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Old 06-03-2007, 11:20 AM
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Building a 671 SBC Motor

Hey, Just wondering if anyone can give me some tips on what to buy for a 671 build on a 350. Heads, rotating assembly, etc....

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Old 06-03-2007, 02:15 PM
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Blown motor builds are really straightforward, I'll touch on them here. Forged pistons @ about 8-1 compression, Prefered four bolt main, less so a steel crank..unless very high power levels are sought, Use a "blower" cam (very little overlap) .
You need to think about pump gas or racing gas & how it relates to how much boost you will be running (Overdrive or underdrive....pulley diameter relationships).
Full kits are readily available or you can piece together the appropriate parts by using these simple guidlines. How much power you want will ultimately determine all these different things.
_________________________________________________R ick.........
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Old 06-04-2007, 08:12 PM
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Use a double key blower drive hub instead of a balancer and a double key crank. The stock balancer and single key are not strong enough for a blower drive.

http://blowerdriveservice.com/catego...pany_id=101011
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Old 06-06-2007, 08:00 PM
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thanks for your help its greatly appreciated. if i want to run pumped gas would it be o/d or u/d?? also do i run closed chamber heads?
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Old 06-06-2007, 08:12 PM
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I don't think it matters on what heads you run as long as the compression ratio is around 8 to 1.
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Old 06-13-2007, 08:50 PM
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Recommendations from BDS.....
ENGINE BLOCKS BDS suggests that the engine block be in good condition and not excessively over-bored. Two bolt mains are adequate for most mild applications with boost levels up to 7lbs. Four bolt mains are recommended and are considered a must for high performance systems. O-ringing is recommended for engines running 12lbs. of boost or more. When rebuilding, the block should be thoroughly checked as you would in any high performance engine build up.
CRANKSHAFT Steel Cranks are recommended whenever possible and are a requirement for high performance engines spinning high RPM's. Cast cranks are only recommended when the boost levels are below 7 lbs and the engine is limited to 6000 RPM. When rebuilding, cranks should not be less than a 10/10 grind with 2 keys 3/16's" and ¼ " 180 degrees from each other and should have all the trick work as you would for any high performance engine.
RODS Most factory rods will work well for mild blower systems up to 8-10 lbs. of boost. Factory and after market steel rods with heavy duty rod bolts are recommended and required for high performance applications. After market aluminum rods are recommended and required for high performance applications. The rods should be magnafluxed for cracks, shot peened, beams polished, balanced, and bushed to size for full floating pins.
PISTONS Factory cast pistons are recommended but may be used in very low boost (3-5 lbs.) applications. Forged, low compression pistons (7-8:1) are the best choice for performance applications. Higher compression ratios are not recommended because of overheating and excessive final compression ratios (see the final compression ration chart). Pistons should use full floating pins and double spiro locks or buttons for high performance applications. For street applications. Standard rings will perform well on pump gas,for high performance engines, we recommend stainless steel rings. In cases where alcohol is used, the compression ratio of the engine should be between 10-12:1.
HEADS Factory heads work well in most blower applications. The heads should be in good condition or have a three angle valve job. After market heads will provide increased performance. Stainless steel valves are recommended. Head modifications (porting, polishing, etc.) are not required unless high performance is the desired result. Resurfaced or shaved heads can cause problems with the blower and manifold. The secret to horsepower is cylinder head air flow. More air flow equals more horsepower.
CAMS Choosing the proper camshaft would be the most important requirement for a blower motor. An improper cam will cause a variety of problems that can easily be avoided by following a few simple guidelines. Hydraulic cams are recommended if you intend to drive the vehicle frequently, requiring little or no maintenance, and the maximum engine RPM's are kept around 6500 or lower. Roller rocker arms are recommended. Flat tappet and roller cams are recommended for high performance applications especially where the engine will see high RPM's. Exact camshaft specifications vary depending on the performance level you wish to attain. BDS offers ten different types or stages of cam grinds specifically made for blower motors. Refer to camshaft specs listed in tech info for BDS' individual engine camshaft specifications and their intended uses.
