409 with 671 blower - Impala Tech
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post #1 of 10 (permalink) Old 01-24-2010, 11:32 PM Thread Starter
 
 
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409 with 671 blower

Hello,
I recently bought a car at an auction with a 409 and 671 blower and will not be able to touch it for about a month. I have never owned a supercharged car before and have many newb questions to ask about it before i even start. I have been doing a lot of reading but not necessarily finding the info that I am looking for. I am wanting a occasional weekend street/strip car that is reliable but obviously still make decent power. I was not able to talk to the seller and thus do not know much about the motor. I know that it has aluminum heads but don't know that much more than that because i have not had a chance to open the motor up. I am not sure exactly what heads i have. It has a turbo 400 with a 2500 stall converter. It has two eddy 650 carbs with a 1.5" spacer. I am hoping to use 93 to 95 octane gas. I am curious about what cam duration separation and lift would be optimal. I am hoping for 8:1 static compression ratio. What cc dished pistons should i get. With that setup how many lbs of boost would i be able to run and it still be reliable and drivable? What would this make my effective compression ratio? What would be a good drive ratio? I was thinking about 8-15% underdriven. Does that sound right? Can you give me a ballpark on hp and torque? Do you have any guess on what this guy would have put on there. Are there more or different questions i should be asking? Thank you for your help. Feel free to ask me more about more info because i am sure i left something out.
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post #2 of 10 (permalink) Old 01-25-2010, 12:45 PM
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you may want to post this over at 348-409.com They know a lot about the 409 engine.

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post #3 of 10 (permalink) Old 01-25-2010, 02:39 PM
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I bought my Nova from a guy with a '63 gasser Impala that had a blown 409 in it. It isn't gray primer from Pa. is it?

Don't listen to me, what do I know? I've only been doing this for 30 years.
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post #4 of 10 (permalink) Old 01-25-2010, 05:59 PM
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Welcome to the Team Chris!

Well for openers there is no such thing as a dished piston with a 409. They use a flat head with overhead valves (no combustion chamber in the head, it was in the piston). The only aluminum head made for the 409 to my knowledge is the new Edelbrock RPM head. You might have gotten lucky and also have an aftermarket World casting Merlin block for your 409 which when combined with a Scat or Eagle crank and rods would mean there isn't a Chevrolet part in your 409 with the whole motor being an aftermarket piece. The 6-71 would be a little small in my opinion for a motor displacing 409 cubes and capable of twisting up to 6,700 RPM. It can keep up but it means twisting the rotors faster than is optimal to keep heat down. I would have favored the more common 8-71 often used with big blocks to move the same quantity of air with larger pulleys to keep the rotor speed down.

The only thing unusual about a blower (aside from the fact the hood won't close without a hole in it) are the carbs that sit on top of the blower. They have to be boost referenced and tuned for the motor and blower combination. Ignition has to be boost referenced as well to pull out ignition with boost to keep from getting into detonation. I assume it is running now so I would leave it alone until you know more about tuning a blown motor.

Big Dave
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post #5 of 10 (permalink) Old 01-25-2010, 10:37 PM Thread Starter
 
 
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Well for openers there is no such thing as a dished piston with a 409. They use a flat head with overhead valves (no combustion chamber in the head, it was in the piston). I have read that I just didn't process it, that makes a lot of sence. Thats nice to know about the heads. I have never dealt with a 409. How many bolt main does an original 409 have. I thought it was 2. How many does the merlin have. I read on Car Craft that custom machined billet steel center main caps use stock outboard bolt locations and tap into beefy block webbing for inboard fasteners. http://www.carcraft.com/techarticles...placement.html Can you explain this too me please. I have been reading up on tuning but am getting a little confused can you give me a basic description for some ground work to be able to understand what i am reading about. I have posted this on 348-409.com but have not received much of a response. Nope it is not in an impala at all actually I just figured you guy were probably more knowledgeable about the 409 than most forums.
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post #6 of 10 (permalink) Old 01-26-2010, 07:32 AM
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409 like all of GM's motors up until the Marl IV BBC had two bolt mains.



