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Old 12-24-2010, 09:55 PM
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Engine 305, Adjust help

This was my first time swaping a cam. I left the heads stock on my 305, but i installed the following:
-Pistons Std. Bore Size 3.766 Comp.. Height 1.54 Pin Diameter 0.9273 offset
-Comp Cams Magnum Camshaft, Hydraulic Roller Tappet, Advertised Duration 280/280, Lift .525/.525.
-Rocker Arms, Stud Mount, Self-Aligning, Roller Tip, Steel, 1.52 , 3/8 in. Stud
-Valve Springs, Dual, 1.430 in. Outside Diameter, 322 lbs./in. Rate, 1.150 in. Coil Bind Height.
-COMP Cams Pro Magnum Hydraulic Roller Lifters.
-Carburetor, Quadrajet Stage 2, 750 cfm, 4-Barrel, Spread Bore, Single Inlet.
-TH 350 Shift Kit, Transpak, GM, TH350C
this is my first time working with this kind of modification and I spent a several days looking for answers about the adjustments or settings for this configuration. I would like to know if the combination mentioned is correct? If you have any comments or some knowledge about this, for example advance time adjust, I would appreciate your opinion. Thanks in advance.
P.D.I would like install a Ignition Box, MSD 6AL-2, Digital CD, with Rev Limiter
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Old 12-24-2010, 10:22 PM
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In general, that cam is ill suited for the 305. The cam requires more compression than you're going to have using that set of heads. The torque converter needs to be a higher stall than stock, the rear gear ratio needs to be lower than stock.

The heads will need to be modified to use those valve springs- the spring seat is for 1.25" OD springs. You'll need long slot rockers, possibly the pushrod holes opened up unless these are rail rockers, the guide boss might be too tall, the stock spring retainers won't work, the pushrods may need to be a different length to maintain the correct valve train geometry.

Here's a list of some areas that need to be verified to be OK. It is not all-inclusive:
  • Springs for installed height
  • Coil bind (+/= 0.040” between all coils)
  • Retainer to seal/ valve guide boss at full lift
  • Retainer to rocker
  • Trunnion to stud boss
  • Piston to valve clearance (both before and after TDC on overlap)
  • Push rod to guide slot in head (if used)
  • Can use EITHER self aligning rockers OR pushrod guide slots in head OR guide plates, not two or more of these
  • ”Rails” of self adjusting rockers (if used) to retainer
  • Rocker to valve tip (by adjusting guide plates if used)
  • Rocker slot to stud (if using stamped/cast rockers w/pivot balls)
  • Proper geometry (push rod length)
  • Rocker/polyloc to valve cover or baffle
  • Camshaft endplay
  • Distributor shaft and gear endplay
  • Distributor gear material compatible with cam material
  • Hydraulic lifter preload
  • Timing set phasing
  • Cam gear to crank gear alignment
  • Cam gear end play if roller cam
  • If using the SBC OEM roller lifters and retainers (aka dogbones), lobe lift must be kept below 0.354” so the lifters don’t lose contact w/the retainers
  • Cam lobe to connecting rod on strokers

These things have to be physically checked, you should not take it on faith or by figuring it out on paper.
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Old 12-25-2010, 05:36 AM
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I`ll follow that with the break down of, the problem you`ll have with it is, too much cam, not enough engine. The 305 has a small bore, which restricts breathing potential. The pistons you have use a 1.54 compression height, which puts them counting the deck clearance and gasket thickness about .070 down in the hole at top dead center which is going to give it sloppy combustion characteristics and combined with that cam, the compression ratio is knocked down to about 7.5:1. The proper compression height for 305, 350 and 400 engines is 1.560. Rebuilder pistons that are cheap and sold in high volume to shops are 1.540. The carb is too big for a 305. a 600 is all a 305 needs. Out of the hole it`ll be a turd and won`t start to make power until about 3500 RPM, then it won`t be much as the small bore and valve size restricts breathing. Just saying, your combo is somewhat mismatched.
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Old 12-25-2010, 11:23 AM
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thanks guys, as I said previously I am new in this type of modifications,the engine is running, it feel great but not perfect,i adjusted initial advance time to 10 degrees, the mixture air/fuel is too rich, you can smell gas,
the intention of a big carburetor and a big cam is to place turbo, What would be my best choice for a turbo?
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Old 12-25-2010, 02:50 PM
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you actually should have a diffrent camshaft for a turbo.. dual pattern with a 112/114* LSA will be best.. as far as what turbo, that depends on if your going twin or single and intercooled or not.. the carb is fine. Chevy put Q-jets on 305's from the factory
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Old 12-25-2010, 03:16 PM
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they also put Q-jets on 6 cylinders, but a Q-jet is a air dam carb, it only gives what the engine demands. The Q-jets small primaries made it work on a variety of engines which was the whole point.
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Old 12-25-2010, 08:23 PM
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What cam LSA? What head castings?

Generally, I don't have as big a problem with your combination as the others do, depending on what your actual compression ended up at. I also doubt that you'll get the typical dual plane intake/spread bore/quadrajet working well at all without boost, and worse with boost.

Without more info we're really all just guessing, but I would guess that you probably need more timing down low and less up top, you might even be happy with locked out timing somewhere in the 20-25* range, plus or minus depending on your actual comression

What turbo/turbos are you planning on running?
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