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  #1 (permalink)  
Old 02-26-2009, 12:46 AM
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fabricating performance parts for a 305 GMC V6

first, i guess i should have a small introduction. i'm currently a college student studying to be an auto tech and welder (duel major) and working on a 1961 gmc with a 305 v6 engine. OK... now I've done some searching and haven't found any real definitive stuff on fabbing a sheet metal intake(mostly leave it to the pros...). now i know this engine is not going to have massive HP but i would like some performance gains and i would really like an upgrade form the one barrel holly carb that is on it now.

now from the little i have gathered the flanges seem to be the hardest parts to get a hold of, lucky me my school has a sweet machining course with a CNC mill as well as an excellent auto cad program, so i have had the flanges made (used the new intake gaskets as a template). i have also used the old intake manifold to make a jig. im going to be using mild steel and MIGing it:
1. because i have access to the welder
2. its easier
3. weight is not a factor
4. its not a bleeding edge, need to milk out another 5hp project
5. they don't make after market performance parts for a 305 V6
6. it looks wicked cool

that all being said how can i net the most gain from this, i am thinking about a cross flow design the only considerations i have are the total height with a carb and air cleaner and the width of the plenum since it has the plugs in the valley. i would like to be able to change them without pulling the intake.

now what considerations should i take in regards to the runner length?
can the be too long?
too short?
i will most likely be running a 2 or 4 barrel carb but nothing too radical, recommendations(low profile will probably be necessary)?
is there such a thing as to much flow?
i'm probably going have the manifold powder or ceramic coated once i have it finished, any preference or performance issues i should take into account?

im looking for a net of ~20hp but anything over stock will be awesome
also i want to eventually convert it over to EFI should i make considerations for the placement of the injector bungs?

i have looked at http://theoldone.com/ and they did give me some info but i was wondering if any one here had anything more. also i have used http://www.6066gmcguy.org/ as basically my bible concerning this engine/truck combo

now i know this is rather jumbled but im just trying to see if i have overlooked or have completely missed something due to ignorance

thank you in advance
five-oh

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Old 02-26-2009, 02:29 AM
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Not to sound negative, but I doubt fabbing a intake would be of much help. The heads have to be able to flow, they, with the cam are the heart of making power and along with them goes compression etc.
These brute engines use very heavy pistons and the crank is also heavy. They were made for pretty much torque use. But, torque is what moves the vehicle, and some diesels are indeed very fast even so they too use rather heavy pieces. You could likely go in and see if there`s any flow gains to be had by porting the heads but there likely isn`t a larger cam for this engine unless you have it custom ground ($$$) Next you can likely fabricate a set of headers for it also which would be of help. But the limiting factor is, your trying to hot rod a engine that wasn`t intended to be a hot rod. Not saying it can`t be done though.
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Old 02-26-2009, 12:18 PM
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OK a cam is probably not going to happen ... i am in college after all. headers are going to be next semesters project. i have not been able to do much on the truck since i am having to pay my way through school so this was a relatively cheep and hopefully easy project for me to do ~60 bucks for the metal, had the flanges cut out for free through the college machine shop, and basically all i have to do this semester in my auto class is build it (its a high performance/ modification class, all it has to do is work and boosts the trucks performance or aesthetics). porting the heads? possibly latter. when i have money, time and space to take apart the engine and rebuild it
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Old 03-11-2009, 09:57 AM
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'kay so its been about 2 weeks and not much has been posted to kinda help out with this soooo how should i design the runners should i leave them rectangular or i was thinking having them tapper into a triangle, going of course backwards from the heads to the plenum this should give me more space to have the runners cross under the plenum, this a good or a bad idea? also i need a suggestion for a short 2 or 4 barrel carb ~400cfm and relatively cheep also an electric choke would b a very high priority on it. also does anyone know more about the larger V6's from this period? do they have a more radical cam and would if fit in the smaller V6's or would they just bore/stroke them and leave a virtually identical cam in it? i was thinking maybe using the cam from either the 351, 351e, 351m, or the 379 because from the limited engine data i do have i would feel safe hazarding that its the same block just bored on the 351's and then bored and stroked on the 379 but i would really love confirmation.
also i was rather bothered by the "trying to hot rod a engine that wasn`t intended to be a hot rod" i would like to point out that the Model T's were never meant to be rodded either and now they are one of the most iconic hotrods out there, its a rare car show that doesn't have at least one T bucket in it. and most glaringly in modern times the honda civic. the civic was supposed to be an efficient people mover, now they are probably the most heavily modded cars on the road today. i dont know maybe i was reading into it too much but i kinda felt like he was trying to politely tell me to just buy a ZZ350 and move on
also i started a journal here

