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Old 03-27-2008, 06:00 AM
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Camshaft Options for SBC 350

I am rebuilding a 1988 GM 350 motor. I had to go .060 over to get the cylinders right. I'm looking for a reliable, smooth running motor that I can get some reasonable gas mileage out of using regular fuel. I have refreshed the heads and will be using stock valves and valve springs. New pistons of course mounted to the original rods. The rods are in great shape and match original specs. The crank has been polished and balanced. I am seriously considering a stock cam to get the reliability and smoothness I desire. I have been told I could go to a more aggressive cam for better torque without compromising smoothness or mileage. But I don't have the experience to be able to judge what lift or duration would be a decent option. Any suggestions on how to go about evaluating different cams that would perform to my expectations?

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Old 03-27-2008, 06:21 AM
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carb or fuel injection?
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Old 03-27-2008, 07:50 AM
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Any Small duration with high lift cam will give you better gas mileage over the stock.
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Old 03-27-2008, 08:12 AM
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Look for a cam with an intake duration in the low 200's (200 to 206) and exhaust duration between 206 and 212 at 0.050". Try to use about 6 more degrees on the exhaust as compared to the intake to help the exhaust flow. Stock exhaust ports and stock exhaust manifolds tend to flow poorly. If you plan to use a carb get a cam with 108 to 110 lsa. efi does better with 112 to 114 lsa.
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Old 03-27-2008, 09:21 AM
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Also, try to find the most lift for a cam with a max of about 210/218 adv. duration. Anymore and you'll start to see milage suffer though power will still increase. For smoothness I would go with a 112 LSA... you'll loose a tiny bit of power but it really helps smoothe the engine.
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Old 03-27-2008, 02:17 PM
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Cam and head gaskets will be the two most important choices you will make for this motor. The cam must be chosen with the correct intake closing point to support the static compression ratio of the motor and the gasket thickness will determine the squish (piston crown to cylinder head clearance with the piston at TDC)

You are in the same position most guys are when building a motor, starting off choosing parts without knowing the particulars of the motor. To choose the correct cam, you must know the exact static compression ratio of the motor. To choose the correct gasket, you must know the piston deck height (distance from the piston crown to the block deck surface with the piston at TDC).

You'll need the piston deck height first, so measure the depth of the piston in the bore. This will tell you the correct gasket thickness to use so you can add the gasket cc's to your formula to find SCR. You'll be shooting for a squish (piston deck height added to gasket thickness) of 0.035" to 0.045".

To find the SCR, you'll need 5 values. Cylinder cc's, combustion chamber cc's, piston deck height cc's, gasket cc's and piston eyebrow or dish cc's. I'll start you off with the cylinder, 738 cc's.

This might be a good opportunity to clarify the difference in deck height nomenclature for some of you.
BLOCK DECK HEIGHT: the measurement from the centerline of the main bearing bore in the block to the block deck where the head gasket sits. As produced by the factory, the block deck height on all small block chevies is 9.025".
PISTON DECK HEIGHT: the measurement from the top of the piston crown (the tallest part of the crown, right next to the cylinder wall) to the block deck where the head gasket sits. This measurement can vary considerably depending on the true rod journal throw from centerline, the true center to center length of the rod, the true compression height of the piston (centerline of the wrist pin to the crown) and the true block deck height. Piston deck height on motors from the factory might range between 0.025" and 0.035" for instance.

Last edited by techinspector1; 03-27-2008 at 02:52 PM.
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Old 04-01-2008, 11:03 AM
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techinspector1 has managed to convince me that I need to go to cam school for a year to begin to understand what he is telling me. I understand the basic mechanics and even the timing and lift concepts discussed. But I'm a simple thinker. All I was trying to learn was a coulpe of brands and part numbers of mild cams for a basicly stock motor that has been punched out 60 thousands. Don't get me wrong, I truely appreciate the technical reply. I just have to take some time to absorb it so I can apply it wisely. Thanks for the compressed lesson.
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Old 04-01-2008, 05:52 PM
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Another set of variables to consider are the weight of the vehicle, torque converter stall speed, and tire size/axle ratio.

You can put a lot more cam in the engine if its a light vehicle with high stall converter and low axle ratio (high number). I have a GMC 3/4 ton pickup (heavy) with a stock TH350, and the cam duration of 222/222 at .050 is really too much cam. It would be a lot better in a lighter vehicle or with a high stall converter. I'm looking at moving to about 206/212, or even 195/202 to get more low end torque and better gas mileage.

Bruce
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