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Old 09-01-2011, 08:24 AM
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sbc 350 torque

I've got a 350 from a 1980 k20. Motor dates to 1975. 3970010 casting bored .030 over with 345np flattop pistons. Heads are 3998991. Four bolt main block.
I am looking for torque and mileage. I have a set of 305the heads with casting 14014416 with stock valves. I also have been looking at camshafts and summit 1101 has caught my eye.
What should I do?
Kent

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Old 09-01-2011, 09:14 AM
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Double whammy. First off, depending on which 345 pistons you have there`s two problems.
First off, the older 345 pistons were "rebuilder" pistons. a stock 350 piston has a 1.560 compression height. The rebuilder pistons have a 1.540 compression height. This puts the piston way down in the bore at top dead center, killing compression and quench action. Due to a lack of quench it has awful burn characteristics and to get the engine to have any throttle response you have to advance the hell out of the timing.
However, with the small chambered 416 head the compression ratio will be in the high 9`s, but due to lack of quench and the sluggish combustion it`ll be a detonation monster, you can cut down on the spark advance to keep it out of detonation but because of the awful burn, the result will be it won`t have much power.
Next is, if your using the newer 345 pistons that claim to have the correct compression height of 1.560 the compression ratio with the small chambered 416 heads will put the compression ratio over the 10:1 mark which is too high for 93 octane pump gas and the result will be to run it you`ll have to slow the timing way down and this will kill any power the engine has or it`ll detonate itself to destruction in short order.
If you want mileage then I would have started with a late 80`s 305 out of a firebird, camaro, caprice, etc. The passenger car 305`s came with factory roller cams. Or I would have gotten a later Vortec 305 which also came with roller cams. You can also build 350`s for mileage, but it takes somewhat careful selection of parts. However, do a check, bring a piston to top dead center, then measure how far down in the bore it is then tell us the number you got.

Last edited by DoubleVision; 09-01-2011 at 09:19 AM.
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Old 09-01-2011, 11:01 AM
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The pistons are new. The distance at top dead center is 0.040 and is that the same as deck clearance? The 345np pistons have valve reliefs. What would the effective dome volume be?
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Old 09-01-2011, 01:00 PM
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.040 below deck is not good. There is no "Dome" so I don`t know what you mean. If you mean how many cc`s do the reliefs take it`s 5.
Now lets add it up, .040 below deck, plus .041 thickness head gasket, that`s .081 quench distance, and there is no quench past .060.
I just added up compression ratio on a calculator with the specs given and I get 10.1:1. The only way you could make this work with 416 heads is you`d have to run 93 octane fuel, plus you would have to be very conservative on the spark timing curve. You would have to get the spark advance dialed in perfectly to keep it out of detonation, but of course with less spark timing it`s not going to make much power. What does the rest of the combo look like? What intake? What carb? Headers?
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Old 09-01-2011, 06:02 PM
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My other parts include holley 650 vacuum sec, edelbrock performer intake, headers which I haven't bought yet. The head gasket I have crushed is 0.039. The truck has 4:10 gears and running 33" all terrains.
What do I need to do in order to get good fuel mileage and a lot of torque? What is a good compression trio to shoot for and what do I need to do to get it? Budget isn't necessarily an issue but it is going in an old truck not a race car so I don't want to get too wild.
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Old 09-01-2011, 08:58 PM
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1.21 giga-watts???!!!!
 
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Weld some eyebolts to the 305 heads and use them as boat anchors. Get some 062 or 906 Vortec heads. Put a cam in with around 225 duration at .050 and .470 lift, dual plane manifold, 750 cfm carb of your choice, headers, and your putting out around 420 lb. ft of torque at the flywheel. .045 in the hole? Have the block decked or everything else you do will just be a bandaid.
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Old 09-02-2011, 11:36 PM
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Any input on milling the 3998991 heads to get my CR up?
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Old 09-04-2011, 05:57 AM
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Doublevision is giving you good advice. I started down the exact same path you're on and learned a lot along the way. I have since shelved, sold or gave away everything I originally bought and started over. I'm now using a 350 1 Pc. rear main roller cam block with K-B 118 22cc D-cup pistons, L30 305 Vortec heads, the GM '395 RamJet 350/HT383/Marine hydraulic roller cam with Doug Thorley 1-5/8" Tri-Y headers. Static compression will be 9.17 to 1 with .043 quench. Estimated flywheel HP is 335 @ 5000 with 410 FtLbs. Flywheel TQ. at 2,500 only dropping to 400 @ 4,000. That's with a 590cfm TBI unit. It is replacing a 305 in an '88 C1500 RCLB. I've already had the 700R4 rebuilt and replaced the 2.73 open with a 3.73 posi. My mileage with the 305 stayed the same before and after the gear swap.

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Originally Posted by cool rockin daddy
Weld some eyebolts to the 305 heads and use them as boat anchors.
Why weld them? Just screw them into the accessory bolt holes...............
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Old 09-04-2011, 08:07 AM
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1.21 giga-watts???!!!!
 
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Yeah, after I posted I realized you didn't have to waste good weld rod and could screw the eyebolts in.
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