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Old 03-14-2008, 04:21 PM
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350 SBC head numbers don't match

Oh boy.
I think I opened up a can of worms here, but this is my issue: I have a '34 Ford and was told it had a 350 bored .060, Corvette 2.02 heads, cam is 284 duration - 0.480 lift (that's all I know), 9:1 compression with Rochester 2G tri-power. The cam just seems too big for cruising, it idles like a dragster and bucks in first gear but runs great at 2500+ rpms. I replaced the ignition, rebuilt & adjusted the carbs, all else seems OK. It just seems to have a big cam. The plugs have a nice light brown color but the exhaust smells very rich.

So, I decided to pull the cam to see what it was and replace it with a milder one then as I checked the casting numbers on the heads...they don't match!

1) 3973487: '68-'72 350, 1.94/1.5 OR 2.02/1.6 & 75cc chamber

2) 3973493: '71-'72 400, 1.94/1.5 & 76cc chamber

I found several websites with info, but the bottom line is I don't know if my heads match or not. Worst case is one side is a 1.94 intake and the other is a 2.02?

Any advice is appreciated.
Thanks,
DB

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Old 03-14-2008, 05:22 PM
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Without knowing the history of the heads. It would be impossible to tell valve size from the outside. Neither head had factory 2.02 intake valves. Neither head is unlead gas compatible and will have seat problems with more than light use.

The reason for the poor performance and rich gas. Is probably the cam. A lot of those old ".480" cams were long duration, borderline stock convertor, bad idle quality, soggy bottom turds, if used in a basic stock engine/drivetrain situation.
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Old 03-15-2008, 10:36 AM
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Update after I removed the cam...

The Crane cam # is 10018, 286H06: Lift = 465, ADV duration = 286, Duration = 230/230.

Basic recommended RPM = 3200-6200 (rough idle) which is much too big for my cruising needs.

I'll leave these heads on for now and see how it performs after I decide on the right replacement cam (I'll research this board for cam recommendations, any cam tips for a 350 would be appreciated)

Thanks,
DB
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Old 03-15-2008, 12:50 PM
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Its all in the setup:
If you were to recurve the distributor with a spark timing curve that works with that cam and then modify the metering rod step up springs in your carbs so that they stay down at idle with the big cams reduced idle manifold vacuum, you would find that cam is a lot lot easier to live with.
That cam need a lot of initial spark advance at idle 24deg at idle works well.
This requires that you modify , limit the amount of mechanical advance travel in the distributor for about 10-12deg so you get a curve with 24 initial base and 34-36deg total mechanical advance all in by 2800-3400rpm with a smooth fast curve. This involves more than just swapping advance springs.
You have to modify the advance limit bushing and/or advance pin slot length.
Then you can add 10 to 15deg of vacuum advance for cruising using ported vacuum. (adjustable vacuum can)
The manifold vacuum at idle with a cam such as that will be much less than a "stock motor". This requires that you change or modify the carbs primary metering rods power mode step up springs to account of the reduced idle manifold vacuum. (Less spring tension) You need to look at your springs and see if a spring from a Qjet or Edelbrock performer carb will work as a replacement to control the rods. (lighter spring tension to allow the low(er) manifold vacuum to hold the rods down in the jet at idle)
Edelbrock carb tuning service parts.
Measure the idle manifold vacuum (vacuum gauge) and pick a power step up spring with a lower rated opening point based on manifold vacuum at idle so the spring cannot lift the rods at idle. I suggest a 3.5" or 4.5" rated spring
Before reinstalling the carbs, reset the throttle opening (idle speed adjustment on all three carbs so that the idle transfer slot is just barely exposed under the edge of the throttle plate (.020-.030")
(The final adjustment my take some experimentation but must be within the critial, narrow "sweet spot" or the idle and off idle response will be all wrong.)
If the carbs throttles are too far open at idle the idle quality will be way off.

Once you've done these mods the smelly exhaust and poor throttle response will be gone. That cam needs a looser than stock torque converter. A 3000-3500 stall 10" converter is best.
If you have a "stock converter" you would want to change it to a high stall.
Needs a fairly high rear gear ratio unless the car is very light.

The fact that you are having diveabilty problems with a cam such as the one you have is very common, especially if you don't know what needs to be done to dial it all in.
Once all dialed in as I have outlined, the motor will have a rough but rock steady and clean idle at around 800rpm. The throttle response will be very good and it will not tend to "load up" on ya or "buck". The fuel mileage while cruising will be much better than before.
If your primary carb has a 3/8" PCV port on the base then you want to hook up and use a functional PCV valve and valve cover vent on it. Its part of the carbs orignional design and if you eliminate the PCV, you throw off the factory idle calibration in a few important ways.
The motor will work much better with a functional PCV.

Cylinder heads:
Remove the valve covers and look for push rod guide plates. A clue to the valve size. Other than that the heads are more or less functionally equal.
I would not loose sleep over it. If these 76cc heads are combined with dished pistons then the cr will be too low for best performance with that crane cam.
9.5 to 10:1 is best. A 305 head swap is a easy effective affordable solution.

If you don't want to make these changes to make your car work with the cam you have now, then just replace it with this cam+life set
It has much higher manifold vacuum, much smoother idle and low speed response. Should not require modding your carbs. Works with a stock torque converter. Can use a moderate spark advance recurving for best performance. The idle sound is "noticable" but certianly not "rough". The price is right.
Very good moderate street power and torque with very good cruising manors.

If you are willing to make the necessary tuning changes than you'd find the Crane cam you have now to be much more cruiser friendly even with the rough racey idle, but all the changes I outlined are nessessary to bring it all together. It's all in the set up.

Last edited by F-BIRD'88; 03-15-2008 at 01:31 PM.
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Old 03-16-2008, 07:15 PM
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F-BIRD'88, thanks for the great detailed info. I wish I was smart enough to make all of the required changes (or have you as a neighbor!). I'll go with plan B and install the smaller cam and keep the same heads (PS: they don't have push rod guide plates).

It's interesting the part about running the old cam timing at around 24 degrees initial. I thought something was wrong with my timing light or the degree tab on the timing cover. It just wouldn't run at 10-12 like every other SBC I had so I did run it around 24 and it made me nervous. Good call, at least I'm not too crazy.

http://www.hotrodders.com/gallery/sh...hp/photo/33578
http://www.hotrodders.com/gallery/sh.../cat/500/page/
DB
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Old 03-16-2008, 08:44 PM
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Gotta love that Tri-power induction
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Old 03-16-2008, 09:15 PM
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We had a similiar situation with mismatched bbc heads but they were 7 or 8 cc's off, could never tune that motor to run as it should. Doesn't sound like that is your problem but the standard issue 400 head has 194 intake and 160 exhaust valve which was unique to the big inch sb, not sure that would be the issue but worth noting.
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