If you wish to purchase your cam from one of the many fine camshaft manufacturers, we suggest using our camshaft specs as a guidline. Extremely high lift and long duration cams are recommended for high RPM, high performance racing only.
The lobe center of the cam will play an important role in determining the performance characteristics of an engine. Wide lobe centers (112 to 114 degrees etc.) will create higher cylinder pressure providing more horsepower with cooler burning fuel such as alcohol and methanol. We have found 110° lobe centers to produce the best overall power on gasoline.
Whatever cam you choose, make sure that it will operate and perform properly in the RPM range required for your application.
CARBS AND FUEL INJECTION The overall performance of the entire engine package will be determined by the fuel induction system. Carbs work very well in most applications as long as the carbs have been calibrated or blue printed by BDS or another reputable company. Refer to the carburetor section for help in determining the correct carbs for your needs.
Mechanical fuel injection will provide greater performance and throttle response than carbs. However, these mechanical injection systems can be quite temperamental and are recommended for the experienced racing enthusiast only.
BDS Electronic Fuel Injection offers you the best of both worlds. Retaining all of the drive characteristics of a carburetor system with the performance and looks of mechanical fuel injection system. We deliver the 'Best of the Best', Performance-Looks-Economy with the driveability and ease of operation in a single package. For more information, please refer to the Electronic Fuel Injection section and/or EFI catalog.
IGNITION The ignition system and advance curve are very important to a blower motors longevity and performance. The general rule for ingnition timing in a blower motor is as follows: Initial advance at idle should be set at 16-24 degrees with the total advance of approximately 32-36 degrees, all in by 2500-3000 RPM. It is very important to verify the advance curve. Locked out magnetos or distributors are recommended for racing applications only. Improper curves may cause a variety of problems including overheating. Spark plugs should be one or two heat ranges colder than the recommended stock factory plug (never use extended tip spark plugs). The colder plugs need to be used due to the higher cylinder pressure created by supercharging; higher cylinder pressure means more heat. Ignition management systems that can vary the timing according to engine requirements are a good idea to help keep the engine from killing itself with detonation and to keep performing at its maximum.
COOLING SYSTEMS The cooling system for a blower motor should be in a good general operating condition. Inadequate air flow across the entire radiator at low speeds is one of the most common causes for overheating. Mechanical fans and shrouds are highly recommended. In a recent study of electric fans, especially anything from 18 to 20 amps with a 3000-4500 CFM capapbility, these fans seem to work efficiently on blower engines, but it may still require some experimenting with location to find the best operating position. A 180 degree thermostat is recommended. Water flow restrictors may also be used, however, you will have to experiment to find the size that works best with your system. Stock factory water pumps are recommended and required in most applications. After market "High Performance" water pumps work best in the mid to upper RPM ranges and therefore may not have adequate water flow at lower RPM's to keep a blower motor cool. Three core radiators or larger are recommended for most applications. Higher performance engines will require better cooling systems because of the additional heat generated by these types of engines.
EXHAUST SYSTEMS Exhaust systems are very important to the overall performance of the blower motor. The blower forces more air into the engine than it would normally take therefore the engine must be able to get rid of more air through the exhaust. Small restrictive exhausts will cause excessive back pressure, robbing the engine of power and causing additional heaing problems as well as unusually high boost readings. Large free flowing exhaust and headers are recommended choices.
FUEL REQUIREMENTS The fuel requirements for a blower motor may vary greatly depending on the application and engine/blower specifications. Unleaded fuel is okay as long as the engine is setup for unleaded fuel. The "Final Compression Ratio" (see chart) of the engine/blower combination is the determining factor in fuel octane requirements. As a general rule, the maximum final compression ratio should not exceed 12.4 to 1 for 92 octane fuel. Octane boosters and higher octane racing fuel will allow you to run a higher final compression ratio. Final compression ratios should not exceed approximately 24-26:1 for racing gas.
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