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GMC V6 and V12 Engines Engine repair and rebuilding

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  #31  
Old June 4th, 2016, 03:42 PM
bigblockv6 bigblockv6 is online now
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Default Re: 305E Intake mod.

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Originally Posted by bigblockv6 View Post
I've never had a detonation problem with the HEI. Also I always wanted to find spark plugs for my 478 that's in my 68 now that would have the .060 gap so I found AC 44XLS6 were compatible.
A 750 cfm carb would be too much carburetion for a 305 V6, 650 cfm would be the maximum
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  #32  
Old June 4th, 2016, 07:46 PM
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Default Re: 305E Intake mod.

Ah ok, That's a big gap! I think mine are at .040 or .045. Just pulled some plugs again after hauling a couple heavy loads of lumber....load on truck and load on trailer....plugs are still looking new. Did notice some carbon build up on one around the ridge....but nothing to be concerned about. I'll post some pictures of the load in my build thread. I was joking about a 750. haha The 600 is plenty for that motor, not enough RPM to justify the air....but those jugs sure draw some vacuum!
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  #33  
Old June 4th, 2016, 08:12 PM
bigblockv6 bigblockv6 is online now
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Default Re: 305E Intake mod.

600CFM definitely sounds good for the 305, is there a noticeable increase in acceleration when the secondaries kick in? When I was 17 years old I tried a Holley 780 dual feed carb on the 68 with that same adapter you first used, vacuum leaks, fluctuating idle and excessive gas consumption. No way would it work so back went on the Holley 500 2bbl
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  #34  
Old June 4th, 2016, 09:02 PM
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Default Re: 305E Intake mod.

You can feel them kick in I know that. lol Mainly depends on RPM....I'll gun it from 2000 to 3000 and that's about it. But it pulls much better now that the manifold is open! Here's a picture of the load in tow this morning. lol Unloaded those by hand......
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1964 GMC 1500 305E/sm420 4 barrel intake mod and dual exhaust.
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1966 Chevy C10 292 hotrod 6/TKO600
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  #35  
Old June 8th, 2016, 04:19 PM
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Default Re: 305E Intake mod.

Wow... quite the load there TJ!
I'd like to toy with the idea of adding a SMALL 4 bbl carb (Carter AFB or maybe Rochester Quadrajet, maybe Holley Street Avenger) to the 351, but on a 351, 600 cfm would really be the useable limit. These engines were intended for low rpm, they have no harmonic balancer even. Doing mods than are only beneficial at 4000+ rpm will tear up these engines.

I think a 351 may be able to get even better fuel mileage if I were to run a small 4 bbl on it.

600 cfm on a 305 is just overkill. You'd likely get better low end response with a smaller carb. Having your cam reground to add about .020" lift and running custom higher compression pistons to be able to run magnum heads on a 305 (otherwise the big chambers will drop your cr below useable), that would require a little more carburetion, and make great power.

The thing that always gets me in the hot rod automobile community vs vintage motorcycle community, is that the bigger automotive engines mask mixture issues more than a smaller bike engine, so people never bother changing out carb jetting to fine tune the engines!

I have a wide and o2 sensor and AFR gauge/controller tailpipe sniffer setup nowadays. I have yet to put it on the GMC, but will when I run the 351C with the Holley 500 conversion. I hope to add an X pipe to the exhaust at some point, and maybe get some custom headers made. Dual exhaust is overrated, that's why H pipes became common and now the superior X pipe dual setup is popular. Same reason everyone started making 4 into 1 headers for 4 cylinder race bikes following Pops Yoshimura's lead...
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  #36  
Old June 8th, 2016, 04:29 PM
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Default Re: 305E Intake mod.

Quote:
Originally Posted by Chuck78 View Post
Wow... quite the load there TJ!
I'd like to toy with the idea of adding a SMALL 4 bbl carb (Carter AFB or maybe Rochester Quadrajet, maybe Holley Street Avenger) to the 351, but on a 351, 600 cfm would really be the useable limit. These engines were intended for low rpm, they have no harmonic balancer even. Doing mods than are only beneficial at 4000+ rpm will tear up these engines.

I think a 351 may be able to get even better fuel mileage if I were to run a small 4 bbl on it.

600 cfm on a 305 is just overkill. You'd likely get better low end response with a smaller carb. Having your cam reground to add about .020" lift and running custom higher compression pistons to be able to run magnum heads on a 305 (otherwise the big chambers will drop your cr below useable), that would require a little more carburetion, and make great power.

The thing that always gets me in the hot rod automobile community vs vintage motorcycle community, is that the bigger automotive engines mask mixture issues more than a smaller bike engine, so people never bother changing out carb jetting to fine tune the engines!

I have a wide and o2 sensor and AFR gauge/controller tailpipe sniffer setup nowadays. I have yet to put it on the GMC, but will when I run the 351C with the Holley 500 conversion. I hope to add an X pipe to the exhaust at some point, and maybe get some custom headers made. Dual exhaust is overrated, that's why H pipes became common and now the superior X pipe dual setup is popular. Same reason everyone started making 4 into 1 headers for 4 cylinder race bikes following Pops Yoshimura's lead...
Thanks for the info Chuck! The overall performance was greatly increased, even with an over kill carb. The motor has no issues burning the fuel with stock points and a pertronix coil. I've lost a tad bit of low end, but not enough to where it's really noticeable. When towing I can shift at 2700 RPM and still pull through the gears whereas before I had to shift over 3000. That load I towed there I never shifted over 3000 rpm with! I may try getting a 500 edelbrock and jetting it down a bit, or a quadra bog. But I just recently pulled some plugs again and they still look really good! Gas mileage I got a rough estimate of 10. lol But I have been doing a lot of towing and stopping and going. No real highway driving. Part of that problem to would be the 4.11 gearing and 29 inch tires. I plan on 31's or 32's in the near future as I finally got a set of 16 inch rims. Even the inside of the exhaust is not as black as before. I've been playing with the idea of getting that MSD fuel injection unit. lol
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1964 GMC 1500 305E/sm420 4 barrel intake mod and dual exhaust.
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1977 Chevy C20 572 BBC/TH400
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  #37  
Old July 6th, 2016, 10:28 PM
Clyde the 65 Clyde the 65 is offline
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Default Re: 305E Intake mod.

Can this mod be completed on a 1960 single barrel intake?
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  #38  
Old July 6th, 2016, 10:44 PM
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Default Re: 305E Intake mod.

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Originally Posted by Clyde the 65 View Post
Can this mod be completed on a 1960 single barrel intake?
I don't believe so. Best thing to do would be to find either a 305 two barrel intake or a 351 magnum intake. The magnum is way easier to modify so that is why I went with it.
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1964 GMC 1500 305E/sm420 4 barrel intake mod and dual exhaust.
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1964 Chevy C20 292/SM420
1966 Chevy C10 292 hotrod 6/TKO600
1977 Chevy C20 572 BBC/TH400
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  #39  
Old July 6th, 2016, 11:34 PM
Clyde the 65 Clyde the 65 is offline
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Default Re: 305E Intake mod.

Thx, I have an extra 2 barrel and can do the mod on that. I guess the old 1 barrel is paperweight material now.
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  #40  
Old July 7th, 2016, 12:15 AM
bigblockv6 bigblockv6 is online now
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Default Re: 305E Intake mod.

The 1bbl manifold does not have a big enough plenum to accept any kind pf modifications. The best is the 66 and up Magnum manifold used on 351E,M,401M, 432 and 478M engines because theses particular manifolds have a 4bbl plenum that just needs to be opened up.
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