Last edited by GMCfive-0V6; 03-11-2009 at 10:14 AM.
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Old 03-11-2009, 10:30 AM
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It wasn`t my intention to make you feel that way but you can take it anyway you like. If the engine was meant to be a hot rod then the aftermarket would have picked up on making hot rod parts for it.
Comparing flat head engines and honda ricer engines to what you have is apples to oranges. I never said buy any engine, you can run whatever you like and it makes no difference to me what so ever.
Flat heads and honda ricer engines both can be modded quite a lot and still get there doors blown off by almost a stock small block.
If you want to focus getting power out of your 305 V6 then do as I suggested and focus on the torque the engine makes. Thats why this engine had a 1 barrel carb with long runners as well as a heavy rotating assembly, it was designed to make torque, not horsepower. Torque is what moves the vehicle, torque is what shreds tires. I`ll take a low end torquer engine any day over a 10,000 RPM high revver.
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Old 03-11-2009, 03:29 PM
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ok maybe it wasn't you but it has been the sentiment that i have been getting "rip the thing and drop a 350 in it" ,that kinda thing from friends and family it was kinda getting mildly irritating, i'm sorry i took it out on you. back to the topic tho the runners on the stock intakes were not very long i "should" be getting ~2 inches longer runners with the cross flow design i hope to be using. the one friend i have who is mildly supportive in this endeavor said i had asked the right questions now i just need the right answers
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Old 03-11-2009, 07:25 PM
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305 GMC preject.

There was a time when "hotrodders", had to virtually make a lot of their own parts; I think it was called "the old days". Some of them used odd engines as a starting point. The question about the old GMC has come up many times on this site. The way I see it, you're doing this for a grade in school, and also to be different. More "power" to you, and not in the just the vernacular either. You're not going to build any kind of screamer, and you already know that. First off, you're working on the intake and exhaust systems, freeing up things there can only help, everyone does that. An engine is an air pump, and the more air you can move, the better. Later, you can delve into the engine and heads, on the camshaft ( someone can regrind it ), do some portwork and angle milling ( be careful here with your use of steel to make your intake,as milling the heads also means milling the intake ). I'll bet this truck's exhaust will have a truley unique sound. Now, if it were me, I would conscider the turbocharging route, and even use the stock intake and exhaust manifolds. But, that's just me; good luck with your project, and see if you can find a pair of those "kilt like" painted valve covers. I looked at your intake manifold in progress, keep it up.

It looks to me like the stock intake manifold is of the open plenum design, it looks surprisingly a lot like the Pontiac 301 carbed intake, and looks to have siamesed ports also, like the Pontiac 301, save for yours being for a single throat carb. Maybe start with making an open plenum manifold, it would probably be the easiest.

Last edited by junior stocker; 03-11-2009 at 11:30 PM.
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Old 03-11-2009, 08:46 PM
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I sold a open plane aluminum SBC intake that had been bead blasted to 'newness" and was even Zyglo'ed for cracks, I couldn't give it away.

I finally had a guy pick it up for $50. Look around you will find similar deals for used AL intakes.