It wasn't until 1968 that the SBC got it's first set of factory four bolts with the introduction of the large journal crank in the 350. An aftermarket four bolt main conversion uses the factory two bolt location and adds two bolts at a 45° degree angle that is threaded into the pan rail bulkhead.



Because there is water in this area the bolts can not be drilled too deep (but anything beyond the thickness of the bolt doesn't add much more in terms of clamping power).



The factory doesn't do this as the threads are difficult to start at that angle and most mechanics assemble anything and everything with an impact in their hand. The Merlin uses four bolts set vertically not at an angle, but they also add more material to the bottom of the block around the bulk heads, and set the registers deeper in the block (it has more of a lip than a stock block).

Once you leave normally aspirated engines behind and resort to a power adder your tuning skills need to improve as well as the heat in the combustion chamber (what makes power in our heat pumps we call engines) increases the chance of detonation is also increased. Detonation is auto ignition or dieseling and destroys pistons (but if you make the pistons strong enough as in a diesel engine it wouldn't matter). A gasoline engine is light because they do not use one inch thick piston tops (they are usually thinner than an eighth of an inch thick). The gasoline block is also lighter, using less metal in the bores and bottom end than a diesel block. This is why gasoline powered flight was the first choice of aviation and not the diesel engine (though the Nazi's during WWII used diesel powered bombers to compensate for a lack of gasoline).

Big Dave
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post #7 of 10 (permalink) Old 01-26-2010, 11:44 PM Thread Starter
 
 
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Is there an easy way to tell the difference between the GM block and the Merlin block.
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post #8 of 10 (permalink) Old 01-27-2010, 12:16 AM Thread Starter
 
 
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How much hp and torque can that GM block hold. How about the merlin?
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post #9 of 10 (permalink) Old 01-27-2010, 08:44 AM
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The GM block will have a GM casting numbers on it, and GM date codes that you can look up on line at MorTec.com. The World Merlin block will say Merlin above the timing cover and it is painted blue from the foundry, not Chevy Orange. The Merlin block has only been in production for about a half a year so I would think the seller would mention that in the ad for sale since the Merlin is a stronger casting than the factory block.

Almost all of the passenger blocks that have been hot rodded have failed in service. Just about the only blocks left are truck blocks that have survived up until now as they were cast just like the passenger car block (shared the same dimensions); but cast out of a tougher high nickel content nodular iron. The Merlin block is both beefier (more material in the main webs where the 348-409 blocks failed), and cast out of high nickel content material.

Big Dave
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post #10 of 10 (permalink) Old 01-27-2010, 11:05 AM
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Chris it sounds like you're on the right track for most of your comments here. I've run a 671 on a 355 small block for 20 years on the street with occasional strip use. Your compression estimate is about where you want it. Same for the drive ratio. Depending on the chamber cc's of the heads, you should be pushing @ 10 psi of boost at 12% under drive. That's about where you want to be on a street engine. Roots type blowers have a hard time making more boost since they start to create heat with added boost which begins to diminish power. Hopefully the previous owner installed thick top blower pistons like the ones TRW offers. They are also coated. Another good tip when building a blower motor is to go with a plasma-moly top piston ring for the added heat you'll deal with. This ring requires a special cylinder hone.

If you have aftermarket heads on it you are in a good place as this is a big area for making more HP on a blown engine. I've been told you want to avoid a polished port though as the fuel tends to puddle up as opposed to remaining more atomized with a slightly rougher port finish.

The cam is another big area to build HP on a blown engine. As far as cam grind; talk to Dyers, BDS or Weiand for suggestions as it depends on a couple of parameters. A blower cam should have long duration with lift being less of a concern. You have to watch your overlap though.

There are some good books out there about street supercharging and they will include chapters on how to build/tune the carbs if they're not already done. Again that is another area that a blower shop can help you with. Basically speaking, the carb areas that need attention are jetting, power valves, some air bleeds and if you run vacuum secondaries it's best to run a balance tube between to the diaphrams. Also as mentioned you have to set up the curve on your distributor very carefully to avoid detenation. Roots blowers are always creating boost and have no lag time like a turbo when you mash the pedal. You also want to avoid vacumm advance on a blown motor. MSD is manditory for a nice fat spark. The more advanced units can set up timing retard based on RPM's and boost.

-Scott
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