're probably looking at a square-bore Holley. If it's a more or less stock 389 though, I'm not sure your engine will be actually able to use more than that. Depending on which afb you've got (GeeTee, help me out here) some of them were capable of up to 650 cfm.

There is such a thing as "too big". A carburetor needs a certain minimum air flow velocity in order to function properly. That's why putting something like a 1050 Dominator on that engine would be a disaster. A 389 wouldn't be able to pull enough air to make it work. The trick is to hit the sweet spot where the carb is "big enough" to not restrict air flow to the engine, and the engine is capable of generating enough air flow velocity to make the carb work right.

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post #8 of 10 (permalink) Old 01-22-2014, 10:45 AM Thread Starter
 
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Thanks again Bear, I thought I read somewhere that the stock one is 500 cfm, but really don't know.
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post #9 of 10 (permalink) Old 01-22-2014, 11:30 AM
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You can "rule of thumb" it fairly easily. On one revolution of the engine, only half of the cylinders suck in air (the other half are on their power strokes).
So, for one revolution, a 389 that is running at 100% volumetric efficiency and also creating zero vacuum (neither of which can actually occur at the same time) will pull in at most 389/2= 194.5 cubic inches of air. Let's impose a redline of say, 6000 rpm just to be generous. At 6000 rpm (again at zero vacuum and 100% VE) it's moving 6000*194.5 = 1167000 cubic inches of air, or 1167000 / (12*12*12) = 675.35 cubic feet per minute. Again that's with zero restriction (zero vacuum) and 100% volumetric efficiency. Now we already know we need SOME vacuum for the carb to function at all, so we have to have at least a tiny bit of restriction/vacuum, and we also know that these engines only run at maximum VE at a very narrow, specific rpm (usually very close to where they make maximum torque). Let's be (very) generous and say the 389 is still at 80% VE at 6000 rpm. That drops the maximum "air capacity" down to 540 cfm and we're still not considering that we need some vacuum to make the carb work.

Things aren't that simple still, though. Carburetor manufacturers understand that they need vacuum to work, so the CFM measurements/ratings are taken while the carb is operating at a specifc standard vacuum. (I forget what the standard is, or even if all manufacturers use the same standard.) Often, the carb will continue to function and feed fuel at a lower vacuum than what is used to obtain the rating though (up to a point).

The above is why some engines will benefit (make more power) with a carb that is rated slightly larger than what they theoretically "need". Putting a "larger" carb on the engine will create less of a restriction (vacuum) which will allow the engine to move more air (and make more power), in other words, allow it to get "closer" to its theoretical maximum air handling capacity at that rpm - and as long as there's still enough "vacuum signal" to cause the carb to accurately meter and feed fuel, we'll make more power. Go "too big" through and things go down the tubes (or rather, fail to go down the tubes ) in a hurry. You'll find that the car becomes very difficult if not impossible to tune, especially at idle, because the carb isn't getting enough vacuum signal to operate consistently. It'll probably bog like a big dog whenever you hit the throttle because the off-idle transition ports won't work right, if at all. It'll be an all around miserable experience.

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post #10 of 10 (permalink) Old 01-23-2014, 06:38 PM
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My personal experience: the carter AFB that came on these early GTO's works very well. Better than an Edelbrock carb, which is a poorer quality copy of the AFB. With the stock AFB intake, I have run Holley 750 carbs with a short spacer plate and power was increased...pretty noticeably. Gas mileage also decreased quite a bit. My 2 cents: stick with the Carter and clean it up and detail everything. Or, swap on a '68-'72 Quadrajet intake and carb and have power and economy, as Mr. Bear stated. I've had a lot of these cars, with 389 power and still have one, so I kind of know what works and what doesn't by trial and error over the years.
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