This fits into the "redesigning the wheel" category and although you have the time and inclination is not worth the hassle and medicore performance gains you will see with a fabbed intake manifold. Lots of testing went into the design of an intake manifold and you don't have resources to duplicate the effort.
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Old 03-12-2009, 12:10 AM
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I've looked at your proposed runner shape and while I can understand your wanting to lengthen the runners for increased torque at low rpm the shape you have planned is not going to work well because you are smaller at the carb end than you are at the cylinder head. This is the exact opposite of what you want. A very simple, overly-crude analogy is to look at a funnel--works good as intended, but worthless used in reverse. The runner should be at its largest cross section as it leaves the common plenum under the carb and gradually taper smaller as it gets closer to the intake valve (this is a very shallow taper, 2 to 4 degrees). Your probable best form is to imagine the factory intake if made out of rubber, hook onto the carb pad, and pull it up 3 to 4 inches. There is no reason to stay as low as the factory intake is unless you don't have the hood clearance to go taller.
I don't recall the name of it, but there is an enthusiast/GMC truck forum that specializes in these engines, you might try asking over at the H.A.M.B. forum.

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Old 03-12-2009, 02:35 AM
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Two words. Ram Induction.
http://www.300f.com/ram.htm
http://www.chrysler300club.com/uniq/...aboutrams.html
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Old 03-16-2009, 03:55 PM
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ok first ram induction = very sweet unfortunatly i dont have the RPMs to keep the Fuel suspended that long in the runner
next forced induction would be one of those things i may do latter and yah it does have a very unique exhaust note already, 2 to 1 going throuch a glass pack. once i get the headers made next year im going to go for true duel with cherry bombs
and last i cant go to a jegs catalog and point one out and say "i want that one" wait two weeks and bolt it on, happy as can be with my 1000+ hour dyno proven manifold. in my case there is no "wheel" to redesign here im just looking for some hints tips generalizations or basic formulas (even complex formulas, since i did manage to get through my colledge algebra course without much trouble). even a " i dont know anything more than you do kid, but give them hell" would be welcome anyway i have revised my runner shape and after spring break im going to start cutting out parts and tack welding posting new picks to my journal in a few
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Old 03-16-2009, 06:04 PM
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Ram induction is an excellent solution, IMHO. The formulae are available to figure the correct dimensions.

As for fuel atomization, correctly designed, this will not be an issue that can't be overcome. After all, the 413 in the Old Man's '61 New Yorker never (well, not often, anyway) saw the high side of 3K rpm, yet was strong. It's all in the dimensions. And adequate preheating of the intake.

If need be, electronic port injection FI could be adapted from a modern V6. Many old mechanical FI manifolds have in recent years been adapted for street use.
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Old 04-19-2009, 12:43 AM
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posted more pics in my journal, ive kinda neglected my forum here i decided to pass on the tunnel ram setup have been working my bum off i feel confident that i will have this done by the end of the semester.
any one know of a good company to do a ceramic coating and can anyone recommend a good carburetor roughly 400CFM preferably a 4barrel or even a 500cfm i can de-tune
just remember I'm on a college student budget
thanks in advance
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Old 04-21-2009, 02:21 PM
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i have a 63 with the same engine. hopefully all goes well for you. you know you could have just replaced your intake with a 351 v6 gmc intake for some extra umph. i hear the holley 4412 works great with these eingines its a 500 cfm and should bolt up to the stock intake with an adapter. check out this link http://www.6066gmcguy.org/holley.htm

i was looking for a 351 intake to install with my new carb. maybe i can get one from you if your interested in fabricating another one. good luck
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Old 04-22-2009, 08:02 PM
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well an intake from the 351 woulda been pretty good but they are kinda rare in my neck of the plains, besides this is wayy cooler. since im doing this in my auto shop and don't actually own my own welder it would end up being a while for me to make ya one, first i need to see if this one does anything... then i can go from there and honestly i wouldn't mind making another 1, so if ya still don't have one in in a year or two when i'm outa college ill see what i can do
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