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'39 Buick Street Rod, intake question


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Let me start by saying that this 1939 Buick Century car project was not initiated by me. But I will finish it one way or another. The story... Someone has transplanted a Chevy 350 Corvette engine with earlier camel hump heads and a Holly Contender intake and Rochester Quadrajet. While I like the beast, the engine has a Comp Cam 12-213-3 grind which has a large valve overlap resulting with 6 in of Hg vacuum and a very lumpy idle which I do not like. In changing the cam to a more moderate grind, I have noticed that the Holly intake does not accommodate for any coolant flow from the rear of the cylinder heads. Is this a shortcoming or is it OK? Other Chevrolet engines that I have worked on had intakes with coolant flow front and rear. Advice and comments are appreciated, The intake is a Holly Contender 300-38. The new cam is a Comp Cam 12-21-2. New lifters and push rods also. Hushpower dual exhaust mufflers follow the headers. Should be nice when finished.

Edited by Brad30 (see edit history)
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Hey Brad, appreciate you letting us know that you didn't initiate the project, especially the Chevy engine part. :) If you determine the engine won't work for you, just think how cool a Nailhead would look sitting in that 39's engine bay. In fact I think it would look so cool!!! that I will give you a good clean (albeit disassembled) 364 Nailhead if you will make the swap and come down to Buick Gardens and pick it up. If not I understand and look forward to hearing more. I had a '65 Corvette 396 engine (insurance rated at 425 HP) swapped into my black on black '67 Chevelle SS back in the day. With posi traction, had to take a night job to afford tires.

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Small block Chevy manifolds,most of them,block the rear coolant passages.I have seen some that people modify by drilling and tapping holes for a crossover pipe/hose between the rear coolant passages.Yours will be fine.

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The cylinder head castings are the same for both sides. With the Chevy distributor in the rear of the engine AND using the intake manifold as a mounting point, there's not really enough real estate back there to have coolant flow through the intake manifold back there. Hence the intake manifold gasket sets with block-off "tin" plates at the rear of the gasket.

You'll need to find out what the QJet's number is to see what it originally came off of. No doubt, with that low manifold vacuum at idle, there must have been a huge mis-match of the throttle plate location (% open) compared to the idle and off-idle transition ports in the throttle body itself. Having a stock-type "tight" torque converter will be a negative in this situation, too!

If the "new" cam is, indeed, the Comp 280H Magnum cam, compared to the 292H Magnum which was "not workable", it's NOT much better. In order to use a reasonably stock Q-Jet carb would need something more like the 268H High Energy cam. From my friends' experience, the 268H HE cam is a great cam in a Chevy 350 with headers. In one '79 Corvette (the only year with 3.55 gears as standard), the 268 cam worked great with the slightly-deeper than normal rear axle ratio as the car was cruising on the highway in the "meat" of the torque curve when "cylinder filling" was more optimum. End result, very little loss in fuel cruise economy with much better performance.

Personally, I'd recommend the 0-9895 Holley 4175 spreadbore as it was orig for the L82 Corvette engine (with a cam much closer to what you're going to end up with) and should require less additional tuning to work with your "smaller big cam". PLUS it's much easier to tune and modify than the QJet.

With the 110 degree Lobe Centers, compared to the typical stock cam's 114 degrees, you'll need to put a quicker curve into the distributor, just have the whole mechanical advance thing start at about 1200 engine rpm and be "all in" by about 3500rpm. Many aftermarket kits can accomplish this for you, but the stock curve will NOT work and make you wonder what you did to end up with a significant performance DECREASE.

I had a '77 Camaro with the stock 305 in it, into which I upgraded to the Comp 260H HE cam equivalent. I added a Holley 28-Z single plane/divided plenum/Z-Line intake with a 600cfm Holley 4160 (emissions spec for a '76 Chevy 350). After I got the ignition advance where it was working decently-well, I then sought to get the idle speed/mixture better finessed. At idle, in gear, it had 10.5" Hg, which was just above the first-step enrichment of rthe Holley's 2-stage power valve. I played with it and got a little improvement, but not much. Fuel economy for this upgraded combination dropped only about .4mpg, average, from what it was with the factory 2bbl and cam.

Later, like about 250K miles later, I put the 0-9895 on it, back when it was in the "first gen calibration" specific to the OEM application. Fuel economy immediately went up 2mpg+, such that average mileage was now 20-21mpg, with it hitting 23.5mpg on a 60 mile highway loop. Idle speed and mixture could be set to what seemed to work better.

Now that a similar carb is sitting on top of a 350 in that same car, same cam, I have no doubt the idle vacuum is better. I never did check it though, as it seemed to work better. The later 0-9895 carb has now been changed to allow for more of an "aftermarket" high performance use rather than the OEM-spefic "replacement" carb for the orig ('79?) L-82 Corvette application. Idle mixture/speed calibrations were also aided by replacing the orig THM350 by one (an OEM-GM close-out sale "crate" trans) spec'd for a '81 Corvette L82/Z-28 applications with the V-6 torque converter in it.

IF the Holley intake is the "-38", rather than the "-36", you're going to have one heck of a mis-match, it seems to me. While many of the forum operatives speak of the -36 in the same revered tones as the orig Chevy Z-28 high-rise aluminum intake, the -38 is no more than a less expensive "4bbl upgrade" version of the GM "normal" 4bb low-rise intake manifold. The later would have a spreadbore or dual pattern for the carb mounting, but the former would have only the "holley" pattern on it. GM Perf Parts has a dual-plane intake they use on the ZZ_ crate motors, which is supposed to make the same power as the high-rise Z/28 intake, but in a low-rise configuration . . . it has a GM part number cast into the front section.

The "double hump" heads were nothing special either. If they've been upgraded to the 2.02/1.60 valve sizes, with the intake side of the chamber "clearanced" to unshroud the 2.02 intakes, then they might be better, but still nothing to be able to use the "smaller big cam" you're putting in it. Not enough "high lift flow" unless they've been extensively worked, if I remember correctly.

With those "earlier" heads, the (apparently) low-rise intake, the QJet of undermined origin . . . you'd be much better of with nothing larger than a Comp 272H-size cam. If it's that lumpy idle you're after, the 268H HE can do that if you set the base idle down to about 500rpm, from the more-normal 700rpm. One of my associates put the 268H HE in a '70 Monte Carlo he was driving. At idle, it sounded really radical, although he claimed it was the 268H HE. When I adjusted the idle speed up to about 650, the idle got as smooth as could be . . . with just the idle speed change. I was surprised! He kind of frowned . . . he liked it the other way, but now we knew how it all happened, which was good.

Sorry if I might have "clouded-up" your project, but I'd hate to see it not come out great for you.

Take care,

NTX5467

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