Carb Adapter

[Mlcdc1]

Gave up on tuning the quadrajet and bought an Edelbrock 1906 for my 1977 C15 with 350 engine. Looks like there are a couple of options available for the adapter to fit it to my stock spreadbore intake. Any recommendations or opinions?

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[AuroraGirl]

Gave up on tuning the quadrajet and bought an Edelbrock 1906 for my 1977 C15 with 350 engine. Looks like there are a couple of options available for the adapter to fit it to my stock spreadbore intake. Any recommendations or opinions?

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show us your intake opening on your iron intake
you should also invest in the proper fuel line adapter so you can eliminate rubber hose on top of the engine, and a phenolic spacer. you also want to regulate fuel pressure to 6.5 psi or whatever. edelbrock fuel pump does this internally but a regulator would work. not having a return line with this change is a mistake.

 

[Ricko1966]

You should have a dual plane manifold,not an open plenum manifold,verify this. If it is true dual plane I'd run the 4 hole adapter. Running the open adapter will probably sacrifice some low end drivabilty.

 

[Mlcdc1]

Thanks for the info. It is the original stock manifold so it is a dual plane manifold. Mostly just cruising around vehicle - not a daily driver

 

[Confused]

On a stock cast iron intake they recommend this;

https://www.edelbrock.com/carb-adapter-for-4150-square-bore-to-stock-quadrajet-thermo-quad-spread-bore-2696.html

Or if you have some extra cash look for an Edelbrock 2701 new or used.

https://www.edelbrock.com/performer-eps-intake-manifold-2701.html

 

[AuroraGirl]

On a stock cast iron intake they recommend this;

https://www.edelbrock.com/carb-adapter-for-4150-square-bore-to-stock-quadrajet-thermo-quad-spread-bore-2696.html

Or if you have some extra cash look for an Edelbrock 2701 new or used.

https://www.edelbrock.com/performer-eps-intake-manifold-2701.html

nice intake, but it sounds like he wants to keep his stock intake

A phenolic gasket, fuel line adapter, regulator, return fuel line, and maybe even heat shield (also sold in addition to phenolic gasket) are great ways to make it work best but he may be willing to deal with the temperamentalness of it without that all

 

[bucket]

You should have a dual plane manifold,not an open plenum manifold,verify this. If it is true dual plane I'd run the 4 hole adapter. Running the open adapter will probably sacrifice some low end drivabilty.

Personally, I do not like the 4-hole adapters. The tiny and wide spread primary bores do not line up well with the square bore primary butterflies, especially before the butterflies hit wide open. I don't have the means to test it, but it has to really kill velocity.

I once ran a square bore Holley on a bone stock '84 305 and did not notice any loss in low end driveability with the open type adapter that I used.

Also, it's kind of apples to oranges, but have read and watched a lot of dyno testing of carb spacers on relatively mild small blocks. Generally, the open type spacers seem to work better with a dual plane intake and the 4-hole spacers often work better with a single plane intake. It tends to be backwards from what one might often think.

 

[bucket]

Another option is to modify a 4-hole adapter into a split open. It could be the best of both worlds. I made this one to test with, but there was an issue with my replacement carb and I never got back to it.

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[75gmck25]

I also recall an Engine Masters episode where they found an open hole spacer on a dual plane was a good combination. It got a little more power in the upper range, without losing any low end. You want a phenolic spacer or a wood spacer, not aluminum, to provide heat separation.

Also remember that the Edelbrock 1406 (and I assume the 1906) has a fuel fitting on top, and an electric choke, that gets in the way of a stock air cleaner base. That is why folks often use a 1/2” or 1” spacer under the stock air cleaner. Edelbrock does sell one full sized air cleaner that works without a spacer, but I’d have to look at mine to see the exact part number. The goofy looking small mesh air cleaners may also work, but they aren’t the best choice for flow.

 

[bucket]

I also recall an Engine Masters episode where they found an open hole spacer on a dual plane was a good combination. It got a little more power in the upper range, without losing any low end. You want a phenolic spacer or a wood spacer, not aluminum, to provide heat separation.

Also remember that the Edelbrock 1406 (and I assume the 1906) has a fuel fitting on top, and an electric choke, that gets in the way of a stock air cleaner base. That is why folks often use a 1/2” or 1” spacer under the stock air cleaner. Edelbrock does sell one full sized air cleaner that works without a spacer, but I’d have to look at mine to see the exact part number. The goofy looking small mesh air cleaners may also work, but they aren’t the best choice for flow.

Do you mean the "fly's eye" air cleaners? I think they look cool, but they are a terrible fire hazard if there's ever a backfire.

 

[Ricko1966]

Personally, I do not like the 4-hole adapters. The tiny and wide spread primary bores do not line up well with the square bore primary butterflies, especially before the butterflies hit wide open. I don't have the means to test it, but it has to really kill velocity.

I once ran a square bore Holley on a bone stock '84 305 and did not notice any loss in low end driveability with the open type adapter that I used.

Also, it's kind of apples to oranges, but have read and watched a lot of dyno testing of carb spacers on relatively mild small blocks. Generally, the open type spacers seem to work better with a dual plane intake and the 4-hole spacers often work better with a single plane intake. It tends to be backwards from what one might often think.

I read a very good,very in depth article years ago about true dual plane manifolds,carb spacers the whole deal. I've tried to find it and will continue to. It was a real world test that documented drivability and mpg on a test mule run through the same challenges . It was very interestin,again I'll see if I can find it again. As for just horsepower I'd agree an open spacer is the ticket,but the mpg difference or the ability to climb certain grades without downshifting was significant between the open vs. Divided. Hmm now you have me thinking. I've never considered a carbureator adapter as a performance modification,always thought of them as a budget method to swap on a carb,on a beater. I always pick an appropriate intake manifold. But an open adapter would increase plenum volume so probablywould work better for WOT performance

 

[bucket]

I read a very good,very in depth article years ago about true dual plane manifolds,carb spacers the whole deal. I've tried to find it and will continue to. It was a real world test that documented drivability and mpg on a test mule run through the same challenges . It was very interestin,again I'll see if I can find it again. As for just horsepower I'd agree an open spacer is the ticket,but the mpg difference between the open vs. Divided was significant.

That's actually one part of the test that I did, with the open adapter on the stock 305. I recorded my daily driven mileage for a year or more before the swap, as well as a year or more after the swap. Now granted, I also changed the carburetor, but I went from a Quadrajet to a Holley which should have hurt mileage. But I averaged 14.5 mpg, just like I did before.

I'll get a picture of the airflow issue with a square bore carb on a 4-hole, spread bore adapter soon.

And yes I know we are kind of having two conversations at once, lol. One about adapters and one about spacers.

 

[ChuckN]

Lots of good info here. Keep in mind that Edelbrocks use no more than 5-6 psi of fuel pressure.


I splurged and bought all the accoutrements from Edelbrock to avoid having to piece everything together. Edelbrock fuel pump (psi set for carb from factory) an AN line/braided hose that threads onto a hard line with an integrated fuel filter.

You obviously could do it much cheaper.

 

[Ricko1966]

That's actually one part of the test that I did, with the open adapter on the stock 305. I recorded my daily driven mileage for a year or more before the swap, as well as a year or more after the swap. Now granted, I also changed the carburetor, but I went from a Quadrajet to a Holley which should have hurt mileage. But I averaged 14.5 mpg, just like I did before.

I'll get a picture of the airflow issue with a square bore carb on a 4-hole, spread bore adapter soon.

And yes I know we are kind of having two conversations at once, lol. One about adapters and one about spacers.

And you're writing while I'm editing,see the brain juices are flowing now.

 

[Confused]

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I have used many of these to put a Holley/Edelbrock on a spread bore performer intake on my big block Dodge motors. Some say they are better on aluminum intakes but for what they cost it may be worth a try. A 1/2 phenolic or wood spacer on top of it may help with fuel percolation.